Automatic Starter for Split-Phase Motor

Automatic Starter for Split-Phase Motor: 

Automatic Starter for Split-Phase Motor

The Split Phase Motor: 

The Split Phase Motor Split phase induction motors are most widely used type of single phase motors. They are made in larger sizes than shaded pole type motors.

Automatic Starters: 

Automatic Starters These are used in split-phase motors, especially when the latter are large and must be started frequently; moreover, such equipment may be provided with more than one push-button station, sometimes remotely and conveniently located, as well as some form of electrical braking.

Two typical arrangements:: 

Two typical arrangements: Starter for Two-value Capacitor Motor Starter, with Dynamic Braking, for Split-Phase Motor

Starter for Two-value Capacitor Motor: 

Starter for Two-value Capacitor Motor This type of split phase motors have two capacitors of different values in the auxiliary winding. The motors develop a high starting torque with one capacitor of a higher value and gives a quite running performance with the other capacitor of comparatively lower value.

The Circuitry: 

The Circuitry

Description of the circuit: 

Description of the circuit In the circuit shown an electrolytic capacitor has been used for starting purpose and an oil capacitor has been used for continuous running operation. During starting both the capacitors are in the circuit. When the motor picks up speed the electrolytic capacitor get disconnected cue to opening of a relay contact.

Operation: 

Operation When the START-push button is pressed the M contactor is energized which in turn energizes both the main and the auxiliary windings due to closing of contacts M 2 and M 3 . A thermal timer TR is also energized along with the contactor.

Cont.: 

Cont. This thermal timer consists of a heater coil wound on a bimetallic strip. After a pre-set delay the bimetallic strips bends and closes contact TR 1 . Closing of contact TR 1 causes energization of relay R which then gets hold through its own contact R 2 . at the same time relay R disconnects the electrolytic capacitor from the auxiliary winding circuit due to opening of its normally closed contact R3. The thermal timer gets de-energized as soon as relay R is energized due to opening of its contact R 1 .

Dynamic Breaking of Split Phase Motors: 

Dynamic Breaking of Split Phase Motors A full wave rectifier is used to obtain dc supply form 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 contactors. A rheostat is provided in the dc circuit to control the time required for the motor to stop. Less is the value of rheostatic 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 Circuitry: 

The Circuitry

Operation: 

Operation When the STOP-push button is pressed, contactor M gets de-energized and closes its contact M 2 . Contactor DB gets energized through the back contact of the STOP-push button, closed contact M 2 . 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.

Cont.: 

Cont. 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 TR 2 and thus de-energizes contactor DB and gets re-set. The time allowed to stop the motor, as explained earlier, can be varied by adjusting the rheostat R.