Electric devices are generally composed of the following sections:
Special motor, characterized by strong overload capacity, large starting torque, small moment of inertia, short - time, intermittent work. 
The deceleration mechanism is used to reduce the output speed of the motor. 
The travel control mechanism is used to regulate and accurately control the opening and closing position of the valve. 
A torque limiting mechanism used to adjust the torque ( or thrust ) and to make it do not exceed a predetermined value.
The interlocking mechanism of manual or electric switching mechanism for manual or electric operation. 
The opening indicator to show the position of the valve in the opening and closing process. 

1. Select electric device according to the type of valve
1). Angle travel electric device ( angle < 360 degrees ) is suitable for butterfly valve, ball valve, plug valve, etc. 
The rotation of the output shaft of the electric device is less than one week, that is, less than 360 degrees, and usually the opening and closing process control of the valve is realized for 90 degrees. This type of electric device according to the different installation interface is divided into direct - direct, the base of two cranks. 

A ) Direct - attach: refers to the form of direct attached installation of the output shaft and stem of the electric device. 
B ) Base crank type: refers to the form of the output shaft connecting the stem through the crank. 

2. Multi - rotary electric device ( angle > 360 degrees ) is suitable for gate valve, globe valve, etc. 
The rotation of the output shaft of the electric device is greater than one week, that is, it is greater than 360 degrees, so it is generally necessary to realize the control of the on-off process of the valve. 

3. Straight stroke ( linear motion ) is suitable for single - seat regulating valve, two - seat regulating valve, etc. 
The motion of the output shaft of the electric device is a linear motion, not a form of rotation. 

4. Determine the control mode of the electric device according to the production process control requirements
1). Switch type ( Open - loop control )
The switch type electric device generally realizes the open or close control of the valve, the valve is either in the full open position or in the full closed position, this kind of valve does not need to accurately control the medium flow. 
Particularly noteworthy is that the switch type electric device can be divided into split structure and integrated structure because of different structural forms. This must be explained in the selection, otherwise it will often happen in the field installation and the control system conflict and so does not match the current image. 

A ) Split structure ( commonly referred to as common type ): the control unit is separated from the electric device, and the electric device cannot be controlled by the valve alone. the control unit must be added in order to realize the control. generally, the external controller or control cabinet is used for matching. 
The disadvantage of this structure is that it is not easy to install the whole system, increase the wiring and installation cost, and easy to appear failure. when the fault occurs, it is not easy to diagnose and repair, and the cost performance is not ideal. 

B ) Integrated structure ( usually referred to as integral type ): the control unit and the electric device are encapsulated in one, and the external distribution control unit can be operated on the spot without the external control unit, and the remote control information can be operated on only the relevant control information. 
The advantage of this structure is to facilitate the overall installation of the system, reduce wiring and installation costs, easy diagnosis and troubleshooting. But the traditional integrated structure products also have a lot of imperfect places, so the generation of intelligent electric devices. 

2). Regulating type ( Closed - loop control )
The adjustable electric device not only has the function of the switch - type integral structure, but also can control the valve precisely and regulate the flow of the medium. 
A ) Control signal type ( current, voltage )
The control signal of the regulating electric device generally has the current signal ( 4 ~ 20ma, 0 ~ ma ) or voltage signal ( 0 ~ 5v, 1 ~ 5v ), and the type and parameters of the control signal must be clearly defined. 
B ) Forms of work ( electrically operated, turn off )
The operation mode of the regulating type electric device is generally electric type ( take the control of 4 ￣ 20ma for example, the electric opening type refers to the valve closed, 20ma corresponding to valve opening ), and the other type ( 4 - 20ma control as an example, the electric open type refers to the 4 signal to the valve opening and 20ma corresponding to the valve closing ). 
C ) Loss of signal protection
Loss of signal protection means that when the control signal is lost due to a fault, the electric device will control the valve to set the protection value, and the common protection values are fully open, fully closed, and maintain in situ three conditions. 

5.Electric devices classified according to the use of the environment and explosion-proof grade
According to the requirements of the use of environment and explosion-proof grade, the electric device of the valve can be divided into common type, outdoor type, flameproof type, outdoor flameproof type, etc. 
Determine the output torque of the electric device according to the torque required by the valve
The torque required by the opening and closing of the valve is determined by the user or by the manufacturer itself. the torque is determined by the valve diameter, working pressure, etc., but the torque of the same valve manufactured by different manufacturers is different, even if the same valve manufacturer produces the same valve torque, the torque selection of the actuator is too small to open close valve, so the electric device must select a reasonable range 

6. Correct selection of the basis for valve electric device:
Operating torque: the operating torque is the main parameter of the selection of valve electric device, the output torque of the electric device shall be 1.2 ~ 1.5 times the maximum torque of the valve operation. 
Operating thrust: the main structure of the valve electric device is two kinds: one is not to configure the thrust plate, the direct output torque; The other is to configure the thrust plate, the output torque is converted to the output thrust through the stem nut in the thrust plate.