How does a generator not stop itself?
How does a generator not stop itself?


It does, under a later assumption that you made:


  What happens if we suddenly remove the applied voltage and externally spin the motor at a constant speed in the same direction (CCW) … assuming we've shorted the circuit, \$I = E/R_{windings}\$ …


Shorting the terminals of a generator/motor results in maximum opposing torque. The mechanical energy is converted into heat in the motor windings. (This can be a way of braking a machine driven by an electric motor. However, see below.)

If, instead, you leave the circuit open, then there is no current, hence approximately no torque, and the back-EMF maintains a voltage across the open terminals. (This is a normal and non-destructive situation, just like an outlet with nothing plugged in.) An ideal model with no losses would keep spinning at the same speed without any mechanical or electrical power input.

In practice, if you are using a generator to do some work then there is some load on it that is neither an open circuit nor a short circuit, and so the behavior will be intermediate between these two cases.



Dynamic braking (which includes regenerative braking) is the practice of using an electric motor to stop a vehicle or machine. The simplest way to do it is to connect a suitable resistor between the terminals; then, unlike the short-circuit case, most of the energy is dissipated in the resistor, and the braking is not quite as strong as in a short circuit. Regenerative braking uses a rechargeable battery instead (with intermediate power converters managing the voltage/current); just as the motor can be a generator, the battery can absorb instead of supply power, as long as the intermediate systems are designed to allow this.