Tuesday, June 16, 2009
Wednesday, May 21, 2008
NFO Sinus Inverter VS PWM Frequency Converter
Prepared by Taisir Khalil
Positive sequence harmonics (7,13,19,25 etc.) create a magnetic field in the direction of rotation. The magnetic field developed by the fundamental harmonic (50 Hz) must be in the direction of rotation. Otherwise, the motor would run backwards. Therefore, the fundamental is a positive sequence harmonic. Negative sequence harmonics(5,11,17 ,23 etc.) develop magnetic fields in the opposite direction of rotation. This reduces torque and increases the overall current demand required for a given load.
The presence of harmonics in the output voltage of frequency converter results in excessive heat of the AC motor. A load designed to pull 25 amps at full load may draw 30 amps if the harmonic distortion is high. This additional current can lead to insulation damage and possibly a catastrophic failure
http://www.pdma.com/VFDtest.html
For example, the 5 th harmonic , is consuming electrical energy and wasting it as a heat in the motor with out giving any useful work, the system needs to consume more energy to balance its negative effect as the 5 th harmonic is trying to rotate the motors shaft in the opposite direction of the motors movement.
Negative effect of harmonics in the output signal of frequency converter
1- Motor heat , vibration and noise – short motor life
2-Common mode current
-Bearing currents .
-Instrumentation reference to ground
3- Torque ripple
Negative effect of pulse wave form in the output signal of frequency converter
1- Voltage wave reflection in long cables ( above 25-35 m)
That’s extremely important to be avoided in applications like :
- HVAC systems , heating, ventilation and air conditioning
- elevators
- Pumps
- Traction and conveyer systems
- Low voltage regulated power supply to medium voltage motor.
- Test floor and test stand applications.
- Shore to ship power for non-50 Hz systems.
Matching Frequency converter with AC electric motor
1- conventional PWM type F.C with used motor
Reflective wave between the PWM type F.C and the motor ( high dV/dT) in additional to harmonics content will damage the motor.
High dV/dT Effects
The rapid rate of voltage rise (dv/dt) at the leading edges of each output pulse of the PWM inverter, produces an uneven distribution of voltage within the motor windings. The result is a concentration of the voltage at the particular points of the winding causing abnormal stress leading to breakdown of the insulation. This phenomena has been described as "first coil breakdown" and is well documented.
Reflections in Long Lines & Cables
A long cable, in addition to resistance, has distributed inductance and capacitance, producing effects similar to a transmission line .
The high frequencies present in the output of PWM wave forms cause reflections in long conductors connecting the motors to the drives . Harmful effects with conductors as short as 10 meters have been observed. However, the effects are most severe with cables of lengths greater than 50 meters leading to the doubling of the applied voltage. This translates to voltage peaks approaching 1130 volts in 400 volt systems.
Cable Input & Output Voltage - Using No Filter
On the output of PWM inverters the voltage peak due to reflections in long cable lines can be 200% ofthe DC bus voltage value..
The combination of these two effects stresses the winding insulation considerably beyond design limits and has been known to shorten the insulation life and in some instances leads to early catastrophic failure of motors.
Possible solutions :
To add ( dV/dT) on the out put of the PWM type F.C or/and to derate the motor , either else , to get less work from the motor .
- The harmonics content, will continue to reflect its negative impact (as mentioned above) on the motor as heating ,vibration , noise , possible bearing damage and less efficiency.
For centrifugal fans and pumps , even a minor change in motors operating speed translates into a significant change in imposed load and annual energy consumption. Fan and pump “affinity” laws indicate that the power loading upon a motor by centrifugal loads varies as the third power or cube of its rotational speed. For example a seemingly minor 20rpm increase in motors rotational speed from 1450 to 1470 rpm .- can result in 3.8% increase in the load placed upon a motor driving a fan or a pump. In contrast, the quantity of air or water delivered varies linearly with speed .
http://www1.eere.energy.gov/industry/bestpractices/pdfs/mc-2463.pdf
Fans exhibit the best savings for the reasons shown:
1-In variable speed drive (VSD) systems, Volume is proportional to speed, while power is a function of speed cubed
2-Thus for half volume, we need half speed and one eighth power
3-Significant noise reduction is another strong benefit in many systems
Calculation example for 15 kW derated motor to 9 kW
If the (used) derated 15 kW motor will run with 17 A( rated current for 9 kW motor) its speed will be about 2960 rpm. In the same situation for 9 kW motor , it will run with rated speed 2949 rpm . the difference in speed rotation can be translated to 2.5 % energy consumption
Please see motors data used for calculation.
http://www.transdrive.co.uk/leroy_somer_2pole_selection_chart.htm
As shown above , derating the motor in retrofitting project and using patched solution like dV/dT filter to protect the standard motor will cost more money and reflects in less reliability . The best solution in this case is to feed the used motor with pure sine wave current and voltage without any derating , This solution is real and could be found by using NFO Sinus Inverter.
1- Conventional PWM – FC with new standard motor in new project
In this case , it is necessary to derate the motor and to add output filter(dV/dT) , or to use sine wave former on the out put of PWM-FC without derating of the standard motor.
From easy calculation, its clear that such patched solution is more expensive and less reliable than using NFO Sinus F.C with pure sine output waves forms signals.
Below is shown an example of price level for such solution :
25 Dec. 2007
To make a complete analyze of the drive system, it is necessary to study the mutual influence between its parts , those parts are:
1-Motor ( AC motor)
2-Frequency converter
3- Network supply
Mainly, in all applications of frequency converters , it is known that there is a negative impact on the network when frequency converter is connected to it, and there is standards in Europe to regulate that negative impact included in EN 61000-6-3 directive for using electrical equipment in residential, commercial and light industry environment, EN61000-6-2 in industrial environment.EN60601-1-2 in hospitals.
For this reason it is necessary to include EMC filter in the drive system, that filter can be added between the drive and the connection point to the network ( mainly , all drives manufacturers must to do that) or to have it built in the drive as NFO Drives AB does
AC Motor
The motor could be old (used) as in retrofitting projects or new as in any project will be planned .In new project ,the motor could be chosen standard type or inverter duty type.
The inverter duty type motor is constructed to withstand voltage spikes up to approximately 2 times its nominal DC voltage say 1130V for nominal AC voltage 400V .
Frequency converter could be PWM type converter ,PWM type + patched solution as (dV/dT filter, Sine wave filter or sine wave former) or NFO Sinus Inverter
Character of wave form on the output of PWM type Frequency converter
PWM- FC has a wave form that appears as a choppy squared-off wave when viewed through an oscilloscope .
Such output voltage , will be a source of harmonics currents as it is shown on the following fig.
Positive sequence harmonics (7,13,19,25 etc.) create a magnetic field in the direction of rotation. The magnetic field developed by the fundamental harmonic (50 Hz) must be in the direction of rotation. Otherwise, the motor would run backwards. Therefore, the fundamental is a positive sequence harmonic. Negative sequence harmonics(5,11,17 ,23 etc.) develop magnetic fields in the opposite direction of rotation. This reduces torque and increases the overall current demand required for a given load.The presence of harmonics in the output voltage of frequency converter results in excessive heat of the AC motor. A load designed to pull 25 amps at full load may draw 30 amps if the harmonic distortion is high. This additional current can lead to insulation damage and possibly a catastrophic failure
http://www.pdma.com/VFDtest.html
For example, the 5 th harmonic , is consuming electrical energy and wasting it as a heat in the motor with out giving any useful work, the system needs to consume more energy to balance its negative effect as the 5 th harmonic is trying to rotate the motors shaft in the opposite direction of the motors movement.
Negative effect of harmonics in the output signal of frequency converter
1- Motor heat , vibration and noise – short motor life
2-Common mode current
-Bearing currents .
-Instrumentation reference to ground
3- Torque ripple
Negative effect of pulse wave form in the output signal of frequency converter
1- Voltage wave reflection in long cables ( above 25-35 m)
That’s extremely important to be avoided in applications like :
- HVAC systems , heating, ventilation and air conditioning
- elevators
- Pumps
- Traction and conveyer systems
- Low voltage regulated power supply to medium voltage motor.
- Test floor and test stand applications.
- Shore to ship power for non-50 Hz systems.
Matching Frequency converter with AC electric motor
1- conventional PWM type F.C with used motor
Reflective wave between the PWM type F.C and the motor ( high dV/dT) in additional to harmonics content will damage the motor.
High dV/dT Effects
The rapid rate of voltage rise (dv/dt) at the leading edges of each output pulse of the PWM inverter, produces an uneven distribution of voltage within the motor windings. The result is a concentration of the voltage at the particular points of the winding causing abnormal stress leading to breakdown of the insulation. This phenomena has been described as "first coil breakdown" and is well documented.
Reflections in Long Lines & Cables
A long cable, in addition to resistance, has distributed inductance and capacitance, producing effects similar to a transmission line .
The high frequencies present in the output of PWM wave forms cause reflections in long conductors connecting the motors to the drives . Harmful effects with conductors as short as 10 meters have been observed. However, the effects are most severe with cables of lengths greater than 50 meters leading to the doubling of the applied voltage. This translates to voltage peaks approaching 1130 volts in 400 volt systems.
Cable Input & Output Voltage - Using No Filter
On the output of PWM inverters the voltage peak due to reflections in long cable lines can be 200% ofthe DC bus voltage value..
The combination of these two effects stresses the winding insulation considerably beyond design limits and has been known to shorten the insulation life and in some instances leads to early catastrophic failure of motors.
Possible solutions :
To add ( dV/dT) on the out put of the PWM type F.C or/and to derate the motor , either else , to get less work from the motor .
What’s wrong with such solution?
-The filter(reactor) seems to be that it will protect the motor from the high speed rise of the voltage ( high dv/dt), but the voltage value ( surge voltage) will continue to stress the motors windings with a fraction of the duoble DC voltage ( say 1130 V for 400 AC voltage), the reactor will limit the speed range regulation of the AC motor, applying vector control with feed back will be more complicated .More investment and more losses.
- The harmonics content, will continue to reflect its negative impact (as mentioned above) on the motor as heating ,vibration , noise , possible bearing damage and less efficiency.
For centrifugal fans and pumps , even a minor change in motors operating speed translates into a significant change in imposed load and annual energy consumption. Fan and pump “affinity” laws indicate that the power loading upon a motor by centrifugal loads varies as the third power or cube of its rotational speed. For example a seemingly minor 20rpm increase in motors rotational speed from 1450 to 1470 rpm .- can result in 3.8% increase in the load placed upon a motor driving a fan or a pump. In contrast, the quantity of air or water delivered varies linearly with speed . http://www1.eere.energy.gov/industry/bestpractices/pdfs/mc-2463.pdf
Fans exhibit the best savings for the reasons shown:
1-In variable speed drive (VSD) systems, Volume is proportional to speed, while power is a function of speed cubed
2-Thus for half volume, we need half speed and one eighth power
3-Significant noise reduction is another strong benefit in many systems
Calculation example for 15 kW derated motor to 9 kW
If the (used) derated 15 kW motor will run with 17 A( rated current for 9 kW motor) its speed will be about 2960 rpm. In the same situation for 9 kW motor , it will run with rated speed 2949 rpm . the difference in speed rotation can be translated to 2.5 % energy consumption
Please see motors data used for calculation.
http://www.transdrive.co.uk/leroy_somer_2pole_selection_chart.htm
As shown above , derating the motor in retrofitting project and using patched solution like dV/dT filter to protect the standard motor will cost more money and reflects in less reliability . The best solution in this case is to feed the used motor with pure sine wave current and voltage without any derating , This solution is real and could be found by using NFO Sinus Inverter.
1- Conventional PWM – FC with new standard motor in new project
In this case , it is necessary to derate the motor and to add output filter(dV/dT) , or to use sine wave former on the out put of PWM-FC without derating of the standard motor.
