PTO Generator

I've been given an old power take-off (PTO) style generator for my farm tractor. It has a gear box, and two 120 VAC generators each rated at 7500 watts. The generators are connected through what appear to be two large transformers and then connect to a single 60 amp two pole circuit breaker, and from there go to a 240 VAC outlet and a 120 VAC duplex outlet. The unit is old, and was either made or sold by Niagra Generator, and the individual generators have Generac labels on them, but I haven't been able to find any information concerning a model number, or a schematic or anything. I don't even know how old it is.

When running at normal speed, I can read 120 VAC from either generator, but when I read across the breaker I only read 50 to 55 VAC, instead of 240 volts. I can run a 120 volt load no problem, I have connected 600 watts, but I can't get 240 out of it.

Any ideas, questions, suggestions, comments, theories, guesses and hints are welcome.
Thanks!
Jerry

I thought the way they did '220' (or 240) in the States was actually 2x 120v legs, not actually a 220v line. But if the thing was actually set up to give an actual 240v, it almost sounds like one of the generators is about 60 degrees out of phase, 60 electrical degrees depending on how many poles the generator actually has, not 60 shaft degrees.

Quote:

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Originally Posted by skimask

I thought the way they did '220' (or 240) in the States was actually 2x 120v legs, not actually a 220v line. But if the thing was actually set up to give an actual 240v, it almost sounds like one of the generators is about 60 degrees out of phase, 60 electrical degrees depending on how many poles the generator actually has, not 60 shaft degrees.

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Right! Think center tapped transformer with the center tap grounded (earth).
Think you are right about a phase error too.

Hi Ski and Joe,
You're correct, it is two separate 120 volt 'legs' connected together to produce 240 VAC. I read 120 volts on each generator, but not 240 volts across both. I'm going to split them up and put a 24 volt transformer on each output and connect that up to the scope. I suspect that when I split them I'm going to find out that only one is actually putting out 120 volts, and that it was feeding the other one through the transformer. If the other one is putting out less voltage out of phase that would explain it.

I have seen a lot of PTO generators before that produce 240 or 208 volts, and then drop this to 120 through a transformer, they are pretty common on farms in the U.S. but I've never seen one like mine before.

I'll let you know what I find next!
Thanks,
Jerry

gear or belt driven ? the gens have to be locked together at same phase angle or you can't join the legs that way. if gear then you can shift teeth position until the 240 is actually the addition of the 2 120's. if belt driven, you can never guarantee same phase angle so you may have to do something else with output loads.

don's 2-cents

A little slow on the comback here, I'm sorry amgen, I didn't get your question until today. Each generator is 115 vac @7500 watts. They are gear driven to provide the correct phasing. My guess is that one set is working and backfeeding 115 to the second. I can get 115vac out both gensets, but only 115 7500 watts. I'm going to set it up and disconnect the transformer and verify the both units are putting out. Then I'll get it working. I have other gensets, but this will be my largest, aNd most usefull. I want to then monitor output and automate the tractor regulation setup.
Jerry

Niagara Generator has a home page and appears to have a customer service number. Sears looks like it sells parts for some Niagara/Generac generators. Contacting either may yield a manual or operating instructions. That might get you rotational position without having to measure phases. I'd bet there are some markings on the shaft/housing to ensure correct phasing. I also wonder if the xformers tie may be corroded. That would be easy to check by measuring resistance between neutrals.

If you go the transformer route to compare phases I'd caution you to be very careful. I don't think there will be a GFI and if so it won't quit until those 60 amp breakers trip. It could tingle a bit. I don't see any way to do it differently though if there are no markings to ensure correct phasing.

so, if there are 2 independent windings which can be hooked series or parallel, test phasing as follows,
if you are sure there are no other connections on output legs, you can join 1 connection and meter the legs to determine the phasing and output voltages for different hookups. careful

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