When you see the term "thermal protection" or "thermally protected" 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 protector consists of one or more heat-sensing elements built into the motor or motor-compressor, plus an external control device. The thermal protection is present to turn the motor off when excessive heat is generated within the motor circuitry. This safety feature halts the temperature climb before it can burn up the motor.
Thermal protectors generally 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 this 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 device 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. For example, there may be an obstruction on the load attached to the motor, leading to excessive load that causes heat to build up in the motor.
Examples of Motors With Thermal Protectors
A sump pump is a motor for which this scenario often occurs. If a sump pump pumps water filled with debris, pieces of debris may get caught in the impeller and lock the rotation of the pump motor, causing it to overheat very quickly. On a pump equipped with thermal protection, the device will trip and shut down the electrical flow to the motor windings. This will allow the motor to cool down and may well save it from complete failure. Alerted to the problem because the motor shuts down, you may be able to clear the impeller and keep your sump pump motor operating for quite some before it needs replacing.
The same scenario may be true on other electric motors that process variable loads, such as garbage disposers, washing machines or vacuum cleaners. Without thermal protection, such motors can be more prone to burn-out.
Thermal protection is generally a good feature to look for when buying equipment with electrical motors. By protecting the motor from overheating, it can greatly extend the life of the motor.