Please take a look on the following link:
http://pdf.directindustry.com/pdf/mte/mte-corporation-catalog/17137-27790.html
From easy calculation, its clear that such patched solution is more expensive and less reliable than using NFO Sinus F.C with pure sine output waves forms signals.Below is shown an example of price level for such solution :
For 15 kW motor, adding sine wave former will consume additional 650W in every hour of its work !!( about 850 USD/ year)
In addition , using sine wave former will limit the speed range and make it more complicated to apply Vector control .
Typical relative costs-Drive and preventative measures ( Motor = 100%)
Reference:http://www.gambica.org.uk/pdfs/Report1_3rd%20Edition.pdf
1- Using conventional PWM-F।C with inverters duty motor
In this case, there is no need for oversizing the motor , no need for dV/dT filter , but the PWM – FC still continue to deliver square waves form output signals to the motor, and include harmonics content, which will be reflected in heat losses equal to (5-8) % as wasted energy of the motors power rating .
Other wise , it needs to apply patched solution and to add Sine Wave Former, which means, that the same shown above calculation will be applied !!
At NFO Drives AB , we have the right solution , 
A small additional capital investment in NFO Sinus Inverter means :
1- Elimination of motor overheating, vibration and noise ( increase motor life)
2- Elimination of harmonic content on the output of NFO sinus ( pure sine wave) ,no losses ,no need for output filters
3- No oversizing of old (used) or new standard AC motor.
4-Elimination of voltage wave reflection ,no voltage spikes in the output of NFO Sinus Inverter (no need for dV/dT filter)
5- No bearing currents ( we give 5 years guarantee for any new motor)
6- Elimination of torque ripple.
7 – Elimination of instrumentation reference to ground. ( RSD or earth leakage protection)
8-Economic solution to protect the motor and save electric energy.
Conclusion.
It is a fact that PWM- FC s can damage the insulation system of your motor. However, with better understanding, you should be able to prevent the infant mortality that so many have seen. Unless you’re looking for a reason to replace the motor, you should not install a PWM FC to an aged class B insulation system. Drive manufacturers of PWM- FC have increased effectiveness through PWM technology to deliver close to a clean current waveform to your motor. But that current comes with the expense of very fast rise time (dV/dT) and harmonics content.
Looking for a patched solution by adding sine wave output filter will protect the motor , but it will result in more capital investment and continuous additional consumption of electric energy , which could cost, in one year, more than 35% of the total purchasing price of the drive.
To avoid all above mentioned headaches and to save your money , NFO Drives AB offer you the solution by using NFO Sinus F. C and the particular, expensive part in it is the (NFO Sinus- Switch) which delivers a clean sine(pure sine wave form) voltage to your motor.
Weather your project is new or retrofitting type , your motor premium or standard one . In all cases with NFO Sinus F.C , your application will work in perfect way for long time with minimum preventive maintenance and saving your money and effort !!
Thursday, May 01, 2008
NFO’s unique technology
Conventional technology generates AC with variable frequency by using Pulse-Width Modulation, (PWM). All competitors use PWM-technology in all competing products. With PWM, the voltage is rapidly switched into short pulses with variable width in such a way that the average of the voltage will approximately be equal to what the motor recieve from a sinus curve. This leads to several drawbacks.
NFO Drives AB is the only company that uses patented technology to generate a pure sinusoidal voltage output directly, without any filters.
Complete absence of radio interference
With a pure sinusoidal voltage, problems with radio interference and ground currents are completely eliminated. The risk for insulation break-through and damage to bearings from electro-erosion are reduced to a minimum. Switching losses in the cables are completely eliminated. The only limitation to cable length is set by its resistance.
PWM-pulses are reflected at the end of cables. Such reflections may cause voltage spikes with a voltage up to twice the DC link voltage 1130V in 400 v system. The cable insulation and the stator winding may be damaged, especially in old cables not designed to withstand this kind of over-voltage. Older motors may not have sufficient insulation. In some motors, the bearings may rapidly be damaged by electro-erosion.
PWM systems will cause severe interference with radio / TV, mobile phones, wireless networks and similar systems. To avoid this, external filters and shielded cables will be required whenever conventional frequency converters are used. A shielded cable will increase the load, and the converter shall have to be placed near the load, not where the user might prefer to have it, for instance together with other electrical installations in the basement.
NFO’s Natural Field Orientation (NFO) makes it possible to control the speed of an asynchronous motor exactly to the required rpm, which otherwise would require an additional sensor and extra control electronics. With NFO, the motor will deliver its rated torque from start and at any speed. In many applications this will make it possible to select a smaller and less expensive motor than when PWM is used.
Certified EMI performance
Mobile phones, personal computers, the Internet, Satellite-TV and wireless local area networks are becoming ubiquitous. To safeguard the proper function of such devices, the European CE-mark, the EMC-Directive (EMC- Electromagnetic Compatibility) and the U.S. FCC regulations were adopted. They define the maximum amount of Electromagnetic Interference, EMI, electronic equipment is allowed to emit, and also how much EMI the equipment must withstand without impeding its functionality.
NFO’s frequency converters are interference-free. The NFO Sinus Inverter G2 are the only converters that are certified according to the Medical Directive for hospital equipment. Most manufacturers are not even certified for use in office buildings or light industry. In contrast to NFO devices, additional measures to reduce interference become mandatory.
Wednesday, April 30, 2008
Motor noise driven by conventional Frequency Converter
All motors generate audible noise when operating. The AC power applied to the motor is an important factor determines how much noise is generated.
The noise originates from the motor cooling fans, the motor bearings, and the humming of the stator laminations excited by the applied power.
Motors sound is regulated by NEMA MG1-1993 standard ,Part 12.53 covers machine sound of medium induction motors. Part 12.53.1 explains that although these standards define the acceptable sound power level of motors.
IEEE Standard 85-1973 is a test procedure for measuring airborne sound of rotating electrical machinery. It applies to unloaded motors mounted in controlled environments operating at rated speed and voltage. It recommends that the user and tester agree upon the following:
1-Mounting – Errors : will be introduced into the sound measurement if the motor vibrations cause the base or floor to vibrate.
2. Method of Loading : The connected load induces error by contributing to the overall sound measured.
3. Background Noise : Any background noise in the frequency range of interest contributes to the overall sound measured inducing error.
4. Accuracy of Measurements : The type of equipment used and how it is used will yield different results for the same machine under the same load.
5. Power Input Requirements : The referenced standards are for a motor running with sinusoidal power at full voltage and rated speed. If other conditions are to be measured, a complete description of those conditions needs to be agreed upon.
6. Interpretation of Data : Both user and tester need to understand what is being measured, how it is being measured, external influences, and the accuracy of measurements in order to obtain useful data.
Since most users are interested in the actual operating conditions of their facility, IEEE 85-1973 would apply only in conjunction with NEMA MG3-1974. The NEMA MG3 standard gives users tools to estimate sound levels in commercial and industrial environments. It leads a user through steps to calculate the sound pressure levels workers may be exposed to after taking measurements of individual motors per IEEE 85 and applying the proper correction factors or adjustments.
Audible motor noise
The audible noise produced by a motor originates from its stator core laminations. The stator core is made up of thin laminated metal sheets. When a 50 Hz sine wave voltage is applied to a motor, a magnetic flux is induced in the stator core. This magnetic flux causes the stator to vibrate 50 times per second producing a low pitch noise similar to that of a transformer.
When a motor is powered from an adjustable frequency drive using a PWM (Pulse Width Modulated) output waveform, the audible noise produced by the stator laminations has a different sound quality than with sine wave power.
The adjustable frequency drive produces an output voltage waveform made of high frequency pulses. The frequency of pulses is determined by the carrier frequency of the selected adjustable frequency drive. The motor stator core laminations vibrate at the carrier frequency changing the pitch of the audible noise. Whether the actual power level of the noise is increased due
to a PWM waveform will depend upon the level of the applied excitation voltage.
There are several solutions offered in the industry today to reduce audible motor noise when operating from a PWM adjustable frequency drive.
Some of these are:
1. Motor Location - In HVAC and pumping applications, the motor should be located in an equipment room away from personnel. Motor location is typically not a concern in industrial applications because of the other ambient noise associated with the driven machinery.
2.Totally enclosed non-ventilated or totally enclosed fan cooled motors will operate more quietly than open drip proof motors. The audible noise of first two types motors( totally enclosed non ventilated or fan cooled) motors is more contained in the motor housing compared to the open drip proof motor style construction.
3.Installing a reactor on the output of the drive will reduce the audible motor noise when low leakage reactance motors are used.
4.Select a drive that automatically adjusts its output voltage level to the motor load. The electrical motor audible noise will be reduced by lowering the effective motor voltage applied. This reduces the motor flux and resulting force on the stator laminations.
5.Select a drive that randomly modulates the carrier frequency 1 kHz above and below the center frequency. This improves the sound quality of the motor by not allowing the stator laminations to vibrate at a distinct pitch which the human ear can easily detect. It also reduces the possibility of the motor mechanically resonating at the carrier frequency which would amplify the audible noise.
6.Select a drive rated for low noise applications. These types of drives typically operate at a higher carrier frequency than other drives. The higher carrier frequency reduces motor current harmonics that contribute to stator lamination vibration and increased motor audible noise.
Selecting the proper motor type, its location, and a low noise type adjustable frequency drive will help reduce audible motor noise levels.
The NFO Sinus Inverter offer an effective solution to audible motor noise concerns. It uses a high carrier frequency(20-200KHz) output and a patented sine switch to produce a pure sinusoidal motor voltage & current waveforms -practically with out harmonics -and ensures that the stator laminations will not vibrate at a distinct pitch. That technique makes NFO Sinus inverter a unique choice for variable torque applications where audible motor noise is a concern.
ref. for motor noise:
http://ecatalog.squared.com/pubs/Motor%20Control/AC%20Drives/8839PD9702.pdf
When Ac motor is driven by conventional Frequency Converter - it is really noisy
Try to hear ( On the video )the sound (http://www.youtube.com/watch?v=uFeroZiGpU8)
And now compare the sound( on the video) ,when AC motor is driven by NFO Sinus Inverter From NFO Drives AB(http://www.youtube.com/watch?v=iccAqyb0KOk)
NFOrives AB / Sweden http://www.nfodrives.se/
In addition , please take a look on NFO Sinus test before delivery to customers:
http://www.youtube.com/watch?v=6qdOjAQs21k
The noise originates from the motor cooling fans, the motor bearings, and the humming of the stator laminations excited by the applied power.
Motors sound is regulated by NEMA MG1-1993 standard ,Part 12.53 covers machine sound of medium induction motors. Part 12.53.1 explains that although these standards define the acceptable sound power level of motors.
IEEE Standard 85-1973 is a test procedure for measuring airborne sound of rotating electrical machinery. It applies to unloaded motors mounted in controlled environments operating at rated speed and voltage. It recommends that the user and tester agree upon the following:
1-Mounting – Errors : will be introduced into the sound measurement if the motor vibrations cause the base or floor to vibrate.
2. Method of Loading : The connected load induces error by contributing to the overall sound measured.
3. Background Noise : Any background noise in the frequency range of interest contributes to the overall sound measured inducing error.
4. Accuracy of Measurements : The type of equipment used and how it is used will yield different results for the same machine under the same load.
5. Power Input Requirements : The referenced standards are for a motor running with sinusoidal power at full voltage and rated speed. If other conditions are to be measured, a complete description of those conditions needs to be agreed upon.
6. Interpretation of Data : Both user and tester need to understand what is being measured, how it is being measured, external influences, and the accuracy of measurements in order to obtain useful data.
Since most users are interested in the actual operating conditions of their facility, IEEE 85-1973 would apply only in conjunction with NEMA MG3-1974. The NEMA MG3 standard gives users tools to estimate sound levels in commercial and industrial environments. It leads a user through steps to calculate the sound pressure levels workers may be exposed to after taking measurements of individual motors per IEEE 85 and applying the proper correction factors or adjustments.
