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
AC Drive & Automation
