1. Selection of thermal relay
The object of protection of the thermal relay is the motor. Therefore, when selecting the motor, it is necessary to understand the technical functions of the motor, the starting conditions, the nature of the load, and the allowable overload of the motor.
(1) Long-term stable operation of the motor The thermal relay can be selected according to the rated current of the motor. Take the thermal relay setting current 0.95 ~ 1.05 times or the median value is equal to the motor rated current. When using, set the thermal relay’s setting current to the rated current of the motor.
(2) The insulation level and structure of the motor should be considered
Due to the different insulation levels of the motor, its ability to tolerate temperature rise and overload is also different. Under the same conditions, the higher the insulation level, the stronger the overload capacity. Even if the insulation material used is the same, but the motor structure is different, there should be differences in the choice of thermal relays. For example, a closed-type motor dissipates less heat than an open-type motor, and its overload capacity is lower than that of an open-type motor. The setting current of the thermal relay should be selected as 60% to 80% of the rated current of the motor.
(3) The starting current and starting time of the motor should be considered
The starting current of the motor is generally 5-7 times of the rated current. For motors that are not frequently started and run continuously, the thermal relay can be selected according to the rated current of the motor when the starting time does not exceed 6s.
(4) If the thermal relay is used as the motor phase protection, the connection of the motor should be considered
For a Y-connected motor, when a certain phase is disconnected, the currents of the remaining unphased windings and the current superheating relays increase in the same ratio. General three-phase thermal relays can provide phase-failure protection for Y-connected motors as long as the tuning current is reasonable. For the Δ-connected motor, when the phase is disconnected, the current flowing through the non-phase-breaking windings and the current flowing through the over-temperature relays have different ratios of increase. In other words, the current flowing through the thermal relay cannot reflect the overload current of the winding after the phase-break. Therefore, the general thermal relay, even if it is a three-phase type, cannot provide sufficient phase-off operation for the three-phase asynchronous motor of the delta connection method. protection. At this time, JR20 type or T series thermal relays with differential phase failure protection mechanism should be used.
(5) should consider the specific work situation
If it is required that the motor not be allowed to stop casually to avoid economic loss, only the occurrence of an overload accident can be considered to allow the thermal relay to trip. At this time, the setting current of the selected thermal relay should be larger than the rated current of the motor.
The thermal relay is only suitable for overload protection of motors that do not start frequently and start at light loads. For motors with frequent positive and negative reversals and frequent switching, such as hoisting motors, thermal relays should not be selected for overload protection.
2. Installation of thermal relay
The direction of the installation of the thermal relay, the use environment, and the cable used will all affect the operation function. Attention should be paid to the installation.
(1) Mounting direction of thermal relay
The installation direction of the thermal relay is easily overlooked. The thermal relay heats the current through the heating element and pushes the action of the bimetal. There are three methods of convection, radiation, and conduction in heat transfer. Convection is directional, and heat is transmitted from bottom to top. When placed, if the heating element is under the bimetal, the bimetal will heat fast and the operation time will be short; if the heating element is next to the bimetal, the bimetal will be slower and the thermal relay will take longer time to operate. . When the thermal relay is installed together with other electrical appliances, it should be installed under the electrical appliance and away from other electrical appliances for more than 50mm to avoid being affected by the heating of other electrical appliances. The installation direction of the thermal relay shall be in accordance with the specifications of the product specification to ensure that the thermal relays have the same function when used.
(2) Use environment
Mainly refers to the ambient temperature, it has a greater impact on the speed of the thermal relay action. The temperature of the medium surrounding the thermal relay should be the same as the temperature of the surrounding medium of the motor, otherwise it will destroy the adjusted coordination situation. For example, when the motor is installed at a high temperature and the thermal relay is installed at a lower temperature, the action of the thermal relay will lag (or the operating current is large), and its action will be advanced (or the operating current is small). For thermal relays without temperature compensation, the thermal relay and the motor should be used in locations where the ambient temperature does not differ significantly. Temperature compensated thermal relays can be used in locations where the thermal relay and the motor have certain differences in ambient temperature. However, the effects of changes in ambient temperature should be minimized.
(3) connection line
In addition to the electrical conduction, the thermal relay connection also serves as a thermal conductor. If the connecting wire is too thin, the heat generated by the connecting wire will be transmitted to the bimetal and the heat generating element will dissipate less along the wire, thus shortening the tripping time of the thermal relay; conversely, if the selected connecting wire is too thick , will extend the thermal relay tripping action time. Therefore, the cross-section of the connection conductor should not be too thin or too thick. It should be selected as far as possible in the specification or similar cross-sectional area.
3. Adjustment of thermal relay
Before putting it into operation, make sure to adjust the thermal relay setting current to ensure that the thermal relay’s setting current matches the rated current of the motor being protected. For example, for a 10kW, 380V motor rated current 19.9A, you can use JR20-25 type thermal relay, heating element setting current of 17 ~ 21 ~ 25A, first set at 21A according to the general situation, if you find regular advance action, The motor temperature rise is not high, you can set the current to 25A to continue observation; if the motor temperature rises at 21A, and the thermal relay lag action, you can change to observe at 17A, in order to get the best match.