Audible motor noise
The audible noise produced by a motor originates from its stator core laminations. The stator core is made up of thin laminated metal sheets. When a 50 Hz sine wave voltage is applied to a motor, a magnetic flux is induced in the stator core. This magnetic flux causes the stator to vibrate 50 times per second producing a low pitch noise similar to that of a transformer.
When a motor is powered from an adjustable frequency drive using a PWM (Pulse Width Modulated) output waveform, the audible noise produced by the stator laminations has a different sound quality than with sine wave power.
The adjustable frequency drive produces an output voltage waveform made of high frequency pulses. The frequency of pulses is determined by the carrier frequency of the selected adjustable frequency drive. The motor stator core laminations vibrate at the carrier frequency changing the pitch of the audible noise. Whether the actual power level of the noise is increased due
to a PWM waveform will depend upon the level of the applied excitation voltage.
There are several solutions offered in the industry today to reduce audible motor noise when operating from a PWM adjustable frequency drive.
Some of these are:
1. Motor Location - In HVAC and pumping applications, the motor should be located in an equipment room away from personnel. Motor location is typically not a concern in industrial applications because of the other ambient noise associated with the driven machinery.
2.Totally enclosed non-ventilated or totally enclosed fan cooled motors will operate more quietly than open drip proof motors. The audible noise of first two types motors( totally enclosed non ventilated or fan cooled) motors is more contained in the motor housing compared to the open drip proof motor style construction.
3.Installing a reactor on the output of the drive will reduce the audible motor noise when low leakage reactance motors are used.
4.Select a drive that automatically adjusts its output voltage level to the motor load. The electrical motor audible noise will be reduced by lowering the effective motor voltage applied. This reduces the motor flux and resulting force on the stator laminations.
5.Select a drive that randomly modulates the carrier frequency 1 kHz above and below the center frequency. This improves the sound quality of the motor by not allowing the stator laminations to vibrate at a distinct pitch which the human ear can easily detect. It also reduces the possibility of the motor mechanically resonating at the carrier frequency which would amplify the audible noise.
6.Select a drive rated for low noise applications. These types of drives typically operate at a higher carrier frequency than other drives. The higher carrier frequency reduces motor current harmonics that contribute to stator lamination vibration and increased motor audible noise.
Selecting the proper motor type, its location, and a low noise type adjustable frequency drive will help reduce audible motor noise levels.
The NFO Sinus Inverter offer an effective solution to audible motor noise concerns. It uses a high carrier frequency(20-200KHz) output and a patented sine switch to produce a pure sinusoidal motor voltage & current waveforms -practically with out harmonics -and ensures that the stator laminations will not vibrate at a distinct pitch. That technique makes NFO Sinus inverter a unique choice for variable torque applications where audible motor noise is a concern.
ref. for motor noise:
http://ecatalog.squared.com/pubs/Motor%20Control/AC%20Drives/8839PD9702.pdf
When Ac motor is driven by conventional Frequency Converter - it is really noisy
Try to hear ( On the video )the sound (http://www.youtube.com/watch?v=uFeroZiGpU8)
And now compare the sound( on the video) ,when AC motor is driven by NFO Sinus Inverter From NFO Drives AB(http://www.youtube.com/watch?v=iccAqyb0KOk)
NFOrives AB / Sweden http://www.nfodrives.se/
In addition , please take a look on NFO Sinus test before delivery to customers:
http://www.youtube.com/watch?v=6qdOjAQs21k
Tuesday, April 29, 2008
Articles - good to read
1)Problems associated with using conventional PWM type frequency converter
http://www.pdma.com/VFDtest.html
2 ) Earth leakage protection devices to be used with drives - problems & solutions http://www.telemecanique.com/85256E540060851A/all/852566B70073220C85256E130056BB1C/$File/vvg998gb.pdf
3)NFO sensorless vector control is similar to (ABB http://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/Documentation/files/PRoducts/DTCTechGuide1.pdf, Emotron http://www.emotron.com/en/company/innovations/Direct-Torque/) DTC ( Direct Torque Control) - It is built in the same way based on having mathematical module of AC motor in the inverters memory, auto tuning parameters of the AC motor connected to the inverter ( once only) ,directly measuring and comparing the electromagnitic state of the motor and controlling the motors variables of torque and flux.
It works in the following way: Before running autotuning,it needs to enter the nominal motor data, parameters P-nom,U-nom,F-nom,N-nom,I-nom,and cos f. These are usually shown on the motor plate,and must be entered for the connection for which the motor is to be used(Y or D).
Once you have entered the parameters, you can run the Tuning command,which has to be confirmed to run. The motor parameters are then recorded and saved to the respective motor parameters.
http://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/Documentation/files/PRoducts/DTCTechGuide1.pdf
4) EMC Filter Guide
http://www.reo.co.uk/files/corel_designer_9_0_-_emc_draft_book.pdf
5) Motors windings Insulation problems in AC drives with PWM type frequency converters
http://www.vonroll.com/downloads/ANISIDM.pdf
6)Motor insulation Voltage Stresses under IGBT inverter operation
http://www.gambica.org.uk/pdfs/Report1_3rd%20Edition.pdf
7)Bearing currents under PWM inverter operation
http://www.gambica.org.uk/pdfs/Report2_2nd_ed.pdf
8) Предотбращение аварий двигателя при его подключении к Преобразобателя Частоты на IGBT
http://www.regr-is.ru/pdfs/nagr.pdf
http://www.pdma.com/VFDtest.html
2 ) Earth leakage protection devices to be used with drives - problems & solutions http://www.telemecanique.com/85256E540060851A/all/852566B70073220C85256E130056BB1C/$File/vvg998gb.pdf
3)NFO sensorless vector control is similar to (ABB http://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/Documentation/files/PRoducts/DTCTechGuide1.pdf, Emotron http://www.emotron.com/en/company/innovations/Direct-Torque/) DTC ( Direct Torque Control) - It is built in the same way based on having mathematical module of AC motor in the inverters memory, auto tuning parameters of the AC motor connected to the inverter ( once only) ,directly measuring and comparing the electromagnitic state of the motor and controlling the motors variables of torque and flux.
It works in the following way: Before running autotuning,it needs to enter the nominal motor data, parameters P-nom,U-nom,F-nom,N-nom,I-nom,and cos f. These are usually shown on the motor plate,and must be entered for the connection for which the motor is to be used(Y or D).
Once you have entered the parameters, you can run the Tuning command,which has to be confirmed to run. The motor parameters are then recorded and saved to the respective motor parameters.
http://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/Documentation/files/PRoducts/DTCTechGuide1.pdf
4) EMC Filter Guide
http://www.reo.co.uk/files/corel_designer_9_0_-_emc_draft_book.pdf
5) Motors windings Insulation problems in AC drives with PWM type frequency converters
http://www.vonroll.com/downloads/ANISIDM.pdf
6)Motor insulation Voltage Stresses under IGBT inverter operation
http://www.gambica.org.uk/pdfs/Report1_3rd%20Edition.pdf
7)Bearing currents under PWM inverter operation
http://www.gambica.org.uk/pdfs/Report2_2nd_ed.pdf
8) Предотбращение аварий двигателя при его подключении к Преобразобателя Частоты на IGBT
http://www.regr-is.ru/pdfs/nagr.pdf
Monday, April 21, 2008
Line & load Reactors in AC Drives
Adding reactor in a DC bus circuit of an AC drive, simply limits the rate of change of current in the circuit. Since current in an inductor wants to continue to flow at the given rate for any instant in time. That is to say, an instantaneous increase or decrease in applied voltage will result in a slow increase or decrease in current.
Conversely, if the rate of current in the inductor changes, a corresponding voltage will be induced. If we look at the equation V= L (di/dt) for an inductor where V is voltage, L is inductance and (di/dt) is the rate of change of current in amps per second, we can see that a positive rise in current will cause a voltage to be induced.
This induced voltage is opposite in polarity to the applied voltage and proportional to both the rate of rise of current and the inductance value. This induced voltage subtracts from the applied voltage thereby limiting the rate of rise of current. This inductance value is a determining factor of the reactance. The reactance is part of the total impedance for an AC circuit. The equation for the reactance of an inductor is XL = 2ПFL. Where XL is inductive reactance in Ohms, F is the applied frequency of the AC source and L is the inductance value of the reactor. As you can see, the reactance and there for the impedance of the reactor is higher with a higher inductance value. Also, a given inductance value will have a higher impedance at higher frequencies. Thus we can say that in addition to limiting the rate of rise in current, a reactor adds impedance to an AC circuit proportional to both its inductance value and the applied frequency.
Side-Effects of adding a Reactor:
Since a reactor is made of wire (usually copper) wound in a coil, it will have the associated losses due to wire resistance. Also, if it is an Iron core inductor (as in the case of most reactors used in power electronics) it will have some “eddy current” loss in the core due to the changing magnetic field and the iron molecules being magnetically realigned. In general a reactor will add cost and weight, require space, generate heat and reduce efficiency.
Sometimes the addition of a line reactor can change the characteristics of the line you are connected to.
Other components such as power factor correction capacitors and stray cable capacitance can interact with a line reactor causing a resonance to be set up. AC drives have exhibit a relatively good power factor and do not require the use of correction capacitors. In fact, power factor correction capacitors often do more harm than good where AC drives are present. For the most part, power factor correction capacitors should never be used with a drive. You may find that the addition of a reactor completes the required components for a line resonance where none previously existed, especially where power factor correction capacitors are present in such cases either the capacitor or the inductor must be removed.
Furthermore, reactors have the effect of dropping some voltage, reducing the available voltage to the motor and or input of the motor drive.
A Reactor at the Input to reduce Harmonics:
Most standard “six pulse” drives are nonlinear loads. They tend to draw current
only at the plus and minus peaks of the line. Since the current wave-form is not sinusoidal the current is said to contain “harmonics”. For a standard 3 phase input converter , using six SCR’s or six diodes and a filter capacitor bank as shown in figure below , the three phase input current may contain as much as 85% or more total harmonic distortion. Notice the high peaks
Fig 1
If there was ever a mandate to install an input reactor, it may be on a small drive where the transformer feeding it might be 20 times or more of the current or power rating of the drive. In some cases a large transformer (one with a low source impedance and or high short circuit capability) feeding a relatively small drive can result in overheating of the drive internal DC capacitor bank. When an NTC (negative temperature coefficient) pre-charge system is used, a large transformer feeding the drive can result in excessive inrush and clear line fuses or damage the drive. An input line reactor here will help. In this case, the reactor reduces harmonic current but the real reason for its’ presence is to limit the peak current that will flow at the input and in the capacitor bank.
Note : A reactor does not fix grounding issues nor does it provide isolation. Keep in mind that while a reactor provides some buffering, it does not provide isolation and can not take the place of an isolation transformer. If isolation is needed, an isolation transformer must be used. Also, it must be stated that while a reactor can provide light buffering from a short duration (less than 1 ms) transient condition, it will not fix a high line condition or protect against line swells (high line for several line cycles). Nor should it be expected to protect against high energy short duration events such as lightning strikes.
Reactors at the drive output to increase load inductance:
Applying a reactor at the output of a drive is sometimes necessary. Again, all of the “side-effects” s previously stated hold true. And yes, there are a few instances when it may be necessary to add load impedance by inserting an output reactor .If the motor has a “low leakage inductance” a reactor can help bring the total load inductance back up to a level that the drive can handle. In the days of the “Bipolartransistor” drive, carrier frequencies rarely exceeded 1.5Khz. This meant that the transistor “On time” was much longer. This allowed current to ramp up higher, limited by the load or motor inductance. The result of a low inductance motor was huge ripple current that sometimes ran into the current limit of the drive causing poor performance or tripping. For the most part, the higher carrier frequencies and correspondingly lower ripple current of today’s IGBT (Isolated Gate Bipolar Transistor) drives have eliminated the need to add inductance to the load.
