PWM Issue With Motor Speed

[adamey]

I built a PWM test circuit using a PICAXE and a Pololu motor driver carrier (12A cont, 30A peak capability).

I first tried it with a small 12VDC motor (window motor from a car) with no load and at 20KHz it worked great from 10% to 90% duty cycle.

I then tried it with a radiator fan motor (which draws about 5A continuous and peaks around 20A). At 10% the motor will not start to turn unless I manually push it, and then it turns very slowly. At 20% it will run, but not well. At 30% and above it runs fine and speeds change until I hit 70%. Anything above 70% does not increase motor speed, and at 90% it actually turns slightly slower.

Tried it with lower frequencies and although the fan runs better at lower duty cycles, it still seems to top out at around 70%.

I thought I was being so smart by building a test rig so I could experiment with various frequencies/duty cycles to see how the motors respond, and I just ended up getting myself confused.

 

[BrownOut]

Some motors are wound to limit thier own RPM. You evidently reached the max for the fan motor at about 70%, and using more width didn't make it faster, because you already hit the motor's limit. Don't be confused. You've learned that not all motors well lend theirselves to PWM control. Remember something has to limit the speed of a DC motor when not externally controlled.

 

[Dx3]

adamey...Thank you for posting your results. I learned a couple of things from this.
Even when you think it has all gone wrong, there are parts that are interesting and educational.

 

[BrownOut]

It also dawned on me that some motors should be loaded to be properly tested. If you coupled a small electric generator connected to a high power resistor, that might be a better test. Different value resistors will allow for different loading. Or use a big wire wound rheostat.