How to Wire an Industrial Control Motor

Option 1: With a Toggle Switch

Let’s consider a simple three-phase, 480V AC motor control circuit consisting of a contactor, overload block, control power transformer, power fuses or (you may use circuit breakers) and the associated components as illustrated in the figures below:

The schematic diagram for the same circuit is shown below:

The control power transformers steps down the 480V AC to provide 120 volt AC power for the contactor coil to operate on. The overload (OL) contact is wired in series with the contactor coil so that a thermal overload event forces the contactor to de-energize and as a result interrupt power to the motor even if the control switch is still in the “ON” position. The overload heaters appear in the schematic diagram as pairs of back to back “hook” shapes, connected in series with the “T” lines of the motor. Note, the overload (OL) heater elements do not directly interrupt power to the motor in the event of an overload, but rather signal the “OL” contact to open up and de-energize the contactor.

If this motor control circuit is used in an automatic control system, the toggle switch would be replaced by another relay contact, which is actually controlled by the status of a process or a process switch or maybe the discrete output channel of a programmable logic controller (PLC).

The toggling-style of switch is required in order for the motor to continue to run after a plant operator actuates the switch. The motor runs with the switch in the closed state and stops when the switch opens.

Option 2: With a Latching Circuit

We can wire the circuit described in option 1 but instead of using a toggle switch, we use a latching circuit that permits the use of momentary contact switches (one to start, and one to stop). Such a kind of latching circuit is shown below:

The schematic diagram with a latching circuit is illustrated below:

In the above circuits (Fig 1.2 and Fig 1.3), an auxiliary contact actuated by the motor contactor is wired in parallel with the “Start”  pushbutton switch, so that the motor contactor continues to receive power after the plant operator releases the  pushbutton switch. This parallel contact is at times referred to as a seal-in contact, latches the motor in an “ON” state after a momentary closure of the “Start” pushbutton switch.

A normally-closed “Stop” switch provides a means to “un-latch” the motor circuit. Pressing this pushbutton switch stops current in the coil of the contactor, causing it to de-energize, which then opens the three motor power as well as the auxiliary contact that used to maintain the contactors energized state.

You may also read:

• Industrial Motor Control Using Contactors
• How to Perform a Brake Test on a DC Motor
• Power MOSFET Motor Control
• Fuses & Circuit Breakers vs. Overload Heaters for Industrial Motor Protection
• Speed Control of a DC Compound Motor