Star-Delta Starter wiring diagram
Introduction:
According to the 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 the case of a phase induction motor starter is to provide smooth starting, 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 starters available for starting three-phase induction motors. 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 windings 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 must be brought to terminations outside the casing of the motor.
Fig. 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 poles 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 connects or disconnects the main three-phase supply (L1, L2, and L3) to the motor terminals T1, T2, and T3.
Fuses F1, F2, and F3 Protect the motor against overload. If the motor draws more than the rated current the fuses are blown out.
Contactor K1 is the main contractor, K4 is the Delta contactor, and K3 is the Star contactor. All these three contactors are three pole contactors. As shown in Fig. there is a mechanical interlock between the Star contactor and the Delta contactor to avoid unwanted operation.
In Star contactor K3, one of the sides of all three poles is short-circuited, and the other side is connected to motor terminals T3, T2, and T1 as shown in fig. to make star configuration.
The star and delta contactors are mechanically interlocked i.e., if one of them is closed the other contactor cannot close. This is done to avoid a dead short circuit in case both the contactors close simultaneously. Electrical interlocking has also been provided, by using the contactor´s control contacts.
To make Star-Delta configuration the ends of all stator windings must be brought to terminations outside the casing of the motor.
Star-Delta starter control circuit diagram:
As shown in the fig. Star-Delta starter control circuit consists of the Start Push button S1, Stop push button S0, Timer T1, main contactor K1, Star contactor K3, and Delta contactor K4.
starting sequence; after pressing the start push button S1, single-phase supply(230v) activates the timer T1, timer coil is energized, after energizing the coil NO contact is closed and NC contact open. This energized star contactor K3 coil and motor get connected in star connection.
After some time interval motor attains the rated speed and the timer T1 switches the circuit 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.
Stop sequence; once you press the stop push button S0 the circuit is disconnected from the supply voltage. So the delta contactor K4, main contactor K1, and Timer T1 are de-energized. Motor speed gradually reduces and finally stops.
Star-Delta starter working principle:
When we press the start push button S1, the motor is connected in a star configuration. In star connected state Voltage applied to the motor winding is reduced to 1/√3 of the line voltage VL.
When the motor attains the rated speed of a full R.P.M timer T1 coil is activated. Timer contacts disconnect star contactor K3 first and connect delta contactor K4 into the circuit, which means now the motor is connected in a delta configuration.
Star state:
Phase current= Line current
Phase voltage = Line voltage/√3
Delta state:
Phase current = Line current/√3
Phase voltage = Line Voltage
