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re-polarizing a generator

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Andrew Mace

Andrew Mace

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I have a good generator running on a positive ground car. I need to "borrow" it temporarily to put on a negative ground car. Can any harm be done by polarizing it to negative ground now and then re-polarizing it back to positive ground later, once my other generator is fixed? Sorry to ask such a naive question, but this is one area of electricity that I never have fully understood! :confuse:
 
I have used (for 10+ years) a generator that began as positive ground, was switched by brute force (PO installed the battery backwards and drove for a couple of years) to negative ground, then polarized again by me to positive ground.

I don't think it has any lasting effect on the unit... possibly just rearranging a few electrons the way your wife has you rearrange the living room every few years.
 
Of course, depending on what you have installed on the car, for a temporary use you might be able to rotate the battery 180° and just hook it up as negative ground for the interim.
 
The car with the "bad" generator is my "new" '67 Sports 1200 convertible. It is currently negative ground. I'm not sure it was from the factory, but it does have a 1980s Honda radio installed, hence my leaving it negative ground. The generator I'm borrowing is the original (rebuilt two years ago) one from my positive ground '62 Herald 1200 sedan!
 
Won't hurt the generator at all, Andy, assuming you remember to polarize the field and not the armature :smile:

When you first start the engine, there is almost no field current to the generator. The voltage regulator contacts have the field connected to the armature, but the cutout is open, isolating both from the battery (except for a very small current through the red warning light). However, there is some residual magnetism left in the outer housing of the generator, which is enough to generate just a little bit of voltage. That little bit gets fed back to the field coils, making a stronger magnetic field, which produces more voltage, etc. and it quickly comes up to where the cutout contacts close.

When you "polarize" the generator, you are setting up that residual magnetic field with the proper polarity for your electrical system. When you polarize the other way, the magnetic field gets flipped around as well. You can flip it as often as you like, won't hurt anything at all.

However, I would suggest remembering to polarize <span style="text-decoration: underline">before</span> starting the engine. Otherwise the cutout contacts may close with 24 volts across them, which isn't exactly good for them :smile:
 
Also an old quick trick to check the generator to see if it is working (generator is out of the car or no belt on) is to hook a 12v battery (correct polarity) to the generator, if it spins it is okay.

Marv
 
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