Note : This is a main part of an article pubished by (Rockwell Automation-Mequon Wisconsin) with changes.
Conversely, if the rate of current in the inductor changes, a corresponding voltage will be induced. If we look at the equation V= L (di/dt) for an inductor where V is voltage, L is inductance and (di/dt) is the rate of change of current in amps per second, we can see that a positive rise in current will cause a voltage to be induced.
This induced voltage is opposite in polarity to the applied voltage and proportional to both the rate of rise of current and the inductance value. This induced voltage subtracts from the applied voltage thereby limiting the rate of rise of current. This inductance value is a determining factor of the reactance. The reactance is part of the total impedance for an AC circuit. The equation for the reactance of an inductor is XL = 2ПFL. Where XL is inductive reactance in Ohms, F is the applied frequency of the AC source and L is the inductance value of the reactor. As you can see, the reactance and there for the impedance of the reactor is higher with a higher inductance value. Also, a given inductance value will have a higher impedance at higher frequencies. Thus we can say that in addition to limiting the rate of rise in current, a reactor adds impedance to an AC circuit proportional to both its inductance value and the applied frequency.
Side-Effects of adding a Reactor:
Since a reactor is made of wire (usually copper) wound in a coil, it will have the associated losses due to wire resistance. Also, if it is an Iron core inductor (as in the case of most reactors used in power electronics) it will have some “eddy current” loss in the core due to the changing magnetic field and the iron molecules being magnetically realigned. In general a reactor will add cost and weight, require space, generate heat and reduce efficiency.
Sometimes the addition of a line reactor can change the characteristics of the line you are connected to.
Other components such as power factor correction capacitors and stray cable capacitance can interact with a line reactor causing a resonance to be set up. AC drives have exhibit a relatively good power factor and do not require the use of correction capacitors. In fact, power factor correction capacitors often do more harm than good where AC drives are present. For the most part, power factor correction capacitors should never be used with a drive. You may find that the addition of a reactor completes the required components for a line resonance where none previously existed, especially where power factor correction capacitors are present in such cases either the capacitor or the inductor must be removed.
Furthermore, reactors have the effect of dropping some voltage, reducing the available voltage to the motor and or input of the motor drive.
A Reactor at the Input to reduce Harmonics:
Most standard “six pulse” drives are nonlinear loads. They tend to draw current
only at the plus and minus peaks of the line. Since the current wave-form is not sinusoidal the current is said to contain “harmonics”. For a standard 3 phase input converter , using six SCR’s or six diodes and a filter capacitor bank as shown in figure below , the three phase input current may contain as much as 85% or more total harmonic distortion. Notice the high peaks
Fig 1
If a line reactor is installed as in figure 1, the peaks of the line current are reduced and somewhat broadened out. This makes the current somewhat more sinusoidal, lowering the harmonic level to around 35% when a properly sized reactor is used. This effect is also beneficial to the DC filter capacitors. Since the “ripple current” is reduced. The capacitors can be smaller, run cooler and last longer. Though harmonic mitigation is an important reason to use a line reactor, most drives at the 10 kilowatt rating and above include a “DC link choke” as seen in figure 1. The link choke is a reactor put in the DC bus between the Rectifier bridge and the capacitor bank. It can provide the necessary harmonic mitigation and since it is in the DC bus, it can be made smaller and cheaper than the 3 phase input reactor.
Small Drives may need an Input Reactor:
Generally drives less than 10 kW do not have a dc link reactor. And in most cases that’s not a problem since any harmonic current distortion would be small when compared to the total load of the facility. If many small drives are required for a process, an input reactor is a valid method in reducing harmonics. In the case of many small drives, it is often more economical and practical to connect a group of 5 to 10 drives through one large three phase reactor as shown in figure 2.
Small Drives may need an Input Reactor:
Generally drives less than 10 kW do not have a dc link reactor. And in most cases that’s not a problem since any harmonic current distortion would be small when compared to the total load of the facility. If many small drives are required for a process, an input reactor is a valid method in reducing harmonics. In the case of many small drives, it is often more economical and practical to connect a group of 5 to 10 drives through one large three phase reactor as shown in figure 2.
Fig 2
If there was ever a mandate to install an input reactor, it may be on a small drive where the transformer feeding it might be 20 times or more of the current or power rating of the drive. In some cases a large transformer (one with a low source impedance and or high short circuit capability) feeding a relatively small drive can result in overheating of the drive internal DC capacitor bank. When an NTC (negative temperature coefficient) pre-charge system is used, a large transformer feeding the drive can result in excessive inrush and clear line fuses or damage the drive. An input line reactor here will help. In this case, the reactor reduces harmonic current but the real reason for its’ presence is to limit the peak current that will flow at the input and in the capacitor bank.A Reactor as a line voltage buffer:
In some cases, other switch gear on the line such as contactors and disconnects can cause line transients, particularly when inductive loads such as motors are switched off. In such cases, a voltage spike may occur at the input to the drive that could result in a surge of current at the input. If the voltage is high enough, a failure of the semiconductors in the DC converter may also result. Sometimes a reactor is used to “Buffer from the line”. While a DC link choke, if present, will protect against a current surge, it cannot protect the converter from a voltage spike since a link choke is located after the converter (refer to figure 1). The Semiconductors are exposed to whatever line voltage condition exists. For this reason a reactor at the input to the drive may be of some help, but a better solution would be to attenuate the voltage spike at the source with a snubber circuit.
In some cases, other switch gear on the line such as contactors and disconnects can cause line transients, particularly when inductive loads such as motors are switched off. In such cases, a voltage spike may occur at the input to the drive that could result in a surge of current at the input. If the voltage is high enough, a failure of the semiconductors in the DC converter may also result. Sometimes a reactor is used to “Buffer from the line”. While a DC link choke, if present, will protect against a current surge, it cannot protect the converter from a voltage spike since a link choke is located after the converter (refer to figure 1). The Semiconductors are exposed to whatever line voltage condition exists. For this reason a reactor at the input to the drive may be of some help, but a better solution would be to attenuate the voltage spike at the source with a snubber circuit.
Figure 3 shows both methods being used to protect the drive input semiconductors.
Figure 3
Figure 3
Note : A reactor does not fix grounding issues nor does it provide isolation. Keep in mind that while a reactor provides some buffering, it does not provide isolation and can not take the place of an isolation transformer. If isolation is needed, an isolation transformer must be used. Also, it must be stated that while a reactor can provide light buffering from a short duration (less than 1 ms) transient condition, it will not fix a high line condition or protect against line swells (high line for several line cycles). Nor should it be expected to protect against high energy short duration events such as lightning strikes.Reactors at the drive output to increase load inductance:
Applying a reactor at the output of a drive is sometimes necessary. Again, all of the “side-effects” s previously stated hold true. And yes, there are a few instances when it may be necessary to add load impedance by inserting an output reactor .If the motor has a “low leakage inductance” a reactor can help bring the total load inductance back up to a level that the drive can handle. In the days of the “Bipolartransistor” drive, carrier frequencies rarely exceeded 1.5Khz. This meant that the transistor “On time” was much longer. This allowed current to ramp up higher, limited by the load or motor inductance. The result of a low inductance motor was huge ripple current that sometimes ran into the current limit of the drive causing poor performance or tripping. For the most part, the higher carrier frequencies and correspondingly lower ripple current of today’s IGBT (Isolated Gate Bipolar Transistor) drives have eliminated the need to add inductance to the load.
Refer to the comparison in figure 4.
Fig 4
In some rare cases where a strange motor configuration or a motor with 6 or more poles is used, the motor inductance may be too low and a reactor may be needed.
Running multiple motors on one drive may also result in a low inductance load and the requirement of an output reactor.
Reactors at the drive output to reduce the effect of reflected wave:
A reactor at the output of a drive is sometimes installed in order to prevent a reflected wave voltage spike when long motor leads are required. This is not always a good practice. Though the reactor will slope off the voltage rise time providing some benefit, It is not likely to limit the peak voltage at the motor. In some cases, a resonance can be set up between the cable capacitance and reactor that causes even higher voltages to be seen at the motor. In general, a motor terminator is a better solution. If a reactor is installed at the output, it is most likely part of a specially designed “reflected wave reduction” device that also has damping resistors in parallel. If a reactor is used at the output, it should be located as close to the drive end as is possible. Figure 5 shows the motor voltage before and after the installation of a reactor. The DC bus voltage is shown for reference.
Notice that the rise times are different, the peak voltage is about twice the DC bus voltage regardless of the use of a reactor.
For this reason , the best solution in this case s to feed the motor with sine wave output voltage- such solution is available from NFO Drives AB http://www.nfodrives.se/
The product is called NFO Sinus inverter
Fig 5
Output voltage of NFO Sinus Inverter
Since a current regulated drive requires “voltage margin” to regulate current, the output voltage is already limited by about 5%. Adding a reactor at the output will drop the voltage even further. A reactor at the output of this type of drive may not be a problem so long as the application can run without full motor voltage near full speed (typically 45to 50 hertz). In some cases a specially wound motor may be used to compensate. For example a 460 volt 150 amp motor may be rewound as a 400 volt 175 amp motor.
Sizing a reactor:
The first rule is make sure you have a high enough amp rating. In terms of the impedance value, you will usually find that 3% to 5% is the norm with most falling closer to 3%. A 3% reactor is enough to provide line buffering and a 5% reactor would be a better choice for harmonic mitigation if no link choke is present. Output reactors, when used, are generally around 3%. This % rating is relative to the load or drive where the reactor impedance is a % of the drive impedance at full load. Thus a 3% reactor will drop 3% of the applied voltage at full rated current. To calculate the actual inductance value we would use the following formula. L =X L/(2ПFL) Where L is inductance in Henrys, XL is inductive reactance or impedance in Ohms and F is the frequency. In general Frequency will be the line frequency for both input and output reactors.
Example of calculation:
l. If a 3% reactor was required for a 100 amp 480 volt drive, a 100 amp or larger current rating would be required. The drive impedance would be: Z=V/I or 480/100 = 4.8 ohms. 3% X 4.8 ohms = 0.114 ohms inserting this 0.114 impedance in the equation for inductance we get a value of about 300 Microhenrys .
Summary:
Reactors can prevent certain problems when they applied properly.
For the most part, a reactor at the input or output is not automatically required. Reactors can be helpful in providing some line buffering or adding impedance especially for drives with no DC link choke. For small drives they may be needed to prevent inrush or provide reduction in current harmonics when many small drives are located at one installation. At the output they should only be used to correct low motor inductance and not as a motor protection device.
Use a reactor:
To add Line Impedance.
To provide some light buffering against low magnitude line spikes.
To reducing Harmonics (When no link choke is present).
To compensating for a low inductance motor. Only as part of a filter for reflected wave reduction.
Sizing a reactor:
The first rule is make sure you have a high enough amp rating. In terms of the impedance value, you will usually find that 3% to 5% is the norm with most falling closer to 3%. A 3% reactor is enough to provide line buffering and a 5% reactor would be a better choice for harmonic mitigation if no link choke is present. Output reactors, when used, are generally around 3%. This % rating is relative to the load or drive where the reactor impedance is a % of the drive impedance at full load. Thus a 3% reactor will drop 3% of the applied voltage at full rated current. To calculate the actual inductance value we would use the following formula. L =X L/(2ПFL) Where L is inductance in Henrys, XL is inductive reactance or impedance in Ohms and F is the frequency. In general Frequency will be the line frequency for both input and output reactors.
Example of calculation:
l. If a 3% reactor was required for a 100 amp 480 volt drive, a 100 amp or larger current rating would be required. The drive impedance would be: Z=V/I or 480/100 = 4.8 ohms. 3% X 4.8 ohms = 0.114 ohms inserting this 0.114 impedance in the equation for inductance we get a value of about 300 Microhenrys .
Summary:
Reactors can prevent certain problems when they applied properly.
