Electric motor controls. Article:

How does a Star-delta starter works.


What is a Star-Delta Starter.

A Star-delta starter is a method of starting larger induction electric motors usually of 4kw and above.

In star-delta starter the electric motor first run in star and then switched to delta after reaching 80% of its full speed.

The Stardelta starter consist of two circuits, the power and control circuit. The power circuit provide power to the motor and the control circuit controls the operations of the motor.

Power Circuit.

star-delta starter power circuit

Control Circuit.

star-delta starter control circuit

How does a Star Delta starter works.

A Star-Delta Starter consist of:

1.Start button - a normal open contact button which is pressed for the electric motor to run. 2.Stop button - a normal close contact button which is pressed to stop the electric motor. 3.Circuit breakers - are either manually operated to switch off the circuit for maintenance or they automatically trip off, breaking the circuit in event of a fault. 4.Overload relay - they break off the circuit in event of overload which may cause overheating of the electric motor winding.

Overload is condition in which current flowing in a circuit exceeds the amount required. Some overloads may not cause tripping of circuit breakers, so they is a need of an overload relay.

5.Contactors (Main, delta and star) - A contactor is a device which when current flow through its coil, closes the contacts of power circuit for electric motor to run and vice versa. 6.Timer - anables the electric motor to switch from star to delta. 7.Electric motor - is a device which convert electrical energy into mechanical energy.

With the circuit breakers in ON position, to start the electric motor, the start button is pressed, this anables current to flow through the coils of main and star contactor coils.

When current flow through the coils, the power contacts of the contactors are pulled to close and power reach the electric motor and the motor begin to run in star.

After a preseted time is reached, the timer open the star contact and closes the delta contact.

This stops current from flowing through the coil of star contactor and anables current to flow through the delta contactor coil, thus opening the power contacts of star contactor and closing the power contacts of the delta contactor and the motor switch to delta.

The main contactor contact labeled "hold in" which is in parallel with the start button is for hold in. This contact is a normal open contact, it is put in close position when main contactor coil is energized.

Since the Start button is a normal open contact, soon after pressing it, it retains to its normal open position, current will then continue to flow to the energized contactor coils through this Hold in contact.

To stop the electric motor, the stop button is pressed, this stops current from flowing through energized contactor coils, causing opening of contactor power contacts and the electric motor stops running.

Advantages and Disadvantages of Star Delta starter.


The starting current of larger electric motors is greatly reduced If properly installed, the response time to fault and operations is very short. It offers protection to electric motors by automatic switching off the electric motor in event of fault. It provide a safe invironment to people working with the electric motor.


It is a complex circuit, difficult to understand and to install. It needs more components than other starting methods, hence more expensive method. Difficult to trace a fault.

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