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correcting the frequency of a induction motor generator

Posted: Wed Jul 23, 2008 11:37 am
by social misfit
having tried an induction motor as a generator i have found the frequncy to be 83 + Hz is there a way of correcting this to 50Hz as a motor its running speed is 2800 rpm i would have thought running at 3000 rpm would be right as its a 2 pole motor?

Posted: Wed Jul 23, 2008 10:10 pm
by NickJ
This is very curious! It should be the case that a bit over 3000 should give you 50 hz at low load for a 2 pole motor. So say 3100 or something like. Where 83Hz comes from I have no idea! I could understand 100.

My only suggestion is that either you have a faulty frequency meter / rev counter or the motor (assuming here that it is self exciting with capacitors) is producing a waveform sufficiently distorted with harmonics to give a dodgy reading on your meter. I would suggest an oscilloscope (if you havent tried already :) )

If you put it on a scope the waveform should look like a sine wave but if it isnt that may be your problem.

Fascinated to hear


Posted: Thu Jul 24, 2008 10:32 am
by social misfit
i used a plug in power meter to check it which was reasonably accurate on house mains it would fluctuate between 49 an 50 Hz which im informed by electricity company is normal.
have got a rev counter coming shortly so will be able to check speed of generator accurately as have been using pulley diameters for calculating speed from a known motor speed.

Posted: Thu Jul 24, 2008 4:23 pm
by NickJ
I suspect that the plug in meter may not be accurate on the waveform from your generator. If your rev counter shows the speed of the machine to be accurate then I would try and borrow an oscilloscope to check the waveform and frequency. If you then find the frequency is wrong some serious head scratching needed :)

Posted: Thu Jul 24, 2008 7:42 pm
by microman
Calculating rpm from pulley sizes is not likely to be accurate because the effective diameter is rather indeterminate especially for small pulleys.

If you can't get hold of a 'scope to look at the waveform you might try listening to the output via a suitable step down transformer and loudspeaker (or perhaps a pick-up coil made from a few turns of wire held near the output fed into your audio gear would do) and comparing with the 50Hz mains.

If your generator really is 83Hz it will sound obviously higher in pitch (somewhat over a fifth if you're musically inclined).