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 diagrambelow 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.
The previous article which covered the star delta motor control power circuit for industrial process control presented an electrical diagram showing the interchanging motor wiring configuration for a star connection and a delta connection using magnetic contactors as motor starters. Exploring further with reference to the previous star delta power circuit diagram, if you manually trace each line connections going to the motor you will begin to understand how the following star and delta motor wiring connections are achieved.

Motor Terminals Before Wiring Connection

Normally, an AC induction motor have six wiring terminals, U1, V1, W1 for the primary terminals, and U2, V2, W2 for the secondary terminals. The start coil where the motor's coil winding starts is marked by a dot (see drawing on the right) to represent each primary starting ends of the three internal motor coils. These starting coil windings are designated with the number 1 following each three phase terminal which are alphabetically represented with U, V, W, so then the primary terminals are named U1, V1, W1. Correspondingly, the other end of each coil is designated with the number 2 to represent the completed end of the coil where each winding comes to a finish, each of these finish ends which are designated with the number 2 are also alphabetically and numerically represented to identify them as the secondary terminals U2, V2, W2.

In the field of industrial process automation control technology, it has always been a common safe practice before doing the actual wiring installation of the electric motor to firstly check for the continuity on both ends of each coil using an ohm meter to verify that U1 meets with U2, and V1 meets with V2, and W1 with W2.

Motor Connected in Star (Wye) Configuration

The AC induction motor's wiring terminals can be configured in either the star connection or the delta connection. The star wiring configuration is connected by shorting all the secondary terminals U2, V2, W2 together while connecting all the primary terminals U1, V1, W1 to the supply source voltage L1, L2, L3. This is achieved by the star delta control circuit which initially activates the main contactor and the star contactor simultaneously while leaving the delta contactor inactive during the startup run sequence of the motor from standstill.

Motor Connected in Delta Configuration

The delta wiring configuration of the motor, which resembles a triangle, is connected by joining U1 and V2, V1 and W2, W1 and U2, while also connecting each of these joined terminals to each of the supply source voltage L1, L2, L3. The delta configuration is achieved by energizing the main contactor together with the delta contactor while disengaging the star contactor.

Learn more about the proper phasing sequence in accordance to a conforming orderly configuration for a star delta motor control wiring configuration.