The Contactor: A Relay That Remembers to Stay Off

A motor's supply is too heavy for a hand switch to make and break safely, and a machine that starts by itself when the power returns can kill. The contactor answers both: a heavy relay whose main contacts switch the motor, controlled by a start button, a stop button and one of its own auxiliary contacts that holds it in once started. That one extra contact — the seal-in — turns the relay into a memory. Press Start and it latches. Press Stop, or lose the supply for a moment, and it drops out and stays out until someone presses Start again. The same circuit, with an overload relay added, is the motor starter on almost every machine. This rung simulates two- and three-wire control through a power cut, works out why a contactor holds at a lower voltage than it needs to pull in, and wires a working start/stop station on the bench.