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.