Motor Starters and Contactors
11 Overview
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A motor starter is a device that is designed to safely start and stop a motor, and provide running (overload) protection.
A starter can be a simple two-wire manually operated switch or a more complex magnetically-operated forward/reverse motor starter. Regardless of its design, all motor starters must be used within their voltage and power ratings and must provide overload protection. One of the ways that motor starters provide overload protection is through the use of overload relays (OLR).
Common Ratings
There are three significant electrical ratings for motor starters: voltage, horsepower, and ampacity (current value).
The voltage rating of a motor starter must be rated for at least the operating voltage of the motor that it controls. This rating is specified on the motor and starter nameplates.
The ampacity rating of a starter usually refers to the overload protection device. The selection of the proper overload heater depends on the FLA (full-load-amps) current drawn by the motor. FLA is usually specified on the motor’s nameplate and most manufacturers supply a table for heater-size selection.
Ambient temperature can also have an effect on overload heater sensitivity. Precautions should be taken when operating motor starters in very high or low ambient temperature environments.
The horsepower rating of a starter must meet or exceed the horsepower rating of the motor it controls at its listed operating voltage.
Horsepower is always given at a specified voltage. For example, a starter rated 3 horsepower at 240 V must not be used to control a 3 horsepower, 120 V. Instead it would most likely be rated for only 1.5 horsepower at 120 V.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.
A device for making or breaking the connection in an electric circuit.
The maximum amount of voltage that a fuse, circuit breaker, switch-gear or motor starter can handle. The voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.
The rate at which work is done. It is measured in watts (W), or joules per second (J/s).