DC motor ideal frequency

[skychas]

I have a quick question that I cant get my head around, I am not a motor boffin but know electronics quite well so please bare with me.
I are working on a hobby with a friend of mine to build an underwater scooter using a DC motor. Obviously when underwater, power is at a minimum. I have heard that there is an optimal frequency to run a DC motor using PWM. Is this true and if so are there any calculations to work this out?
Thanks Charlie

 

[buzzp]

PWM will not reduce the power consumption for the same job. However, it is an efficient way to vary the speed without to many losses (other methods to reduce speed are generally more lossy than PWM). This will cause other potential problems like transients and such.

By adjusting the frequency you will increase or decrease the average value (DC) the motor sees. The frequency will vary depending on the speed you want to go and the amplitude of the pulses.

 

[skychas]

Thanks for the reply,
the frequency was not the duty cycle or rms voltage, it was the actual frequency of the "square wave". I have heard from some sources that you need to use the time constant calculation taking into account the inductance and resistance of the motor which works out at about 2kHz ish (right in the audio frequency range), the other figure quoted to me was about 20kHz.
Looked all round the web and not quite sure what to believe

 

[Lewish]

I would worry more about the power losses in the switches that create the PWM, than I would about the motor itself. The higher your PWM frequency, the more losses in your switch.