The term thermally protected in motors, pertains to a thermal protector placed internallly within a motor or motor-compressor to protect the motor and motor windings components from dangerous overheating that can cause motor failure.
This overheating generally occurs when the motor is overloaded, a bearing seizes up, something locksd the motor shaft from turning, or the motor simply fails to start properly. A failure to start may be caused by faulty start windings in a motor.
The thermal protector consists of one or more heat-sensing elements integral with the motror or motor-compressor and an external control device. The thermal protection is in place to turn the motor off when excessive heat is generated within the motor circuitry and keep it from burning up the motor.
These thermal protectors are reste when the motor cools to a safe operating temperature. There is usually a visible red button located on the wiring side of the motor, opposite of the motor shaft, but not always. In some cases, these buttons are reset manually, but not always.
Although having a motor shut down because it tripped a thermal lmit is inconvenient, it certainly is better than having to replace a motor. Just think of it as a second chance to find out what the problem is with the motor or the connected devices or load attached to it.
You see, just because a motor fails to start or overheats during operation, doesn't mean that the motor is necessarily at fault. There could be an obstruction on the attached load to the motor, causing excessive load on the motor. This in turn causes excessive heat to build up in the motor and hopefully, the thermal protector trips the motor circuit to save the motor.
A good example of this is a sump pump, that probably everyone has used at one time or another. Imagine that the sump pump is pumping dirty water with sticks and who knows what all out of a sump pump hole. Suddenly, a piece of a stick gets caught in the impeller of the pump and locks the rotation of the pump motor. It keeps the motor from turning and causes the motor to heat very quickly. Suddenly the thermal protector trips and shuts the circuit down to the motor windings. This allows the motor to cool and saves the motor. Unfortunately, the water is no longer pumping, but it obviously wouldn't have anyway and you'd likely be changing the sump pump out, rather than just cleaning out the obstruction. That's not to say the pump motor hasn't gone bad, as they odten fail from hours of use.