induction generator torque-speed relation when not sync'd to grid? 6

[electricpete]

This question arises from discussion in the thread on degraded voltage. Please be assured that if you respond I will not argue with you ;-)

An induction generator can apparently operate without being interconnected to another power source, provided that capacitors are attached to the terminals.

I assume that voltage can be controlled by adjusting the terminal capacitance.

What determines and/or controls real power and torque of the generator? It seems like the induction machine torque-slip characteristic is irrelevant if there is no "syncronous" frequency and therefore no slip. What determines the generator torque-speed characteristic?

 

[redtrumpet]

Very interesting discussion in the other forum, electricpete. I will now live in fear of being called a WOFFLER.

 

[Lewish]

Electricpete,
The output voltage is not a linear increase but is a smooth curve. There are no jumps in it.
If you have access to Matlab and the Power System Blockset, you can produce the curve by inputting a negative torgue value to the asynchronous machine model. I don't have any way to reproduce it here.
Yes, voltage out does depend on the capacitor values, but the dependency is less with increasing RPM. You need enough to excite the motor when starting, but not too much or your voltage will rise too quickly as you increase your RPM.

 

[Lewish]

One other point worth mentioning, maybe, is that if you are interested in achieving the maximum efficiency when using an induction motor as a generator, you will need to turn the motor about 10 per cent faster than normal. That is, if you have a 60Hz motor with a nominal RPM rating of 1750 and an efficiency rating of 84 per cent, if you turn it at 1850 RPM you will produce 60Hz power but at only about 80 per cent efficieny. To get up to the 84 per cent rating, you will have to increase the speed until you are outputing about 65-66Hz. But, in so doing, your output voltage will rise well above the nameplate rating, so you will need to reduce the excitation capacitance or, you know what will happen.

 

[jbartos]

Comment: The previous posting content is consistent with the posted Prof. M.G. Say book content pertaning to induction generators, especially the low efficiency.