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If a starter were a simple resistor, Ohm's law would say that applying twice the voltage would result in twice the current (not half). Four times the electrical power would then be flowing through the starter. But a starter is not a resistor. It is a series wound DC motor (or a permanent magnet DC motor). The faster it spins, the more back EMF it develops. This tends to raise the effective resistance of the motor and reduces the current flow. The differnce in current draw between full stall and rated speed for most DC motors of these types is about 5 to 1. If the engine is stuck, and the starter motor is fully stalled as it attempts to spin, the current draw will be maximum. Under these conditions, the motor does behave like a resistor. Apply twice the voltage and the motor will draw twice the current, and likely burn out if forced to do this for very long. If the engine to be started is free to turn, applying a higher voltage to the starter will cause the starter to spin faster. Current draw will be somewhat higher than with the rated voltage and the same load, but less than when operated on its normal voltage but with the starter stalled, or turning against a very stiff engine. The no-load speed of the starter will be higher with a higher voltage. Should the bendix disengage and the starter freewheel, there is the potential of damage to the armature from centrifugal force. So the operator has to be more careful when applying a higher than rated voltage to the starter. Extended cranking, putting the vehicle in gear and attempting to move it with the starter, and operating with a starter drive that sometimes doesn't "catch", are things you can get away with with the normal rated voltage, but could lead to damaging the starter when operating with a higher voltage. I use 12 volts on the 6 volt starter on my 9N tractor and it works quite well. I would hesitate to apply 24 volts to a 12 volt starter, but would think it would be OK at least for a while.
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