I've built induction generators, I was a kid growing up around a repair shop, with a whole salvage yard full of parts to use. I used an old 5 HP 3600 rpm 3 phase motor and a Briggs & Stratton model 19 I took off of an old paving roller. I belt drove it about 125% overdrive so I could get the motor to spin at it's syncroness speed + the slip.
It put out nothing until I added some caps that I salvaged from some old high bay fixtures. ...
Joe - Your experment is fascinating. I've been around only a few induction alternators - all were connected to an external grid. I've never seen one driving into a load with no external source to stabilize the frequency. And since you actually have done this, seen this, I've got a few questions.
Here's my limited knowledge on induction alternators:
As mentioned earlier (by yourself and others): To act as an induction alternator, an induction motor requires reactive power, roughly the same as would be required to supply magnetizing current for operation as an induction motor. The reactive power is either supplied by an external source (such as the utility or a parallel synchronous generator) or can be supplied by capacitors.
In order for an induction alternator to generate, the rotor has to be turning faster than synchronous speed. It has to have slip - for a power output roughly equal to motor nameplate horsepower the slip would have to be about 5%. A two pole motor would have to turn about 3800rpm if it were driving into a 60hz grid.
If I am good up until now, here's my questions:
1. If the induction alternator is driving into a load, (no grid) what provides the 60hz for the alternator to slip against?
2. When you did this experiment were you connected to a load? Or were you just measuring the voltage output open circuit (open circuit except for the caps)?
3. Probably no chance you had a frequency meter connected - did you? I can't figure out how the alternator picked out what the output frequency would be. Induction motors don't have any inherent synchronous speed. A two pole, 60hz, 3450rpm motor is plenty happy at 30hz and 1750rpm.
I start looking at Torque-Speed curves, slip ratios, power output, voltage, current and it really gets confusing - Especially with no grid to stabilize the voltage or frequency.
Any insight you could throw my way will be graciously appreciated.
ice