Short Braking

In succession to the principle of rotation and the principle of power generation of a brushed DC motor, short braking is here explained. This is part of a series of the principles of operation of brushed DC motors, and so they should all be understood conjointly.

Short Braking of a Brushed 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.

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.