For the most part, a reactor at the input or output is not automatically required. Reactors can be helpful in providing some line buffering or adding impedance especially for drives with no DC link choke. For small drives they may be needed to prevent inrush or provide reduction in current harmonics when many small drives are located at one installation. At the output they should only be used to correct low motor inductance and not as a motor protection device.
Use a reactor:
To add Line Impedance.
To provide some light buffering against low magnitude line spikes.
To reducing Harmonics (When no link choke is present).
To compensating for a low inductance motor. Only as part of a filter for reflected wave reduction.
Note : This is a main part of an article pubished by (Rockwell Automation-Mequon Wisconsin) with changes.
Thursday, April 10, 2008
PWM frequency Converter with Earth leakage protection relay
Leakage current
Frequency Inverters are using high- speed switching devices for PWM control.
When a relatively long cable is used for power supply to an inverter , current may leak from the cable or the motor to the ground , because of its capacitance,adversely affecting peripheral equipment.The intensity of such a leakage current depends on the PWM carrier frequency, the lengths of the input and output cables,etc. , of the inverter, and it is related as by product to the nature of DC chopped voltage fed to the AC motor.
To prevent current leakage , it is recommended to use pure sine wave voltage inverters as NFO Sinus from NFO Drives AB
or to take the following measures , as a part solution.
Types of leakage current
1- Leakage due to the capacitance between the ground and the noise filter (EMC filter)
2- Leakage due to the capacitance between the ground and the inverter itself
3- Leakage due to the capacitance between the ground and the cable connecting the inverter and the motor.
4- Leakage due to the capacitance of the cable connecting the motor and any inverter in another power distribution line .
5- Leakage due to the grounding line common to the motors.
6- Leakage to another line because of the capacitance of the ground.
Those mentioned above leakage currents may cause the following troubles
Malfunction of a leakage circuit breaker in the same or another power distribution line
Malfunction of a ground-relay , installed in the same or another power distribution line.
Noise produced at the output of an electronicdevice in another power distribution line
Activation of an external thermal relay installed between the inverter and the motor , at a current below the rate current value.
Measures to eliminate the leakage currents or to reduce their effects
The best solution to eliminate the leakage currents is to feed the AC motor with pure sine wave voltage , but additional measures can be used against the effects of leakage currents as follows:
1- Measures to prevent the malfunction of leakage cicuit breakers
(1)-Decrease the PWM carrier frequency of the inverter, but that results in increasing the harmonic content in the output voltage signal fed to the motor and will last in additional motor heating.
(2)-Use radio- frequency interference- proof earth leakage circuit breaker( ELCBs) as ground-fault interrupters , not only in the system into which the inverter is incorporated but also in other systems ( that is expensive) . When ELCBs are used, the PWM carrier frequency needs to be increased to operate the PWM frequency inverter – this measure is in opposite to (1) mentioned above , so it is necessary to choose optimal carrier switching frequency !!
(3)- When connecting multiple inverters to a single ELCB, use an ELCB with a high current sensitivity ( expensive) or reduce the number of inverters connected to the ELCB.
2- Measures against malfunction of ground – fault relay :
(1) decrease the PWM carrier frequency of the inverter (results in additional heating of the driven motor)
(2) Install ground-fault relays with a high – frequency protective function ( means – grounding wire of each system separately to the grounding point).
(3) Ground ( shield) the main cicuit wires with metallic conduits – additional cost
(4) Use the shortest possible cables to connect the inverter to the motor , other wise it needs dV/dt , or sine wave filter
(5) If the inverter has a high-attenuation EMI filter ,turn off the grounding capacitor detachment switch to reduce the leakage current – Doing that leads to a reduction in the noise attenuating effect .
Ground fault
Before beginning operation , thoroughly check the wiring between the motor and the inverter for incorrect wiring or short circuit. Do not ground the neutral point of any star- delta connected motor.
Radio Interference ( noise produced by inverters)
All frequency converters PWM types produce noise ( NFO Sinus from NFO Drives AB exception – it is pure sine wave voltage output) and sometimes affects near by instrumental devices ,electrical and electronic systems, etc. The effects of noise greatly vary with the noise resistance of each individual device ( p.s EN 61000-6-3 directive for using electrical equipment in residential , commercial and light industry environment, EN61000-6-2 in industrial environment , EN 60601-1-2 in hospitals), its wiring condition, the distance between it and the inverter , etc.
Measures against noises
According to the route through which noise is transmitted, the noises produced by PWM frequency converter are classified into transmission noise , induction noise and radiation noise.
Examples of protective measures
separate the power line from other lines, such as weak- current lines signal lines , and install them apart from each other.
Install a noise ( EMC) filter in each inverter. It is effective for noise preventation to install noise filters in other devices and systems , as well. But the best is ti include the noise (EMC) filter inside the inverter itself.
Shield cables and wires with grounded metallic conduits ( expensive , not needed with NFO sinus inverter), and cover electronic systems with grounded metallic cases.
Separate the power distribution line of the inverter from that of other devices and systems.
Install the input and output cables of the inverter apart from each other.
Use shielded twisted pair wires for wiring of the weak- current and signal circuits,and always ground one of each pair wires.
Ground the inverter with grounding wires as large and short as possible, separately from other devices and systems
It is recommended to have built in noise ( EMC) filters ,which significantly reduce noise.
Frequency Inverters are using high- speed switching devices for PWM control.
When a relatively long cable is used for power supply to an inverter , current may leak from the cable or the motor to the ground , because of its capacitance,adversely affecting peripheral equipment.The intensity of such a leakage current depends on the PWM carrier frequency, the lengths of the input and output cables,etc. , of the inverter, and it is related as by product to the nature of DC chopped voltage fed to the AC motor.
To prevent current leakage , it is recommended to use pure sine wave voltage inverters as NFO Sinus from NFO Drives AB
or to take the following measures , as a part solution.
Types of leakage current
1- Leakage due to the capacitance between the ground and the noise filter (EMC filter)
2- Leakage due to the capacitance between the ground and the inverter itself
3- Leakage due to the capacitance between the ground and the cable connecting the inverter and the motor.
4- Leakage due to the capacitance of the cable connecting the motor and any inverter in another power distribution line .
5- Leakage due to the grounding line common to the motors.
6- Leakage to another line because of the capacitance of the ground.
Those mentioned above leakage currents may cause the following troubles
Malfunction of a leakage circuit breaker in the same or another power distribution line
Malfunction of a ground-relay , installed in the same or another power distribution line.
Noise produced at the output of an electronicdevice in another power distribution line
Activation of an external thermal relay installed between the inverter and the motor , at a current below the rate current value.
Measures to eliminate the leakage currents or to reduce their effectsThe best solution to eliminate the leakage currents is to feed the AC motor with pure sine wave voltage , but additional measures can be used against the effects of leakage currents as follows:
1- Measures to prevent the malfunction of leakage cicuit breakers
(1)-Decrease the PWM carrier frequency of the inverter, but that results in increasing the harmonic content in the output voltage signal fed to the motor and will last in additional motor heating.
(2)-Use radio- frequency interference- proof earth leakage circuit breaker( ELCBs) as ground-fault interrupters , not only in the system into which the inverter is incorporated but also in other systems ( that is expensive) . When ELCBs are used, the PWM carrier frequency needs to be increased to operate the PWM frequency inverter – this measure is in opposite to (1) mentioned above , so it is necessary to choose optimal carrier switching frequency !!
(3)- When connecting multiple inverters to a single ELCB, use an ELCB with a high current sensitivity ( expensive) or reduce the number of inverters connected to the ELCB.
2- Measures against malfunction of ground – fault relay :
(1) decrease the PWM carrier frequency of the inverter (results in additional heating of the driven motor)
(2) Install ground-fault relays with a high – frequency protective function ( means – grounding wire of each system separately to the grounding point).
(3) Ground ( shield) the main cicuit wires with metallic conduits – additional cost
(4) Use the shortest possible cables to connect the inverter to the motor , other wise it needs dV/dt , or sine wave filter
(5) If the inverter has a high-attenuation EMI filter ,turn off the grounding capacitor detachment switch to reduce the leakage current – Doing that leads to a reduction in the noise attenuating effect .
Ground fault
Before beginning operation , thoroughly check the wiring between the motor and the inverter for incorrect wiring or short circuit. Do not ground the neutral point of any star- delta connected motor.
Radio Interference ( noise produced by inverters)
All frequency converters PWM types produce noise ( NFO Sinus from NFO Drives AB exception – it is pure sine wave voltage output) and sometimes affects near by instrumental devices ,electrical and electronic systems, etc. The effects of noise greatly vary with the noise resistance of each individual device ( p.s EN 61000-6-3 directive for using electrical equipment in residential , commercial and light industry environment, EN61000-6-2 in industrial environment , EN 60601-1-2 in hospitals), its wiring condition, the distance between it and the inverter , etc.
Measures against noises
According to the route through which noise is transmitted, the noises produced by PWM frequency converter are classified into transmission noise , induction noise and radiation noise.
Examples of protective measures
separate the power line from other lines, such as weak- current lines signal lines , and install them apart from each other.
Install a noise ( EMC) filter in each inverter. It is effective for noise preventation to install noise filters in other devices and systems , as well. But the best is ti include the noise (EMC) filter inside the inverter itself.
Shield cables and wires with grounded metallic conduits ( expensive , not needed with NFO sinus inverter), and cover electronic systems with grounded metallic cases.
Separate the power distribution line of the inverter from that of other devices and systems.
Install the input and output cables of the inverter apart from each other.
Use shielded twisted pair wires for wiring of the weak- current and signal circuits,and always ground one of each pair wires.
Ground the inverter with grounding wires as large and short as possible, separately from other devices and systems
It is recommended to have built in noise ( EMC) filters ,which significantly reduce noise.
Power Factor improvement capacitors
It is not recommended to install power factor improvement capacitors on the input or output of the inverter .
Installing a power factor improvement capacitor on the input or output side causes current containing harmonic components to flow into the capacitor, adversely affecting the capacitor itself or causing the inverter to trip.To improve the power factor,install an input AC reactor or a DC reactor on the primary of the PWM frequency converter.
Installation of input reactors
These devices are used to improve the input power factor and suppress high harmonic currents and surges. Install an input AC reator when using frequency converter under the following conditions:
(1) When the power source capacity is at least 10 times or more greater than the frequency converter capacity .
(2) When the PWM (FC)frequency converter is connected to the same power distribution system as a thyristor – committed control equipment.
(3) When the FC is connected to the same power distribution system as that of distorted wave-producing system, such as arc furnaces and large-capacity frequency converters .
It is not recommended to install power factor improvement capacitors on the input or output of the inverter .
Installing a power factor improvement capacitor on the input or output side causes current containing harmonic components to flow into the capacitor, adversely affecting the capacitor itself or causing the inverter to trip.To improve the power factor,install an input AC reactor or a DC reactor on the primary of the PWM frequency converter.
Installation of input reactors
These devices are used to improve the input power factor and suppress high harmonic currents and surges. Install an input AC reator when using frequency converter under the following conditions:
(1) When the power source capacity is at least 10 times or more greater than the frequency converter capacity .
(2) When the PWM (FC)frequency converter is connected to the same power distribution system as a thyristor – committed control equipment.
(3) When the FC is connected to the same power distribution system as that of distorted wave-producing system, such as arc furnaces and large-capacity frequency converters .
Wednesday, April 02, 2008
VFD Need for Output Filtering
Eliminating Motor Failures Due to IGBT-Based Drives when Connected with Long Leads
The application of new generation Variable Frequency Drives, (VFD's), utilizing Insulated Gate Bipolar Transistors, (IGBT's), in the inverter section with motors connected by long leads has been a source for concern and expense. Motors controlled by VFD's installed some distance away often fail due to high voltage-induced insulation breakdown.
Nature of the problem .
