Bimetallic strip thermal relays are simple in structure, easy to use, and inexpensive. When selected properly, they can provide excellent protection, and are therefore widely used by many manufacturers for overload protection of low-voltage asynchronous motors. They are often combined with AC contactors to form a comprehensive electromagnetic starter. Motor overload can be caused by many factors, such as phase loss, mechanical overload (locked rotor), undervoltage, excessive operating frequency, and obstructed cooling paths, but phase loss is the most common fault in practice. During operation, the winding insulation of a motor has a certain temperature rise limit; that is, to ensure normal operation, the winding insulation has a time-dependent permissible overload characteristic. For a series motor, this is a permissible overload characteristic curve band. The protective characteristic of the thermal relay should be below this characteristic band (Figure 1). The thermal relay should reliably operate and disconnect the motor before the winding insulation reaches a dangerous temperature. In summary, the basic requirements for a thermal relay are that it can reliably protect the motor from burnout due to overload, while ensuring normal operation and starting of the motor, and fully utilizing the motor's overload capacity. The following is a brief introduction to the types, protection characteristics, and selection of thermal relays. (1) Types and protection characteristics The basic series of thermal relays currently produced in my country include JR16, JR14, JR15, etc. “皿” is the Chinese pinyin name of the thermal relay, and the number following it is the design serial number. Those ending with …D are three-phase type with phase loss protection device. JR16 and JR14 series thermal relays are all three-phase type, and the operating mechanism is a reed or tension spring type (JR14) instantaneous trip mechanism. The current specification ranges from 0.25 to 160 amps, and there are 20 thermal elements. In addition to being used for overload protection of motors, they can also be equipped with phase loss protection device. JRt type thermal relay is two-phase type, without phase loss protection device. The operating mechanism is also a reed instantaneous trip mechanism. The current specification ranges from 0.25 to 150 amps, and there are 20 thermal elements. The above thermal relays are all temperature compensated type, ranging from -25℃ to +40℃. Within its operating temperature range, the protection characteristics are minimally affected by temperature changes. Thermal relays typically have one normally closed and one normally open control contact. The normally closed contact has a breaking capacity of 380V, 2A, and the normally open contact has a maximum closing current of 380V, 5A. After the thermal relay trips, the user can choose automatic or manual reset as required. When ordering a thermal relay, the user should specify the model, current rating, and thermal element specifications. For example, JR16-20/3D, 7.2-11A indicates a JR16 series thermal relay, 20A rating, with phase loss protection, and a thermal element current of 7.2-11A. The protection characteristics of the thermal relay should comply with the requirements of the JB2457-78 standard for "Bimetallic Strip Thermal Relays". (2) Correct selection of thermal relays is extremely important for reliable protection of motors. Actual usage surveys show that a considerable number of thermal relays are improperly selected and fail to provide the necessary protection. Many units simply select thermal relays based on the rated current of the motor. In fact, meeting only this condition is insufficient. For example, a motor with heavy-load starting has a long starting time. If a common type thermal relay suitable only for light-load starting is selected for protection, the thermal relay will still trip during the starting process, even if it matches the rated current of the motor. Therefore, when selecting a thermal relay, it is necessary to understand the model and specifications of the motor, the nature of its mechanical load (duty system), the starting time and starting current multiple under normal load, as well as the environment of the application and the importance of the working machinery, etc. If stopping the motor would cause significant production losses, then a combination of thermal relay and other signals should be considered for protection. Even if it may shorten the life of the motor, the thermal relay should not trip (otherwise the losses would be greater). It should only trip and disconnect the motor in truly dangerous situations. Depending on the different usage methods of the motor, thermal relays can be selected according to the following situations: a. For motors operating continuously or intermittently, the setting current value of the thermal relay should be selected as 0.9-1.05Ie (Ie is the rated current of the motor) according to the motor's operating process and actual load rate. For Y-connected motors, a standard three-phase thermal relay is sufficient. However, for delta-connected motors, a three-phase thermal relay with phase loss protection must be selected. This is because after a phase loss occurs in a delta-connected motor (Figure 2), the current distribution inside the motor windings will change significantly. For example, if a phase is lost while the motor is operating at 58% load, theoretically, the line current will increase to 100%, but some winding phase currents will increase to 115%. Since the thermal relay is connected in series with the motor's main circuit, the current flowing through the thermal relay is the motor's line current. A standard thermal relay will not trip, but some windings may burn out if they are continuously subjected to 115% current. Therefore, a "3D" type thermal relay with phase loss protection should be selected for delta-connected motors. b. Thermal relays cannot be used to protect motors that frequently switch on and off or rotate in both directions. This point should be noted when selecting a thermal relay. To ensure reliable operation of the thermal relay, it should be installed in the normal manner, and the connecting wires should meet the specifications. Before putting it into operation, the current value on the adjustment knob should be checked to see if it matches the rated current of the protected motor. The thermal relay should also be checked regularly during operation. As long as the thermal relay is properly selected and inspection and maintenance are emphasized, the thermal relay can effectively protect the motor.