As the overload protection component of motor, thermal relay should know the technical performance, starting condition, load property and allowable overload capability of motor when selecting.

Everyone is accustomed to choose thermal relay according to rated current of motor, but it is not correct in fact. If a heavy-duty starting motor is protected and only selected according to rated current of motor, the thermal relay will be released when the motor starts, resulting in misoperation. Therefore, when choosing thermal relay, we should pay attention to the following points.

1. The capacity and voltage of motor are the main basis for selecting thermal relay.

2. Long-term stable motor

Thermal relay can be selected according to rated current of motor. The setting current of the heating relay is 0.95-1.05 times or the median value is equal to the rated current of the motor. When using, the setting current of thermal relay should be adjusted to the rated current value of motor.

3. The insulation grade and structure of motor should be considered.

Due to the different insulation levels of motors, their allowable temperature rise and overload capacity are also different. Under the same conditions, the higher the insulation level, the stronger the overload capacity. Even though the insulating materials used are the same, the structure of the motor is different, and the selection of thermal relays should also be different. For example, the heat dissipation of closed motor is worse than that of open motor, and its overload capacity is lower than that of open motor. The setting current of thermal relay should be 60%-80% of the rated current of motor.

4. Start-up current and start-up time of motor should be considered.

The starting current of the motor is usually 5 to 7 times of the rated current. For motors that do not start frequently and run continuously, the thermal relay can be selected according to the rated current of the motor when the starting time is not more than 6 seconds.

If the load inertia is large and the starting time is long, the setting current of the selected thermal relay should be slightly higher than the rated current of the motor in order to achieve correct protection. If there is a gap between the setting current and the selected value, the adjusting cam can be adjusted to the appropriate value.

5. If the thermal relay is used as the phase-out protection of the motor, the connection method of the motor should be considered.

For Y-connected motors, when a phase is broken, the current of the remaining uninterrupted windings increases in the same proportion as the current of the overheated relay. General three-phase thermal relay can realize Phase-break protection for Y-connected motor as long as the setting current is adjusted reasonably. For the motor with_connection, when its phase is broken, the increase ratio of the current flowing through the uninterrupted winding to the current flowing through the overheated relay is different. That is to say, the current flowing through the overheated relay can not reflect the overload current of the winding after the phase break. Therefore, the general thermal relay, even if it is three-phase, can not provide adequate protection for the Phase-break operation of the three-phase asynchronous motor with the delta connection method. At this time, JR20 or T series thermal relays with differential phase-breaking protection mechanism should be selected.

6. Consideration should be given to specific work conditions.

If the motor is not allowed to shut down casually in order to avoid economic losses, only in case of overload accident can the thermal relay be considered to trip. At this time, the setting current of the selected thermal relay should be larger than the rated current of the motor.

Thermal relays produced in China are basically only suitable for overload protection of motors which start infrequently and start lightly. For the motors with frequent switching between positive and negative switching and frequent switching-off, thermal relay should not be used as overload protection for lifting motors. This must be noted when selecting thermal relays.