FBM201b Foxboro FOXBORO Controller, Shanghai Sijian Intelligent Technology Co., Ltd. is engaged in technical development, technical consultation, technical transfer in the fields of intelligent technology, information technology, and automation technology, as well as the sales of electrical equipment, automation equipment, electronic products, communication products, and electromechanical products. Industrial automation products include: PLC, DCS, sensors, frequency converters, encoders, relays, servo drives, soft starters, touch screens. Brands involved in the products include: Eurotherm, Schneider, Siemens, ABB, SICK, P+F, E+H, Yokogawa, Yamakawa, Mitsubishi, Emerson, Honeywell, FANUC, GE, Eaton Electrical, CT,
FBM201b Foxboro FOXBORO Controller, When a motor is operating, if the load is too heavy, the voltage is too low, or a single-phase open circuit fault occurs, the motor's current will increase, often exceeding the rated current. If the excess is not significant, the fuse element in the circuit will not blow, but over time it will affect the motor's lifespan and may even burn out the motor, thus overload protection is necessary. Thermal relays are used for motor overload protection; they utilize the thermal effect of current to cause a bimetallic strip to bend when heated, activating contacts through a linkage mechanism. Figure 1-23 shows the structure and graphical symbol of a thermal relay.
It consists of a heating element, a bimetallic strip, contacts, and a set of transmission and adjustment mechanisms. The heating element is a small resistance wire connected in series in the main circuit of the protected motor. The bimetallic strip is made by rolling two metal strips with different thermal expansion coefficients together. In the illustrated bimetallic strip, the lower strip has a higher thermal expansion coefficient, while the upper one has a lower one. When the motor is overloaded, the current through the heating element exceeds the set current, causing the bimetallic strip to bend upward and disengage from the latch plate, opening the normally closed contacts. Since the normally closed contacts are connected in the motor's control circuit, their opening de-energizes the connected contactor coil, thereby opening the contactor's main contacts and cutting off the motor's main circuit, achieving overload protection.
The main technical parameter of a thermal relay is the set current. The set current is the current that can pass through the heating element for a long time without causing the thermal relay to operate. When the current through the heating element exceeds the set current by 20%, the thermal relay should operate within 20 minutes. The set current of the thermal relay can be adjusted using the set current knob. When selecting and setting a thermal relay, it is essential to ensure that the set current matches the motor's rated current.
Due to the thermal inertia of the thermal relay, which operates based on heat, even if the current through the heating element briefly exceeds the set current by several times, the thermal relay will not operate immediately. This ensures that the thermal relay does not operate during motor startup due to the high starting current; otherwise, the motor would not be able to start. Conversely, if the current slightly exceeds the set current but persists for a long time, the thermal relay will eventually operate. Thus, the function of a thermal relay differs from that of a fuse: a thermal relay provides overload protection but not short-circuit protection, while a fuse provides short-circuit protection but not overload protection. In a well-designed control circuit, especially for larger capacity motors, both types of protection should be included.