Repolarize generator

T

talwrench

Member
This site has given me a lot of help as a new 1930 pick-up owner. After researching about changing from negative ground back to positive I think I have done it correctly, but would like feed back

When I purchase the truck in January it had and 8 volt battery connected negative ground. It was bad so i purchased a 6 volt battery and connected it positive ground. As i expected, the amp meter now shows discharge when running. So I repolirized the gen. To my surpise the amp meter still shows discharge. When I check out put of gen with my multi meter it seems to be charging positive to ground. DO I JUST NEED TO SWITCH WIRES AT AMP METEr?????? Thanks for the help!! Tim
 
To polarize/flash, start engine and jumper across cutout leads for a nano-second. That is, as long as you're not using a diode or regulator.
I only reverse the ammeter wires if it reads a charge when I turn on a light.
 
Thanks Patrick, I flashed the gen without the engine running. Will try it your way tomorrow. Iam not running reg or diode.
 
If you change from negative ground to positive ground, you will have to reverse the wires on the ammeter.

The generator can also change it's polarity with a redo of the residual magnetic field polarity via a repolarize.

Marc
 
Tim,

What I mean by "polarity" of a generator is : when the generator is turning the output from the generator terminal is positive while the frame is negative OR the generator terminal is negative while the frame is positive.

All DC voltages.

For the Model A as built, the generator put out a negative voltage with respect to the frame as the Model A used a "positive ground" system.

The generator can be made to switch to a positive output voltage with respect to the frame by repolarizing the generator. Also, it can be switched back to the original setup by again repolarizing the generator.

The generator converts rotation to electrical current by having wires (armature) passing through a magnetic field (from the field coils). The direction of the magnetic field determines the "polarity" of the generator. In the normal operation, the initial "start" of generating current happens from a small amount of residual magnetism in the field iron. This produces a small amount of current which flows into the field windings...creating a stronger magnetic field. Then this stronger magnetic field produces even more current which again produces more current....on and on until the system limits itself with the help of a connected battery.

When you polarized the generator, you are putting in a small amount of residual magnetism into the metal by having current flow in the field windings. The direction of the current flow in the field windings while you are polarizing the generator sets the "polarity" of the generator.

Marc
 
If the generator is not polarized with respect to the battery it effectively becomes in series with the battery. Not a good thing and hard on the reverse current relay. It increases the chance to weld points.
 
I think you answered your own question. You checked it with a multimeter and its charging correctly. Just reverse the wires on the ammeter and all should be well.
 
Also check the coil polarity, as that might have been switched. Do a google search for "coil polarity", and the first one to appear will probably be by the Chicagoland MG Club. This takes only a couple minutes to read and tells why you want correct coil polarity.
 
I have a strange thing to add to this.
My Truck was parked for 34 years, left untouched. Once I purchased it I put a battery in it and got it running. I never polarized the generator and it was charging. I tested it by removing the battery cable when it was running and it continued to run without even a miss.
I thought for sure i would have to polarize the generator. I'm not sure why I didn't have to.
 
Looking at the attached picture, when flashing the generator to repolarize, the jumper goes from where to where? What are the two leads referred to above?
 

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A DC current passed through a field coil in proper polar direction is the best way to put some residual magnetism in the pole shoes. We do it to magnaflux all the time. The only problem is that the generator would have to be partially disassembled to do that with a direct connection to the 3rd brush field wire. The output terminal on the generator is connected to the 1st brush so the current would have to pass through the armature and that's not the best way to do it. It's easy on a 2 brush generator. A person also has to be careful not to damage the cut out relay if it still has one or a Fun Projects regulator. They should be isolated. Some folks just motor the generator and hope that it gets a good field going in the pole shoes. It will leave some residual when motoring but I don't know how strong the magnetic gauss would be in the pole shoes. If it works then that may be the hot smokin tip just as long as it doesn't get hot and start smokin.
 
I started my engine and then briefly jumpered as outlined above. No change....still no indication that the generator is working. While the engine is running, is there a way to check if there is output besides looking at the ammeter? I'm thinking that since the generator got really hot the other day that it may be toast.
 
Pull the band off the generator and look inside, see if it looks burnt or threw
any solder off the armature. You could also loosen the fan belt, jump the cutout
like when you polarized it and see if it motors.


Bob
 
Bob C said:
Pull the band off the generator and look inside, see if it looks burnt or threw
any solder off the armature. You could also loosen the fan belt, jump the cutout
like when you polarized it and see if it motors.


Bob
Click to expand...


I removed the band and nothing looks amiss. I loosened the fan belt, turned on the ignition, and jumpered the cutout. Generator did not try to motor.

Figuring I need to get a reconditioned one and go from there.


Thanks.
 
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