Thermal Overload Protector

Single-phase motors provide a bimetal switch for use as a built-in overload device. This overload is mounted in the rear end plate near the centrifugal switch assembly and terminal board. The thermal overload consists of a heater and contacts. When the motor is starting and running, all its current is pulled through the heating element. If the motor draws excessive current, the heating element will become warm enough to cause the bimetal contacts to snap open and de-energize the motor windings. When the bimetal cools down, the bimetal will cool again and snap closed, which will re-energize the motor. If the same fault still exists, the motor will overheat again and continue to cycle off through its overload.

Some overloads don't reset automatically. Instead, they have a reset button that must be depressed manually to close the overload contacts. This necessitates someone going to the motor when it trips the overload and resetting it manually. At that time the motor and its loads should be inspected to ensure that the motor is operating correctly.

Changing Voltage and Speeds of Single-Phase Motors