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Contactors and Motor Starters |
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Introduction
Discrete devices have two states, ON and OFF, while analog devices have or respond an electrical signal varying in value between two end limits.
The abbreviation NO stands for normally open: that is, open in the non-actuated position. When actuated, the NO device will close. The abbreviation NC stands for normally closed. When actuated, an NC device goes from the closed position to the open position. Note that for limit switches there are four symbols. Two are straight NO and NC limit switches. The other two are held closed and held open. The designation held closed or held open means that at the start of a process the limit switch is mechanically held in the actuated position, ON or OFF.
Typical discrete-output devices include relays, indicating lights, contactors, and motors. Such devices are ON when power is applied and OFF, or not actuated, when power to them is OFF.
Analog output devices include positioning motors, meters, positioners, and stepper motors. Even though the stepper motor is not strictly continuous (it moves in small steps), it is considered an analog output device.
Contactors and Motor Starters
Magnetic contactors are electromagnetically operated switches like relays, specifically designed to provide a safe and convenient means for connecting and disconnecting motors. The principal difference between a contactor and a motor starter is that the contactor does not contain overload relays. Contactors are used in combination with pilot control devices to switch lighting and heating loads and to control ac motors in those cases where overload protection is provided separately. The larger contactor sizes are used to provide remote control of relatively high-current circuits where it is too expensive or impractical to run the power leads to the remote controlling location.
