Star Delta Starter-Working principle, Theory, Circuit Diagram

There are many different starting methods used to start a three-phase induction motor like Star delta starter. To start a higher rating 3-phase motor star-delta starter is used. wye-delta starter is useful where all six terminals of the motor are accessible.

In this article, we will discuss the star delta starter working principle with the help of a power and control diagram, Theory,  connection wiring diagram, parts like contactor, mcb, fuses, a timer circuit, and overload relay.

Star-Delta starter is the simplest starting method for reducing the inrush starting current of the Induction motor. The starter can be used with all squirrel-cage induction motors that are delta-connected for normal operation.

Star-Delta starter is the simplest starting method for reducing the inrush starting current of the Induction motor. The starter can be used with all squirrel-cage induction motors where all six winding terminals can be accessible.

The reduction of the high motor current causes a reduction of the starting torque of the motor. Star-delta starting is therefore especially suitable for drives that are not loaded until after starting. The starting time is longer than with direct starting, which is especially apparent when driving larger inertial masses.

To start a large motor we need a reduced voltage starter, to limit the starting current. Star delta starter is a reduced voltage starter that applies 1/√3 voltage during the starting time.

Star Delta Starter(wye-delta) working principle

Wye delta starter works in three states: 

a) Star Connected state

Star connected motor terminals in star delta starter
Star-connected motor terminals
current and voltage in star connection star delta starter
current and voltage in star connection

During the starting time of the star delta starter, the Main contactor and star contactor remain in the close state and complete the power circuit.

During starting, the motor is connected in a star state. In star connected state Voltage applied to the motor winding is reduced to 1/√3 of the line voltage.

See also  DOL Starter Working Principle(Direct Online Starter) and Control diagram

When the motor attains a sufficient speed of full speed i.e. 90% of full R.P.M, the timer connected to the circuit is activated. it disconnects the star contactor first and connects the delta contactor to the circuit, which means It closes the delta contactor.

b) Open state 

Between switching from star to delta, the circuit becomes open and the motor neither remains in star nor in delta state. This state is called an open transition state.

c) Delta state

delta connected motor winding
delta connected motor winding
Current and voltage in delta state
Current and voltage in delta state

After activation of the timer, the motor switched from star to delta state. In the delta connection state, phase voltage is equal to line voltage. Hence full line voltage is applied to the motor winding and the motor runs at its rated full speed. delta-connected motor winding is shown in the figure.

Star Delta Starter Control Diagram

Star delta starter control diagram
Star delta starter control diagram

Star delta starter control circuit diagram consists Timer and pushbuttons for Start and Stop.

At a starting time, after pressing the start push button, the single-phase supply activates the timer, timer NO contact gets closed and NC contact gets opened. this energized star contactor coil and motor get connected in the star.

After some time motor attains 90% of the rated speed and the timer circuit switches Starter from the star transition state to the delta state, full line voltage gets applied to the motor and the motor continues to rotate at its full speed.

Star Delta Starter Power Circuit diagram

star delta starter power circuit diagram
star delta starter power circuit diagram

The theory of working is explained as follows

1) During the working of the starter, two contactors remain closed. These two contactors are the main contactor and the delta contactor. 

2) The third contactor is the star contactor and it takes part only during the starting time of the motor and carries the star current when the motor is in the star state. 

3) The current in the star state is 1/3 of the current in the delta state. Hence contactor rating is one-third of motor rated current. 

See also  What is Soft Starter,Working Principle,diagram,advantages

4) During starting the Main contactor KM3 and Star contactor KM1 are closed initially.

5) After some time, the timer in the circuit gets activated, it opens star contactor and closed delta contactor. 

6) The switching of star state to delta state is done by using a timer, which connected in the wye-delta starter control circuit. 

Components of Y-Δ motor starter


In Star Delta Starter 3 contactors are used. Main contactor, star contactor, and delta contactor. A contactor is a heavy-duty relay with a high current rating, used to power up the electrical motor. The current rating for contactors varies from 10 amps to several hundred amps. The high-current contactor is made from an alloy containing silver. Arcing during the switching operation of the contactor causes the contact to oxidize. However, silver oxide is still a good conductor.

Overload protection is provided along with the contactors to start the motor. A contactor is not used to interrupt a short circuit current, Unlike the circuit breaker used. Contactor size varies from small to large for high current appliances.

Overload relay (OLR) 

The majority of winding failure is because of overloading, Operation on unbalanced supply voltage, or single phasing due to phase loss which leads to excessive heating to degradation of winding insulation, to this electrical motor requires overload protection, to prevent damage from over-loading the motor, or to protect form sort circuit or internal winding fault of electric motor. All these conditions are prevented by using a thermal overload relay


The function of a timer in the star-delta starter is to switch the contactor from star to delta after attaining sufficient speed up to 90% of the motor’s full speed. 

Fuse unit 

The fuse unit is used to protect the motor from over current or short circuit conditions.

See also  What is Auto transformer Starter Working principle, diagram,advantages

The main purpose of the fuse is to protect the motor and is composed of an alloy that has a low melting point. A strip of a fuse is placed in series with the motor circuit. The working principle is that if the current is in excess then the strip would melt and break the circuit and isolate the motor from the supply.


A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload/short circuit. Its main function is to break the current flow after a fault is detected. On the Other hand, a fuse, which operates once and then must be replaced, and a circuit breaker can be reset to begin normal operation.

To protect the motor from short circuit conditions and avoid damage to motor winding MCB is used in the star delta starter motor circuit.

Start push button (NO) 

This is normally an open(NO) type push button, used to start the motor.

Stop push button (NC)

 This is an NC-type push button and is used to stop the motor.

Advantages of Star delta starter

  • Star-Delta starters are popular due to their low price.
  • There are no limits to the number of times they can be operated.
  • The starting current is reduced to approximately 1/3 of the rated motor current.
  • Produce high torque per ampere of line current.

Disadvantages of Star delta starter

  • Star delta starter can only be useful to motors where the six motor terminals can be accessed.
  • The supply voltage must be the same as the rated motor voltage for the Delta connection.
  • Due to the starting current being reduced to approximately 1/3 of the nominal current, the starting torque is also reduced to 1/3.

2 thoughts on “Star Delta Starter-Working principle, Theory, Circuit Diagram

Leave a Reply

Your email address will not be published. Required fields are marked *