Precautions when selecting a thermal relay

2021-04-25

(1) Consider the reset form

Thermal relays generally have two forms of reset, manual reset and automatic reset. The conversion of these two reset forms can be accomplished by means of the adjustment of the reset screw. When the thermal relay is shipped from the factory, the manufacturer is set to an automatic reset state. In use, whether the thermal relay should be set to the manual reset state or the automatic reset state depends on the specific conditions of the control loop.

Under normal circumstances, the principle that the protected motor should not be automatically restarted after the thermal relay protection action is followed, even if the thermal relay is automatically reset, otherwise the thermal relay should be set to the manual reset state. This is to prevent the motor from repeatedly restarting the damaged device if the fault is not eliminated. For example: The manual control and manual stop control circuit of the button control can be used to set the thermal relay to the automatic reset mode; the automatic start circuit with automatic component control should set the thermal relay to the manual reset mode.

(2) Consider the insulation level and structure of the motor

Due to the different insulation levels of the motor, the ability to withstand temperature rise and withstand overload is also different. Under the same conditions, the higher the insulation level, the stronger the overload capability. Even if the insulation materials used are the same, the motor structure is different and should be different when using a thermal relay. For example, closed motor heat dissipation is worse than open type motor, its overload capacity is lower than that of open type motor, and the rated current of thermal relay should be selected as 60% to 80% of rated current of motor.

(3) Selection of thermal relays with different rated current levels but the same range of thermal components

For example: JR16 series of thermal relays, in the rated current of 20A and 60A, the thermal element rating has an adjustment range of 14 ~ 16A, in this case, should check the ambient temperature of the thermal relay and the environment of the protected motor temperature. When the ambient temperature of the thermal relay is higher than the ambient temperature of the protected motor by 15 °C, the thermal relay of the larger rated current class should be used; when the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15 °C or less When using a small thermal relay with rated current rating. In addition, the load of the motor and the range of adjustments that the thermal relay may require should also be considered.

(4) Consider the connection of the motor

If a thermal relay is used for motor phase loss protection, the connection of the motor should be considered. For a Y-connected motor, when a phase is broken, the current of the remaining unphased windings is the same as the current of the flow superheat relay. The general three-phase thermal relay can realize phase failure protection for the Y-connected motor as long as the rated current is adjusted reasonably. For the motor of the delta connection method, when the phase line is broken, the current flowing through the uninterrupted phase winding and the current increase ratio of the flow superheat relay are different. That is to say, the current flowing through the superheat relay cannot reflect the overload current of the winding after the phase failure. Therefore, the general thermal relay, even if it is a three-phase type, cannot provide sufficient phase-breaking 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.

(5) Consider the specific work situation

For motors with long-term stable operation, thermal relays can be selected according to the rated current of the motor. Take the rated current of the thermal relay from 0.95 to 1.05 times or the intermediate value is equal to the rated current of the motor. When using, adjust the rated current of the thermal relay to the rated current of the motor.

If the motor is not allowed to stop immediately, so as to avoid economic loss, only the thermal relay should be considered to trip.

At this time, the rated current of the selected thermal relay should be larger than the rated current of the motor.

It is used to adjust the rated current when the overload protection of the motor is repeated for a short time. More tests and adjustments can be made in the field to obtain more reliable protection. The method is: firstly adjust the rated current of the thermal relay to be slightly smaller than the rated current of the motor, and if it is found to operate frequently during operation, gradually increase the rated value of the thermal relay until the operation requirement is met.

Motor protection during special work. Motors that are working in positive, reverse, and frequent switching operations should not be protected by thermal relays. The ideal method is to protect it with a temperature relay or a thermistor embedded in the winding.

(6) Selection of connecting wires

The connecting wire at the outlet end should be selected according to the rated current of the thermal relay. Too thick or too thin will also affect the normal operation of the thermal relay.

If the connecting line is too thin, the heat generated by the connecting wire will be transmitted to the bimetal, and the heat generating component will radiate less heat along the wire, thereby shortening the tripping time of the thermal relay; otherwise, if the connecting wire is too thick, This will extend the tripping time of the thermal relay.

For a thermal relay with a rated current of 10A, the cross-sectional area of ​​the connecting wire of the outlet end should be 2.5mm2 (single-strand copper core plastic wire), 4A2 for 20A (single-strand copper core plastic wire), 16A for 60A It is suitable (multi-strand copper core rubber cord), 150A is 35mm2. Suitable (multi-strand copper core rubber cord). Because the wire material and thickness will affect the amount of heat transferred from the junction of the thermal element to the outside. The wire is too thin, the axial thermal conductivity is poor, the thermal relay may move in advance; if the wire is too thick, the axial heat conduction is fast, and the thermal relay may lag. The connecting wires of the outlet end of the thermal relay generally use copper core wires. If an aluminum core wire is used, the cross-sectional area of ​​the wire should be increased by 1.8 times, and the wire end should be tinned.

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