How to short a dc motor?

In a brushed DC motor, in order to quickly stop the rotor when it is rotating due to inertia after having turned off power, the brushes can be shorted (short-circuited) to apply braking.

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When the brushes are disconnected from the power supply, and with the coils (rotor) still rotating counterclockwise, the brushes are shorted together.

In state ①, as explained in the previous article on the principle of power generation, a positive (+) emf occurs in the left-side brush relative to the right-side brush, and so because the brushes are shorted to each other, current flows. As a result, the outside of coil A is N, while the outsides of coils B and C are S.

When the motor has transitioned to state ② as well, current flows similarly, so that the outside of coil B is S, and the outsides of coils A and C are N.

Thus, when the brushes are shorted together in this way, a rotation force that is opposite the current rotation direction occurs (solid black arrows), resulting in braking to stop the original rotation. This is called short braking.

Because the larger the current flowing, the greater is the force stopping the rotation, at high rotation rates a strong braking force is applied, but as the rotation rate falls the braking is weakened, and when rotation stops, the force becomes zero.

Shorting a motor by itself is no problem. It only becomes a problem if external forces turn the motor shaft fast enough to cause too much dissipation in the motor. When the motor is shorted, all the mechanical power from external things driving its shaft ends up getting dissipated in the motor as heat. As long as the motor can handle that heat, there is no problem. It should take no power electrically to keep the motor shorted.

The short circuit is caused by a buildup of dirt, copper particles, and debris between commutator segments. The segments are then shorted together resulting in arcing between the brushes. A flashover can be catastrophic to the motor and motor brushes.

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How to short a dc motor?