DC motor response to undersized power leads

[Tmoose]

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

 

[iop995]

Cooper resistance increase with temperature and for a fixed voltage source and motor constant speed, current in circuit will decrease; depending of cable area, lenght, battery internal resistance and motor resistance, cable may blow up or if area is high enough will go up in temperature with time.

 

[Skogsgurra]

That guy may be right - under the assumption that the motor is a shunt wound motor. Then, the extra voltage drop in the cables reduces excitation and can cause armature current to rise. If it is really bad, you MAY see some heat in the cables.

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

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Compositepro]

A 12 volt DC winch motor would most likely be permanent magnet, but that's not too important. The motor current will be a function of the supply voltage minus the the counter emf of the motor and the total circuit resistance. If the wire are "undersized" that means that the wire resistance is a significant part of the circuit resistance.

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.

 

[LionelHutz]

The simple way to explain it is that the current and heating would remain fairly stable if the winch still gets enough current/voltage to do it's job. The cables would really start to heat up if the winch stopped moving. However, I doubt most people would stand there for minutes holding the winch switch in the FWD position if the winch isn't turning so melting the wires isn't very likely.