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Archive for July 10th, 2011

Stepper Motor Back EMF

Some simple measurements using that Pololu driver in its default mixed decay mode and that Arduino sync generator. The captions give the operating conditions; basically, I’m varying the rotation speed by cranking the signal generator driving the Pololu board.

At 1 rev/s, it’s about as good as it gets:

Back EMF - 9V 400mA 1 RPS

Back EMF - 9V 400mA 1 RPS

At 5 rev/s, the driver has trouble getting current out of the winding:

Back EMF - 9V 400mA 5 RPS

Back EMF - 9V 400mA 5 RPS

At 10 rev/s, things are getting ugly:

Back EMF - 9V 400mA 10 RPS

Back EMF - 9V 400mA 10 RPS

At 20 rev/s, the back EMF has pretty much taken control of the current and the driver is going along for the ride:

Back EMF - 9V 400mA 20 RPS

Back EMF - 9V 400mA 20 RPS

At 25 rev/s, the driver produces only occasional dents in the waveform:

Back EMF - 9V 400mA 25 RPS

Back EMF - 9V 400mA 25 RPS

At 25.3 rev/s, the motor stalled. Even with no back EMF (what with the rotor being stopped and buzzing in frustration), the driver can’t force the current to behave:

Back EMF - 9V 400mA 25.3 RPS

Back EMF - 9V 400mA 25.3 RPS

I don’t have any way to measure the motor’s output torque, but at 1500 RPM there won’t be any worth mentioning.

For what it’s worth, 25 rev/s means the driver is handling 40 k steps/sec = 25 ┬Ás/step. The motors in a Thing-O-Matic run at 3 rev/s to move the XY stages at 100 mm/s, so scale what you see here accordingly.

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