[Kris Verdeyen]

An easy way to think about a motor is as two parts in series: a back EMF and a winding resistance. The winding resistance is what you read on an ohmmeter if you put it across the motor leads - for something like the FP motors, it's very low, around 0.1 Ohms or so, and essentially doesn't change with respect to the motor's velocity. The other part of the model, the back EMF, is basically just another voltage source in series with the resistor. It is proportional to the speed of the motor, so at no load, when the motor is hauling, the back EMF is very close to 12V, but at stall, the back EMF drops to 0V.

This means that at no load, the current through the motor is equal to (12V supply - almost 12V back EMF)/ 0.1 Ohms

or, in other words, not very much, because the back EMF is almost equal to the supply voltage.

But, at stall, the back EMF drops to 0, so the 12V from the battery is sunk completely across the motor winding resistance, which gives a current on the order of 120 Amps.


That's how the math goes, but as for the physical origin of the back EMF, that comes from the spinning motor wanting to act like a generator. When you drive a motor, either electrically or by spinning it with your hands, it causes a coil of wire to move through a magnetic field. Whenever that happens, it causes a current in the wire, which manifests itself as back EMF. The winding resistance, on the other hand, simply comes from the resistance of the wire in the coils itself.