Electrical Engineering Knowledge Sharing Hub: Starting Methods of 3 phase IM- Star-Delta,Auto transformer

Star Delta Starter

Principle of operation

Star Delta Starters can be seen in 3 states.
1) Star connected state.
2) Open state.
3) Delta connected state.
Main and star contactors are normally closed and complete circuit at the starting time. Voltage is reduced to 1/√3 of the Line Voltage through every winding in the star connected state. When motor reaches a better rotational speed, like 90% of full r.p.m ,the starters timer connects the Delta connector after disconnecting Star connector. During the state transition the circuit becomes open and at that time the motor doesn’t stay in any of those two states. This is mentioned as open transition switching because of that. Winding voltage becomes equal to the line voltage in Delta connected state.
At the beginning if the application is light loaded, this starting method is used If the motor is heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.

Circuit/Connection diagrams

Switching "Start push button" completes Star circuit by switching ON main contactor "K1M" and Star contactor "K2M" through a timer. Now motor keeps on running in Star configuration. Once circuit is completed and main contactor "K1M" is activated, one of its "NO" contact point, connected parallel to "Start push button", becomes close and keeps circuit complete by providing a continuous holding even after Push button is released
Motor remains in Star state configuration till "timer" remains close. After a time delay is reached, timer becomes open, thereby de-activating Star contactor. Immediately after that another switch becomes close, thereby activating Delta contactor "K3" to complete Delta circuit.

Characteristic curves during starting

Torque Vs Speed

Current Vs Speed

Commercial products

The MCW 37kW Open Chassis Star-Delta Starter is manufactured from three 415V coil contactors, a timer, and a thermal overload relay (Trip Class 10).

Applications

Compressors need low torque to accelerate to the full speed
Pumps for hydraulic elevators.
Motor applications which need low acceleration

Auto transformer starters

Principle of operation

The principle of auto-transformer leads to the concept of Auto transformer starter which consists of additional flexibility compared to the other starters. Motor directs line voltage using some relay arrangement method in DOL starter. Starting voltage is a fixed reduced value in star delta starter. The specialty of the auto transformer is the starting voltage is reduced and also can vary in a range which is done by studs connected to different tapings. The versatility to the starter is provided by this method which enables connecting motors with different ratings by controlling the output voltage. This is a result of the adjustable starting voltage at the beginning.

There are some main components of auto transformer starters.

1) Three phase auto-transformers.

2) Contactor with movable rod.

3) Handle arrangement.

The line voltage at the primary windings of auto transformers will be connected by moving the operating handle because of the arrangement of movable rod. In addition the secondary windings of the transformer and the motor terminals are connected using that way. Due to the requirements of starting torque and starting current the tapings can be seen varied because the square of the transformation ratio decides this ratio.

Movable handle H controls assembly of all switches and the double throwing switch is connected to every transformer each. All the switches connected with respective contactors and the motor begin running with a low voltage when the handle is moved.

For the maximum performance the motor should fed with full load current after the safe starting of the starter. The handle H should be put into the run position immediately when motor accelerates and get a speed of 80% of its rated value. Full rated voltage is applied at motor terminals and auto-transformer is detached from the circuitry in that way.