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.
(A) Control Circuit of the WYE-DELTA starter.
(B) Power Circuit of the WYE-DELTA starter.
(C) WYE connection of the motor windings.
(D) Delta connection of the motor winding.
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