Question on a-circuit 2 brush generator

Wpiescik · Oct 31, 2022

I tried to test an a circuit generator, but thought it was a b-circuit. I disconnected both harness wires and jumped d+to DF when idling. I got a couple of volts.

I removed the generator and took out to the local shop, and they told me that I had burned out the field.

I can't understand how this would burn out the field. When I look at the diagrams on this site, it looks like the DF terminal is connected to the field coil, routed around the magnets the terminates at the output brush, which should be +voltage, therefore no short. Why would it burn out?

I confess, this is not a tractor at all but a vintage sports car, but this forum seems to be the most knowledgeable about these old generators.

JMOR · Oct 31, 2022

(quoted from post at 13:38:19 10/31/22) I tried to test an a circuit generator, but thought it was a b-circuit. I disconnected both harness wires and jumped d+to DF when idling. I got a couple of volts.

I removed the generator and took out to the local shop, and they told me that I had burned out the field.

I can't understand how this would burn out the field. When I look at the diagrams on this site, it looks like the DF terminal is connected to the field coil, routed around the magnets the terminates at the output brush, which should be +voltage, therefore no short. Why would it burn out?

I confess, this is not a tractor at all but a vintage sports car, but this forum seems to be the most knowledgeable about these old generators.

f all connections were as it reads, true, it does not make sense, a I see it. No greater field current that in normal operation. Field will draw only the amount of current that the winding resistance allows, no matter which end the windings are fed from. You probably told them what you had done before asking for a test.

Steve@Advance · Oct 31, 2022

You didn't burn it out.

It may have an open field, not necessarily burned out, but maybe a bad connection.

Or it may have been open before you tried to test it.

I would get it back and take it apart, see if it is an A or B, look it over and find the problem. You
need to know in order to get the right regulator.

Remember, generator shop employees can be just as confused as the rest of us! They hate to see
generators come in because they often come back. They price it high hoping you'll go away!

How correct does this need to be? Maybe an alternator disguised as a generator would be in your future?

John T · Nov 1, 2022

Regardless if its an A or B circuit, when Full Fielded and calling for MAXIMUM charge the Field coils (if
alls well) cant draw more current than V/R IE Battery voltage divided by field coils resistance (Example 12
Volts/4 Ohms = 3 field amps) ... I can envision a short somewhere in the fields (especially where the 2
splice in the middle) as drawing excess current and burning open, but NOT if full fielded.

John T

Wpiescik · Nov 1, 2022

Thank you! It was driving me crazy to think I had done this. The only thing I could think of was that the generator had flipped polarity, but even then there shouldn't be a problem.

Thanks for weighing in. Conveniently, the same generator change available first sale on the Alfa Romeo for extraordinariky short money, did I bought that for replacement, parts or spare. ($80!)

JMOR · Nov 1, 2022

(quoted from post at 10:08:11 11/01/22)
Thank you! It was driving me crazy to think I had done this. The only thing I could think of was that the generator had flipped polarity, but even then there shouldn't be a problem.

Thanks for weighing in. Conveniently, the same generator change available first sale on the Alfa Romeo for extraordinariky short money, did I bought that for replacement, parts or spare. ($80!)

ctually, now that you mention it.....applying voltage to the Field terminal (where is would normally receive ground via VR) would polarize the generator opposite normal and result in reversed voltage output at ARM terminal. As a result, if it did in fact generate, that would double the voltage applied to the field & double the current. However, even double current for 30 seconds or so would be very unlikely to damage the field windings.

Question on a-circuit 2 brush generator

Wpiescik

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JMOR

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Steve@Advance

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John T

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Wpiescik

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JMOR

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