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With the development of technology, the application range of motor is more and more extensive, especially the use of motor in daily life and production is also more and more important. But the motor burn accident also happens, in real life, the motor burn is mainly caused by the following blocked coil overheating, the load is too large motor overload, lack of phase and other reasons. Installing a heat protector is one of the effective ways to protect the motor. Thermal protector, also called temperature switch, temperature control switch, etc. It is a kind of temperature switch with bimetal sheet as a temperature sensing element, when the electrical appliance is working normally, the bimetal sheet is in a free state, the contact is in a disconnected state, when the temperature rises to the action temperature value, the bimetal element is heated to produce internal stress and rapid action, open the contact, cut off/connect the circuit, so as to play a thermal protection role. Wait until the ambient temperature drops to thereset temperature of the protector, the contact will re-close the connected circuit, and the motor will work again.

How to choose the right protector is very important, otherwise not only cannot play a protective role, but will cause the motor cannot work because of the wrong selection, or even burn.

1. Voltage & Current determination

  • Motors have different voltage ranges 380V 230V 220V 110V, DC motors will also have 36V 24V 12V, etc., according to different working voltages to choose different rated voltage protectors, in principle, the actual application voltage is less than or equal to the rated voltage of the protector, for example, 380V motor cannot choose rated voltage 220V protector, and rated voltage 220V protector can be used in 110V motor;

  • The current of the motor is divided into three kinds of starting current, normal working current and blocked current, the rated current of the protector should be greater than the maximum value of the three currents, and need to leave a margin. For example, a motor normal working current 1A, starting current 1.5A, blocking current 2A, then the rated current of the selected protector is at  least 2A, and it is best to choose the rated current of 3A or 5A to retain the margin. Some motor starting current is greater than the rated current of the protector, but the starting time is shorter, and the normal working current and blocking current are less than the rated current of the protector, in this case, you can also choose this protector. Because the starting current is greater than the rated  current, but due to the short time, the thermal effect of the current has not had time to make the double piece heat leading to protection of the protector, so as not to cause misaction.

2. Choose to protect the temperature

  • The choice of protection temperature is related to three factors: insulation level of enameled wire, installation position and protector type.

Temperature grade of the insulation






Max. temperature(℃)






Winding temperature rise limit value (K)






The protection temperature is lower than the maximum allowable temperature, for example, the maximum temperature ofF class is 155℃, and the actual selection can be 145℃ or 150℃.

  • The installation position is divided into built-in inside the coil and bundled outside the coil, because when the motor is blocked and forwarded heat, the overheating phenomenon spreads from inside and outside. If the protector is buried in the interior, the protection temperature is slightly lower than the highest temperature of the enamelled wire; If it is tied outside the coil, consider the temperature difference between the inside and outside, and choose a lower temperature protector. For example, the insulation level is F class enameled wire, coil inside and outside the temperature difference of 20℃, then choose 155-20=135℃ or so of the protector;

  • Protector is divided into pure temperature type and temperature current double protection type two, the differencebetween the two is that the former in actual use of the motor current does not pass through the bimetal sheet, no current thermal effect, the protector will not jump protection in advance; The latter motor current through the bimetal sheet, there is a current thermal effect, the protector will be disconnected protection in advance. In the actual selection process, the pure temperature protector does not need to consider the current thermal effect, and the actual jump temperature is consistent with the rated disconnect temperature; And the temperature current double protection type should consider the current thermal effect, according to the temperature value of the jumpin advance, increase the temperature of the protector. For example, when the lock current 5A causes the protector to jump 10℃ in advance, the actual selection of 145℃ of the protector needs to be increased by 10℃ to select 155℃ as the disconnect temperature.

3. Selection of the protector life

Different motor lock-in test time different requirements for the life of the protector are not the same, such as normal household motor lock-in 18 days, the number of cycles of normal action reset will not exceed 2000 times, the rated voltage current life of more than 2000 times can be selected protector; Some pump motors only do 3 days of blocking, in principle, the life requirement is hundreds of times.

4. Selection of the protector for sealing performance

Motor coil in the production process will dip paint to enhance the insulation performance of the process, dip paint is divided into drop paint, immersion, vacuum into the paint and other processes, especially the vacuum into the paint  pressure is large, insulating paint is easy to enter the inside of the protector, resulting in the failure of the protector. The ST01 series of An Electronics has a very high sealing, under the condition of -0.08MP, the paint dipping rate can reach 3%; In the case of -0.03MP, the paint rate can reach less than one in ten thousand.


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Sales:Don Qu

Phone:+86 183 5813 1344

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