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What is Star Delta Starter
When an induction motor is started directly on line, it takes a starting current 6(six) times the full load current. For large motor the high starting current causes voltage drop in the power system which may trip other motors in the systems. The utility companies restrict the rating of the motor which can be started directly on line. To reduce the starting current of an induction motor the voltage across the motor need to be reduced. This can be done by autotransformer starter, star-delta starter or resistor starter. Now-a-days VVVF drive used extensively for speed control serves this purpose.
For star-delta connection the motor windings are connected in star during starting. The connection is changed to delta when the motor starts running. The starting current and starting torque of DOL started and start-delta connected motors are as follows:
Starting method Starting current Starting Torque
DOL 6I 2T
Star-delta (1/root3)^2x6I (1/root3)^2x2T
Thus it can be seen that the starting current and starting torque are both reduced. The motor should be capable to start at such reduced torque with load.
Arrangement in the motor line
The star-delta starter with overload relay, including a thermally delayed overcurrent relay are situated in the cables leading to the motor terminals U1, V1, W1 or V2, W2, U2. The overload relay can also be operated in a star circuit as it is usually connected in series with the motor winding and the relay current flowing through it = rated motor current × 0.58.
Arrangement in the mains supply line
Instead of the arrangement in the motor line, the overload relay can be placed in the mains supply line. The section shown here indicates how the circuit differs from that on Automatic star-delta starters . For drives where the F2 relay trips out when the motor is starting in the star circuit, the F2 relay rated for the rated motor current can be switched in the mains line. The tripping delay is thus increased by approximately four to six times. In the star circuit the current also flows through the relay but here the relay does not offer full protection since its limit current is increased to 1.73 times the phase current. It does, however, offer protection against non-starting.
Configuration in the delta circuit
Instead of the arrangement in the motor line or mains supply line, the overload relay can be placed in the delta circuit.When heavy, long-starting procedures are involved (e.g. for centrifuges) the F2 relay, rated for relay current = rated motor current × 0.58, can also be connected in the connecting lines between delta contactor Q15 and star contactor Q13. In the star circuit no current then flows through relay F2. This circuit is used wherever exceptionally heavy and long starting procedures are involved and when saturable core current transformer-operated relays react too quickly.
Fig(Star Delta Diagram)
* Low cost and simple
* Torque too high – causes snatch
* Torque too low – motor stalls
* Transition peak up to 20 x In
* Motor can stall in transition