Archive for June 27th, 2011

Stepper Motor Winding Current Rise Time

Here’s how the stepper drive voltage affects the current rise, using that kludge to sync the scope on one of those motors with L=2.6 mH and R=2.2 Ω. The peak winding current is 1 A, so the first step current-limits at 200 mA.

At 9 V:

Current Rise - 9 V 1A 3 RPS

Current Rise - 9 V 1A 3 RPS

At 18 V:

Current Rise - 18 V 1A 3 RPS

Current Rise - 18 V 1A 3 RPS

Knowing the rise time and current change, you can calculate the actual voltage across the inductor using:

VL = L di/dt

With 9 V drive the motor sees:

4.4 V = 2.6 mH x 220 mA / 130 us

With 18 V drive the motor sees:

14 V = 2.6 mH x 240 mA / 45 us

So, in round numbers, the driver MOSFETs, winding resistance, and all the crappy solderless breadboard connections soak up about 4 V of the available supply voltage. There’s some back EMF in there, too, but I haven’t measured that part of the puzzle yet.

The motor is turning at 3 rev/s in 1/8 microstepping mode, so each microstep is:

200 us = 1/(3 rev/s x 1600 step/rev)