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question-onan 15 kw pto generator

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Old 02-23-2006, 11:47:03 AM
NDcardoc NDcardoc is offline
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Question question-onan 15 kw pto generator

hi all have a question on a 15 kw pto drive generator. model#15.0uf3 s/153e.
it won't put out any voltage. i inspected the brushes and all is clean and free, the slip rings (3) are clean, and all the connections are tight also. the 2 circut breakers are making contact, the fuse is good also. there is a field boost timer on the control panel- i put that on while it was spinning and some smoke came from the diode that is connected to the timer. any help in explaining what that timer is for would be apprecieated. also this generator has been setting for quite a few years, does the fields have to be recharged or something like that? thanks for your time.
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Old 02-23-2006, 03:20:41 PM
Jim Rankin Jim Rankin is offline
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Default Re: question-onan 15 kw pto generator

I would start by ohming the field. Lift the brushes and put a piece of paper or thin cardboard under each brush to keep it from touching the slip ring. Then set the meter on a low scale-ohms and check that the field is continuous from slip ring to slip ring and not continuous from slip ring to shaft. You should expect something like a few ohms through the field.

If this checks out fine, then I would remove and check the diode that smoked I'll look this afternoon, I have what is bound to be a similar set, but do not have a diagram or manual on it. It has the same field boost timer.

You could also ohm the stator to make sure there is no short to ground or open circuit in the windings. You will have to remove any exciter connections to the stator before you can check the stator. You will be unable to detect a turn to turn short circuit in either the stator or field with a multimeter so this isn't an exhaustive check, but might find something not so obvious as a burned winding.

One other thing, when you say the generator produces no voltage? how little could you detect? "residual"voltage will be 20V or less. Since it smoked the diode, it must be doing something, but if it really has no voltage, you could try flashing it with a battery by momentarily applying 12v or less to the brush leads. Look for + and - markings on the leads. If you do this while running at rated rpm it should make voltage on the output leads---BE CAREFUL. This can also be done with the alternator stopped. If it makes voltage while flashing, the basic alternator is good, look for trouble in the exciter. You can also check for equal voltages between each circuit in the stator. If three phase check all combinations of phases (A-B, B-C,C-A) and they should be equal either with residual voltage or with flashing voltage applied to the field. You can also check each phase to neutral if there is one. If single phase 120/240 check the two hots to neutral they should be equal that way and have 240 between them.
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Old 02-23-2006, 07:11:44 PM
NDcardoc NDcardoc is offline
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Question Re: question-onan 15 kw pto generator

Thanks so much for your reply. now i am wondering if that black bullet shaped thing with a stiff wire comming out each end really is a diode. I took it out and did a check on it similarly to one i do to an automotive diode. both readings were .143 ohms both directions. also, what is the purpose of the field boost timer and does the generator have to be running when it is turned on? Thanks again, db
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