How to learn Star Delta Motor Control – A Basic Guide to Learning Star Delta Motor Controller

The most common motor control method for reducing induction motor starting current in any industrial application is the star delta connection. Star delta motor connection normally incorporates the usage of electromechanical devices which primarily consist of magnetic contactors, a timer and a thermal overload relay.

Due to the low cost and availability of these electromechanical devices for this type of application, the star delta connection method is still, by far, the most widely used electrical control for reduced start up current of 3 phase induction motors which is widely known in any industrial application.

A typical star delta control circuit wiring diagram is shown on the attached CAD drawings. Refer to the attached star delta electrical diagrams, it is intended as a supplemental aid for the visual comprehension of the following explanation.

Star delta motor connections is interchangeably executed automatically by the magnetic contactors which are actuated by the star delta control circuit illustrated in the diagram. The sequence of operation for the motor to start running from the star connection to the delta connection is initiated upon pressing the ON push button switch.

A star configuration of the motor's wiring terminal is initially connected, this is achieved by completing the control circuit with the closing of the ON push button switch, which in turn actuates both the main magnetic contactor and the star contactor and of course including the timer, while leaving only the delta contactor open or deactivated, awaiting for the timer to reach a preset time period.

Pay special attention that upon releasing your finger from the ON push button switch after pushing it ON, that the interlocking contact of the main contactor (which is connected in parallel to the ON push button switch) will instantly seize control to take over the place of the ON push button switch to provide a close contact to maintain a complete circuit after releasing your finger from the ON push button switch.

Once the timer is up after reaching the specified preset time period, the associated timer contacts in the circuit will change condition opposite its normal state. The normally close timer contact will change from close to open to release the star contactor coil, while the normally open timer contact will close to activate the delta contactor coil, hence, providing full voltage to the motor to achieve the required full power for the load.

Illustrated on the power circuit diagram above, the 3 phase line voltage L1, L2, L3 is supplied from the main circuit breaker down to the main magnetic contactor and finally to the three primary terminals of the motor coils U1, V1, W1, while shorting the other three secondary terminals of the motor terminals U2, V2, W2 with the closing of the star contactor.

Take note that the control circuit of the star delta motor controller is specifically configured in the manner so as to achieve the operational transfer sequence that provides the motor terminal with a switching interchange from star connection to delta connection.

The main magnetic contactor is maintained energized all throughout the running duration of the motor while interchanging only the activation between the star contactor and the delta contactor. Once the timer reaches the specified time period, it activates and energizes the delta contactor to close its contact while at the same time disconnecting the star contactor. This second sequence would eventually disconnect the motor terminals from its previous state of star connection and reconfigures the motor terminals to a delta connection, connecting the reference voltage L1 to the motor terminal U1 and V2, reference voltage L2 connected to motor terminal V1 and W2, then reference voltage L3 connected to motor terminal W1 and U2.

Observe carefully from the control circuit diagram the presence of interlocking contacts placed before the coils of both the star contactor and the delta contactor. These interlock switches serves as safety precautionary features so that one cannot actuate without the other one deactivated, meaning, there is a circuit permission involved between the star and delta contactors, so that the delta contactor will not activate without the star contactor deactivated, and conversely, the star contactor will not also activate without the delta contactor deactivated, this configuration is intended to avoid simultaneous activation of both star and delta contactors, which can destroy the motor.

At anytime within the duration when the motor is running, the motor's control circuit can easily be disconnected by pressing the OFF push button switch, this deactivates or opens all of the three units of magnetic contactors in order to immediately remove the supply voltage to the motor which will slow down the motor until it stops to a full standstill once its rotating inertia runs out.

Another disconnecting contact connected in the control circuit is the thermal overload relay which serves as an automatic contact that will disconnect the circuit upon detection of a motor overload, which means that when the load carried by the motor exceeds its rated capacity, the high current of the motor heats up the metallic plates in the thermal overload relay which mechanically releases its contact to open the motor's control circuit.

Industrial Motor Control is an indispensable part of Industrial Process Automation Technology.

Further to the star delta motor controller is the Direct On LIne (DOL) motor controller which is covered in the Basic Motor Controller Guide for Electrical Motor Controls.

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How to learn Star Delta Motor Control – A Basic Guide to Learning Star Delta Motor Controller by ianjonas is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.