To start up and shut down a three-phase AC induction motor, any three-pole switch with a suitable current rating will suffice.
Simply closing such a switch to send three-phase power to the motor will cause it to start up, while opening the three-pole switch will cut power to the motor to make it turn off.
If we desire to have remote start and stop control over a three-phase motor, we need a special relay with switch contacts big enough to safely conduct the motor’s inrush current over many start and stop cycles.
Large, high-current-rated electromechanical relays built for this very purpose are commonly referred to as contactors in industry.
A schematic diagram of a three-phase contactor connected to a three-phase motor (with fuses for overcurrent protection) is shown here:
Energizing terminals A1 and A2 magnetizes the electromagnet coil, causing all three switch contacts to simultaneously close, sending three-phase AC power to the motor.
De-energizing the coil causes it to de-magnetize, releasing the armature and enabling a return spring inside the contactor to snap all three contacts to the open (off) position.
A contactor rated at 75 horsepower (at 480 volt AC 3-phase power) is shown here, both assembled and with the top cover removed to reveal the three sets of high-current electrical switch contacts:
Each phase switch contact is actually a series pair of contacts that make and break simultaneously with the actuation of a ferrous armature attracted by an electromagnet coil in the base of the contactor assembly.
The operation of the three contact sets may be seen in this pair of photographs, the left-hand image showing the contacts in their normal (open) state, and the right-hand image showing the contacts closed (the armature “pulled in”) by the force of my finger:
Of course, it would be very dangerous to touch or manually actuate the contacts of a motor starting relay with the cover removed as shown.
Not only would there be an electric shock hazard from touching any one of the bare copper contacts with your finger, but the arcing produced by closing and opening such contacts would pose arc flash and arc blast hazards. This is why all modern motor contactors are equipped with arc shield covers.
The actual switch contact pads are not made of pure copper, but rather silver (or a silver alloy) designed to survive the repeated arcing and blasting action of large AC currents being initiated and interrupted.
Below the main power connection terminals (L1-L3, T1-T3) on this contactor hide two small screw terminals (commonly denoted A1 and A2) providing connection points to the electromagnet coil actuating the contactor:
Like most three-phase contactors, this one’s coil is rated for 120 volts AC. Although the electric motor may operate on three-phase, 480 volt AC power, the contactor coil and the rest of the control circuitry operates on a lower voltage for reasons of safety.
Like all electromechanical relays, motor contactors use a low-power signal to control higher-power electric current to the load.
This “amplifying” action enables relatively small control switches, PLCs, and relay circuits to start and stop relatively large (high-current) electric motors.
Credits : by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 License