Why LT motors are delta connected and HT motors are star connected?
Reason is techno commercial.
1. 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.
2.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. 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.)
3.Also as current is less copper (Cu) required for winding will be less.
4.LT motors are delta connected.
1.Insulation will not be problem as voltage level is less.
2.Starting current will not be problem as starting power in all will
be less. So no problem of voltage dips.
3.Starting torque should be large, as motors are of small capacity.
Comparison of star and delta motor starting
LT motors have winding delta connected.
1. In case it is having star delta starter than they are started as
Star connected motor.
2. After it attains 80% of synch speed the changeover takes place from star to original configuration delta.
3. In star the voltages across the windings are lesser that is 1/1.732 times that available in delta so current is limited.
4. 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. So cables for motor are sized for this current that is what it draws in delta connection.