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What You Need to Know About Thermal Relays 1. Selection of Thermal Relays The devices that thermal relays are designed to protect are motors; therefore, when selecting such relays, it is necessary to take into account the technical specifications of the motor, its starting conditions, the nature of the load, as well as the motor’s allowable overload capacity. (1) For motors that operate stably over a long period, a thermal relay can be selected based on the motor’s rated current. The value should be 0.95 to 1.05 times the setting current of the thermal relay, or the intermediate value, and it should equal the rated current of the motor. When in use, adjust the setting current of the thermal relay to the rated current value of the motor. (2) The insulation class and structure of the motor should be taken into consideration. Since the insulation classes of motors vary, their allowable temperature rise and capacity to withstand overloads also differ. Under the same conditions, the higher the insulation class, the greater the overload capacity. Even if the insulating material used is the same, differences in motor design require different choices when selecting thermal relays. For example, enclosed motors have poorer heat dissipation than open-type motors, and their overload capacity is lower than that of open-type motors; the setting current of the thermal relay should be selected at 60–80% of the motor’s rated current. (3) The motor’s starting current and starting time should be taken into consideration. The motor’s starting current is generally 5 to 7 times the rated current. For motors that are started infrequently and operate continuously, a thermal relay can be selected based on the motor’s rated current, provided that the starting time does not exceed 6 seconds. (4) If a thermal relay is used for phase loss protection of a motor, the motor’s wiring configuration must be taken into consideration. For motors connected in a Y configuration, when one phase breaks, the current flowing through the remaining intact windings increases in the same proportion as the current flowing through the thermal relay. A standard three-phase thermal relay, provided that the setting current is adjusted properly, can provide phase loss protection for motors connected in a Y configuration. For motors connected in a delta configuration, when a phase is disconnected, the increase ratio of the current flowing through the intact phase windings to the current flowing through the thermal relay is different. In other words, the current flowing through the thermal relay cannot reflect the overload current of the windings in the event of a phase loss; therefore, conventional thermal relays, even those designed for three-phase use, cannot provide adequate protection against phase loss operation in three-phase asynchronous motors connected in a Δ configuration. At this time, thermal relays of the JR20 type or T series, which are equipped with differential phase-loss protection, should be selected. (5) The specific operating conditions should be taken into account; if it is required that the motor must not stop operating arbitrarily to avoid economic losses, then the thermal relay should only be allowed to trip in the event of an overload condition. At this time, the setting current of the thermal relay should be slightly higher than the motor’s rated current. Thermal relays are only suitable for providing overload protection for motors that are started infrequently and under light load. For motors that frequently switch between forward and reverse rotation as well as those that experience frequent on-off cycles, such as crane motors, thermal relays are not suitable for overload protection. 2. Installation of thermal relays: The orientation in which the thermal relay is installed, its operating environment, and the wiring used all affect its performance, so care should be taken during installation. (1) Installation direction of the thermal relay: The installation direction of the thermal relay is easily overlooked. A thermal relay generates heat as current passes through a heating element, which in turn causes the bimetallic strip to move. Heat transfer occurs through three methods: convection, radiation, and conduction. In this case, convection is directional, with heat being transferred from bottom to top. When installing, if the heating element is located below the bimetallic strip, the bimetallic strip heats up quickly and its response time is short ； If the heating element is located next to the bimetallic strip, the bimetallic strip heats up more slowly, resulting in a longer operating time for the thermal relay. When a thermal relay is installed together with other electrical appliances, it should be placed below those appliances and at least 50 mm away from them, to avoid being affected by the heat generated by the other appliances. The installation direction of the thermal relay should follow the specifications in the product manual to ensure consistent operating performance of the thermal relay during use. (2) The operating environment mainly refers to the ambient temperature, which has a significant impact on the speed at which the thermal relay operates. The temperature of the medium surrounding the thermal relay should be the same as that of the medium surrounding the motor; otherwise, the properly adjusted alignment will be disrupted. For example, when the motor is installed in a high-temperature area and the thermal relay is installed in a lower-temperature area, the operation of the thermal relay will be delayed (or the operating current will be higher) ； Conversely, its operation will occur earlier (or the operating current will be low). For thermal relays without temperature compensation, they should be used in locations where the difference in ambient temperature between the thermal relay and the motor is not significant. For thermal relays with temperature compensation, they can be used in situations where there is a certain difference in ambient temperature between the thermal relay and the motor, but efforts should be made to minimize the impact of changes in ambient temperature. (3) In addition to conducting electricity, the connection wires of the thermal relay also serve to conduct heat. If the connecting wire is too thin, the heat generated by the wire will be transferred to the bimetallic strip; coupled with the fact that the heating element dissipates little heat outward along the wire, this shortens the tripping time of the thermal relay ； Conversely, if a too-thick connecting wire is used, it will prolong the tripping time of the thermal relay. Therefore, the cross-sectional area of the connecting wire should not be too small or too large; it is advisable to use an area specified in the instructions or one that is similar to it. 3. Before adjusting and putting the thermal relay into use, it is necessary to adjust its setting current to ensure that it matches the rated current of the motor being protected. For example, for a 10kW, 380V motor with a rated current of 19.9A, a JR20-25 type thermal relay can be used; the setting current for its heating element is 17–21–25A. As a general rule, it is set to 21A. If the relay trips prematurely frequently while the motor’s temperature rise remains low, the setting current can be increased to 25A and monitored further ； If the motor temperature rises at 21A and the thermal relay operates with delay, monitoring at 17A can be used to achieve the best coordination.