Dynamic braking of 3-phase motors by capacitors

When a series capacitor is introduced in one of the supply lines of a 3-phase induction motor working under normal load conditions, the torque developed by the motor is considerably reduced. If the speed falls below a certain value, the machine develops braking torque and decelerates to standstill and may even run in the opposite direction. This aspect was touched upon in a previous paper1 and is now investigated further, both theoretically and experimentally. Applying the method of symmetrical components, the current and torque equations are obtained for the machine during the braking period. It is suggested that the driving torque developed by the machine at high speeds, even with the introduction of the series capacitor, can be avoided either by working the machine as a self-excited induction generator or by introducing a suitable series reactor in another supply line. These methods of dynamic braking are compared with the conventional plugging method.