Most large motors are three-phase AC, or alternating current, motors. This means three power cables, called L1, L2 and L3, power the motor. These cables can be quite thick. Running three thick cables from a switch to the motor is impractical. A contactor uses a small control voltage to control the electrical flow through the main power lines. The technical educational division of the Siemens Electric Company illustrates this by wiring a small electromagnetic coil to a switch. When the switch is turned on, the coil energizes, and pulls together three large contacts, effectively closing the main power circuits. The large contacts, individually wired to L1, L2 and L3, carry the high voltage to the motor. Because this requires only a small control voltage, the corresponding control wires can be very thin.
Many contactors have a thermal overload built right into the case. This is a dual function of the contactor. If the motor becomes excessively hot, perhaps due to overloading or a short, the thermal overload "kick in" and break the circuit. In effect, the thermal overload is an internal circuit breaker.
In many motor control applications, to stop and reverse requires the motor. For example, you may have to reverse a conveyor belt to remove a stuck component. On the control panel, another control push-button switch is labeled "Reverse." The reverse button is wired into the contactor. When pushed, the contactor internally reverses L1 and L3, causing the motor to spin backward.
Sometimes, you only want the motor to run a little bit, and then stop. A push button on the control panel is labeled "Jog." When you push the button, the motor runs. When you remove your finger, the motor stops. This is called jogging and is another function performed by using a contactor. Jogging is constantly used by automation systems such as aligning a soda bottle precisely so a nozzle can fill it with liquid. A motor drives the bottle conveyor, and the conveyor has to move just a little bit.
Most control panels that control contactors have five push buttons labeled "Run," "Jog Forward," "Reverse," "Jog Reverse" and "Stop." The "Run" button starts the motor. The "Jog Forward" button runs the motor forward only when your finger is on the button. The "Reverse" button reverses the motor's rotation. The "Jog Reverse" button runs the motor, but in reverse rotation, and only when your finger is on the button. The "Stop" button is a general stop, which turns the motor off. If you have to jog or reverse, you first stop the motor by pushing the "Stop" button, and afterward use the jogging or reversing buttons. The control panel wiring has safety interlocks wired in, so you can't hit the "Run and "Reverse" buttons without stopping first.