A simple tool to assist with contactor sizing for star-delta motor starting applications.

A star-delta starter is a device commonly used to reduce the starting current of large three-phase induction motors. During startup, the motor windings are initially connected in a star configuration, which lowers the voltage applied to each winding to 58% of the line voltage. This results in a starting current that is approximately one-third of the direct-on-line (DOL) starting current, significantly reducing the inrush current.

After the motor reaches a certain speed, around 80% of full speed, or after a set time, the configuration switches to delta for normal operation, allowing the motor to run at full power. This method not only minimises electrical and mechanical stress on the motor and connected equipment, but also helps prevent voltage drops in the electrical network during startup. Star-delta starters are a simple, energy-efficient and reliable solution for starting large motors smoothly while protecting the electrical system and motor components.

The calculation for sizing the correct switchgear for a star-delta starter involves determining the motor's full load current (FLC) in the delta configuration. The star contactor should be sized at least one third of FLC. Both the main and delta contactors should be sized at least 58% of FLC. The thermal overload setpoint should be set to 58% of FLC.

Using a star-delta starter reduces the startup current by a factor of three, however it should be considered that startup torque is also reduced by a factor of three.