Star-delta starter sizing
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 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 minimises electrical and mechanical stress on the motor and connected equipment, and helps prevent voltage drops in the network during startup.
Inputs
or derive from nameplate
Result
- Motor current
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- Main contactor current
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- Delta contactor current
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- Star contactor current
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- Thermal overload setpoint
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Sizing the switchgear for a star-delta starter starts from 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, and 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 — but bear in mind that startup torque is also reduced by a factor of three.