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).
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.