DYNAMIC BREAKING OF SPLIT PHASE MOTORS

The principle of dynamic breaking discussed earlier in connection with poly phase induction motors, also applies well to split phase motors. The procedure, as already explained, is to disconnect the stator windings from ac supply while the motor is running and instead, connect dc supply across the windings. Full wave rectifier is used to obtain dc supply from the available ac source. Precaution must be taken to avoid, simultaneous energization of the stator windings from both ac as well as dc supply. This is taken care of by providing electrical and/or mechanical interlocking of the contactors. A rheostat is provided in the dc circuit to control the time required for the motor to stop. Less is the value of rehostatic value of resistance, more quickly will the rotational energy get converted to electrical energy and get dissipated as heat and hence less will be the time required for the motor to stop. The control diagram for dynamic breaking of a split phase motor is shown in Figure.

Figure: Control circuit for dynamic breaking of a split phase induction motor

When the STOP-push button is pressed, contactor M gets de-energized and closes its contact M₂. Contactor DB gets energized through the back contact of the STOP-push button, closed contact TR₂ and contact M₂. Timer TR also gets energized along with contactor DB. The instantaneous contact of relay TR, i.e, TR₁ closes and thus both contactor DB and relay TR remain energized. Contactor DB energizes a bridge rectifier which feeds dc current into the stator winding of the motor. Current to be fed is adjustable through a rheostat, R. Due to feeding of dc current the motor slows down and comes to a quick stop. After a pre-set delay which would match with the time taken by the motor to stop, the timer TR operates and opens its delayed contact TR₂ and thus de-energizes contactor DB and gets reset. The time allowed to stop the motor, as explained earlier, can be varied by adjusting the rheostat R.