Motor Control Operation and Circuits: WYE-DELTA REDUCE VOLTAGE STARTER

Some motors starters at high current more than several times its current at full load condition. The effect of this to other is necessary tripping of circuit breakers.

One way of reducing the high starting current of a motor is the start the motor with the low voltage at its winding. After a few seconds, when the motor is already running at its stable condition, the rated full voltage is applied to its winding. This system is called reduced voltage starting.

One method of reducing the starting current of a motor is to connect the winding of the motor into WYE connection during the starting process and when the motor is running, the winding are connected into Delta, see figures 9(c) and (D). Note that at WYE connection the voltage across the motor windings is 220 volts, Figure below (D).

CONTROL OPERATION

Figure below (A) and (B) shows the power and control circuit of a “WYE-DELTA Reduced Voltage Starter”. A time delay relay (Timer) is in the control circuit. The timer TR will delay the opening and closing of its normally closed contact TR (8-5) and normally open TR (8-6) respectively when its coil terminals TR (2-7) is energized, the duration of delaying I set by adjusting a knob in the timer TR.

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Referring to figures below (A) and (B), pressing the start push button will energized M1 contactor, timer TR, delay relay, and S contactor. Contactor M1 (13-14) will closed to maintain control circuit. Contact S (11-12) will open to prevent unnecessary energizing of contact motor M2. Contacts M1 (1-2,3-4,5-6) and S (1-2,3-4,5-6) will closed simultaneously to connect the motor windings (T1-T4,T2-T5,T3-T6) into WYE connection see figure below (C).

After a few seconds the normally closed the normally closed contact TR (8-5) will open and normally open contact TR (8-6) will close. Contactor S will be de-energized to open its contacts S (1-2,3-4,5-6) and closed its contact S (11-12). Also contactor M2 is energized when TR (8-6) closes. Contacts M1 (1-2,3-4,5-6) will closed. At this contacts M1 (1-2,3-4,5-6) remains closed. At this condition the motor winding (T1-T4,T2-T5,T3-T6) is change into delta connection as shown in figure below (D).

Pressing the Stop push button will de- energized contactor M1,timer TR, and contactor M2. This will bring the motor into halt.