The three “opposed-question-mark” shaped devices in series with each phase going to the motor are called overload heaters. One contactor may have several auxiliary contacts, either normally-open or normally-closed if required. The auxiliary contact is often used in a relay logic circuit, or for some other part of the motor control scheme, typically switching 120 Volt AC power instead of the motor voltage. The lowest contact is an “auxiliary” contact which has a current rating much lower than that of the large motor power contacts, but is actuated by the same armature as the power contacts. Perhaps the most common industrial use for contactors is the control of electric motors.The top three contacts switch the respective phases of the incoming 3-phase AC power, typically at least 480 Volts for motors 1 horsepower or greater. Contactors typically have multiple contacts, and those contacts are usually (but not always) normally-open, so that power to the load is shut off when the coil is de-energized. When a relay is used to switch a large amount of electrical power through its contacts, it is designated by a special name: contactor. Time delay relays are commonly used in large motor control circuits to prevent the motor from being started (or reversed) until a certain amount of time has elapsed from an event.ġ0.2 Contactors All About Contactors Figure 10.5.Continuous motor operation with a momentary “start” switch is possible if a normally-open “seal-in” contact from the contactor is connected in parallel with the start switch so that once the contactor is energized it maintains power to itself and keeps itself “latched” on.
If we use contacts that delay returning to their normal state, these relays will provide us a “memory” of which direction the motor was last powered to turn. Let’s begin by adding a couple of time-delay relay coils, one in parallel with each motor contactor coil. What we might like to have is some kind of a time-delay function in this motor control system to prevent such a premature startup from happening. If the fan was still coasting forward and the “Reverse” pushbutton was pressed, the motor would struggle to overcome that inertia of the large fan as it tried to begin turning in reverse, drawing excessive current and potentially reducing the life of the motor, drive mechanisms, and fan. This could be problematic if an operator were to try to reverse the motor direction without waiting for the fan to stop turning. If our hypothetical motor turned a mechanical load with a lot of momentum, such as a large air fan, the motor might continue to coast for a substantial amount of time after the stop button had been pressed. Let’s consider another practical aspect of our motor control scheme before we quit adding to it. The “Stop” switch, having normally-closed contacts, will conduct power to either forward or reverse circuits when released. Now, if either forward or reverse circuits are latched, they may be “unlatched” by momentarily pressing the “Stop” pushbutton, which will open either forward or reverse circuit, de-energizing the energized contactor, and returning the seal-in contact to its normal (open) state.