Star Delta Starter? Working principle,Theory, Circuit Diagram

Star delta starter working principle theory

There are many different starting methods are used to start a three-phase induction motor like Star delta starter. One of them, popular and cheapest is a Direct online starter. But its use limited for smaller HP motor, to start higher rating 3 phase motors 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 star delta starter working principle with help of power and control diagram, Theory,  connection wiring diagram, parts like contactor, mcb, fuses, a timer circuit, overload relay.

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

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 starting current. Star delta starter is a reduced voltage starter that applies 1/√3 voltage during the starting time.

Types of Star-Delta Starter

1) Manual star delta starter

2) Semi automatic star delta starter

3) Fully automatic star delta starter ( wye-delta) 

  

Star Delta Starter(wye-delta) working principle :

Wye delta starter works in three states: 

a) Star connect state

Star connected motor terminals

current and voltage in star connection

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

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

When motor attains sufficient speed of full speed i.e 90% of full R.P.M timer connected in circuit activated. it disconnects star contactor firstly and connects delta contactor into the circuit, which means It closed delta contactor.

b) Open state :

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

c) Delta state:

delta connected motor winding

Current and voltage in delta state

After activation of timer, motor switched from star to delta state. In delta connection state, phase voltage equal to line voltage. Hence full line voltage applied to motor winding and 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 circuit wiring diagram consist Timer, Push button for Start and Stop.

At a starting time, after pressing the start push button, single-phase supply activates the timer, timer contact  17-18 closed and NC contact 17-28 get open. this energized star contactor coil and motor get connected in star.

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

Star Delta Starter Power diagram and theory :

Power circuit diagram:

star delta starter power circuit diagram

Theory of working explains as follows:

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

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

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

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

1) Contactor :

 In star delta starter 3 contactors are used. Main contactor, star contactor, and delta contactor. A contactor is a heavy-duty relay with the high current rating, used for power up the electrical motor. The current rating for contactor varies from 10 amps to several hundred amps. 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 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.

 2) Overload relay (OLR) 

The majority of winding failure is because of overloading, Operation on unbalance supply voltage or single phasing due to phase loss which leads to excessive heating to degradation of winding insulation, due to this electrical motor required 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. 

3) Timer  

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

4) Fuse unit 

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

The main purpose of the fuse is to protect the motor and is composed of an alloy which 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 in excess then the strip would melt and break the circuit and isolate the motor from the supply.

5) MCB

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 Other hand a fuse, which operates once and then must be replaced, 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.

6) Start push button (NO) 

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

7) Stop push button (NC)

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

Advantages of Star delta starter

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

Disadvantages of Star delta starter

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