Quote:
Originally Posted by Gaz64
Any voltage drop (resistance) in a circuit lowers the current draw. A variable resistor for dash lamp dimming is a example.
A starter motor operating at 10 volts instead of 12 volts is drawing less current, hence lower cranking speed.
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Yes, lower voltage gives lower amp with a resistor, however a motor is an inductive load. I was thinking in terms of an AC motor where the hz is proportional to rpm. If you drop the voltage to an AC motor it will pull more current to make the same power, since the rpm and torque load does not change (in a given service). But a starter is a DC motor. So yes I was wrong, but for a different reason. Your correct, but may have missed a point.
With a DC motor (starter), rpm is proportional to voltage and current is proportional to torque. There is a voltage drop from the resistance of the conductors (wires and frame). So the voltage at the motor is lower, resulting in less rpm. Since it will be turning the engine slower it will take a little less torque to spin it slower. In fact, the motor will pull a little less amps. However there are amps flowing into the resistance in the conductors, heating the wires. The total amp draw at the battery will be higher than what the motor pulls. Too many variables that are unknown to say if the amp draw on the battery will go up, down, or a wash. The hotter the wires get the more the resistance and it kind of turns into a snow ball, where more and more power is going into heating the wires and less and less voltage is getting to the starter motor.
If you do not have heavy enough wires or bad connections, this can happen. When the conductors are all good, the numbers become insignificant and none of this matters.