Take an induction motor. Feed it crappy AC, maybe spiky, maybe square-wavish.
Will it care? One side says the inductance will limit the higher frequency current. Another PoV is the inter-winding capacitance will increase the current, and thus the I^R losses.
What saith the s.e.d wisdom?
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Stepper motors don't much care. Chopped drives commonly feed steppers ~20-80 volt rectangular waveforms.
With typical 20KHz chopping you get a few uS feedthru pulse from the inter-winding capacitances, and hysteresis loss in the iron--lossy if you switch the flux too often--but all-in- all very workable.
An induction motor doesn't have a high impedance at high frequency. Consider the input impedance of an induction motor running at 1/3rd or
1/5th of its normal speed. The input current is very high. This is a lot like what the 3rd and 5th harmonic sees.
The windings look like the primary of a transformer with a nearly shorted secondary. Since as far as the 3rd harmonic is concerned, the slip frequency is very high, the rotor current will be nearly 90 degrees out of phase. This means that although the 3rd harmonic current is high, the power is not being converted into mechanical form.
I've done PWM (rectangular waves) with a PMDC motor, and it worked like a champ. I didn't bother to look at the waveform, because the purpose of the device was a motor that maintained torque down to 0 RPM, which it did, so I shipped it. :-)
Then again, one time I was working with a "Modified Sine" inverter, which produced a string of positive and negative pulses, PWM'd to average out to 120 RMS. They did it the lazy man's way - two output transformers with their secondaries in series; they just drove them with two overlapping square waves.
It ran a bench grinder OK, but weighed almost 100 pounds and cost a fortune.
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