DIY induction motor as generator for cleaner power?

Yes and no.

An inductive machine hooked up and driven that way will look like either a capacitor or an inductor (I can't remember which!), and it will produce power proportional to the drive torque.

So your little clean sine wave inverter will, first, need to have enough current capacity to drive the reactive VARs to the inductive machine, and second, would need to have somewhere to put all the excess energy that would be getting stuffed onto the line any time the mechanical input power exceeded the electric power being used.

I suspect that if you just took a little 1000W Honda generator and hooked it up to a 10 horsepower motor spinning at 3800 RPM that the best you could hope for would be a blown circuit breaker in the generator and no useful power.

Basically, turning an inductive machine at faster than the synchronous RPM works when it's connected to a big grid, but when such critters start dominating the grid then things won't go so well.

You could come up with some exotic VFD that would back-flow power to the electric line, but one way or another you'd need herky enough electronics to take the raw stuff coming out of the generator and turn it into something at 120V, 60Hz.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I think it looks capacitive... At least that's what I recall from a Helium liqufier manual.. I ran it for a while back in grad school. The brake on the expansion engine was such a motor/ generator that sent power back into the grid.

George H.

Many years ago I worked for Cadillac Motor Car in Detroit, who had their own on-site power plant. The coolest thing there was a "rotating capacitor", which was an enormous slow-turning flywheel maybe 20 feet in diameter, axle at floor level so the bottom half of the wheel went below the floor.

They used this to get a better rate from the local utility company, by making the largely-inductive load of the factory look neutral or capacitive to the utility.

Best regards,

Bob Masta DAQARTA v8.00 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE 8-channel Signal Generator, DaqMusiq generator Science with your sound card!

If you have a synchronous machine with a variable field you can trim whether the thing looks inductive, capacitive, or neutral by trimming the intensity on the field winding.

So it could be done with "just" a honkin' big synchronous motor and a flywheel. I don't know if devices were made specifically for the task, though.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

As far as I know this was specifically for PFC. (They also had a separate conventional coal-fired generator for power backup.) Back then, if not still today, factories almost always presented inductive loads, and there were no industries or customers that provided loads that were naturally capacitive. So the utility company was happy to offer reduced rates to get PFC.

Best regards,

Bob Masta DAQARTA v8.00 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE 8-channel Signal Generator, DaqMusiq generator Science with your sound card!

Could be a device to guarantee power when switching power sources to the plant as well. There's alot energy in a large flywheel.

They make "small" as in not 20 foot diameter inertial UPS devices just for this purpose- a cheap way to store less than a minute of power. If you lose even a cycle or two of power, gas discharge lighting shuts off and has to cool for x minutes to restart. that's a showstopper in large factory.