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Originally Posted by gc02
I think what you're seeing is caused by low a PWM switching frequency. If a speed controller's PWM period is too long, longer than the current rise/fall time caused by inductance, you don't get the smoothing you would with a higher PWM frequency.
Best way to check if the duty cycle is linear would be to use an oscilloscope.
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I have a frequency counter with a duty cycle function as well as a scope. When i get home I will definately repeat my tests measuring duty cycle. I will also note the output frequency. Even if you assume that my pwm vs voltage graph is entirely bogus, that still doesn't explain the pwm vs speed graph which looks just like it. One of the less obvious reasons for the pwm vs. speed graph in the first place is that a motor under no load converges on a speed porportional to its input voltage. A PM DC brush motor that is under no mechanical load will act as a linear voltage to speed transducer and likewise a motor under no electrical load will act as a speed to voltage transducer. The second graph was partially to confirm the first.