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Selection and Precautions of Thermal Protector for Electric Motors
Category: Company News
Date: 2023-03-31
Click: 832
Author: 恒新精密电器

1.  Selection of Thermal Protector Type:

1.1. Motor Power

Select a suitable volume of thermal protector based on the power and volume of the motor. For small motors, priority should be given to binding and fixing factors, and smaller thermal protectors should be selected; For large motors, attention should be paid to the current and service life of the contacts.

1.2. Installation Position

The external type of thermal protector, which should choose the shell insulation, current and temperature dual protection function.

For the buried type, the thermal protector with metal casing and Insulating sleeve shall be selected, or the thermal protector with high temperature resistant and insulating casing shall be selected.

1.3. Winding Shaping Method

For the production process of binding the thermal protector first and then shaping the winding, the thermal protector with shell anti-deformation should be preferred.

1.4. Dipping Method                                                                                                             

If vacuum immersion paint or thermal immersion paint process is required, a thermal protector with good air tightness should be selected.

1.5. Motor Type

For small power single-phase capacitor motors, pure temperature type of thermal protectors or current & temperature composite type of thermal protectors can be selected. For series excited motors or motors with rapid changes in thermal overload and large changes, current & temperature composite thermal protectors should be selected.


2.  Selection of temperature of thermal protector

2.1. The thermal protector can meet the requirements of motor output rated power under various rated power supply conditions without causing overheating protection of the thermal protector.

2.2. The thermal protector can reliably protect the motor under conditions of slow changing thermal overload and fast changing thermal overload, and the maximum temperature rise of the winding after thermal protection does not exceed the temperature limit of the insulation grade.

2.3. Generally, the selection of the operating temperature of the thermal protector is determined by the motor manufacturer based on experience and test results when the above two conditions are met. If the temperature selection of the thermal protector is too low, it will cause the motor to be protected in advance within the allowable overload range, and the motor cannot fully exert its efficiency; If the temperature of the thermal protector is too high, the insulation life of the winding will be reduced.


Recommended Optional Temperature

Insulation Level






Operated Temperature of Thermal Protector






Note: The thermal protector disconnection temperature should take into account the thermal inertia of the motor after power failure.


    3.  Selection of Overcurrent Characteristics of Thermal Protectors

With the further improvement of the requirements for thermal protectors, it should not only protect against temperature, but also have a temperature current composite protection function. The critical tripping current and rated tripping time can be selected according to the curve provided by the thermal protector manufacturer.


    4.  Common Problems in the Use of Thermal Protectors

    On the premise of reasonable selection of thermal protectors, the following problems may occur during incoming material acceptance and use:


    4.1. The user detects that the disconnection temperature is in error with the factory standard.

    Reason: The temperature in the thermostat box is not uniform, the temperature fluctuation is large, the heating rate is too fast, and the load current is too large.


    4.2. The winding temperature exceeds the specified maximum temperature during the motor locked-rotor test

    Reason: The installation position of the thermal protector is unreasonable, and the contact area of the temperature sensing surface of the thermal protector is too small.


    4.3. Winding burnout during motor overload or locked rotor test.

    Reason: The binding position of the thermal protector is not between the main winding and the secondary winding, and the disconnection of the thermal protector cannot cut off the circuit between the capacitor and the secondary winding.


    4.4. After the motor stator is dipped in paint, the thermal protector is found to be blocked.

    Reason: The selection of thermal protectors is unreasonable. When the paint liquid enters the interior of the thermal protector after vacuum or thermal immersion paint, or the thermal protector deforms during shaping, the sealing performance is damaged, and then the paint liquid enters the interior of the thermal protector.


     4.5. Thermal protector and winding interturn short circuit

    Reason: During the binding process of the charged metal shell of thermal protector, the insulating sleeve broke, resulting an inter-turn short circuit between the thermal protector and the winding enameled wire.


    4.6. The thermal protector does not work with the enameled wire and motor lead wire after arc welding.

    Reason: Arc welding is a process that uses high current to generate high temperatures in a short period of time to partially melt metals and weld them together. During the welding process, if high current passes through the thermal protector, the thermal protector will be damaged.

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