Power Circuit of a Star Delta or Wye Delta Electric Motor Controller - A basic how to guide

Reduction of the AC induction motor's starting inrush current is achieved by implementing an interchanging star delta or a wye delta connection of the motor, this can be done by incorporating a configured star delta connection of the motor by using magnetic contactors or magnetic starter switches. The diagram below illustrates this type of motor switching connection used in every electrical industry involving industrial process automation control technology.

Star Delta Motor Control Power Circuit

The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.

The main contactor connects the reference source voltage L1, L2, L3 to the primary terminal of the motor U1, V1, W1.

The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.

The star delta power circuit presented here is controlled by the star delta control circuit which provides the command for the required operational sequence, and from which, after a specified time interval wherein the motor has finally developed sufficient inertia with a reduced inrush starting current, the control circuit automatically transfers command from the start sequence to the run sequence. This is achieved by reaching the specified time delay from a timer that provides the switching from the star (start) contactor to the delta (run) contactor, wherein the star contactor is deactivated and activating the delta contactor while maintaining the main contactor energized all throughout the entire running duration of the electric motor. A full line voltage is connected to the motor during the final stage of delta (run) sequence, which also supplies the line voltage L1, L2, L3 to the secondary terminal of the motor U2, V2, W2, hence achieving the full power and torque required for maximum efficiency of the motor capacity.

The thermal overload relay acts as the motor overload detector, which deactivates the control circuit when over current is detected from the motor.

The next article Star Delta or Wye Delta Motor Wiring Configuration provides a detailed presentation of the resulting actual wiring connection of the motor terminals configured by the above star delta power circuit.