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Electric valve working principle and control principle diagram

by:Sino Valves     2021-02-28
Electric valve is a unity composed of valve electric device and valve. It is not only a piping component-a valve, but also an automation component-an electric actuator. Therefore, in addition to meeting the requirements of the production process for valves, it must also meet the requirements of the production process for automatic devices. The structure of the electric valve will vary with its various parts. Here, only the typical structure is combined to discuss the function and mutual relationship of its various components. Figure 1-1 is a typical structure block diagram of an electric valve. Electric valves use electric motors as prime movers. Usually a specially designed three-phase asynchronous motor is used. The motor is designed according to the short-time work system, without heat dissipation equipment, and has soft or softer mechanical characteristics. For occasions requiring high reliability, series-excited DC motors are used. For occasions that require changing the speed, a variable-speed three-phase asynchronous motor is used. The motor drives the opening and closing parts of the valve after decelerating through the main transmission mechanism. The main transmission mechanism has many structural forms. But the transmission mode is nothing more than spur gear transmission, worm gear transmission, spur gear planetary transmission, cycloid pin-tooth planetary transmission and harmonic transmission. The most common main transmission mechanism is a combination of spur gear transmission and worm gear transmission. The torque output by the main drive mechanism is converted into thrust through the trapezoidal thread, and the valve opening and closing parts (gate valve and stop valve) that drive linear motion are removed. Usually the stem nut for torque-to-thrust conversion is set on the stem as a part of the valve. At this time, the electric actuator outputs torque to drive the stem nut. However, sometimes the trapezoidal thread (stem nut) is also installed in the electric actuator so that the electric actuator directly outputs thrust. For valves with the opening and closing parts rotating (ball valves and butterfly valves), the output shaft of the main transmission mechanism must pass through the secondary reducer to drive the valve opening and closing parts. The secondary reducer usually adopts planetary gear transmission, worm gear transmission and screw rocker transmission. The secondary reducer is usually installed in a separate housing and then spliced u200bu200bwith the electric actuator. The main transmission mechanism is equipped with a torque limit mechanism, a stroke control mechanism and a valve position measuring mechanism. The torque limiter can be used to limit the output torque of the electric actuator. When the torque output by the main drive mechanism reaches the setting value of the torque limiting mechanism, the torque limiting mechanism pushes the torque switch and sends a signal to the control circuit to cut off the power of the motor. The stroke control mechanism can be used to determine the opening and closing position of the valve. When the stroke (valve opening) of the main drive mechanism reaches the setting value of the stroke control mechanism, the stroke control mechanism will push the stroke switch and send a signal to the control circuit to cut off the power of the motor. The stroke control mechanism can also send a signal according to the preset stroke value for use by other automation devices (for example, a program-controlled device); the valve position measuring mechanism can provide the valve position signal in the form of an analog quantity for the operator to remotely monitor the valve opening , Or for other automatic devices (regulators). In order to manually operate the electric valve when necessary, a manual-electric switching mechanism and a hand wheel are also installed in the main transmission mechanism. Manually switch the switching mechanism to the manual side and use the handwheel to operate the electric valve. When electric, the switching mechanism will automatically switch to the electric side as soon as the electric motor rotates, and it is operated by the electric motor (called semi-automatic switching mechanism). For some electric valves, the manual-electric switching mechanism is fully manual, that is, it is necessary to manually switch the switching mechanism to the electric side when it is electric. On electric valves with small operating torque, the handwheel can be directly connected to the output shaft through a switching mechanism. On electric valves with large operating torque, the handwheel is connected to the output shaft after deceleration, so that it has a certain reduction ratio during manual operation. The control circuit of the electric valve can start the motor to operate the valve according to the command sent by the operator from the control panel or the command sent by the automation system. When the torque or stroke of the valve reaches the preset value, the control circuit can cut off the power of the motor according to the signal sent by the torque limiting mechanism and the stroke control mechanism. The control circuit also has a protection function. When the motor is overloaded or other faults occur, it can automatically cut off the power supply and send out an alarm signal. When necessary, the control circuit can also have more complex functions. For example, in a pulse-modulated variable-speed electric valve, the control circuit changes the width of the control pulse to meet the motor's variable speed requirements.
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