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Author Topic: Odd voltages from single phase generator  (Read 7280 times)

bmarx

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Odd voltages from single phase generator
« on: August 16, 2011, 11:48:23 PM »
Just bought a Basant 8 hp single cylinder "Lister" engine with  6 kw "single phase" 120 X 2 generator.  Arrived in good shape from Agra, India.  However when generator is spun at proper 1500 rpm (belt drive from engine at 800 rpm), the output a.c. generator voltages are as follows.  Total voltage is about 240 v, but when measured from neutral to each outer wire, each of these is about 165 v.  This generator has four slip rings.  Schematic diagram from India is incomplete.  Seems to follow the diagram taken from website   www.kissangroup.com  (look under "user guide" for the schematic for "single phase 230 v) which actually shows a three phase setup, I think.  Can it be that my generator is actually wound as 2 of three phases?  Anyone else ever have this problem?  Interesting dilemma.

toydiesel01

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Re: Odd voltages from single phase generator
« Reply #1 on: August 21, 2011, 12:31:48 AM »
hi, what do you get across each leg not to neutral but the 165 and the 165 ?  do you have a way of finding out the frequency of the generator?
in the dog house how is it jumped?

bob f

bmarx

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Re: Odd voltages from single phase generator
« Reply #2 on: August 21, 2011, 05:55:25 AM »
Right now generator is at 50 Hz, its designed freq, but I've also adjusted the freq to 60 Hz, while down adjusting the total voltage.  The odd voltage ratios continue without change.   Not sure if I understand your second question.  With the two windings tied to neutral, each measures about 165 v, while across the outer wires, total is 240 v.  I've tried connecting a simple incandescent bulb to each line, and it is very, very bright--as about to burn out--confirming the measured RMS ac voltage--much higher than the expected 120 v.  I've also connected a 240 v water heater across the outer leads, with significant load on the diesel engine, and the coil heats up appropriately.  I've had a couple of engineers suggest the only solution will be to assume the generator is a "delta" unit, and use a transformer with a secondary with center tap, and so get around the high voltages when neutral is used. 

buickanddeere

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Re: Odd voltages from single phase generator
« Reply #3 on: August 21, 2011, 03:24:12 PM »
  Sounds right for a center tapped Delta.
  If you need 120/208 with this gen. A "Y" transformer will be required to make a "neutral" at the center point .

bmarx

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Re: Odd voltages from single phase generator
« Reply #4 on: August 22, 2011, 03:21:31 AM »
Well, I may have to get a "Y" transformer as you say....   by the way, where is a good place to look for such a transformer?   I guess I remain curious about the odd voltages of this generator that came "off the shelf" from a factory in India.  I thought there might be some other guy out there that had experienced the same problem.  Thanks for the comments.

toydiesel01

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Re: Odd voltages from single phase generator
« Reply #5 on: August 26, 2011, 12:22:37 AM »
I agree looks like delta,  but some of these use what is called a floating neutral, which gives you crazy voltage readings.  Which I had.
1 test thing to try is connect a 120 volt light to neutral and ground of the gen head-- the light should not light----if no light then connect a wire to neutral and ground on the gen head  check voltage again each outside wire to neutral should now be close to 120v


NOTE if you connect a light to neutral and ground and the light lights up bright then do not connect a wire to neutral and ground

I had a hard time trying to understand the floating neutral concept,went to places that rewire motors and they didn't know what to think. Finally talked with an old electrical engineer who really had to study the prints.

SEEM like it was missing a wire somewhere

AFTER I connected a wire to neutral and ground of the genn head,  it acted normal and has been good since

hope this helps

bmarx

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Re: Odd voltages from single phase generator
« Reply #6 on: August 28, 2011, 04:40:34 AM »
Thanks for the suggestion regarding the ground wire.  However I had already tested for that, and have grounded the neutral.  No change in overall voltages.

Looks like I will need to use the transformer pathway.  Again, anyone with suggestion where I can get a "Y" transformer for 6 KW?

buickanddeere

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Re: Odd voltages from single phase generator
« Reply #7 on: August 28, 2011, 07:33:18 PM »
  Maybe cheaper and fewer headaches to just purchase a ST Gen head from fleabay.  #330600887179

toydiesel01

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Re: Odd voltages from single phase generator
« Reply #8 on: August 31, 2011, 12:06:21 AM »
I like Georgia generator, their shipping charges are very low and they are there for you after

piperpilot3tk

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Re: Odd voltages from single phase generator
« Reply #9 on: August 31, 2011, 01:02:30 AM »
Tom from Georgia Generator is a great guy. I drove to Milledgeville and picked up my ST8 directly from Tom.  He was as nice and honest of a guy as they come.

bmarx

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Re: Odd voltages from single phase generator
« Reply #10 on: September 01, 2011, 03:06:08 AM »
Thanks for the suggestions everyone.  May have to get another generator.  We'll see.

An experienced engineer wrote me this--and wonder what everyone thinks?

What you describe would be consistent with
an alternator with two windings 90 degrees apart where the voltage vectors either side of neutral would add up to 165 volts times the square root of two or about 233v.. This could be confirmed easily with a dual channel o' scope.
Now it doesn't make sense that they would design with 165v either side of neutral at 90 degrees apart. It would make more sense if it had 120v either side of neutral at 90 degrees apart, but still not useful for a 240/120v system. That makes me think there is a wiring error in the alternator. It probably has four or more stator coils that are connected in series (to make a set of two coils). If one of the four stator coils was connected in reverse by mistake I believe you could get a 90deg phase difference instead of a 180degree difference that the set of two coils should have. To check that would mean opening up the alternator to see if the stator coil wires are easily accessed and changed. A motor rewind shop could probably check it out and fix it if indeed that is the case.
Of course a center tapped 240v transformer would be a way to work around the problem and get back to single phase 240/120 (more correctly 180deg two phase). If you knew someone in the power company, an old "pole pig" fed backwards would do the job.

So this looks like my next step, to take the generator down to a motor rewind shop and see if they can find the stator causing the problem.