Control 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

An electrical schematic diagram for the power circuit of a reversible star delta or wye delta motor controller was tackled in our previous article, where it showed an illustration of the wiring combinations using three (3) phase circuit breakers, magnetic contactors and thermal overload relay to achieve the different variation of motor terminal connections in star and delta configuration in relation to the three (3) phase supply source voltage, hence the term power circuit diagram.


As with all motor controllers, the power circuit is always accompanied with its associated control circuit which is an integral part of the motor controller that provides operator command access in order to control the power circuit either in running or stopping the motor or selecting the forward or reverse rotation of the motor.

The electrical diagram below shows an schematic illustration of a forward reverse star delta or wye delta control circuit used in every electrical industry involving industrial process automation control technology.

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

Depending on the choice of the operator, starting the circuit is selected via the forward or the reverse push button switches. When the forward push button switch is selected, the command is transferred to the relevant forward function of the circuit which activates the forward main coil (blue). The relevant auxiliary contacts of the forward main coil (blue) would then be actuated opposite its normal state, that is, a normally open contact would eventually switch to a close contact whereas a normally close contact would switch to an open contact.

The forward main contact (blue) closes to activate the star coil (brown) together with the timer coil (green), hence placing the motor terminal in the power circuit connected to a star configuration. After an elapsed time is reached, the timer contact (green) will release the star coil (brown) and transfers the command to the forward delta mode as a result of a completed circuit with the closing of the timer contact (green) along with the awaiting initially closed forward main contact (blue), thereby permitting power flow to the forward delta coil (dark blue). This final stage places the motor terminal in the power circuit configured to a delta connection.

Another forward main contact (blue) is connected in parallel across the forward push button switch to serve as a latching switch that keeps the forward main coil maintained energized after the operator releases the forward push button switch.

Conversely, when the reverse push button switch is pressed by the operator, all the related contacts of the reverse main coil (red) is activated which holds the reverse main coil maintained activated after releasing the reverse push button switch, then energizes the star coil (brown) and the timer coil (green), then switches to the reverse delta mode (orange) after the specified time set on the timer expires.

To manually interrupt the circuit, the operator only needs to press the stop push button switch to deactivate any energized coil in the control circuit to instantaneously isolate the power source to the motor. Another interrupting contact is the thermal overload contact (purple) which serves as an automatic shutoff switch that instantaneously opens the control circuit to shut down the motor when overload current greater than the rated capacity is detected from the electric motor.

Interlock contacts are also strategically placed on the control circuit as a protective measure. An auxiliary contact from the star contactor (brown) is connected in series before the two delta contactor coils to isolate both the forward delta coil and the reverse delta coil when the star contactor coil is active. Likewise, auxiliary contacts from both the forward and the reverse delta coils are also placed in series before the star coil to isolate power to the star coil should any one of the two delta coils be active to ensure the absence of power flow to the star coil when  the control circuit is actively in the delta mode.

Auxiliary contacts from both forward and reverse coils are also placed opposite each coil so that one cannot activate without the other one deactivated first, meaning, the normally-closed forward main contact (blue) connected before the reverse main coil (red) will not permit the activation of the reverse main coil (red) while the forward main coil (blue) is activated, conversely, the normally-closed reverse main contact (red) connected before the forward main coil (blue) will not also permit the activation of the forward main coil (blue) while the reverse main coil (red) is activated.

The same condition applies to both the forward and the reverse delta coils. The opposing interlock contacts placed before the forward and the reverse delta coils also provides protective action by preventing the simultaneous activation of the forward and the reverse delta coils at the same time, that is, the normally-closed reverse main contact (red) connected before the forward delta coil (dark blue) will not permit the activation of the forward delta coil (dark blue) while the reverse main coil (red) is active, and the normally-closed forward main contact (blue) placed before the reverse delta coil (red) will not also permit the activation of the reverse delta coil (red) while the forward main coil (blue) is active.

To facilitate for a further understanding about the concept of the forward reverse star delta electric motor control circuit, a flowchart diagram is provided to serve as a schematic representation of the sequence of operation of the reversible star delta electric motor control circuit.

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