Star-Delta Starters Working Principles

1 T: 01246 455565 F: 01246 455244 W: E: Units 1-6 Ambrose Buildings, Broombank Road, Chesterfield, Derbyshire, S41 9QJ VAT Reg: 598 6371 75 Company Reg: 3191383 Star-Delta Starters Working Principles Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed This is the reduced voltage starting method.

2 Voltage reduction during Star-Delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. Scheme - Working Principle of Star-Delta starter After a period of time the windings are reconfigured as delta and the motor runs normally. A dual starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply.

3 Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below 5 hp ( kW). Reduced-voltage Starters are employed with motors above 5 hp. Although dual motor Starters are available for motors less than 150kW on 400 V and for motors less than 1 MW on kV. Supply reliability and reserve power generation dictates the use of reduced voltage or not to reduce the starting current of an induction motor the voltage across the motor needs to be reduced. This can be done by 1.

4 Autotransformer starter 2. Star-Delta starter 3. Resistor starter Currently invertor drives used extensively for speed control also serve this purpose. In a dual starter the motor is directly fed from the line; and in star delta starter the motor is started initially from star and later during running from delta. This is a starting method that reduces the starting current and starting torque. The Motor must be delta connected during a normal run, in order to be able to use this starting method. Recon Electrical Ltd, Units 1-6 Ambrose Buildings, Broombank Road, Chesterfield, Derbyshire, S41 9QJ VAT Reg: 598 6371 75 Company Reg: 3191383 The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a start.

5 Star-Delta Starters are probably the most common reduced voltage Starters . They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply. The Star-Delta starter is manufactured from three contactors , a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1 3 = (58%) of the current in the line.

6 This connection amounts to approximately 30% of the delta values. The starting current is reduced to one third of the direct starting current. There are two contactors that are closed during run, often referred to as the main contactor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third of the motor rating.

7 In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by the timer (K1T) built into the starter . The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states: 1. OFF State. All contactors are open 2. Star State. The Main and the Star contactors are closed and the delta contactor is open.

8 The motor is connected in star and will produce one third of DOL torque at one third of DOL current. 3. Open State. The Main contactor is closed and the Delta and Star contactors are open. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor and behaves like a generator. 4. Delta State. The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available. This type of operation is called open transition switching because there is an open state between the star state and the delta state.

9 Recon Electrical Ltd, Units 1-6 Ambrose Buildings, Broombank Road, Chesterfield, Derbyshire, S41 9QJ VAT Reg: 598 6371 75 Company Reg: 3191383 Recon Electrical Ltd, Units 1-6 Ambrose Buildings, Broombank Road, Chesterfield, Derbyshire, S41 9QJ VAT Reg: 598 6371 75 Company Reg: 3191383 The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit. When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.

10 When Star Auxiliary Contactor (1) (which is placed on Main Contactor coil circuit ) become NO to NC it s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor s Main and Auxiliary Contactor Change its Position from NO to NC. This sequence happens in a friction of time. After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.