When a motor starter closes and starts a motor, the sudden application of the full voltage to a motor which is still stationary leads to a high current peak. Starting the motor with a star delta connection is the least expensive and simplest way of reducing this current peak.
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A standard motor has three phase windings in the motor connected together internally; the three phases are brought out to three terminals. This motor can only be switched on or off. A motor for star delta starting, has both ends of each winding brought out to six terminals. The windings can thus be connected first in a star or Y configuration for reduced current starting; when the motor has run up to speed, it changes to a delta connection for full load operation.
A standard full load motor starter has a single contactor which closes the circuit and connects the three phases to the three motor terminals. A star delta starter has three contactors. One contactor connects one set of three motor terminals to the three phases and the other two contactors are connected to the other set of three terminals, representing the other end of each of the windings. One of these two contactors is wired so it short-circuits the winding ends, giving a star or Y connection. The other contactor is wired so the windings are connected across the phases in a standard delta connection.
Star Delta Operation
When the motor starts, the two contactors providing the Y connection are closed. A phase to ground voltage of about 58 per cent of rated phase-to-phase voltage is applied to each winding, producing a torque of about 65 per cent of rated torque allowing the motor to start with a reduced current. When the motor has picked up speed, the Y contactor is opened and the delta contactor is closed, applying full phase-to-phase voltage to the windings and allowing for full rated operation.
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