Can a 6 volt positive ground generator be used in a 12 volt negative ground system? The question pertains to converting a 6 volt positive ground automobile to a 12 volt negative ground system. The majority of online automotive forums state that the field coils must be changed when converting to 12 volts. My understanding of a DC generator is that it will continue to increase voltage output until regulated, either by a voltage regulator or by the battery. So I would think only the voltage regulator would have to be changed when converting to a 12 volt system. The field coils will reverse polarity when polarized on the first connection of the 12 volt battery correct?

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  • Can you make an apple into an orange? Not to sound trite but the generator, chassis is its ground, so only the 'hot' wire needs to run to the battery. You would have to tear apart the generator to reverse the connections from the rectifiers, usually 3-phase, and ground the negative output. Good luck with that... – Sparky256 Jan 16 at 0:16
  • I suspect that using the original field coils will mean that a higher idle speed will be required to keep the battery from discharging at idle. If the field current is increased too much, the field will burn out. – Charles Cowie Jan 16 at 0:19
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    @Sparky256: In US English usage, a "generator" has brushes, an "alternator" has rectifiers. They didn't have silicon diodes in 1936... – TimWescott Jan 16 at 1:29
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    Assuming the iron doesn't saturate, if the generator is wound for 6V and you try to run it on 12, you'll need to double the field current. You can do that, because you have twice the voltage to force current through the field coils -- but those poor coils will be dissipating four times the heat due to \$I^2 R\$ losses. You want to have the thing rewound so that half the field current generates the same field, and that same field generates twice the voltage. Find a shop that rewinds generators, or if your car is common enough, just buy a 12V from someone who does that sort of thing. – TimWescott Jan 16 at 1:34
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    Note that reversing the polarity is a snap, because it's got separate field excitation. Change the polarity if the field, and the 'lectricity coming out of the thing changes polarity. – TimWescott Jan 16 at 1:36

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