Door electric device is to achieve valve programmable, automatic control and remote control indispensable equipment, the movement process by stroke, torque or axial thrust to control the size. Because the working characteristics and utilization of the valve electric device depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or the device, the correct selection of the valve electric device prevents overloading (the working torque is higher than the control Torque) is crucial. Usually, the correct choice of valve electric device is based on the following: Operating torque: The operating torque is the most important parameter to select the valve electric device. The output torque of the electric device should be 1.2 ~ 1.5 times of the maximum torque of the valve. Operation thrust: There are two kinds of main structure of the valve electric device: one is do not configure the thrust plate, the direct output torque; the other is configured thrust plate, the output torque through the thrust plate of the stem nut is converted to output thrust. The number of turns of the output shaft: The number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem and the number of threads, and is calculated according to M = H / ZS (M is the total rotation that the electric device should satisfy The number of turns, H is the valve opening height, S is the stem thread pitch, Z is the stem thread number). Stem Diameter: For multi-swing rod valves, the valve stem can not be assembled into an electric valve if the maximum stem diameter allowed by the actuator can not pass through the stem of the valve. Therefore, the hollow output shaft of the electric device must have an inner diameter greater than the outer diameter of the stem of the rod valve. For some rotary valves and multi-turn valve in the dark rod valve, though not to consider the passage of the stem diameter, but also should fully consider the stem diameter and keyway size, so that the assembly can work properly. Output speed: If the valve opening and closing speed too fast, easy to produce water hammer phenomenon. Therefore, should be based on different conditions of use, select the appropriate opening and closing speed. Valve actuators have special requirements that they must be able to limit the torque or axial force. Valve actuators are usually torque-limiting couplings. When the electric device specifications are confirmed, the control torque is also determined. Generally run within a predetermined time, the motor will not be overloaded. However, the following situations may lead to overloading: First, the power supply voltage is low and the required torque can not be obtained, so that the motor stops rotating. Second, the torque limit mechanism is incorrectly set to be greater than the stopping torque, Resulting in continuous generation of excessive torque, the motor stops rotating; Third, intermittent use, the heat savings, exceeding the allowable temperature rise of the motor value; Fourth, for some reason the torque limiting mechanism circuit failure, so that the torque Large; Fifth, the use of ambient temperature is too high, relative to the motor thermal capacity decreased. In the past the way to protect the motor is to use fuses, overcurrent relays, thermal relays, thermostats, etc., but each has its own advantages and disadvantages. This variable-load device for electric devices, there is no absolute reliable protection. Therefore, we must take a variety of combinations, summed up in two ways: First, the increase or decrease of the motor input current to judge; the second is to determine the motor itself fever. Either way, regardless of the type of motor thermal capacity to be given a given amount of time margin. Normally, the basic protection methods of overload are as follows: the overload protection of the motor in continuous operation or inching operation adopts the thermostat; the protection of the motor is locked and the thermal relay is adopted; for the short-circuit accident, the fuse or overcurrent relay is adopted.