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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 locks 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. 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.
Positive Temperature Coefficient (PTC) Thermistors
Positive Temperature Coefficient (PTC) thermistors exhibit increasing electrical resistance with increases in environmental temperature and decreasing electrical resistance with decreasing temperature.
These imbedded thermistors are unreliable, the tension of the windings often breaks them and they go open circuit. They are usually wired to a special box that detects when they go over resistance (from memory 600 ohms?) and is part of the start/overload contactor circuit. This should be right after the motor before the VFD or any other AC source. you can check them with a DVM for resistance.
Frank
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