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# Selection of Thermal Relay

The current flowing into the heat element generates heat, and the bimetal piece with different expansion coefficients is deformed. When the deformation reaches a certain distance, the link is pushed to make the control circuit open, thereby causing the contactor to lose power, and the main circuit is broken. Open to achieve overload protection of the motor. This article will show you how to choose the right relay.

As the overload protection component of the motor, the relay has been widely used in production due to its small size, simple structure and low cost.

• Rated voltage: The highest voltage value that the thermal relay can work normally, generally 220V, 380V, 600V AC.
• Rated current: The rated current of a thermal relay mainly refers to the current through a thermal relay.
• Rated frequency: Generally speaking, its rated frequency is designed according to 45~62HZ.
• Setting the current range: The range of the setting current is determined by its own characteristics. It describes that the operating time of a thermal relay is inversely proportional to the square of the current under certain current conditions.

Before putting into use, the setting current of the thermal relay must be adjusted to ensure that the setting current of the thermal relay matches the rated current of the protected motor. For example, for a 10kW, 380V motor, the rated current is 19.9A, JR20-25 type thermal relay can be used, the heating element setting current is 17~21~25A, first set at 21A according to the general situation, if it is found that it often moves in advance, The temperature rise of the motor is not high, the setting current can be changed to 25A to continue observation; if the motor temperature rises at 21A, and the thermal relay lags, it can be observed at 17A to get the best fit.

### Thermal Relay Selection

1. In principle, the safety characteristics of the thermal relay should be as close as possible to or even coincide with the overload characteristics of the motor, or under the overload characteristics of the motor. At the same time, the thermal relay should be unaffected at the moment of short-time overload and starting of the motor (no action).
2. When the thermal relay is used to protect the motor of long-term working or intermittent long-term working, it is generally selected according to the rated current of the motor. For example, the setting value of the thermal relay can be equal to the rated current of the motor of 0.95~1.05 times, or the middle value of the setting current of the thermal relay is equal to the rated current of the motor, and then adjusted.
3. When the thermal relay is used to protect the motor with repeated short-time working, the thermal relay has only a certain range of adaptability. If there are many operations in a short time, it is necessary to use a thermal relay with a speed saturation current transformer.
4. For special working motors with frequent reversal and continuity, it is not advisable to use thermal relays as overload protection devices, but should be protected by temperature relays or thermistors embedded in the motor windings.

### Selection of Thermal Relay

• The motor that works smoothly for many years can select the thermal relay according to the rated current of the motor. Take the thermal relay setting current from 0.95 to 1.05 times or the middle value is equal to the motor rated current. When using, adjust the setting current of the thermal relay to the rated current value of the motor.
• The insulation level and structure of the motor should be considered. Due to the different insulation levels of the motor, its ability to allow temperature rise and overload is also different. Under the same conditions, the greater the insulation level, the stronger the overload capability. Even if the insulation materials used are identical, the structure of the motor is different and should be different when using a thermal relay. For example, a closed motor fan is inferior to an open motor, and its overload capability is lower than that of an open motor. The set current of the thermal relay should be selected to be 60% to 80% of the rated current of the motor.
• The restart current and restart time of the motor should be taken into account. The restart current of the motor is generally 5 to 7 times the rated current. For motors that are not frequently restarted and operated continuously, the thermal relay can be selected according to the rated current of the motor when the restart time is not as high as 6 s.
• If the thermal relay is used as the motor phase loss protection, the connection of the motor should be considered for the Y-connected motor. When a phase is broken, the current of the other unphased windings is exactly the same as the current of the short circuit relay. The general three-phase thermal relay can realize phase failure protection for the Y-connected motor as long as the setting current is properly adjusted. For the Δ-connected motor, when the phase is broken, the current flowing to the uninterrupted winding and the current increasing ratio of the short-circuit relay are different. In theory, the current of the short-circuit relay cannot reflect the overload current of the winding after the phase-off. Therefore, the general thermal relay, even the three-phase type, cannot provide sufficient for the phase-off operation of the three-phase asynchronous motor of the delta connection. protection. In this case, a thermal relay with a differential phase-break protection mechanism such as JR20 or T series should be used.
• It should be considered that if the motor is not allowed to stop at will, so as to avoid economic loss, it is only necessary to let the thermal relay trip when the overload accident occurs again. At this time, the setting current of the selected thermal relay should be larger than the rated current of the motor.