Star-Delta Starter wiring diagram | Control circuit wiring | Working principle

Star-Delta starter

Introduction:
According to electromagnetic induction principle, once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and starts rotating. Of course this means that the three phase induction motor is entirely capable of self-starting.

Need of starter: The need for a starter therefore is not, but in case of three phase induction motor starter is to provide smooth starting, to reduce Heavy starting currents, provide overload and no-voltage protection.

Methods of Starting Three Phase Induction Motors: There are a number of different types of starter available for starting three phase induction motor. They are listed below

1. The Direct On-line Starter.

2. The Star- Delta Starter,

3. Auto-Transformer and

4. Rotor resistance.

Star-Delta Starter:
In this section we are going to study about Star-Delta Starting method.

This is the most common form of starting method used for three phase induction motors. It achieves an effective reduction of starting current by initially connecting the stator winding's in star configuration which effectively places any two phases in series across the supply. Starting in star not only has the effect of reducing the motor’s starting current but also the starting torque. Once up to a particular running speed an ON delay timer changes the winding arrangements from star to delta whereupon full running torque is achieved.

Such an arrangement means that the ends of all stator winding's must be brought to terminations outside the casing of the motor.

Motor terminal 

Major parts of Star-Delta starter:
1. Three pole contactors- a) main, b) star and c) delta contactor’s 3 Nos.
2. On delay timer 1No.
3. Three pole bi-metallic thermal overload release 1No.
4. Fuse/MCCB/MPCB for main control 1No.
5. Fuse/MCB for control circuit 1No.
6. Indication lamp: Start-Green, Stop-Red 1No.
7. Push buttons: Start-Normally open, Stop-Normally close 1No.
8. Three phase induction motor 1No.

Star-Delta starter wiring diagram:
The main circuit breaker C.B Q1 serves as the main power supply switch that supplies electricity to the power circuit. The main circuit breaker Q1 connect or disconnect the main three phase supply (L1, L2, and L3) to the motor terminal T1, T2, and T3.

Fuses F1, F2, and F3 Protects the motor against overload. If the motor draws more than the rated current the fuses are blow out.

Contactor K1 is the main contractor, K4 is Delta contactor, and K3 is Star contactor. All this three contactor´s are a three pole contactor. As shown in the fig. there is a mechanical interlock between Star contactor and Delta contactor to avoid un-wanted operation.

In Star contactor K3 one of the side all the three poles are short circuited, and other side is connected to motor terminals T3, T2, and T1 as shown in fig. to make star configuration.

The star and delta contactor´s are mechanically interlocked i.e., if one of them is closed the other contactor cannot close. This is done to avoid dead short circuit in case both the contactor´s closing simultaneously. Electrical interlocking has also been provided, by using contactor´s control contacts.

To make Star-Delta configuration the ends of all stator winding's must be brought to terminations outside the casing of the motor.

Star-Delta starter power circuit

Star-Delta starter control circuit diagram:
As shown in the fig. Star-Delta starter control circuit consist of Start Push button S1, Stop push button S0, Timer T1, main contactor K1, Star contactor K3, Delta contactor K4.

starting sequence; after pressing the start push button S1, single-phase supply(230v) activates the timer T1, timer coil is energised, after energising the coil NO contact closed and NC contact get open. This energized star contactor K3 coil and motor get connected in star connection.

After some time interval motor attain rated speed and the timer T1 switch the circuit Starter from star transition state to delta state, full line voltage gets applied to the motor and motor continues to rotate at its full speed.

Stop sequence; once you press stop push button S0 the circuit is disconnected from supply voltage. So the delta contactor K4, main contactor K1, and Timer T1 are de-energised. Motor speed gradually reduce and finally stops.

Star-Delta starter control circuit

Star-Delta starter working principle:
When we press the start push button S1, the motor is connected in star configuration. In star connected state Voltage applied to the motor winding is reduced to 1/√3 of line voltage VL.

When motor attains rated speed of full R.P.M timer T1 coil is activated. Timer contacts disconnects star contactor K3 first and connects delta contactor K4 into the circuit, which means now motor is connected in delta configuration.

Star state:
Phase current= Line current

Phase voltage = Line voltage/√3

Delta state:
Phase current = Line current/√3

Phase voltage = Line voltage

Star-Delta connection