Motor "singing" (high frequency noise) using VFD

Bernie -

I don't know for sure why the engineers decided to use audible frequencies for their pulse rate on motor drives but the low-end DC PM machines like Sieg sure do - It's right around 1000 Hz. I don't think the technology of the day allowed for higher PWM frequencies, because it would require much faster switching times for the Mosfets and fast recovery diodes. A lot of the newer ones seem to be 25-30khz so are out of the range of human hearing. Now, the SCR controllers run at 60 Hz so they just 'hum'.....

-Pete
 
You're right, the sound is coming from the motor, but it's caused by the way the VFD is generating the 3 phase. Basicically the VFD is turning the power on and off very fast. This can cause the mechanical portions of the motor to vibrate slightly, and if the frequency of the switching is below 20kHz, that vibration will be audible. A line reactor will smooth that out a bit too.

I may try playing with the switching frequency of mine, I saw it listed as a parameter, but I figured the auto tune would deal with that.
 
To a large extent, it's because of the hysterisis properties of the rotor and stator materials... What's optimal for a motor is not optimal for the power source so it's a tradeoff. And yes, much if it is in the audible/perceptable range and unless you use some very expensive materials for the cores, it shall remain that way.

Ray

olduhfguy said:
Bernie -

I don't know for sure why the engineers decided to use audible frequencies for their pulse rate on motor drives but the low-end DC PM machines like Sieg sure do - It's right around 1000 Hz. I don't think the technology of the day allowed for higher PWM frequencies, because it would require much faster switching times for the Mosfets and fast recovery diodes. A lot of the newer ones seem to be 25-30khz so are out of the range of human hearing. Now, the SCR controllers run at 60 Hz so they just 'hum'.....

-Pete
Click to expand...
 
A possible cause could be that the windings in the motor are loose and vibrating at the switching frequency. if this is the case then dunking the windings in transformer varnish is about the only cure.
Talk to a motor rewinder if this appears to be the case.

Steve
 
That's about the size of it. If anyone still re-winds motors, it would probably help but, there's also a thermal component. There's a miniscule amount (but it's additive over surface area) of temperature change which, believe it or not, makes sound as the surrounding air changes density. I've got all kinds of oscillators soldered to boards and you can hear them. Also, some of the noise is coming from the capacitors and inductors inside the VFD unit itself...

stevec said:
A possible cause could be that the windings in the motor are loose and vibrating at the switching frequency. if this is the case then dunking the windings in transformer varnish is about the only cure.
Talk to a motor rewinder if this appears to be the case.

Steve
Click to expand...
 
Thanks guys-

What the heck would that cost?

Dangit- I thought i would get a Baldor motor and be "set"!! It is much louder than the lovely old ones.


Bernie
 
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