Drives and motors often need to be separated by distance. Motors in mines and wells must be controlled above ground: the deeper the well, the longer the leads between the drive and the motor. In some plants, motors can withstand the harsh surroundings. However, sensitive VFD electronics cannot tolerate such environments, forcing long distances between the motor control centers that house the drives and the motors that they control. Conveyors and presses often utilize single drives to operate multiple motors that are positioned along the length of the conveyor. The length of the conveyor often dictates the longest distance between a drive and a motor.Most manufacturers of VFD's publish a maximum recommended distance between their equipment and the motor. The restriction of that maximum distance often makes application difficult, impractical, or unfeasible. Maximum tolerable distances vary by manufacturer, but might be 50 -100 m. Many users of VFD's have elected, or have been forced, to disregard the maximum recommended distance. These users are now replacing or rewinding motors after a 2-week, a 6-week, or a 6-month life span. In some cases, motor failure occurs even though the installation is within, but close to, the maximum recommended distance. Both the cost of these repairs and the downtimes that they demand are mounting quickly.The PWM VoltageVFD's generate the useful "fundamental" voltage and frequency via a modulation technique known as "Pulse Width Modulation (PWM)". For a 480V /400 system, the typical fundamental voltage ranges from 0 to 460/380 V and the fundamental frequency varies from 0 to 60/50 Hz. The inverter circuit "switches" rapidly, producing a carrier upon which is contained the useful fundamental voltage and frequency. The carrier, or switching frequency used for IGBT-based VFD, generally ranges between 800Hz to 15 kHz.Switching time is the time required for the IGBT inverter to transition from the "off" (high impedance) state to the "on" (low impedance) state and visa-versa. For the latest generation of IGBT's, the switching time varies from 100 to 200 nanoseconds,(ns). Because these devices are used in circuits fed by approximately 650 V DC, for a 480V(565 V DC for a 400 V - 50 Hz)system, the rate of change of voltage with respect to time, (dV/dT), can exceed 7500 volts per microsecond, (V/ms).IGBT'sThe relatively recent availability of high voltage, high current IGBT's has led to the wide use of these devices as the main switching element in the D-C to A-C inverter section of 1-phase and 3-phase AC Pulse Width Modulated VFD's. Virtually all of the manufacturers of these types of power conversion circuits have developed, or are developing, product lines that utilize these relatively new devices. One of the main reasons for the widespread use of these devices is their extremely fast switching time. This results in very low device transition losses and, therefore, in highly efficient circuits. In addition, a fast switching time allows drive carrier frequencies to be increased above the audible range. (Slower switching topologies operating at a range of 800Hz to 2kHz often induced irritating mechanical noise in a motor.)
The Reflected Wave Phenomenon
Voltage wave reflection is a function of the voltage rise time, (dV/dT), and of the length of the motor cables which behave as a transmission line. Because of the impedance mismatch at both ends of the cable, (cable-to-inverter and cable-to-motor), some portion of the waveform high frequency leading edge is reflected back in the direction from which it arrived. As these reflected leading edges encounter other waveform leading edges, their values add, causing voltage overshoots. As the carrier frequency increases, there are more leading edges present that "collide" into one another simultaneously, causing higher and higher voltage overshoots. If the voltage waveform was perfectly periodic, it might be possible to "tune" the length of the wire. However, since the width of the pulses varies throughout the PWM waveform, it is not possible to find any "null" points along the lead length where the motor may be connectedwithout the fear of damage.
The Resonant Circuit Phenomenon
Another way to analyze the problem is with respect to system resonance. Because multiple conductor wire runs contain both distributed series inductance and distributed parallel capacitance, the conductors can be viewed as a resonant tank circuit.In those applications where the physical length of conductors connecting the motor to the inverter exceeds 20 m., L and C values combine to form a typical resonant frequency range between 2 to 5 MHz, depending on wire characteristics. If the length is longer than 100 m, the resonant frequency will be lowered to the range of 500 kHz to 1.5 MHz. These self-resonant frequency ranges are at, or below, the high frequency components of the voltage waveform produced by the IGBT inverter. (A spectral analysis of the voltage waveform generated by inverters employing IGBT's would reveal frequency components ranging in excess of 1 to 2 MHz). Furthermore, whenever the self-resonant frequency of the conductors approximates the frequency range of the IGBT voltage waveform, the conductors themselves go into resonance. The conductor resonance then creates a "Gain", or an amplification of the voltage components at, or near, the conductor's natural resonant frequency. This results in voltage spikes at the waveform transition points. These voltage spikes can readily reach levels in excess of 2 to times the DC voltage feeding the inverter.
Voltage OvershootFor a 480 V system
It is common to find voltage spikes at the motor terminals ranging between 1200 to 1550 V. (575/600V systems are even more vulnerable, as peak voltages are further amplified by the higher system voltage.)Also, recall that these voltage spikes can have a rise time, dV/dT, in excess of 7500 V/ms. This can have an extremely detrimental effect on the motor windings and on the insulation system, often causing premature motor failure.
Most motor manufacturers believe that the life of the motor will be greatly extended by limiting both the magnitude of the voltage spikes to levels below 1000V and the dV/dT at the motor terminals to levels less than 1000 V/ms.
Motor Failures
Compare the Voltage Overshoot to a Mini-Dielectric Test.
All manufacturers of motors and of other electromagnetic components, such as inductors, perform one or two dielectric tests on their equipment during the manufacturing stage in an attempt to detect any defects in the insulation system components. For 600V class equipment, these tests consist of applying a relatively high voltage, 2500 to 3000V, for a short period of time. These types of tests stress the insulation system components and, if applied too many times or for too long a period of time, damage the insulation system. When long motor leads create a voltage overshoot, each spike acts like a little dielectric test. If enough of them occur, the insulation system will fail and the motor will need to be repaired or replaced.
Insulation Punch-Through Failures
Seldom, if ever, do large motors fail due to insulation punch-through. This is because they are usually "perfect" wound, which means that the location of each turn of wire in the phase winding is precisely controlled. Therefore, the level of voltage from turn to adjacent turn is controlled. In smaller motors, however, the wire size is quite small and the number of turns is large. Usually, these motors are "random" wound and do not lend themselves to control over the proximity of adjacent turns. Therefore, it is quite possible to have two turns of wire next to each other with a high voltage potential that is close to the maximum allowable limit of the insulation system. Even in the absence of an overshoot voltage, when a high dV/dT is applied, the insulation components may experience punch-through, causing motor failure.
Normally, these types of failures occur within hours or weeks of start-up.
Partial Discharge (Corona Inception) Failures
As the voltage associated with the high dV/dT increases, the likelihood of partial discharge, or "corona", also increases. When corona is present, highly unstable ozone,O3, is generated. This very reactive by-product then attacks the organic compounds in the insulation system. Corona can easily develop whenever the dV/dT and the resulting voltage overshoot are not controlled. Even the larger motors, whose turn-to-turn voltage can be controlled with "perfect" winding techniques, are vulnerable to corona. Overall, this corona effect will lead to motor failure.
Some Techniques for Correction
The addition of a Line Reactor
Applying a line reactor at the drive terminals has been attempted. Unfortunately, adding inductance merely reduces the resonant frequency of the total circuit. Because there are additional losses associated with the inductor, both in the copper and in the core, overall circuit dampening increases. This dampening may reduce the overshoot slightly, but it will also increase the duration of the overshoot voltage, applying additional stress on the motor windings.
Applying a line reactor at the motor terminals has also been attempted. Since line reactors and motors share common construction materials, line reactors applied in front of motors simply become sacrificial lambs. They will eventually fail due to the same voltage-induced stresses.
Carrier-Stripping Filters
A tuned low-pass filter can be designed to remove all carrier frequency voltages. These application-specific, custom filters were originally designed to strip low frequency carrier energy from Bipolar and Darlington transistor-based drives to limit audible motor noise. While this approach removes all frequencies above the fundamental, and affords the ultimate in motor protection, it comes at a severe price. These filters are large, costly, and consume large amounts of power. In addition, they reduce the fundamental voltage due to high inductor insertion losses and force the motor to draw higher fundamental currents to produce rated horsepower. Finally, the specific tuning frequency of a carrier-stripping filter greatly restricts the ability to alter carrier frequencies after installation. This limits fine-tuning of the drive application.
Voltage Clippers, Snubbers, Etc.
These energy-consuming devices must be applied at the motor terminals, which is difficult in most industrial and commercial applications. They require the addition of extra junction boxes or equipment enclosures as well as alterations and additions to the conduit scheme.
The best solution is to feed the AC motor with pure sine wave voltage , and that is available from NFO Sinus Inverter. http://www.nfodrives.se/
To read the original article , please refer to the link:
http://216.85.60.10/qd/Applicat.nsf/bf25ab0f47ba5dd785256499006b15a4/11794a749120b819862566ac0055c8ed?OpenDocument
The application of new generation Variable Frequency Drives, (VFD's), utilizing Insulated Gate Bipolar Transistors, (IGBT's), in the inverter section with motors connected by long leads has been a source for concern and expense. Motors controlled by VFD's installed some distance away often fail due to high voltage-induced insulation breakdown.
Nature of the problem .
Drives and motors often need to be separated by distance. Motors in mines and wells must be controlled above ground: the deeper the well, the longer the leads between the drive and the motor. In some plants, motors can withstand the harsh surroundings. However, sensitive VFD electronics cannot tolerate such environments, forcing long distances between the motor control centers that house the drives and the motors that they control. Conveyors and presses often utilize single drives to operate multiple motors that are positioned along the length of the conveyor. The length of the conveyor often dictates the longest distance between a drive and a motor.Most manufacturers of VFD's publish a maximum recommended distance between their equipment and the motor. The restriction of that maximum distance often makes application difficult, impractical, or unfeasible. Maximum tolerable distances vary by manufacturer, but might be 50 -100 m. Many users of VFD's have elected, or have been forced, to disregard the maximum recommended distance. These users are now replacing or rewinding motors after a 2-week, a 6-week, or a 6-month life span. In some cases, motor failure occurs even though the installation is within, but close to, the maximum recommended distance. Both the cost of these repairs and the downtimes that they demand are mounting quickly.The PWM VoltageVFD's generate the useful "fundamental" voltage and frequency via a modulation technique known as "Pulse Width Modulation (PWM)". For a 480V /400 system, the typical fundamental voltage ranges from 0 to 460/380 V and the fundamental frequency varies from 0 to 60/50 Hz. The inverter circuit "switches" rapidly, producing a carrier upon which is contained the useful fundamental voltage and frequency. The carrier, or switching frequency used for IGBT-based VFD, generally ranges between 800Hz to 15 kHz.Switching time is the time required for the IGBT inverter to transition from the "off" (high impedance) state to the "on" (low impedance) state and visa-versa. For the latest generation of IGBT's, the switching time varies from 100 to 200 nanoseconds,(ns). Because these devices are used in circuits fed by approximately 650 V DC, for a 480V(565 V DC for a 400 V - 50 Hz)system, the rate of change of voltage with respect to time, (dV/dT), can exceed 7500 volts per microsecond, (V/ms).IGBT'sThe relatively recent availability of high voltage, high current IGBT's has led to the wide use of these devices as the main switching element in the D-C to A-C inverter section of 1-phase and 3-phase AC Pulse Width Modulated VFD's. Virtually all of the manufacturers of these types of power conversion circuits have developed, or are developing, product lines that utilize these relatively new devices. One of the main reasons for the widespread use of these devices is their extremely fast switching time. This results in very low device transition losses and, therefore, in highly efficient circuits. In addition, a fast switching time allows drive carrier frequencies to be increased above the audible range. (Slower switching topologies operating at a range of 800Hz to 2kHz often induced irritating mechanical noise in a motor.)
