The Ford Barn

Conaway2 · 12-15-2020, 05:12 PM

I’ve read a lot of posts on this, but the procedure still isn’t completely clear to me.

I’ve just disassembled a generator to re-wrap the field coils with Scotch glass electrical tape, and then cleaned and re-assembled it.

It will motor with my 6 volt 10 amp battery charger attached to it - negative to output post and positive to generator case.

If I connect a multimeter to the unit while spinning it in normal rotation direction, Ican measure 70 mV on it - no cutout attached.

Questions -

is a battery charger enough to polarize the generator ?

If the generator is working, should I see more than 70 mV at the output ?

Thanks - Jim

jrelliott · 12-15-2020, 05:44 PM

When you motored the generator, it should have been enough to get the armature magnetized, probably not as much as shorting across the cutout with piece of wire for a very short time when attached to the battery in the vehicle . How fast did you spin the generator to do your voltage measurement? Did you measure the voltage on the output of the generator instead of the cutout? Have you checked the 3rd brush connection? How clean is the commutator( the copper segments the brushes rub on). The commutator can be polished up with very fine sandpaper or crocus cloth.

Conaway2 · 12-15-2020, 06:04 PM

I polished the commutator, so it looks quite good, and all brush connections are clean. I used a cordless electric drill to spin the armature, so probably not much faster than 100-150 rpm. That may be the reason for the low voltage measurement.

Patrick L. · 12-15-2020, 08:46 PM

As said, adding power to motor the generator should polarize it.
But, to polarize it just jump across the cutout terminals for a nano-second. [ original type cut-out]
Once installed if the generator has low or no output trying adjusting the third brush.

rotorwrench · 12-16-2020, 03:01 PM

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The OP needs to find a friend with a growler so that the armature can be properly tested. Check the ohm readings on the field coils. Make sure wiring or terminals are properly insulated.

Motoring by itself is not always a good indication that the components are all good to go. Polarizing or flashing the field with a DC current should give the pole shoes a good magnetic gauss. If there is no residual magnetism then the generator won't put out enough current to close the cut out switch.

12-15-2020, 05:12 PM	#1
Conaway2 Senior Member Join Date: Apr 2019 Location: Mt. Pleasant, SC Posts: 601	Polarizing a generator I’ve read a lot of posts on this, but the procedure still isn’t completely clear to me. I’ve just disassembled a generator to re-wrap the field coils with Scotch glass electrical tape, and then cleaned and re-assembled it. It will motor with my 6 volt 10 amp battery charger attached to it - negative to output post and positive to generator case. If I connect a multimeter to the unit while spinning it in normal rotation direction, Ican measure 70 mV on it - no cutout attached. Questions - is a battery charger enough to polarize the generator ? If the generator is working, should I see more than 70 mV at the output ? Thanks - Jim

12-15-2020, 05:44 PM	#2
jrelliott Senior Member Join Date: May 2010 Location: Pine, AZ Posts: 794	Re: Polarizing a generator When you motored the generator, it should have been enough to get the armature magnetized, probably not as much as shorting across the cutout with piece of wire for a very short time when attached to the battery in the vehicle . How fast did you spin the generator to do your voltage measurement? Did you measure the voltage on the output of the generator instead of the cutout? Have you checked the 3rd brush connection? How clean is the commutator( the copper segments the brushes rub on). The commutator can be polished up with very fine sandpaper or crocus cloth.

12-15-2020, 06:04 PM	#3
Conaway2 Senior Member Join Date: Apr 2019 Location: Mt. Pleasant, SC Posts: 601	Re: Polarizing a generator I polished the commutator, so it looks quite good, and all brush connections are clean. I used a cordless electric drill to spin the armature, so probably not much faster than 100-150 rpm. That may be the reason for the low voltage measurement.

12-15-2020, 08:46 PM	#4
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: Polarizing a generator As said, adding power to motor the generator should polarize it. But, to polarize it just jump across the cutout terminals for a nano-second. [ original type cut-out] Once installed if the generator has low or no output trying adjusting the third brush.

12-16-2020, 03:01 PM	#5
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,431	Re: Polarizing a generator Sponsored Links (Register now to hide all advertisements) The OP needs to find a friend with a growler so that the armature can be properly tested. Check the ohm readings on the field coils. Make sure wiring or terminals are properly insulated. Motoring by itself is not always a good indication that the components are all good to go. Polarizing or flashing the field with a DC current should give the pole shoes a good magnetic gauss. If there is no residual magnetism then the generator won't put out enough current to close the cut out switch.