Star-delta starting can only be used either if the motor normally operates in a ? (delta) connection or starts softly or if the load torque during Y starting is low and does not increase sharply. On the Y step the motors can carry approximately 50 % (class KL 16) or 30 % (class KL 10) of their rated torque; The starting torque is approximately 1/3 of that during direct on-line starting. The starting current is approximately 2 to 2.7 times the rated motor current.

The changeover from Y to ? must not be effected until the motor has run up to rated speed. Drives which require this changeover to be performed earlier are unsuitable for star-delta starting.

The ratings given in the selection and ordering data are only applicable to motors with a starting current ratio of I_A ? 8.4 x I_N and using a 3RP15 74 star-delta time-delay relay with a dead interval of approximately 50 ms on reversing.