Generator Electrical Engineering Question

[dangier_VA]

Had an occasion to use my pto generator due to a power outage a few days ago. Sounds like a bearing was failing in the brush end. It is an old Niagara Power (Generac), dual 7500 watt power heads for 15,000 watt capacity. Rated for 115/230 volts, 60 cycles at 3600 rpm (540 rpm pto speed).
In pulling the pto speed increaser apart, the large driven gear that drives each power head came off with the case. In order to not loose the armature locations, I marked them as they were. Both had turned some when the gear case was pulled apart as the teeth are cut an an angle on the gears. When I put the gear case together, my marks were a little off for the armatures to be exactly 180 degrees out to produce 230 volts in tandem. I lined up both armatures exactly 180 degrees out (My marks were about two teeth off on the driven gear). Today I test ran the generator and noticed the following: While running the pto speed to produce 60 cycles, the voltage outputs were 127/129 volts (hot to neutral-both legs), and 250 volts (hot to hot). Too high!!! These readings were taken while unit was under a 5000 watt resistive load.
Now the question--Were the armatures on purpose not at 180 degrees apart to derate the generator to the correct voltage? Guess I need to pull down and slip the gears a few teeth and try again.
I still have the bearing noise.... Bearing housing is worn larger than the bearing external diameter. Tried peening with a punch and need to do more.

 

[dangier_VA]

...Thinking about this a little more. Am I right in thinking that the armatures should be 180 degrees out for max power and the individual voltage regulators should control the voltage output as needed. As long as the rpm is correct for the 60 cps.
I cleaned and checked the wiring connections while I was in there. Could that have changed something?
..Still thinking

 

[KWentling]

I have no experience with a double head generator but I wouldn't think the rotating members would need to be timed. I would assume there is a voltage regulator that has some adjustment. That said I don't think your voltages are high enough to be a concern. I've always heard +/- 10% is acceptable.

Kim

 

[Craig Clayton]

Sir
A few pics would be of interest, you may not be the first to have taken the generator apart so do not rely on what you have found.The worn end bell repair by center punching will not last forever. Use a C3 grade bearing which are for electric motors, they have a more relaxed tolerance and 2RS ( two rubber seals ) are fine, ZZ means steel shields only for high heat.
Craig Clayton

 

[zzvyb6]

How do you know it was 60 Hz? Not by the tractor's rpm indicator, I hope....

 

[dangier_VA]

zzvyv6-Nope, by power/cps meter.

KWentling-The generator gets the 220/240 volts from the paralleling of the power heads with the wiring (one leg from #1/one leg from #2) to the breaker. Would it make sense to get the max use of the magnetic field? The other leg from each power head is grounded.

Craig Clayton-I have pictures of the complete unit (attached). Did not take pictures when it was broken down. I may have to go to a larger bearing and have a machine shop bore out the end bell. Right now I have about .20 slop.

 

[LD1]

I may not totally understan this but I will throw my opinion out there.

Each head is supposed to generate 120v.

It shouldn't even matter if the two heads arent 180 degrees appart, IF you never use 240v from it. The fact that you are getting 250V is die to each head making more than 120V each. Changing the timing will drop the higher voltage reading, but wont be 180 appart, which may be worse for whatever you run than just having 250V. Most everything you buy that is 240V can run on +/- 10%. Which would be 216v-264v and you should be fine. But the root cause of your problem lies in each individual head making more than 120v, and NOT the timing of the two units.

If you have an accurate meter, verify that you are indeed getting 60Hz. If you are getting more than 60, slow the RPM down till you get 60 and then re-check the voltage. Too many RPM's will give you more HZ but will also increase the voltage slightly.

 

[Bill_C]

If you really want to check the "timing", an oscilloscope would easily show the two waveforms.

 

[hr3]

dangier_VA,, Do you have any readings from before you took it apart?? What are the voltages with no load??? Model and ser. num. off unit would be nice too..

 

[zzvyb6]

I second the motion to use an ocilloscope to check this out. Its not the voltage of frequency that you should be concened about, but the phase content. Chances are that there was a hiccup in the sinusoidal waveform at around zero volts that's going to worry the circuit of any motor or trnasformer that its hooked up to. Such a unit will run extra hot because of this waveform deviation and will probably cause premature failure in its windings or worse, a fire.

You may have actually fixed the condition. Just like extra pressure won't hurt a garden hose if its a good quality deal, the extra voltage won't either as long as it doesn't jump the insulation insulation. The glitch in the waveform though will have serious consequences.

Be carefull to make sure the windings and armatures are also symetrical. The gears may not reflect the true symmetry of the two alternators. The gears may have been adjusted to give perfect correlation.