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The selection of thermal relay

1) Long-term stability of the work of motor
According to the motor rated current selection relay thermal relay. Take relay> 0.95 to 1.05 times or intermediate values ​​of the thermal relay setting current equal to motor rated current. Use to relay setting current of thermal relay adjusted to motor rated current value.
(2) should consider the level and structure of the motor insulation
Motor insulation class, the allowable temperature rise and bear the upload capacity is also different. Under the same conditions, the higher the insulation class, overload the stronger. Even if the insulating material is the same, but the motor structure, should also be differences in the selection of relay thermal relay for example, enclosed motor cooling than open motor, its overload capacity than the open-type motors, relays > thermal relay setting current should be chosen as 60 to 80 percent of the motor rated current.
(3) should be considered the motor starting current and starting time
Motor starting current is generally 5 to 7 times the rated current. For frequent starting, continuous operation of the motor, in the case of start-up time not more than 6s, according to the motor rated current selection of relay> thermal relay.
(4) If using relay "thermal relay for motor phase protection, you should consider the connection of the motor
Y-connection motor, when the disconnection of a phase, the remaining off-phase winding current flows through the relay> the same proportion of increase in the current of thermal relay. General three-phase relay thermal relay tuning current regulation is reasonable, is the Y-connection motor for phase failure protection. Flow through the Δ-shaped connection of motor phase wire without breaking the phase winding current flows through the relay "the current increase in the proportion of the thermal relay is different. In other words, the flow through the relay thermal relay> current does not reflect the loss of phase winding overload current relay "> thermal relay, three phase, we can not provide adequate protection phase operation Δ form of the method of three-phase asynchronous motor. At this point, should use this type of the JR20 type or T-series with the differential-phase protection agencies relay "thermal relay.
(5) should consider the specific situation
In this case, select the relay "> thermal relay setting current should be higher than the motor if the requirements of the motor does not allow random shut down, so as not to suffer economic losses, only in the event of overload accident, may consider to allow relay thermal relay trip. The rated current is too large.
Relay "thermal relay is only applicable to infrequent start, light load to start the motor overload protection. Positive reversal frequent conversion, and frequent on-off motor, such as lifting the motor should not be used relay> thermal relay appliances for overload protection.
2. Relay thermal relay installation
Heat following the direction of installation, use of the environment and the cable will affect the movement performance, the installation should attract attention.
The installation direction of the thermal relay (1) relay>
Relay> the installation direction of the thermal relay can easily be overlooked. Relay "> thermal relay is the current through the heating elements heat, promote the bimetal action. The transfer of heat convection, radiation and conduction in three ways. Which convection is directional, the heat since the uplink. Placed when the heating element in the bottom of the bimetal, the bimetal heater, the action time is short; heating element next to the bimetal, the bimetal heat slowly, the relay "> thermal relay electrical action a long time when the relay> thermal relay and other electrical appliances that fit together, should be installed in electrical below and away from other electrical appliances more than 50mm, from other electrical fever. Relay> the installation direction of the thermal relay should be the requirements of the product manual, to ensure that the relay> the action of the thermal relay in the use of performance is consistent.
(2) use of the environment
Mainly refers to the ambient temperature, it relays "> the speed of impact of the thermal relay, relay" thermal relay the temperature of the surrounding medium, and the same temperature as the surrounding medium of the motor, otherwise it will destroy the adjusted with the situation. For example, when the motor is installed at a high temperature at the relay "thermal relay is installed in the temperature lower, relay" thermal relay action will lag (action current); the contrary, its action will advance (or small) operating current.
No temperature compensation of the relay, thermal relay, the relay> thermal relay and motor both ambient temperature differences in small places. Temperature compensation relay "thermal relay can be used to relay"> ambient temperature of thermal relay and motor both have some differences, but it should be to minimize the impact of changes in ambient temperature.
(3) cable
In addition to conductive, the relay "thermal relay cable from the thermal conductivity effect if the cable is too small, the heat generated by the cable reached the bimetal, coupled with the heating element along the wire out less heat, thus shorten the relay> thermal relay tripping time; Conversely, if the cable is too thick, it will extend the time of the tripping of the relay thermal relay connecting wire cross-section can not be too thin or too thick, should be adopted in the provisions of the Manual or similar cross-sectional area.
Adjustment of the 3. Relay thermal relay
Put into use, the relay "thermal relay setting current must be adjusted to ensure that the relay"> thermal relay setting current match with the rated current of the motor to be protected. For example, a 10kW, 380V motor rated current 19.9A, can be used JR20-25 relay> thermal relay, the heating element setting current for 17 ~~ 21 ~ 25A, press the tuning in 21A, if the found that often ahead of time, the motor temperature rise is not high, setting current can be changed to 25A continue to observe; in 21A, the motor temperature rise, while the relay thermal relay hysteresis action, you can change observed in the 17A to get the best co-ordination.


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