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 µs/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.