I have bipolar stepper motor of following specs: 12 V, 0.5 A, 0.43 Nm, 24 Ω, 36 mH.
Back-EMF value is 0.0637 Vrms/RPM, measured by spinning it with power tool and measuring open coil voltage on scope:
I'm using DRV8255 stepper driver with 24V power supply. It follows that EMF reaches 24V with 24/0.0637=376 RPM. According to my understanding, torque at this speed should become 0 and it should be impossible to spin this motor faster with 24V.
Signals on oscillograms, from top to bottom: step signal (driver in 1/16 mode, signal generated from scope wavegen), coil voltage, coil current measured at driver sense pin.
When increasing the speed with motor running unloaded without anything mechanically attached, when the discontinuity in current waveform decreases to 0, motor stalls. This happens around 370 RPM and I think this means that EMF exceeds drive voltage and no more current can be pushed into the coil.
However, this is where my question comes in. I noticed that if I connect the motor to shaft adapter and some bearings (which presumably have some rotational inertia), it can in fact be spun faster than that. Waveform continues to change shape, and current RMS is rising with speed again.
I can keep increasing the speed and the motor finally stalls at about 1200 RPM. How's that possible? What's the principle of operation in the >370 RPM range with only 24V supply?
