During frequent start-ups or high inertia moment of the load the thermal protection of control meters is often not sufficient. In cases like this one the machine can be equipped with motors with thermal protection.

The thermoprotecotor is placed in winding according to one of following systems:

• Thermoprotector with bimetal disc
  When winding’s temperature reaches the same temperature as excitation temperature of bimetal transmitter, its contact (which is usually closed) opens and directly breaks coil of control meter, what results in its opening.
  
• Thermoprotecotor with thermistor (PTC)
  When excitation temperature grows then inner resistance suddenly increases. It results in break of meter coil and its opening. It happens by agency of “detaching device” – we don’t have it our offer.

Thermal protection for motors is useful in following cases

• - overloading caused by increase of moment of force
• - too high or low power voltage
• - work at lack of one phase
• - defective ventilation and/or high environment temperature
• - unsuccessful start-up caused by monophase power supply or excessive moment of force. In cases like those termoprotectors or thermistors are really essential

Exciting temperature of thermoprotectors depends on kind of motor insulation:
155^oC for „F” Class, 180^oC for „H” Class.