The formula is:
rpm = freq x 120 / # of poles
Alvin,
The formula I've used yields the same answer, but is:
rpm = freq (hz) x 60 / (# poles/2)
I suffer a bit from the CRS syndrom, so something that I find helpful is to start from known relationships. A 60 hz, 2-pole motor is 3600 rpm, a 4-pole motor is 1800 rpm, a 6-pole motor would be 1200 rpm, etc. You can back into the formula from this starting point. If you're on a 50 hz system, the common speeds will be: 2-pole, 3000 rpm, etc., but the formula is the same.
Chris, I like your explanation of calculating and plotting current based on stator resistance, and speed. The modeling (what little) I've done is similar, but looking at the 'forcing voltage' over and above the nominal bank, and similarly calculating current (and I^2R losses). If you have the generator to play with, I assume this is a more precise method, particularly for looking at efficiency.
Question: When you compare unloaded vs. loaded, do you put a set, fixed, larger than output load on it, or try to hold the voltage at a set level, or does it meatter?
~kitestrings