The Reflected Wave Phenomenon
Voltage wave reflection is a function of the voltage rise time, (dV/dT), and of the length of the motor cables which behave as a transmission line. Because of the impedance mismatch at both ends of the cable, (cable-to-inverter and cable-to-motor), some portion of the waveform high frequency leading edge is reflected back in the direction from which it arrived. As these reflected leading edges encounter other waveform leading edges, their values add, causing voltage overshoots. As the carrier frequency increases, there are more leading edges present that "collide" into one another simultaneously, causing higher and higher voltage overshoots. If the voltage waveform was perfectly periodic, it might be possible to "tune" the length of the wire. However, since the width of the pulses varies throughout the PWM waveform, it is not possible to find any "null" points along the lead length where the motor may be connectedwithout the fear of damage.
The Resonant Circuit Phenomenon
Another way to analyze the problem is with respect to system resonance. Because multiple conductor wire runs contain both distributed series inductance and distributed parallel capacitance, the conductors can be viewed as a resonant tank circuit.In those applications where the physical length of conductors connecting the motor to the inverter exceeds 20 m., L and C values combine to form a typical resonant frequency range between 2 to 5 MHz, depending on wire characteristics. If the length is longer than 100 m, the resonant frequency will be lowered to the range of 500 kHz to 1.5 MHz. These self-resonant frequency ranges are at, or below, the high frequency components of the voltage waveform produced by the IGBT inverter. (A spectral analysis of the voltage waveform generated by inverters employing IGBT's would reveal frequency components ranging in excess of 1 to 2 MHz). Furthermore, whenever the self-resonant frequency of the conductors approximates the frequency range of the IGBT voltage waveform, the conductors themselves go into resonance. The conductor resonance then creates a "Gain", or an amplification of the voltage components at, or near, the conductor's natural resonant frequency. This results in voltage spikes at the waveform transition points. These voltage spikes can readily reach levels in excess of 2 to times the DC voltage feeding the inverter.
Voltage OvershootFor a 480 V system
It is common to find voltage spikes at the motor terminals ranging between 1200 to 1550 V. (575/600V systems are even more vulnerable, as peak voltages are further amplified by the higher system voltage.)Also, recall that these voltage spikes can have a rise time, dV/dT, in excess of 7500 V/ms. This can have an extremely detrimental effect on the motor windings and on the insulation system, often causing premature motor failure.
Most motor manufacturers believe that the life of the motor will be greatly extended by limiting both the magnitude of the voltage spikes to levels below 1000V and the dV/dT at the motor terminals to levels less than 1000 V/ms.
Motor Failures
Compare the Voltage Overshoot to a Mini-Dielectric Test.
All manufacturers of motors and of other electromagnetic components, such as inductors, perform one or two dielectric tests on their equipment during the manufacturing stage in an attempt to detect any defects in the insulation system components. For 600V class equipment, these tests consist of applying a relatively high voltage, 2500 to 3000V, for a short period of time. These types of tests stress the insulation system components and, if applied too many times or for too long a period of time, damage the insulation system. When long motor leads create a voltage overshoot, each spike acts like a little dielectric test. If enough of them occur, the insulation system will fail and the motor will need to be repaired or replaced.
Insulation Punch-Through Failures
Seldom, if ever, do large motors fail due to insulation punch-through. This is because they are usually "perfect" wound, which means that the location of each turn of wire in the phase winding is precisely controlled. Therefore, the level of voltage from turn to adjacent turn is controlled. In smaller motors, however, the wire size is quite small and the number of turns is large. Usually, these motors are "random" wound and do not lend themselves to control over the proximity of adjacent turns. Therefore, it is quite possible to have two turns of wire next to each other with a high voltage potential that is close to the maximum allowable limit of the insulation system. Even in the absence of an overshoot voltage, when a high dV/dT is applied, the insulation components may experience punch-through, causing motor failure.
Normally, these types of failures occur within hours or weeks of start-up.
Partial Discharge (Corona Inception) Failures
As the voltage associated with the high dV/dT increases, the likelihood of partial discharge, or "corona", also increases. When corona is present, highly unstable ozone,O3, is generated. This very reactive by-product then attacks the organic compounds in the insulation system. Corona can easily develop whenever the dV/dT and the resulting voltage overshoot are not controlled. Even the larger motors, whose turn-to-turn voltage can be controlled with "perfect" winding techniques, are vulnerable to corona. Overall, this corona effect will lead to motor failure.
Some Techniques for Correction
The addition of a Line Reactor
Applying a line reactor at the drive terminals has been attempted. Unfortunately, adding inductance merely reduces the resonant frequency of the total circuit. Because there are additional losses associated with the inductor, both in the copper and in the core, overall circuit dampening increases. This dampening may reduce the overshoot slightly, but it will also increase the duration of the overshoot voltage, applying additional stress on the motor windings.
Applying a line reactor at the motor terminals has also been attempted. Since line reactors and motors share common construction materials, line reactors applied in front of motors simply become sacrificial lambs. They will eventually fail due to the same voltage-induced stresses.
Carrier-Stripping Filters
A tuned low-pass filter can be designed to remove all carrier frequency voltages. These application-specific, custom filters were originally designed to strip low frequency carrier energy from Bipolar and Darlington transistor-based drives to limit audible motor noise. While this approach removes all frequencies above the fundamental, and affords the ultimate in motor protection, it comes at a severe price. These filters are large, costly, and consume large amounts of power. In addition, they reduce the fundamental voltage due to high inductor insertion losses and force the motor to draw higher fundamental currents to produce rated horsepower. Finally, the specific tuning frequency of a carrier-stripping filter greatly restricts the ability to alter carrier frequencies after installation. This limits fine-tuning of the drive application.
Voltage Clippers, Snubbers, Etc.
These energy-consuming devices must be applied at the motor terminals, which is difficult in most industrial and commercial applications. They require the addition of extra junction boxes or equipment enclosures as well as alterations and additions to the conduit scheme.
The best solution is to feed the AC motor with pure sine wave voltage , and that is available from NFO Sinus Inverter. http://www.nfodrives.se/
To read the original article , please refer to the link:
http://216.85.60.10/qd/Applicat.nsf/bf25ab0f47ba5dd785256499006b15a4/11794a749120b819862566ac0055c8ed?OpenDocument
Monday, March 31, 2008
NFO Синус преобразователь частоты
NFO Drives AB является шведская компания, расположенная в Svängsta (на средно юго-восточной побережье Швеции) производит преобразователи частоты с высоким качеством и новым стандартам, называемый NFO Синус.
http://www.nfodrives.se/en/index.htm
Благодаря двумя зарегистрированных патентов, наш продукт имеет следующие преимущества над другими типами ПЧ с ШИМ:
1 - (патент) NFO ориентация натурального поля
Асинхронный двегатель, дает полного крутящего момента на всех скоростях, даже при стационарным положением .
Вы никогда не потеряети контроль над двигателем .
Вы можете управлять несколькими двигателями от одного и того же NFO Синус преобразователя частоты .
Вы можете управлять различными по мощнстями двигателями из одного и того же NFO Синус преобразователя частоты .
Не нужен датчик обратной связи на двигатель .
Очень высокие динамичицкие характериcтики с NFO Синус ПЧ .
Нет разницы между вентилятором и насосом - полный крутящий момент всегда можно получить , при необходимости,
2 - (патент) NFO ( Синус - Переключатель )
Дает чисто синусоидальные выходныие сигнали (тока и напряжения) двигателя
Нет EMC вмешательства .
Нет необходимости в принимении доболнителных филтров как (Sine former), ни dv/dt .
Нет необходимости в принимении доболнителных индуктивностей.
Нет необходимости в принимении специалных EMC шкафов, или корпуов - Простой монтаж.
Нет необходимости в принимении экранированных кабелей- можно исползовать стандартныe кабели .
Нет ограничения на длину кабеля, помимо его сопротивления.
Нет проблем земли текущих токов - может эксплуатироваться с остаточным текущым выключателем.
Нет коммутационного шума переключения в двегателе - тихо работает. Двигатель ни перегревается - из за отсутствия высоких гармоник .
Полный срок сложбы двигателя без проблем в его изоляции
Нет необходимости в свыше калибровки двигателя (15-25%), как это делается при питания его от ПЧ с ШИМ
http://www.nfodrives.se/en/index.htm
Благодаря двумя зарегистрированных патентов, наш продукт имеет следующие преимущества над другими типами ПЧ с ШИМ:
1 - (патент) NFO ориентация натурального поля
Асинхронный двегатель, дает полного крутящего момента на всех скоростях, даже при стационарным положением .
Вы никогда не потеряети контроль над двигателем .
Вы можете управлять несколькими двигателями от одного и того же NFO Синус преобразователя частоты .
Вы можете управлять различными по мощнстями двигателями из одного и того же NFO Синус преобразователя частоты .
Не нужен датчик обратной связи на двигатель .
Очень высокие динамичицкие характериcтики с NFO Синус ПЧ .
Нет разницы между вентилятором и насосом - полный крутящий момент всегда можно получить , при необходимости,
2 - (патент) NFO ( Синус - Переключатель )
Дает чисто синусоидальные выходныие сигнали (тока и напряжения) двигателя
Нет EMC вмешательства .
Нет необходимости в принимении доболнителных филтров как (Sine former), ни dv/dt .
Нет необходимости в принимении доболнителных индуктивностей.
Нет необходимости в принимении специалных EMC шкафов, или корпуов - Простой монтаж.
Нет необходимости в принимении экранированных кабелей- можно исползовать стандартныe кабели .
Нет ограничения на длину кабеля, помимо его сопротивления.
Нет проблем земли текущих токов - может эксплуатироваться с остаточным текущым выключателем.
Нет коммутационного шума переключения в двегателе - тихо работает. Двигатель ни перегревается - из за отсутствия высоких гармоник .
Полный срок сложбы двигателя без проблем в его изоляции
Нет необходимости в свыше калибровки двигателя (15-25%), как это делается при питания его от ПЧ с ШИМ
Sunday, March 30, 2008
Friday, March 28, 2008
Conversation between AC induction motor- (1) and his owner (or Consultant)- (2)
Prepared By Taisir Khalil
17.Nov.2007- Sweden
Variable speed drive (VSD)
AC Induction Motor
C0nversation between AC induction motor- (1) and his owner(or
(1) AC motor - Help !! Help!! Help!!
(2) The owner ( consultant) -Hello , what’s wrong with you? what’s going on ? what’s happening?
1) OOH Sir, I am sorry , I feel my self very bad , I am over heated , I think I have flu !! Actually I am going to die soon!!
2)Don’t be afraid , you are still young , just few years ago you started to work for me , usually you are living 10- 15 years .
1) I agree with you , but 10 C of overheating me will reduce my life to 50% ! and I am very hot. All that started after that moment, when you connected me to PWM frequency converter
! I was happy before that time , working at full speed with full load fed with clean power from the network 400V / 50 Hz.
But ,at that time, you were not satisfied , crying all the time that I am consuming a lot of energy and always running at full speed or standing .
You wanted me to start walk instead of running (even some times very slow) !!
2) yes, my friend , because a lot of applications want to start and stop smoothly and to change their speed according to their needs , like Fans , Pumps and elevators for example ,supplying you with full voltage and nominal current reflected in inrush spikes of voltage and current in the network and you could not handle a lot of important application – that was the reason to connect you to a Frequency Converter ! Do you understand?
1)It is true that your ideas were the reason of my illness and possible close death !!
2) No , no , my friend , don’t worry , I will solve all your problems , but first tell me about them ? I have an idea to call a Doctor from NFO DRIVES AB in Sweden . What do you think?
INFO?? I don’t need information , I know my situation very well , and beside that , how info could help me ?
2) Oh , I am sorry , seems to be you did not hear it well , it is not info ! It is NFO – natural field orientation – it is a patent registered to NFO Drives in Sweden , it helps you to work in a very accurate way !
1)Really ! There was a far talk about it , but no body applied it to me .
May be not to you , personally , but more than 8000 AC motors of your brothers in Sweden are working with it( NFO).
