DC motor response to undersized power leads
DC motor response to undersized power leads
(OP)
I tried a few searches but came up empty.
There is a discussion about the correct sized cable to power a low end 12 volt DC winch to pull stuff onto a trailer. One participant has proposed that if an undersized cable is used the cable will heat up. But he then says " But if the wire starts to heat up, current draw can go up sharply, causing more heat, current goes up more yet, etc, etc and things start melting or the motor burns up. "
I don't follow how greater resistance will cause the motor to draw more amps.
With slightly reduced available voltage, and resulting slightly^2 loss in power, I believe the winch will have to run considerably longer, contributing to overheating of the motor and the cable, but I'd expect the current draw to be reduced.
Is my simplified view in error?
thanks
Dan T
There is a discussion about the correct sized cable to power a low end 12 volt DC winch to pull stuff onto a trailer. One participant has proposed that if an undersized cable is used the cable will heat up. But he then says " But if the wire starts to heat up, current draw can go up sharply, causing more heat, current goes up more yet, etc, etc and things start melting or the motor burns up. "
I don't follow how greater resistance will cause the motor to draw more amps.
With slightly reduced available voltage, and resulting slightly^2 loss in power, I believe the winch will have to run considerably longer, contributing to overheating of the motor and the cable, but I'd expect the current draw to be reduced.
Is my simplified view in error?
thanks
Dan T





RE: DC motor response to undersized power leads
RE: DC motor response to undersized power leads
But, if it is a permanent magnet motor, that will not happen. And not with a series wound motor, either.
But of course, it is always good to have properly sized cables. Anytime, anywhere.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: DC motor response to undersized power leads
Take, for example, the case where the cable resistance is equal to the motor resistance, and the battery resistance is relatively negligible. Under no load the motor would spin fast and draw little current and there is no problem. As the load increases the motor slows down. But, under load, the maximum torque capability of the motor (the stall torque) is half what it would be with no cable resistance. So, with undersized cables the motor is much more likely to stall or run slower and more power is dissipated in the cables. Copper has a fairly small positive temperature coefficient of resistance which will increase the power dissipation in the cable, but this is small factor. The main factor is that a stalled DC motor will act like a piece of wire and the power will be used at the point of highest resistance in the circuit. Another way to look at this is that the motor acts as a high electrical resistance at low load and the effective resistance decreases with load.
I'm sure there is a more elegant way to explain this.
RE: DC motor response to undersized power leads