Amazing bro.i saw in a refinery HV( 11kv and 6.6kv ) motors star connection,LV(415V)motor delta connection.in name plate written starting Star , running delta in LV motors why?
REASON FOR STAR CONNECTION IN HIGH RATING MACHINES: The windings of high voltage rating motors require more insulation. Insulation requirement is directly proportional to voltage level. Also, in 3-ph machines, voltage level is based on phase voltage rating. For 11kV, if you connect in delta, phase voltage = line voltage. So, for 11kV machine, you need to provide insulation for 11kV phase. But when connected in star, phase voltage = line voltage/(root (3)) which is 11kV/root(3) = 6.35kV only. So insulation for 6.35kV is enough (advantageous to us).
REASON FOR DELTA CONNECTION IN LOW RATING MACHINES: In industries, induction motors are preferred to start by Star-Delta method (if application is suitable for such starts) since it is cheaper and simpler. For this starting method, the motor windings should be connected in star while starting and should be connected in delta in running condition. As I said earlier, delta windings cannot be used for high voltage ratings. This is why star-delta starting method is used only for low voltage rating motors.
Simple and clear....
Amazing bro.i saw in a refinery HV( 11kv and 6.6kv ) motors star connection,LV(415V)motor delta connection.in name plate written starting Star , running delta in LV motors why?
REASON FOR STAR CONNECTION IN HIGH RATING MACHINES:
The windings of high voltage rating motors require more insulation. Insulation requirement is directly proportional to voltage level. Also, in 3-ph machines, voltage level is based on phase voltage rating. For 11kV, if you connect in delta, phase voltage = line voltage. So, for 11kV machine, you need to provide insulation for 11kV phase. But when connected in star, phase voltage = line voltage/(root (3)) which is 11kV/root(3) = 6.35kV only. So insulation for 6.35kV is enough (advantageous to us).
REASON FOR DELTA CONNECTION IN LOW RATING MACHINES:
In industries, induction motors are preferred to start by Star-Delta method (if application is suitable for such starts) since it is cheaper and simpler. For this starting method, the motor windings should be connected in star while starting and should be connected in delta in running condition. As I said earlier, delta windings cannot be used for high voltage ratings. This is why star-delta starting method is used only for low voltage rating motors.