Towing

Towing an electric vehicle with a permanent magnet motor can cause major problems. If any E.V. is to be towed, you must disconnect the motors or else jack the machine so that the drive wheels cannot turn.

If a PM magnet motor is rotated manually then it becomes a generator, if it is rotated fast it will generate a lot of power. This power will need to go somewhere, and there are two main scenarios to consider;

Shorted motor

If the controller is one that short-circuits the motor at zero speed, then towing will cause the motor to generate into the short. The motor will generate a  current that depends on the towing speed, and it can easily exceed the motors rated current. Not only will that be like towing with the hand brake on, but the generated current can burn out the relay contacts of the controller and may also damage the motor.

Open-Circuit motor

If the controller does not short out the motor when switched off, the motor will generate a back emf that depends on the towing speed, this can easily be larger that the normal battery voltage, and may even be higher than the safe voltage of the controller.

This excessive back emf will drive a current through the controllers MOSFETs (via the integral, reversed body diode) to recharge the battery. However since it is flowing backwards – the MOSFETs cannot interrupt it and the controller cannot control the current. So it is likely to rise to an excessive level and destroy the controller.

Disconnecting the battery won’t help. Since the back emf generated by the motor is larger than the normal battery voltage it can be higher than the controller can handle and the controller will simply fail.

Note:

Pushing a model loco at walking pace is unlikely to cause problems.