Selection of Thermal Protector in Motor Product Application

With the development of technology, the application range of electric motors becomes more and more extensive, especially the use of electric motors in daily life and production is becoming more and more important. However, accidents of motor burnout also occur from time to time. In real life, motor burnout is mainly caused by the following reasons such as overheating of the locked-rotor coil, excessive load, motor overload, and lack of phase. Installing a thermal protector is one of the effective ways to protect the motor. Thermal protector, also called temperature switch, temperature control switch, etc. It is a temperature switch that uses a bimetal as a temperature sensing element. When the electrical appliance is working normally, the bimetal is in a free state and the contact is in a disconnected state. When the temperature rises to the operating temperature value, the bimetal element is heated to produce internal It acts quickly under stress, opens the contacts, disconnects /connects the circuit, and plays a role of thermal protection. When the ambient temperature drops to the reset temperature of the protector, the contacts will reclose and switch on the circuit, and the motor will work again.

How to choose a suitable protector is very important. Otherwise, not only will it fail to protect, but the motor will not work or even burn out due to the wrong selection.

First, determine current and voltage

Motors have different voltage ranges: 380V, 230V, 220V, 110V, and DC motors, 36V, 24V, 12V, etc. According to different working voltages, select protectors with different rated voltages. In principle, the actual application voltage should be less than or equal to the rated voltage of the protector. For example, a protector with a rated voltage of 220V cannot be used for a 380V motor, while a protector with a rated voltage of 220V can be used in a 110V motor.


The current of the motor is divided into three types: starting current, normal working current and locked-rotor current. The rated current of the protector should be greater than the maximum value of the three currents, and a margin should be left. For example, if a motor has a normal working current of 1A, a starting current of 1.5A, and a locked-rotor current of 2A, the selected protector should have a rated current of at least 2A. It is best to choose a rated current of 3A or 5A, with a margin. The starting current of some motors is greater than the rated current of the protector, but the starting time is shorter, and the normal working current and locked-rotor current are less than the rated current of the protector. In this case, this type of protector can also be selected. Because the starting current is greater than the rated current, but due to the short time, the current heating effect has not had time to make the two-chip heat to cause the protector to protect, so that it will not cause malfunction.

Second, protection temperature selection

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

1. The insulation level corresponds to the maximum operating temperature of the enameled wire, as shown in the figure below

Insulation temperature classClass AClass EClass BClass FClass H
Maximum allowable temperature (℃)105120130155180
Winding temperature rise limit (K)607580100125


The protection temperature shall be lower than the maximum allowable temperature. For example, the maximum temperature of class F is 155 degrees. The actual selection is 145 degrees or 150 degrees.

2. The installation position is divided into the built-in coil and the lashed out of the coil, because when the motor blocks and transfers heat, the overheating phenomenon spreads from the inside to the outside. If the protector is buried inside, the protection temperature can be slightly lower than the maximum temperature of the enameled wire; if it is tied outside the coil, the temperature difference between the inside and the outside must be considered, and a protector with a lower temperature should be selected. For example, if the insulation class is F grade enameled wire, and the temperature difference between the inside and outside of the coil is 20 degrees, a protector with 155-20=135 degrees should be used.

3. The protector is divided into two types: pure temperature type and temperature and current double protection type. The difference between the two is that the motor current does not pass through the bimetal in actual use, and there is no current heating effect, and the protector will not trip the protection in advance; When the motor current passes through the bimetal, there is a current heating effect, and the protector will disconnect the protection in advance. In the actual selection process, the pure temperature protector does not need to consider the current heating effect, and the actual trip temperature is the same as the rated disconnect temperature; while the temperature and current dual protection type needs to consider the current heating effect, and the protector is increased according to the temperature value of the early trip temperature. For example, when the locked-rotor current is 5A, the protector is tripped 10 degrees ahead of time. The actual selection of the 145 degrees protector needs to increase by 10 degrees and select 155 as the cut-off temperature.

Third, the selection of the life of the protector

Different motor locked-rotor test times have different requirements for the life of the protector. For example, if a normal household motor is locked for 18 days, the number of cycles of normal action reset will not exceed 2000 times. Choose a protector with a cycles of more than 2000 times under rated voltage and current. That’s it; some water pump motors can only be locked for 3 days, and in principle, the cycles is only a few hundred times.

Fourth, the selection of protector sealing performance

During the production process, the motor coil will be dipped in paint to enhance the insulation performance. The dipping process is divided into dripping, immersion, and vacuuming into the paint. Especially when the vacuum and the pressure are high, the insulating paint is easy to enter the protector. , Causing the protector to fail. The ST01 series of SAFTTY has extremely high sealing performance. In the case of -0.08MP, the paint dipping rate can reach 3‰ or less; in the case of -0.03MP, the paint penetration rate can reach less than one ten thousandth.