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.
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.