Very simple motor PWM question - 12V motor, 24V supply

[Speakerguy]

I have a PWM + half-bridge driver board from an old project. It has a 15V regulated section for the triangle wave generator and the half bridge driver and the half bridge voltage can go anywhere up to 200V (filter cap / MOSFET limit). It was meant to run off of rectified AC in the US.

Anyway, I have a big 12V gearmotor I need to control the speed of. I can power the above board with a 24V supply to both the 15V linear regulator and directly to the FET's no problem, and I have a stout 24V switcher on hand (13A or so).

1) What PWM duty cycle do I want for 'max' speed (25% or 50%)? The whole average voltage vs average power thing.

2) Will I damage the motor in any way by running it in such a fashion instead of off a 12V supply?

3) It's a half bridge config. Freewheeling diodes are necessary, yes?

4) 1KHz sound about right for a PWM freq?

Thanks for any help.

 

[Sceadwian]

PWM = 50% sound about right to me. Shouldn't damage anything the voltage is low enough the existing insulation should handle it without even noticing.
Half bridge, definitely freewheeling diodes, and make sure they're properly rated.
1khz sounds fine, although you should tune the PWM frequency to a point where the motor doesn't produce noise under load. PWM'ing a motor will cause some kind of buzz, ever motor is gonna have a different acoustical resonating frequency, so play around with it. I'm gonna say you could go as low as 100hz and as high as the motor coils inductance will allow.

 

[medelec35]

I agree with Sceadwian about the noise. A few manufactures drive dc motors at just above audible frequencies. Last pwm driven motor I used a frequency of 10KHz, that worked really well. I started at 2KHz and wine at slow speeds was far to loud.
one manufacture said running motor at 20KHZ, the frequency is fast enough for the motor to see it as DC.

 

[Sceadwian]

It's not so much the primary frequency of the PWM. PWM's are usually square waves which contain a LOT of harmonics, and those harmonics sometimes have a physical outlet. With any motor you should very slowly sweep a wide range of frequencies under various operating conditions to find out what provides the least acoustical output in your situation.