PWM Subharmonics?

[onatirec]

I've been looking at noise and vibrations from a small-medium, 3-phase AC motor and have found peaks at multiples of 4kHz (8k, 12k, 16k)… They show up in a spectrogram just as I would expect PWM signals and their corresponding harmonics to appear.

The PWM is set at 16kHz, and these peaks show up at/around 4k, 8k, etc even when changing the PWM frequency to other values (I've been constrained by multiples of two).

I finally got around to putting a scope on one of the phases and it seems that there are similar artifacts in the phase voltage.

I'm curious as to why these subharmonics are present. All of the sources I've been able to find only mention subharmonics when the ratio of fundamental electric frequency to PWM carrier is on the high side. Here the fe is around 350Hz vs 16kHz for the PWM.

I'm attaching a picture of the scope signal with overlaid FFT (in red). The high peak is at 16kHz and one can see the additional peaks to the left.

I had originally posted in the acoustics forum here:

https://www.eng-tips.com/viewthread.cfm?qid=472468

A spectrogram of the acoustic noise is in the OP there and the 4k, 8k, 12k vertical lines can be seen for more context.

Thanks a bunch.

 

[electricuwe]

One explanation would be as follows:

Due to limited resources of the microcontroller inside the drive, the PWM reference values might be not updated at the switching frequency, but only at a lower rate, maybe at 4 kHz. This would explain low amplitude subharmonics.

Today I think such an approach is unacceptable, but if the controller board was designed long time ago, this might have been used as a workaround to achieve higher switching frequency without investing to much in controller and software.

I would not expect major disadvantages due to this unless your application is very demanding in regard to noise.

 

[waross]

If the cabling between the drive and the motor is able to resonate even slightly at 4 kHz then the 12 kHz may excite lower frequency harmonics.

Bill
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"Why not the best?"
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