What is 'Thermal Protector' on an Electric Motor?
When you see the term “thermal protector” or “electric motor thermal protector” used in the description of an electric motor,
it refers to a device found within the motor or motor compressor that is designed to prevent dangerous overheating that can cause motor failure.
The Purpose of a Thermal Protector
This overheating generally occurs when the motor is overloaded, when a bearing seizes up, when something locks the motor
shaft and prevents it from turning, or when the motor simply fails to start properly.
A failure to start may be caused by faulty start windings in the motor.
The thermal switch for motor consists of one or more heat-sensing elements built into the motor or motor-compressor,
plus an external control device for motor overheat protection. The thermal protection is present to turn the motor off
when excessive heat is generated within the motor circuitry, halting the temperature climb before it can burn up the motor.
These thermal protectors reset themselves once the motor cools down to a safe operating temperature.
There is usually a visible red button located on the wiring side of the motor— usually, though not always, located opposite
of the motor shaft. On motors equipped this way, you must press the reset button to reset and restart the motor.
On other motors without a reset button, the reset happens automatically as the motor cools.
Having a motor shut down because it tripped a thermal limit is inconvenient, but it certainly is better than having to
replace a motor because it overheats. And the shut-down can alert you to problems with the motor or connected devices,
or to the load attached to the motor. When a motor fails to start or overheats during operation, it may indicate that
the motor has reached the end of its useful life and must be replaced. But often the fault does not lie with the motor at all.
There could be an obstruction on the load attached to the motor, leading to excessive load that causes heat to build up in the
motor—tripping the thermal protector that saves the motor.