Electric valve devices have special requirements, that is, they must be able to limit torque or axial force. Usually electric valve devices use torque-limiting couplings. When the specifications of the electric device are determined, its control torque is also determined. Generally, it runs within a predetermined time and the motor will not be overloaded. However, overload may occur if the following conditions occur: First, the power supply voltage is low, and the required torque cannot be obtained, so that the motor stops rotating; second, the torque limit mechanism is set incorrectly to make it greater than the stopped torque. Causes the continuous generation of excessive torque, causing the motor to stop rotating; third, intermittent use, the generated heat accumulation exceeds the allowable temperature rise of the motor; fourth, due to some reason, the torque limit mechanism circuit fails, causing the torque to be excessive Large; Fifth, the use of the environment temperature is too high, relatively reducing the thermal capacity of the motor.
In the past, the method of protecting the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no reliable protection method for variable load equipment such as electric devices. Therefore, various combinations must be adopted, which can be summed up in two ways: one is to judge the increase or decrease of the input current of the motor; the other is to judge the heating of the motor itself. Regardless of these two methods, the time margin given by the motor's thermal capacity must be considered.
Usually, the basic protection methods for overload are: the use of thermostats for continuous or inching operation of the motor; the use of thermal relays for the protection of motor stalls; and the use of fuses or overcurrent relays for short-circuit accidents.
