Is current inverse to voltage in DC motors like in AC motors?

[LarryFine]

I'm having a debate discussion in a motorcycle forum about the current of starter motors with varying battery voltage.

My position is that the torque output is proportionate to electrical power input, just like with 1ph and 3ph AC motors.

The "prosecution" says that DC motors, especially permanent-magnet motors, do not behave the same way as AC's.

As an aside, I thought starter motors were series-wound. I don't know whether that has any effect on the discussion.

So, my question is whether DC motors use more current with a lower voltage? "Does the defense's case hold water?"

 

[winnie]

The speed of a common AC induction motor in normal operation is approximately constant when the voltage changes, so the mechanical power output is approximately constant as the voltage changes. Since power is approximately constant, current varies inversely as voltage.

The speed of a DC motor varies when the voltage changes, so the mechanical output isn't constant. I'd expect a different current to voltage relation.

Jonathan

 

[jaggedben]

DC motors draw more current, and spin faster, the higher the applied voltage. AFAIK it's pretty close to just being Ohm's law, (probably leaving aside the change in resistance if the motor gets jammed or something).

 

[retirede]

The motor construction will also make a difference. Series-wound, permanent magnet, etc.