The composition principle and common model of thermal relay

The heating effect of the thermal relay is to use the current theory of motor overload protection. Motor at work, when the load is too large, the voltage is too low, or a phase circuit fault occurs, the current of the motor will increase, its value is often more than rated current. If more than few, fuse in the circuit of the melt not fuse, but after a long time will affect the life of the motor, and even burning motor, so you need to have overload protection. Thermal relay is used in the motor overload protection, it is to use the current heating bimetallic strip heated bending, automatically by the linkage to make contact action of electrical appliances. Figure 1 - 23 is the structure of the thermal relay and graphic symbol. It consists of heating element, bimetallic strip, contact, and a set of driving and adjusting mechanism. Heating element is a small value of resistance wire, concatenated in the protection of the main circuit of the motor. Bimetallic strip by two different thermal expansion coefficient of metal rolling. Shown in the figure of the bimetallic strip, a lower thermal expansion coefficient is big, the top of the small. By heating element of current when motor overload more than current setting, bimetallic strip heated bending upwards from the gusset plate, the normally closed contact disconnect. Due to the normally closed contact is in the control circuit of the motor, it break the meeting made a contactor coil connected with power, thus contactor main off contact, the main circuit of the motor power, implements the overload protection. Thermal relay, the bimetallic strip cooling over a period of time, press the reset button reset. The main technical data of thermal relay is setting current. Setting refers to the current through heating elements for a long time without causing thermal relay action of maximum current. When the current in the heating element by more than 20% of the setting current value, the thermal relay action should be in 20 minutes. Thermal relay setting current size may be changed by setting current knob. When choosing and setting thermal relay setting current value must be consistent with the rated current of the motor. Action due to thermal relay is heated, thermal inertia is larger, and even by heating element of current in a short period of time several times more than setting current, thermal relay will not immediate action. Only in this way, when the motor starting thermal relay will not action by starting current is big, otherwise the motor will not be able to start. Conversely, if more than setting current is not much, but over time will also be action. Thus, the role of thermal relay and fuse is different, thermal relay can only be overload protection and cannot be short circuit protection, and fuse is only for a short circuit protection and not for a overload protection. In a more perfect control circuit, especially the large capacity motor, both to protect should possess.