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Testing generator out put

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Scott Green

09-17-2000 06:18:54

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I have a used 3500 watt generator which I was asked to look at. It does not seem to put out enough electric power. It will start my circular saw , portable 3/4 horse air compressor , and other hand tools. It does not want to start the air compressor in the 1 -- 1 1/2 horse power range. I even drained the air out of the tank , it still won't start the motor.What kind of a test should I do to see the out put of this generator? How should I go about this test. What do you think the problem could be? And how should I go about fixing it.I've run other 3500 watt generators in the past , and never had problems running anything. This one just doesn't seem to be putting out the power it should. I even had it kick the breaker on the generator last winter , while running my 3/4 HP air compressor. Thanks , Scott

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T_Bone

09-19-2000 14:10:52

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Re: Testing generator out put in reply to Scott Green, 09-17-2000 06:18:54

Hi Scott, The first thing that comes to my mind is the rpm's on the generator set correct for maxmium load? If you try and set engine rpm's to high to start a marginal rated load, then you will be over voltage of the gen set. A typical electric motor will run within 10% of rated voltage with out any problems.

Most cheap generators are way over rated on output.

T_Bone

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larry

09-18-2000 19:42:12

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Re: Testing generator out put in reply to Scott Green, 09-17-2000 06:18:54

Most small generators will have trouble running a compressor due to the type load imposed by the compressor. You could design an idler to go between the drive belt on the motor and the compressor and get the motor running then kick in the compressor. The compressor would need to be converted to a constant run design equipped with an automatic unloader. It is the starting load that is breaking the back of the generator.

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Cal S.

09-18-2000 17:21:48

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Re: Testing generator out put in reply to Scott Green, 09-17-2000 06:18:54

Simple test is to hook up a series of light bulbs that equel the output.Put in a couple that are smaller say total 3300 watts check brightness then put in total of 3500 watts, do they dim at max load?, series or parralel will work.I know dumb but simple if you don't other test equip.

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bbott

09-17-2000 14:11:31

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Re: Testing generator out put in reply to Scott Green, 09-17-2000 06:18:54

Generators are usually tested using a 'load bank' which is a big adjustable resistor bank. Most professional shops that service and sell generators should have one.

I'm not sure how you'd jury-rig one with typical shop equipment.

Did this generator ever run that air compressor ?

That compressor motor may be have a big 'inrush' current demand on startup and that particular generator at '3500W' may simply not be enough whoopie.

Was the generator purchased for the great Y2K crisis ? I have a feeling that some 'marginal' equipment was pushed out the door to take advantage of that marketing 'opportunity'.

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Scott Green

09-17-2000 18:48:10

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Re: Re: Testing generator out put in reply to bbott, 09-17-2000 14:11:31

No , this generator wasn't bought at the Y2K era. It belonged to the company I do work for. It must be around 10-12 years old. No , I never tried to run the one air compressor off this generator before. I'm still open to any ideas that anyone has.

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bbott

09-18-2000 16:13:17

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Re: Re: Re: Testing generator out put in reply to Scott Green, 09-17-2000 18:48:10

I kinda agree with Dean. Depending on the design of the compressor motor you may be asking too much out of 3500W to start a real 1.5 hp motor.

..but you are also reporting that it seems 'weak' ot you in other respects..

I think the only way to find out for sure is to take it to a shop and have them run it on their load bank. It will tell the story of exactly how much the generator is putting out under load.

At any rate, best of luck.

bbott

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Dean

09-18-2000 06:38:29

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Re: Re: Re: Testing generator out put in reply to Scott Green, 09-17-2000 18:48:10

Scott,

Electric motors require 2-3 times their running current to start. I doubt a 3500w generator is going to start a motor that big but you can check at>Link

Dean>Link

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kimk

09-19-2000 17:57:27

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Re: Re: Re: Re: Testing generator out put in reply to Dean, 09-18-2000 06:38:29

Electric motors need 'bout 750watts/hp, so yours needs 'bout 1,125 watts to RUN. Like Dean says start-up needs a lot more. 2 or 3 times would make this 'bout 3,375 watts. Already we are gettin close to the 'new' rating of the generator. Compressors always draw like the devil on start up 'cause they are under max load on the first compression stroke. You can try powering it through #10 wire ( a short run ) to cut down on voltage drop but it all seems kina marginal to me

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Scott Green

09-21-2000 17:30:46

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Re: Re: Re: Re: Re: Testing generator out put in reply to kimk, 09-19-2000 17:57:27

I just want to thank everyone for your input. I picked up some good information here.

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