Power Circuit of a Star Delta or Wye Delta Forward Reverse Electric Motor Controller - A basic industrial process automation control how to do guide for reversible star (wye) delta motor controller

Our previous article covered a typical star delta or wye delta motor controller which is focused on a common circuit intended for a single rotational direction of a 3 phase AC induction motor. But certain star delta motor control applications would also require a reversible motor rotation.


Combining both of our previous articles which covered the star delta motor controller and the forward reverse motor controller, we could come up with a circuit that would provide the required function for a reversible star delta circuit application.

The electrical schematic diagram below is an illustration of a power circuit for a forward reverse star delta motor controller used in every electrical industry involving industrial process automation control technology.

Electrical Schematic Diagram for the Power Circuit of a Forward Reverse Star (Wye) Delta Motor Controller

There are five magnetic contactors shown on the diagram above. The initial operating stage of the circuit is to firstly activate the main contactor together with the star contactor. The star contactor is a common starting device which provides the shorting of the motor's secondary terminal U2, V2, W2 during the startup run of the motor for both the forward and reverse function.

When the time has elapsed wherein the motor has finally gained momentum to attain sufficient holding inertia, the star contactor is released and the command transfers to the final motor run state, activating either the forward or reverse delta contactor, depending on the chosen rotational direction of the motor selected by the operator.

When the forward function is selected to run the motor, the main forward contactor and the star contactor are activated, once the elapsed time is reached, the star contactor releases and the delta forward contactor is activated while maintaining the main forward contactor energized during the entire operation of the motor.

Conversely, when the reverse function is selected to run the motor, the main reverse contactor and the star contactor are activated, once the elapsed time is reached, the star contactor releases and the delta reverse contactor is activated while maintaining the main reverse contactor energized during the entire operation of the motor.

The power circuit diagram above illustrates the corresponding motor connection which follows an orderly wiring configuration conforming to a correct path of phase sequence appropriate for achieving a reduced high transient current spikes during the transition period when the motor connection is transferred from the star mode to the delta mode, this applies to both forward or reverse function of the AC induction motor.

Referring to the vector diagrams presented in the previous article Recommended wiring configuration suitable for reduced changeover current peaks for a star to delta motor connection, the forward (clockwise) and the reverse (counter-clockwise) motor rotation are both configured with appropriate motor connections that produces a minimal Vdiff between the phase voltage sequence and the transition voltage developed during the changeover duration from star to delta configuration.
 
The delta configuration for the forward or clockwise motor rotational direction is achieved with the motor configuration U1-V2-L1, V1-W2-L2, W1-U2-L3 which provides a relatively reduced high inrush transient current during the transition period upon switching of the magnetic contactors from star (wye) to delta motor connection.

Similarly, the delta configuration for the reverse or counter-clockwise motor rotational direction is achieved with the motor configuration V1-U2-L1, U1-W2-L2, W1-V2-L3 which is also the most appropriate configuration for the purpose of providing a suitable reduction of high inrush transient current during the transition period when the magnetic contactors transfers the motor's wiring connection from star mode to delta mode.

The interchanging control for the star to delta and the command selection for choosing between forward or reverse run for the power circuit illustrated above is achieved by means of a corresponding forward reverse star delta control circuit.

Another alternative method of wiring application for a star delta circuit is by means of using a PLC (Programmable Logic Controller) which is diagrammatically explained in the article Star Delta Motor Control Using PLC.