The star-delta starter is suitable for machines that have a small, rugged torque, or that run in a vacuum. Due to the transient regime at the moment of the triangle connection, it may be necessary, above a given power 1 hp motors, to use a variant to limit these transient phenomena. The speed of the electric motor stabilizes when the motor and resistant torque balances, usually between 75 and 85% of the rated speed. The windings are connected in triangle and the motor recovers its nominal characteristics. The transition from the star connection to the triangle connection is controlled by a timer.
The closing of the triangle contactor occurs with a delay of 30 to 50 milliseconds after opening the contactor star, which will avoid a short circuit between phases, since the two contactors can not be closed at the same time. The current through the windings is interrupted by the star contactor opening and resets when the triangle contactor closes. This triangle passage also closes. This triangle passage is followed by a very short but very high transient peak current due to the counter-electromotive force of the motor.