A little direct current can stop a small ac motor in a second! DC applied to the windings will make the motor work as a shortcircuited generator, thus creating a force opposite to its rotation. In the case of larger motors with low resistance windings, a 12 VDC power supply can be used instead of the capacitor, removing the diode and resistor R1. The DC supply must be able to feed the motor windings without being overloaded.