1)Really , I like it , but let me first tell you a bout my illness and troubles which started with PWM – FC
2) Ok . I am listening to you carefully .
1) First of all , you are giving me infected food ( a choppy squared off wave -voltage pulses) full of harmonics , please see the drawings below :
17.Nov.2007- Sweden
Variable speed drive (VSD)
AC Induction Motor
C0nversation between AC induction motor- (1) and his owner(or consultant)- (2)
1)Hello , this is me , i am going to work for you ! are you going to buy me?
2)Yes , yes , but I am not sure in which application ,from the following below to use you!
Fans ,Pumps, compressors and conveyors , cranes or elevators, material handling or machine tools ,and many others.
1) Oh , sir ,don’t worry, no problem , in all those applications I can start to work and you will be happy !!
After a period of time anew changes happened , the AC induction motor (1) started to shout !!
1)Hello , this is me , i am going to work for you ! are you going to buy me?
2)Yes , yes , but I am not sure in which application ,from the following below to use you!
Fans ,Pumps, compressors and conveyors , cranes or elevators, material handling or machine tools ,and many others.
1) Oh , sir ,don’t worry, no problem , in all those applications I can start to work and you will be happy !!
After a period of time anew changes happened , the AC induction motor (1) started to shout !!
(1) AC motor - Help !! Help!! Help!!
(2) The owner ( consultant) -Hello , what’s wrong with you? what’s going on ? what’s happening?
1) OOH Sir, I am sorry , I feel my self very bad , I am over heated , I think I have flu !! Actually I am going to die soon!!
2)Don’t be afraid , you are still young , just few years ago you started to work for me , usually you are living 10- 15 years .
1) I agree with you , but 10 C of overheating me will reduce my life to 50% ! and I am very hot. All that started after that moment, when you connected me to PWM frequency converter
! I was happy before that time , working at full speed with full load fed with clean power from the network 400V / 50 Hz.
But ,at that time, you were not satisfied , crying all the time that I am consuming a lot of energy and always running at full speed or standing .You wanted me to start walk instead of running (even some times very slow) !!
2) yes, my friend , because a lot of applications want to start and stop smoothly and to change their speed according to their needs , like Fans , Pumps and elevators for example ,supplying you with full voltage and nominal current reflected in inrush spikes of voltage and current in the network and you could not handle a lot of important application – that was the reason to connect you to a Frequency Converter ! Do you understand?
1)It is true that your ideas were the reason of my illness and possible close death !!
2) No , no , my friend , don’t worry , I will solve all your problems , but first tell me about them ? I have an idea to call a Doctor from NFO DRIVES AB in Sweden . What do you think?
INFO?? I don’t need information , I know my situation very well , and beside that , how info could help me ?
2) Oh , I am sorry , seems to be you did not hear it well , it is not info ! It is NFO – natural field orientation – it is a patent registered to NFO Drives in Sweden , it helps you to work in a very accurate way !
1)Really ! There was a far talk about it , but no body applied it to me .
May be not to you , personally , but more than 8000 AC motors of your brothers in Sweden are working with it( NFO).
1)Really , I like it , but let me first tell you a bout my illness and troubles which started with PWM – FC
2) Ok . I am listening to you carefully .
1) First of all , you are giving me infected food ( a choppy squared off wave -voltage pulses) full of harmonics , please see the drawings below :
Harmonic content in such wave
Distortion of 5 th harmonic 
Distortion of 5 th harmonic 
Motor’s Voltage near nominal speed
2)Ok , In which way those harmonics are like infection to you ?
1) Ah , I see , did not you know that ? Ok , for example 5th harmonic is trying to rotate my shaft in the opposite direction to my movement , and that’s cost me effort to spend part of my food ( energy) to compensate its negative action , and that’s make me more warm and become overheated .
2) Don’t make me laugh ! Areyou trying to tell me that small 5th harmonic , will make you ( kaboot) !!
1) Not only 5th harmonic, but you can add the effect of its brothers to that !! ; 11 , 17 ,23 and so on…. , all those are trying to rotate me in the opposite direction , and you are paying for them and for there compensation.
In addition , their energy is wasted in heating and did not give any useful work ! even more , other harmonics like 7th , 13 ,19 , 25 and so on… are making my movement pulsating and not smooth.
2)Ok , In which way those harmonics are like infection to you ?1) Ah , I see , did not you know that ? Ok , for example 5th harmonic is trying to rotate my shaft in the opposite direction to my movement , and that’s cost me effort to spend part of my food ( energy) to compensate its negative action , and that’s make me more warm and become overheated .
2) Don’t make me laugh ! Areyou trying to tell me that small 5th harmonic , will make you ( kaboot) !!
1) Not only 5th harmonic, but you can add the effect of its brothers to that !! ; 11 , 17 ,23 and so on…. , all those are trying to rotate me in the opposite direction , and you are paying for them and for there compensation.
In addition , their energy is wasted in heating and did not give any useful work ! even more , other harmonics like 7th , 13 ,19 , 25 and so on… are making my movement pulsating and not smooth.
Motor’s current near nominal speed
2) Ok , for such infection , I will ask to get less useful work from you , or in other words I must to derate you !
1) not only for the harmonics reason , actually , my stators windings ends are becoming very hot , because I am receiving high voltage spikes ,some times double of the DC link voltage 1130 V in 400v system.
My insulation , will not withstand such over voltages spikes , especially when I am located far away ( more than 100feets -35 m) from your friend “ PWM” F.C, or when I am used as a second hand AC motor in any retrofitting project , standard B insulation type !
2)As , I mentioned before , I can derate you ,and use your brother NEMA MG1 Part 31.40.4.2 standards - Inverter grade motor ,insulation systems which are designed to meet and exceed resistance to spikes of 2 times nominal voltage .
1) Of course , you could do that ,but you will continue to pay for the negative effect of high harmonics (5-10)% of the energy fed to me ,it costs you a lot of money ! Take for example my brother 15 kW AC motor and calculate the losses for it, you will pay for 650W wasted energy in every hour of its work, and the efficiency of oversized motor is very poor when you are regulating the speed under 50% of its nominal value ,which always takes place in pumps and fans ,…….. so you are saving from one side and loosing from another !!
Please see an example of efficiency curves of AC induction motors
2) Ok , for such infection , I will ask to get less useful work from you , or in other words I must to derate you !1) not only for the harmonics reason , actually , my stators windings ends are becoming very hot , because I am receiving high voltage spikes ,some times double of the DC link voltage 1130 V in 400v system.
My insulation , will not withstand such over voltages spikes , especially when I am located far away ( more than 100feets -35 m) from your friend “ PWM” F.C, or when I am used as a second hand AC motor in any retrofitting project , standard B insulation type !
2)As , I mentioned before , I can derate you ,and use your brother NEMA MG1 Part 31.40.4.2 standards - Inverter grade motor ,insulation systems which are designed to meet and exceed resistance to spikes of 2 times nominal voltage .
1) Of course , you could do that ,but you will continue to pay for the negative effect of high harmonics (5-10)% of the energy fed to me ,it costs you a lot of money ! Take for example my brother 15 kW AC motor and calculate the losses for it, you will pay for 650W wasted energy in every hour of its work, and the efficiency of oversized motor is very poor when you are regulating the speed under 50% of its nominal value ,which always takes place in pumps and fans ,…….. so you are saving from one side and loosing from another !!
Please see an example of efficiency curves of AC induction motors
The efficiency of the motor is close to its max. value between 75% and 100% of the full rated load
But for the 10% load, the efficiency plummets to 55%. For a 15 kW motor.
2) Ok ,OK ….. I have another solution, it will make you happy!!
1) really , what is it?
2) I will connect you to PWM conventional type - FC through a filter
But for the 10% load, the efficiency plummets to 55%. For a 15 kW motor.
2) Ok ,OK ….. I have another solution, it will make you happy!!
1) really , what is it?
2) I will connect you to PWM conventional type - FC through a filter
In Europe some times , they called it ( Sine Former) 
Elevators
- Pumps
- Traction and conveyer systems
- HVAC systems (heating, ventilation and air conditioning)
But it consumes a lot of energy , comparing that with other solution comes from NFO DRIVES AB by using NFO SINUS Inverter.
- Pumps
- Traction and conveyer systems
- HVAC systems (heating, ventilation and air conditioning)
But it consumes a lot of energy , comparing that with other solution comes from NFO DRIVES AB by using NFO SINUS Inverter.
For PWM – FC connected to a 15 kW motor
Voltage drop = 8% X400 V= 32 V
Losses = 33A X 32V = 1056 W ! it is too much !! ( for VSD with motor Power rating 15kW)
And the ( sine former) is limited up to 100Hz speed change,
In North America , they called it (Motor guard)
Voltage drop = 8% X400 V= 32 V
Losses = 33A X 32V = 1056 W ! it is too much !! ( for VSD with motor Power rating 15kW)
And the ( sine former) is limited up to 100Hz speed change,
In North America , they called it (Motor guard)
It called NFO SINUS Inverter ! and it looks like :
It includes:
1- MOSFET transistors , with switching frequency between (30- 220) kHz …………..IGBT=( 800Hz-15kHz)
2-Sine switch , registered patent to NFO Drives AB, which is forming the output current wave shape and make it pure sine type , some times ,even better than the wave form from the grid !!
It includes:
1- MOSFET transistors , with switching frequency between (30- 220) kHz …………..IGBT=( 800Hz-15kHz)
2-Sine switch , registered patent to NFO Drives AB, which is forming the output current wave shape and make it pure sine type , some times ,even better than the wave form from the grid !!
Please note that NFO Sinus is acting to eliminate the output harmonics content to protect the motor and not to decrease them on the input of the drive
It is a fact that PWM- FC s can damage the insulation system of your motor. However, with better understanding, you should be able to prevent the infant mortality that so many have seen. Unless you’re looking for a reason to replace the motor, you should not install a PWM FC to an aged class B insulation system. Drive manufacturers of PWM- FC have increased effectiveness through PWM technology to deliver close to a clean current waveform to your motor. But that current comes with the expense of very fast rise time (dV/dT) and harmonics content.
Looking for a patched solution by adding sine wave output filter will protect the motor , but it will result in more capital investment and continuous additional consumption of electric energy , which could cost, in one year, more than 35% of the total purchasing price of the drive.
To avoid all above mentioned headaches and to save your money , NFO Drives AB offer you the solution by using NFO Sinus Inverter, and the particular, expensive part in it is the (NFO Sinus- Switch) which delivers a clean sine (pure sine waveform) voltage to your motor.
Weather your project is new or retrofitting type , your motor premium or standard one . In all cases with NFO Sinus Inverter , your application will work in perfect way for long time with minimum preventive maintenance and saving your money and effort !!
Have a nice time
2) Are you satisfied with us ?
1) Sure , with NFO SINUS Inverter, I am happy .
It is a fact that PWM- FC s can damage the insulation system of your motor. However, with better understanding, you should be able to prevent the infant mortality that so many have seen. Unless you’re looking for a reason to replace the motor, you should not install a PWM FC to an aged class B insulation system. Drive manufacturers of PWM- FC have increased effectiveness through PWM technology to deliver close to a clean current waveform to your motor. But that current comes with the expense of very fast rise time (dV/dT) and harmonics content.
Looking for a patched solution by adding sine wave output filter will protect the motor , but it will result in more capital investment and continuous additional consumption of electric energy , which could cost, in one year, more than 35% of the total purchasing price of the drive.
To avoid all above mentioned headaches and to save your money , NFO Drives AB offer you the solution by using NFO Sinus Inverter, and the particular, expensive part in it is the (NFO Sinus- Switch) which delivers a clean sine (pure sine waveform) voltage to your motor.
Weather your project is new or retrofitting type , your motor premium or standard one . In all cases with NFO Sinus Inverter , your application will work in perfect way for long time with minimum preventive maintenance and saving your money and effort !!
Have a nice time
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