Star-Delta Starters

Star-Delta Application Description

Star-delta (or wye-delta) motor starting is used for:

• motors starting on no-load.

• motors having a low load torque.

• motor with load torque increasing with the speed to reduce the stress in the motor and in the distribution network when motor starts.

The characteristics of the star-delta starting method are:

• reduction of the starting current.

• reduction of the voltage drop.

This method of starting is applicable to three–phase motors on which all six stator terminals are accessible and whose delta connection voltage corresponds to the main voltage. It requires the combination of three contactors. It helps to start the motor with reduced voltage due to the star connection of the stator windings:

• the starting torque in star connection is reduced to one third of the direct starting torque, that is about 50% of the rated torque.

• the starting current in star connection is about 1.8–2.6 times the rated current.

The transition from star to delta connection should occur when the machine has run up to its speed. A too rapid build-up in load torque would cause the stabilized run-up speed to be too low and would therefore eliminate any advantage in this method of starting. This is the case with certain machines whose load torque depends on the machine speed (a characteristic of centrifugal machines, for example).

Star-Delta Contactors

The star-delta starter requires the combination of three contactors:

• The line contactor (L, KM2)

• The star contactor (Y, KM3)

• The delta contactor (Δ, KM1)

The line and delta contactors have the same rating and are rated for the motor FLA / √3.

The star contactor is rated for the motor FLA / 3. The size of the star contactor can be

• the same as the size of the line and delta contactors.

• the size under the size of the line and delta contactors.

The star and delta contactors must be mechanically and electrically interlocked to prevent any paralleling, even transitory, of the motor stator connections.

The mechanical interlock depends on the type of the three contactors.

Overload Protection of the Star-Delta Starters

The thermal overload protection of the motor in a star-delta starter can be provided by a TeSys Giga overload relay. The overload relay rating is the same as the rating of the line and delta contactors. The Ir/FLA threshold of the thermal overload protection is set to the motor FLA / √3.

The thermal overload relay can be

• mounted directly under the line contactor (recommended) or

• mounted standalone and connected between the motor stator terminals and the delta contactor.