PWM frequency and motor whine

[idle_techie]

Hello Gurus!
I am an engineer working for an automotive supplier and we are in the process of designing and prototyping an Electric vehicle. We plan to use a BLDC motor for propulsion with a 3 phase PWM controller and I am trying to determine some noise characteristics. The controller we chose has a PWM frequency of 12.5kHz. The maximum RPM of our motor is 8000.
We are concerned about the "motor whine" esp at low RPMs. I dont know the duty cycles of the controller.

Now, since the PWM frequency is constant and the frequency is within the audible band, does it mean that the motor whine will be audible all thro the rpm range? I know the duty cycles will be varied based on required rpm ( low duty cycle = low rpm, high duty cycle = high rpm) , but what does this do to motor noise.

Having to listen to that "motor whine" all thru the rpm range is probably not good either.

I am not an Electronic engineer, so please help out!!

Thanks

 

[crutschow]

If the 12.5kHz is audible from the motor at one speed than it likely will be audible at any speed since the motor speed is unrelated to the PWM frequency. The intensity of the sound may vary with load more so than speed since that affects the PWM duty-cycle and the consequent waveform power.

 

[Warpspeed]

There will be more than one frequency present.

The 12.5 Khz PWM ripple will vary in intensity with load and duty cycle as Carl says, but the three phase motor sinusoidal drive frequency will be changing with motor rpm up and down over a fairly wide operating range.

Unfortunately magnetostriction and the mass and elasticity of steel mean there will always be certain mechanical resonances within the motor, that can create strange chirps and howls at specific rpm as the motor speed and load varies.

Not a great deal you can really do about it directly either, apart from some acoustic screening around the motor.

 

[Boncuk]

How about doubling PWM frequency to 25KHz + sound reducing encapsulation?