Genertor Repair Questions

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iwire

iwire

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Massachusetts
I need some help

I was checking out an older 45 KW generator that is set up for 480Y/277 output.

When I measure the output I find about 50 volts (no load) and this could actually be phantom voltage.

The generator control panel has a breaker labeled 'exciter circuit'.

If I flip it off than on again my meter shows 450 volts on the output for a moment than drops back to 50 volts.

The 'exciter circuit' breaker is not opening after I flip it on I checked that a few ways.

If it helps this is an Onan generator at least 25? years old.

Glad to hear any suggestions (except higher an electrician;)) and if anyone has recommendations for some generator repair / theory books I would be glad to hear them.

Thanks, Bob
 
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If I'm reading this right....... on the output where you should be reading 480Y/277 is where you are checking? And you flip off and on the exciter and you get a reading for a short time? Like it is trying to generate 480Y/277 but then stops? I'm not an expert on them but it sounds like the exciter is tripping or dropping out. I know an expert (onan) and if you give me some name plate info I'll give him a call.
 
I do not have the info handy, I will have it soon.

You are reading it right, given the conditions it seems like the breaker I am opening and closing manually is tripping open right away.

But is not opening once I close it.

Thanks:)
 
I'm thinking the breaker is tripping on a fault, but the handle is not indicating it. Turning it off then on resets it. The fault may be a diode in the exciter circuit. Seems like the 50V is due to residual field
 
Onan is now owned by Cummins. A call to the area rep. was able to get me
the manuals for a 15kw 120/240 generator that is 40 years old (for a price
of coarse). This included all the specs. and parts lists.

Steve
 
Generator

Generator

Iwire , could be control, voltage regulator, take picture
if can of, and post here. of control section.
 
Seems to me like one or more electronic components within the exciter circuit. But since electronics and I are not on speaking terms, I'm not able to help you any further than that.
 
From what I have been able to ascertain, is that field flashing may be required. It is needed when generator voltage does not build up and the generating system (including the voltage regulator) does not have field flash capability.​

The condition is usually caused by insufficient residual magnetism in the exciter and generator fields. A generator that has been out of service for an extended period may lose its residual magnetism and require flashing. The residual magnetism can be restored by flashing the field, thereby, causing a current surge in the generator. ​


Make sure you refer to the voltage regulator manufacturer's literature for procedural instructions because the voltage regulator may contain solid-state components that could be damaged.​

Beyond that, you are outside my area of knowledge.​

 
Field Flashing of Portable Generators

Field Flashing of Portable Generators

Field Flashing of Portable Generators

This tip comes from the Briggs & Stratton Customer Education Department. As an alternative to flashing a rotor winding with a battery applied to the brushes, an electric drill may be used. Follow these steps to flash the generator:

Plug the electric drill into the generator receptacle. (Cordless drills do not work)


If the drill is reversible, move the direction switch to the forward position.


Start the generator


While depressing the trigger on the drill, spin the drill chuck in reverse direction. This will excite the field and the generator will now produce electricity. If spinning the chuck one direction does not work, try spinning the chuck in the other direction as you may have the reverse switch positioned backwards.
Use caution not to get your hand or other materials caught in the chuck. As soon as the field is excited, the generator will produce power and the drill will turn on.

The reason this works is because the electric motor in the drill will act as a small generator when spun backwards. The magnets in the drill's motor induce a voltage into the motor windings, which is fed back through the trigger, cord and into the generators receptacle. From there it goes into the power winding of the stator. The voltage going through the power winding creates a magnetic field, which is intensified due to the iron core of the stator laminations. The rotor intersects this magnetic field as it is spun past the power winding, thus inducing a voltage in the rotor winding. Once current flow is present in the rotor winding the rotor has been flashed.

If flashing the field does not make the generator work, you may have additional problems, besides a lack of magnetism in the rotor. Further testing will be needed. Hopefully, this will give a simple way to field flash your generator if needed - Bruce Perrault
 
kingpb said:
From what I have been able to ascertain, is that field flashing may be required. It is needed when generator voltage does not build up and the

I think since it does generate some voltage without exiter, residual magnetism is sufficient- no need to flash.​
Click to expand...
 
ramdiesel3500 said:
I'm with RayS. ANY voltage reading at all indicates that flashing is not needed!!!
Click to expand...

In this case you could be right, however, in general I have to disagree.

Documentation supports instances where damage to the rotating field caused by extreme temperatures resulted in degradation of the steel, such that it would no longer hold a sufficient residual magnetisim. This was causing the output to be 38 volts at 1800rpm.

It is also a possibility that this generator requires some load before it will produce rated voltage.
 
To make sure I have this right, when you turn the exciter breaker on, you get no-load 277/480 for a short time then it drops to 50v give or take? If that's it, then it sounds like the exciter circuits/regulator is toast (for certain values of toast) or there's an overload in the fields. Forget about "field flashing" this puppy, call the local Onan folks.

BTW, some of the old Audel and Hawkins books have good explanations of generators, albeit a bit dated.
 
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