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

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

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 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.

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.

Figure 3 shows both methods being used to protect the drive input semiconductors.
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.

Note : This is a main part of an article pubished by (Rockwell Automation-Mequon Wisconsin) with changes.

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.

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 .

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/





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