Valve electric device is an indispensable equipment to realize valve program control, automatic control and remote control. Its motion process can be controlled by stroke, torque or axial thrust. Since the working characteristics and utilization rate of the valve electric device depend on the type of valve, the working specification of the device and the position of the valve on the pipeline or equipment, the correct selection of the valve electric device is very important to prevent overload (the working torque is higher than the control torque)& nbsp;  & nbsp; Generally, the basis for correct selection of valve electric device is as follows:  & nbsp; Operating torque & nbsp; The operating torque is the most important parameter for selecting the valve electric device, and the output torque of the electric device shall be 1.2 ~ 1.5 times of the maximum valve operating torque& nbsp;  & nbsp; Operating thrust & nbsp; There are two main structures of valve electric device: one is to directly output torque without thrust disc; The other is to configure the thrust disc, and the output torque is converted into output thrust through the valve stem nut in the thrust disc& nbsp;  & nbsp; Rotation number of output shaft & nbsp; The number of turns of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve rod and the number of thread heads, which shall be calculated according to M = H / ZS (M is the total number of turns that the electric device should meet, h is the opening height of the valve, s is the pitch of the valve rod transmission thread, and Z is the number of valve rod thread heads)& nbsp;  & nbsp; Stem diameter & nbsp; For multi turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the stem of the valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be greater than the outer diameter of the stem of the rising stem valve. For some rotary valves and Non rising stem valves in multi rotary valves, although the passage of stem diameter is not considered, the size of stem diameter and keyway shall also be fully considered during selection, so that they can work normally after assembly.  & nbsp; Output speed & nbsp; If the opening and closing speed of the valve is too fast, it is easy to produce water hammer. Therefore, the appropriate opening and closing speed should be selected according to different service conditions& nbsp;  & nbsp; Valve actuators have special requirements, that is, they must be able to limit torque or axial force. Generally, the valve electric device adopts the coupling to limit the torque. When the specification of the electric device is determined, its control torque is also determined. Generally, it operates within a predetermined time, and the motor will not overload. However, overload may be caused 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 limiting mechanism is wrongly adjusted to make it greater than the stop torque, resulting in continuous excessive torque and stopping the motor; Third, the heat generated by intermittent use exceeds the allowable temperature rise of the motor; Fourth, for some reason, the circuit of torque limiting mechanism fails, resulting in excessive torque; Fifth, the ambient temperature is too high, which reduces the thermal capacity of the motor& nbsp;  & nbsp; In the past, fuse, overcurrent relay, thermal relay and thermostat were used to protect motor, but these methods have their own advantages and disadvantages. There is no absolutely reliable protection method for variable load equipment such as electric device. Therefore, various combinations must be adopted, which can be summarized into two kinds: one is to judge the increase or decrease of motor input current; The second is to judge the heating of the motor itself. Either way, the time margin given by the motor thermal capacity shall be considered. & nbsp; & nbsp; Generally, the basic protection method of overload is: for overload protection of continuous operation or inching operation of motor, thermostat is adopted; Thermal relay is adopted for the protection of motor locked rotor; For short circuit accident, fuse or overcurrent relay shall be adopted.