Why LT motors are delta connected and HT
motors are star connected?
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Reason
is techno commercial.
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In
star, phase current is same as line current. But phase voltage is 1/1.732 times
line voltage.So insulation required in case of HT motor is less.
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The
starting current for motors is 6 to 7 times full load current.So start-up power
will be large if HT motors are delta connected.It may cause instability
(voltage dip) in case small Power system.
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In
starred HT motor starting current will be less compared to delta connected
motor. So starting power is reduced. Starting torque will also be reduced. (It
will not be a problem as motors are of high capacity).
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Also
as current is less copper (Cu) required for winding will be less.
LT motors are delta
connected:
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Insulation
will not be problem as voltage level is less.
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Starting
current will not be problem as starting power in all will be less.
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So
no problem of voltage dips.
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Starting
torque should be large, as motors are of small capacity.
LT motors have winding delta connected:
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In
case it is having star delta starter than they are started as Star connected
motor.
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After
it attains 80% of synch speed the changeover takes place from star to original
configuration delta.
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In
star the voltages across the windings are lesser that is 1/1.732 times that
available in delta so current is limited.
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When
it goes to delta again voltage is full line voltage so current increase even
though it is lesser than the line current it remains higher than the line
current drawn in star connection at reduced voltage.
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So
cables for motor are sized for this current that is what it draws in delta
connection.
