Re: Re: Re: 6v generator to 12v generator

Posted by Bob M on January 24, 2003 at 11:40:11 from (165.170.128.66):

In Reply to: Re: Re: 6v generator to 12v generator posted by Dave on January 24, 2003 at 07:38:27:

Interesting question John! There's not really a definitive answer to what is a generator's 'weak link' after being setup to provide 12 volts. But here's my thoughts:

For the field winding, running over the design current causes higher temperatures in the winding as previously discussed. High temperatures - especially deep in the windings - result in rapid deterioration of the insulation. When the insulation deteriorates enough that a short develops between windings or to a pole shoe, you've got failure. Also higher temperature may cause the solder in soldered connections to slowly creep - especially with the vibration experienced by a generator humg on the side of an engine. When enough solder creeps out or weakens in a connection, the winding goes open and again you've got failure.

The armature on the other hand usually fails when a solder joint at the commutator lets go. These joints are under considerable mechanical stress due to centrifugal force when in operation. So vibration and metal fatigue also may play a role. However armature winding insulation failure can also occur, same as in the field windings.

Field resistance is determined by the wire gage and by the length of wire comprising the winding. Typical rated field current for a 6 volt generator? Depends on the factors above, also on the coil temperature (higher temperature -> increased resistance -> reduced current). However 2-4 amps is what I've typically observed on a 6 volt system.

Regarding relative heating between the field and armature after switching to 12 volts, analysis is a bit more complex for the armature. Granted, the reduced armature current results in reduced heating (P=I^2*R). However armature current is not cut in exactly in half like you'd expect on 12 volts. This due to the increased field magnetic strength caused by the increased field current draw. End result is heating is somewhat reduced, but NOT by a factor of 50%. Further, because the current in each winding reverses polarity with each 1/2 revolution, there's hysteresis and eddy current heating going in the iron parts of the armature. How this affects the total heating in the armature is beyond my analytic capability however (I'm an ME by training, not an EE!)

Bottom line is I guess I'll agree with you: You are probably less likely to experience problems running over current through the field vs the armature. But that's just my guess....

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