Will gear reducer give more torque than pulley system ?

[tropicalson]

I just converted a 3 phase 7.5 hp induction motor into an alternator/generator.i have a 5hp compressor motor that i want to test the alt./gen with.I put a 1.5" pulley on the 5hp motor and a 10" pulley on the alternator/generator. The 5 hp motor spun the generator quite easily and give a RPM of 262 and an output of 209 vdc.I then connected an electric element to the generator and tried to spin it but it would only spin very slowly and then trip the breaker.it seems the 5 hp motor does not have enough torque to spin turn the generator under load. would a gear reducer be able to give more torque than the belt and pulley system using the same 5 hp motor ? or is there anything else that i can use to turn the generator to check it's output at various speeds ?thanks .

 

[jraef]

Just to be clear, you are using a 5HP motor as the prime mover to drive a 7.5HP motor used as an induction generator?

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[waross]

Can you tell us how you converted the motor to a DC generator. I can think of a couple of ways it may be done, but how have you done it. It will have a big impact on our recomandations.
Your 5 hp. motor will output 3.75 kilowatts of mechanical energy. With losses in efficiency you will be able to draw about 3 kilowatts from your generator before you start to overload your 5 hp. motor.
Your gearing ratio sounds extreme, but "it depends" on how you have converted your motor to a generator.
What was the resistance of the electric element you connected?
To answer your original question, I don't think that a change to a gear-reducer will make much difference at this point.
The 5 hp. motor should be good for testing if you keep your load below 3 kilowatts.
yours

 

[jraef]

D'oh!
Missed the DC issue, good catch waross. otherwise that's exactly where I was headed with this too. Changing pulley ratios does not change power, only torque and speed.

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[tropicalson]

The motor was converted by turning down the rotor and adding magnets to it while maintaining the original airgap.It is now a permanent magnet alternator .The idea is to eventually use it as a wind turbine to charge batteries. The output was rectified by a couple of Bridge rectifies to give the DC output.The heating element i used was from an electric stove . I think it's about 2500 Watts.I also tried a 12v battery.The 5 hp motor is rated at 3450 RPM . The lagre pulley was to lower the rpm and hopefully increase the torque or make it easier to turn the generator underload.I want to get about 500 RPM eventually.My main objecttive here right now is to be able to turn the generator under load at different rpm's upto 500.Someone said i would need a motor over 7.5 hp to drive my generator. Any help/advice would be appreciated. Thanks .

 

[waross]

You only need 7.5 hp. (Actually more like 9 or 10 hp when you add losses) if your load is the equivalent of 7.5 hp.
Your 5 hp. motor should do fine for testing if you keep the load down to 3kw as I suggested.
First i suggest looking at your alternator connections. If the windings are in the high voltage configuration change them to the low voltage connections.
Recheck your speeds. The numbers and the ratios you gave work out to 1740 RPM, not 3450 RPM.
Try a smaller load.
There will be some suggestions to use a Variable Frequency Drive. That will work great if you can afford one, or happen to have one available.
itsmoked made a suggestion in a similar post that I will borrow. If you can reconnect to get the voltage down then use some light bulbs for a load.
With the temperature/resistance characteristics of incandescent lamps they will probably not work well in series.
Good luck
yours

 

[electricpete]

I calculated 1750rpm as well. Perhaps you changed the number of poles

Owing to the temperature/resistance characteristics mentioned, the lightbulb will draw much much more current when cold than when hot.

I'm wondering whether over heating element might act the same as well? (don't know). If so that could be contributing to your initial overload. Also your heating element is 2500W at what voltage? If at 120vac, then would be expected to be (209/120)^2 higher at 209vdc.

How did you measure the output dc voltage. (If not filtered, will your instrument correctly interpret the ripple?)

Did the voltage droop under load? You could answer a lot of questions about the loading by monitoring voltage and current in your load (assuming you can measure it accurately.

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[electricpete]

The bottom line -

I agree with waross the first step would be to try a smaller electrical load (higher resistance) and gradually increase load.

One other possibility - did you happen to try to run that machine again with load removed? If not, you might check whether you may have already damaged the "generator" stator windings.

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