Why does the motor protector play an important role in thermal relay?
The thermal time constant of thermal relay is very small, not suitable for large inertia motor loading starting, often used when starting the short circuit method of thermal relay, it not only makes the control system has complicated structure, cost increase, also there are protection of the dead. Some in order to avoid starting misoperation, increase the setting current, the protection of useless, and most of the motor protector adopts saturable current transformer, it generally has the cold state allows the start time is long, the characteristics of thermal overload during rapid movement. This is in line with the actual requirements of the motor protection of industrial and mining enterprises, and more reliable protection of motor overload.
The detecting element of thermal relay is a double metal sheet, because of starting current and overload current impact, it is easy to make double sheet metal fatigue effect caused by scale offset, action is not stable, and the production site are difficult to find, and finally lead to overload operation. The motor protector has no heating problem because of the current transformer used in the detecting element, and its action stability has a qualitative leap compared with the thermal relay.
The more advanced motor protector, the detecting element usually adopts current transformer, and the core structure appears. The utility model has the advantages of convenient operation and complete isolation from the main circuit, and the utility model can not affect the main circuit and improve the reliability of the main circuit. At the same time, the heating problem of the wiring terminal is thoroughly solved, and the interchangeability is good. But at home and abroad generally only large size motor protector adopts core type, and small size can only use wiring type.
The motor protector solves the problem of the single function of the thermal relay, and produces the disadvantage of the power supply. Although the electronic circuit amplifier and driver for power requirements are reasonable, but the application of motor protector brings a series of problems, such as the user must look before using the power supply terminal, understand the supply voltage rating inconvenience. In addition, because the motor protector needs to work continuously, the fault rate is very high because of the fluctuation of the voltage of the power grid, interference and self heating.