Field Windings and Amps

[I'mClueless]

Hi,

I built a DC welding machine that uses a Leece Neville 270 Amp alternator to manufacture the current. I have done a lot research for this project and one of the articles that I read mentioned limiting the field current to 3 Amps in order to make the brushes last longer.

I can see where fusing the circuit to limit the Amps in case of a malfunction, but does not the field windings dictate the Amps?.

Yep,
I'mClueless

 

[MikeMl]

Automotive alternators act like a current amplifier (when turning at a constant rpm) where the output current is proportional to the field current. At a constant applied voltage, the field resistance determines the field current (If = Ef/Rf). If you reduce the field current by putting additional resistance in series with the field, or by reducing the applied voltage, the output current will be reduced proportionally. If you need the full 270A, then you will have to apply the rated field current.