Bad Generator

Bill in IL · Feb 16, 2012

Well I figured out my generator is bad. It motors well but no change in speed when I ground the field terminal. What can be wrong with this thing? Obviously it need the field coils to run as a motor so how can they be bad? Can replacement field coils be bought or would they be rewound at a rebuild shop?

Either way its just for curiosity since tomorrow an alternator is going in its place.

Janicholson · Feb 16, 2012

The answer is that it will motor on the A terminal alone. If the F terminal is grounded, it should slow down substantially due to the fact that the field is now in the circuit, and the magnetism is supposed to make it into a generator. (when shaft power is applied)
If no change takes place, the field is either open (internally disconnected) or if it is a 3 brush design, the third brush might be not contacting.
The motor action relies on the residual magnetism of the field poles, so (though not powerful), it will turn well. Jim

John T · Feb 17, 2012

Sure, old fashion starter genny repair shops sell replacement field coils.

As far as the Motor test I always advise folks if it spins with voltage applied from frame ground to the ARM post the bushings and brushes and armature are at least in decent shape and if it slows wayyyyyyyyy down when you frame ground the Field that tells me the Fields are likely okay.

HOWEVER if theres no speed change with the field grounded as Jim notes the fields are open (often where the 2 splice in the middle) or shorted.

Its not to hard to use an ohm meter to test for open or grounded fields or to test for a shorted armature

John T

Bad Generator

Bill in IL

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Janicholson

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John T

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