What did I do wrong?

[printbus]

I haven't studied the numbers, but I agree that the extruder motor is moving faster than X/Y/Z so a higher impedance seems logical. After pondering this for a while, I now somewhat regret referencing the reprap thread. I think its concept of applying the z=2*pi*frequency*inductance formula to stepper motors is debatable. The formula applies to sine wave signals. For square wave PWM signals like we're dealing with here, the frequency content is really in the rise and fall times of the PWM signal more than the pulse rate. ie, sharper square waves have more higher frequency content. But what do I know - my expertise was in digital circuit design, not motors!

[clough42]

I haven't studied the numbers, but I agree that the extruder motor is moving faster than X/Y/Z so a higher impedance seems logical. After pondering this for a while, I now somewhat regret referencing the reprap thread. I think its concept of applying the z=2*pi*frequency*inductance formula to stepper motors is debatable. The formula applies to sine wave signals. For square wave PWM signals like we're dealing with here, the frequency content is really in the rise and fall times of the PWM signal more than the pulse rate. ie, sharper square waves have more higher frequency content. But what do I know - my expertise was in digital circuit design, not motors!

Actually, I think it's fine. In this scenario, the microstepping of the driver is attempting to create a sine (or triangle) current waveform in the motor coil. The inductor smooths out the current waveform and the driver uses feedback to drive the PWM to get the desired current waveform. It's perfectly valid to analyze the fundamental step signal using the normal inductor equations. I ran this by our motor guy at work (designs laser mirror drive systems) and his position is that the combination of the large inductor and the feedback in the drive pretty much makes the PWM a no-op for this kind of analysis.

I also got some real numbers for the MakerFarm motors--both from Colin and from direct measurement. The DC resistance of the coils is about 18.5 ohms and the inductance is about 35mH.

I'll start a new thread on the MakerFarm motors, just to make this info easier for other people to find.