motors and squarish waves

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.

Cheers, James Arthur

James,

Do you know how an induction motor will handle a square-waveish type DC-AC inverter ? ( Modified Sine Wave type inverter )

don

Heating would be the danger, but I've no experience chopping those motors. Sorry.

James Arthur

Politically correct waveforms are not square, man; they are well-rounded.

No, that's the myth. Deep down, they're all PWMed. ;-)

Tim

-- Deep Friar: a very philos>

It will run hotter than usual.

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

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. :-)

Cheers! Rich

Oopps! Missed the "induction" part.

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.

Cheers! Rich

It seems that no one here has got real data about AC induction motors.

Are there any good books on the design of AC induction motors and their controls ?

Thanks

don

I took two quarters worth of the topic many years ago. It was a senior level course; maybe EE 412.

It, of course, only touched the highlights.