Braking with an electric motor

A typical DC motor may be reasonably well modeled as an ideal motor in series with a certain amount of inductance and resistance. An ideal motor will always have voltage across its leads proportional to rotational speed, and current flowing through it proportional to torque. If a motor with frictionless bearings and 2.4-ohm internal resistance would have a no-load speed of 1200rpm at 12 volts, then such a motor rotated at 1200rpm would generate 12 volts. If the leads are shorted, such a motor would have 5 amps flowing through it, and would generate 5 amps' worth of torque.

As a pretty good approximation, shorting a motor which is spinning at a certain speed will generate braking torque pretty close to what would be obtained by driving a stalled motor with the same voltage as the spinning motor would generate open-circuit.

This is a more popular method of braking a motor than many people seem to realize. The physics behind it boil down to the fact that an electric motor is a generator (the input power is provided as mechanical power from the shaft, e.g. when braking due to inertia) as well and can 'flip' from one operation mode to the other without any further extras. In practical applications this is done either by feeding back the energy stored in the motor into the supply (called regenerative braking), if the supply allows it and the amount of energy saved that way makes it economically worthwhile (the power electronics are more complicated - and expensive - in that case) or you can simply connect a resistor (imaginatively called braking resistor) across the motor during braking. I strongly suggest using a resistor and not simply shorting the motor to avoid overheating or other unwanted side-effects from the big reverse currents that are produced. You must keep in mind that the energy released through braking (accumulated as kinetic energy in the motor plus the mechanical inertia of the load e.g. flywheel) will be transformed into heat and you don't really want it to be dissipated in the motor, but rather in the braking resistor. Otherwise, happy braking! :)