Star Delta Starter – Working, Circuit, Advantages, Disadvantages

Nowadays, In industries, the most common type of electric motor used is Induction Motor. If the motor starts with the direct online method means, it started with the application of maximum voltage, and starting current will be 7 to 10 times of rated current.

So large induction motors do not start directly, if start means it will damage the winding and other parts. Motor needs something else to overcome this problem.

Star Delta starter comes with the need of starting the large indication motors.

Star Delta Starter

Star-Delta starter is a fine method of starting the induction motor which reduces the starting torque and starting current.

Star delta starter design normally consists of 3 contactors, a circuit breaker, and a timer for setting the time for star delta switching.

In Star Delta starter, during normal run-time motor must be connected in Delta connection only.

In Star Delta starter, the received starting current is about only 33 % of starting current during direct online start and the starting torque is reduced to about  33% of the torque available at the direct online start.

Components of Star Delta Starter

Below is the list of components and devices used for the star delta motor starter wiring and circuit.

Miniature Circuit Breaker (MCB)

The MCB is an electro-mechanical device that switches off the circuit automatically if there is any irregularity function occurs.

MCB is used for sensing the over-current caused by a short circuit or any malfunction. It is an automatically operated electrical switch that is used for preventing damage to an electrical circuit as a result of excess current.

MCB was designed in such a case to trip during an overload or short circuit to prevent electrical faults and equipment failure.


The contactor is an electrical device used to make or break contact with a load, in simple terms, it is used for switching an electrical circuit on or off. It is considered as a special type of relay.

They are normally in open contact when energized it provides operating power to the load. They are designed for carrying high currents & the main part of the contactor is the coil that is inside the contactor. The coil is also called an electromagnet.


A simple ON delay timer which is used in Star Delta Starter is practically called Star Delta Timer. Star Delta timer consists of two contacts, one Normally Open (NO) contact and a Normally Closed (NC) contact and power supply connection.

The timer is used here for switching the connection from star to delta automatically. Analog and Digital both types of the timer can be used for star delta starter.

The time range is need to be set manually by the user, depending on Motor power time values may vary.

Push Buttons

Two push buttons were generally used in this starter.

They were:

NO (Normally Open) Push  Button:

It is a type of push button mainly used to start a circuit. In its default state, it makes no contact with the electrical circuit.

When the button is pressed, its state will be changed to closed contact and activates the electrical circuit.       

NC (Normally Close) Push  Button:

It is a type of push button mainly used to stop a circuit. In its default state, it is in contact with the electrical circuit.

When the button is pressed, its state will be changed to open contact and deactivates the electrical circuit.

Measurement Devices

These devices are mainly used for checking the given inputs were correct and the obtaining outputs were correct and also needed for the requirement.

Mostly we want to measure two major factors, which are current and Voltage. Ammeter is used for measuring current and Voltmeter is used for measuring voltage.

Ammeter is connected in series connection whereas Voltmeter is connected in parallel connection for getting the correct value.

Nowadays, Multimeter or Clamp Multimeter where the devices for measuring current and voltages

Working Principle of Star Delta Starter

With a Star Delta motor start, the start-up of the three-phase motor is implemented by a changeover of the winding.

In Star Delta type, all the winding are connected to the mains supply using an automatic changeover switch/contact.

During the operation, the winding of the motor was connected in the delta, which means the winding voltage must be equal to the phase voltage of the three-phase system.

In the star connection, the winding voltage is reduced by a factor of 0.58.

For example, if the rated voltage is 400 V means, in delta connection the voltage will be the same 400. So, starting torque and inrush current in star connection reduced to about a 1/3 of delta connection.

Circuit Diagram of Star Delta Starter

The star delta starter circuit consists of two parts.

They are

  1. Power Circuit
  2. Control Circuit

Power Circuit

Star Delta Starter Power Circuit Diagram

Control Circuit

Star Delta Starter Control Circuit

Star Delta Starter Operation

Power Supply is given and Switch on the MCB.

Star Delta Starter Operation

Then start button is pressed, Main Contactor, Star Contactor, and Timer will get energized.

Since the Main Contactor is latched, even after the release of the Push Button also Star and Main contactor were energized.

For the Control circuit single-phase supply is given, the supply goes through the contactor by controlling the Start Push button and Stop button.

The main contactor is being energized because of the latch and also Star contactor is energized by the closed contacts of the Timer and delta contactor.

Components of Star Delta Starter

Now the motor is Working on Star mode.

Star Delta Starter Working Principle

At the same time, the timer starts counting the time which was set on it after the actual time gets equal to the preset time.

The auxiliary contact gets changed which cuts the star connection makes the motor to run on Delta mode.

Star Delta Motor Starter Circuit

The Main Contactor stage will be the same as earlier only.

Now the motor runs with Delta and Main Contactor which means the motor starts to rotate with its full power.

Star Delta Motor Starter Explanation

Voltage is measured using a multimeter here.

What is Star Delta Starter

Current is measured by using a clamp meter.

In Star mode, the 1/3 of current is reduced on rated current.

Star-delta starter testing

In Delta mode, it attains the maximum current.

Industrial Star Delta Starter for 3-Phase Induction Motor

Advantages of Star Delta Starter

  1. Starting Current and torque is reduced to 33%
  2. Works good in light-loaded conditions also
  3. No Heat will be produced
  4. Less Mechanical Stress.
  5. Automatic changeover makes reduces manual work.
  6. Mechanical stress is less

Disadvantages of Star Delta Starter

  1. During Load start transmission peak can occur.
  2. Starting Time is long compared to DOL starter.
  3. Soft stop is not possible.
  4. More components compared to DOL starter.
  5. Wiring is complex compared to DOL starter.
  6. Starting torque is reduced but cannot be adjusted.
  7. Cost is expensive.

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2 thoughts on “Star Delta Starter – Working, Circuit, Advantages, Disadvantages”

  1. Hi there.
    I think you must correct the connection of the power circuit. When delta contactor is energised, the V2 pole of the motor doesn’t connect with the W1 pole, as it should, but with the U1 as shown now in the schematic….

    Thank you and keep on the good work!


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