When is the star-delta starter used?

When the starting current has to be reduced and starting current and torque values one-third of those obtained with direct-on-line starting are suitable. It is necessary that the motor be designed to operate with the primary winding connected in delta, but with six terminals brought out to allow for connection in star during starting.

The plain star-delta method is used for small and medium-sized motors on light-starting loads, e.g. centrifugal pumps, fans having low inertia, line shafting and motor-generator sets. The Wauchope-type has the same uses but prevents the drop in speed when the stator is disconnected from the supply in changing from star to delta. Switching is done through resistances to maintain continuous line contact. This also obviates the momentary high current when switching from star to delta.