The control circuit discussed here consists of conventional physically hard-wired electrical components, wherein each components are individually wired using electrical cables fastened with screws to wiring terminals located in the electrical components. The introduction of logic controllers, however, has made it possible for such tedious wiring task to be conveniently constructed by drawing graphical representations of the circuit on a computer terminal to assemble the sequential logic of the circuit programmed by using a computer software, which is covered in the article Star Delta Control Circuit Using Programmable Logic Controller.
After pushing the ON push button switch, the auxiliary contact of the main contactor coil which is connected in parallel across the ON push button will close, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch. Another auxiliary contact of the main contactor placed on top of the star contactor coil and the timer coil initializes the control circuit which activates the star connection of the motor through the star contactor coil, and while also awaiting to reach the specified time delay along with the activation of the timer coil during this initial sequence. Once the time delay is reached, the timer's auxiliary contacts will change state opposite its normal condition which would become an open contact (instead of close) to release the star contactor coil while at the same time providing a close contact (instead of open) to activate the delta contactor coil for the second sequence to reconfigure the motor terminal connection from star to delta connection, to place the motor in the run state from start state.
A normally close auxiliary contact from both star and delta contactors are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.
The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch is a manual operator access switch intended for shutting OFF the circuit to stop the motor when necessary. The thermal overload contact is a protective device which automatically opens the circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.
To facilitate for a further understanding about the concept of the star delta electric motor control circuit, a flowchart diagram is provided to serve as a schematic representation of the sequence of operation of the star delta electric motor control circuit.
