Stepper Dynamometer: Mechanical Resonance

Here’s a howling 1200 Hz mechanical resonance excited by a full-step drive at 290 rpm, with the resonance running at 5 cycles for every 4 full steps.:

[Update: top trace is voltage from big stepper-as-generator, bottom is small stepper winding current = 1 A/div.]

Switching to 1/2 microstepping at the same rotational speed pretty well tamps it down:

However, here’s the same resonance excited by 1/2 microstepping at half the rotational speed, oscillating at 9 irregular cycles for every 8 half-steps:

The resonance was strong enough to back the setscrew completely out of the generator end of the shaft, leaving me wondering why the output was dead when the motor was obviously running. Yes, several times…