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)

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  1. #1 by nophead on 27-June-2011 - 08:26

    I expect at that speed the BEMF is significant and the electronics are actually losing a lot less than 4V. Simply hold the shaft to see how much faster it rises.

    • #2 by Ed on 27-June-2011 - 09:58

      It actually makes less difference than I expected; after I stalled the thing out, the increasing-current waveform was about the same. The decreasing-current waveform (on the other side of the sine wave) was much improved, though, which agrees with your observations from a while back.

      More pix on the way…

  2. #3 by John Rehwinkel on 27-June-2011 - 19:32

    Nice “King of the Road” reference. Made me smile.

    • #4 by Ed on 27-June-2011 - 20:23

      My internal sound track has been playing really crusty old songs of late… I hate it when disco come around again, though. [grin]

  3. #5 by Jetguy on 30-June-2011 - 15:30

    Nice work! While I didn’ t “measure” the results when I took my T-O-M to 24 volts on the old high inductance motors, the result was quite similiar as you could tell the bot was responding faster. We had just gone over inductors in my engineering program so I showed the class the difference as it was a very practical example. Your thorough testing is much better than my subjective test.

    • #6 by Ed on 30-June-2011 - 19:44

      you could tell the bot was responding faster

      Going to 24 V gives the driver enough headroom to make the microstepping work on a 14 V motor, which improves everything else: it sounds better, it moves better, it builds better.

      It’d be interesting to re-record those music tracks with microstepping working properly. I think the printer would be much more musical…

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