PWM frequency specs

rjo__

I have a couple of motor control boards, as well as HB-25's for different sized motors ranging from 7.4 volts at a couple of Amps up to 24V at around 15A. The HB-25's are not a problem... give them a pulse and you are done. But they can't handle 24V and they don't have some of the features that I need for the sensored 4 motor implementation at 7.4V. At 12V, they are perfect.

I understand what PWM is... what I'm a befluxed about today is what the specs actually tell us. Specifically... if a board accepts a PWM of say 24Khz... does that mean that I can use any frequency up to 24KHz... or if I want everything to behave as expected I have to use 24KHz?

Leon

Here is a useful app. note on using PWM for motor control:

http://ww1.microchip.com/downloads/en/AppNotes/00905B.pdf

Some experimentation will be required.

rjo__

Thanks Leon,

That did the trick: "The frequency of the PWM waveform is an important consideration. Too low a frequency will result in a noisy motor at low speeds and sluggish response to changes in duty cycle. Too high a frequency lessens the efficiency of the system due to switching losses in the switching devices. A good rule of thumb is to modulate the input waveform at a frequency in the range of 4 kHz to 20 kHz. This range is high enough that audible motor noise is attenuated and the switching losses present in the MOSFETs (or BJTs) are negligible. Generally, it is a good idea to experiment with the PWM frequency for a given motor to find a satisfactory frequency."

And of course one thing leads to another... the note has an important section on Back EMF... "The BEMF voltage is measured between PWM pulses when one side of the motor is floating and the other is grounded. At this instance in time the motor is acting like a generator and produces a BEMF voltage proportional to speed." And in figure 11, it shows a direct connection on to each pole of the motor... That is pretty handy.

I didn't know that either:)

Rich