Operating principle of electric shut-off valve and regulating valve actuator
Time: March 29, 2019 13:03:13 Source: Yonglong Valve Reading: zero second
The electric actuator is the execution unit in the electric unit combined instrument. It is powered by single-phase AC power supply, receives unified standard DC signals, outputs corresponding angular displacement, operates damper, baffle and other regulating mechanisms, and can be equipped with various electric operators to complete the "manual automatic" undisturbed switching of the regulating system, as well as remote manual operation of the regulated object, The electric actuator is also equipped with dual protection of electrical limit and mechanical limit to complete the task of automatic adjustment. It is widely used in electric power, metallurgy, petrochemical industry, boiler system water feeding and damper adjustment.
1. Operating principle of electric shut-off valve and regulating valve actuator
The electric actuator consists of servo amplifier and actuator. The actuator is divided into motor, reducer and position transmitter
The current signal Ii (4-20mA) from the regulator is used as the input of the servo amplifier and compared with the position feedback signal If of the valve. When the difference between the input signal and the feedback signal is not equal to zero, the difference is amplified by the servo amplifier, and then the two-phase servo motor is controlled to rotate in the corresponding direction, and the output shaft is displaced after being decelerated by the reducer; At the same time, the displacement of the output shaft is converted into the feedback signal If of the valve through the position transmitter; When the feedback signal is equal to the input signal, the servo amplifier has no output, the motor does not rotate, and the actuator is stabilized at the position corresponding to the input signal. Displacement of the output shaft of the electric actuator is linear with the input signal.
The electric actuator has three control modes: continuous adjustment, remote manual control and local manual operation.
1.1 Local adjustment mode of electric actuator
When the electric actuator needs local manual operation, when the switch of the electric actuator is placed in the "Manual" position, turn the knob at the end of the motor to the "Manual" position, pull out the hand wheel on the actuator, and shake the hand wheel to achieve manual operation. When local operation is not used, be sure to turn the knob at the end of the motor to the "Automatic" position and push the hand wheel.
1.2 Remote control adjustment mode of electric actuator
When the electric operator's change-over switch is placed in the "Manual" position, it is in the manual remote control state. When operating, as long as the rotary change-over switch is pulled to the "On" or "Off" positions respectively, and the motor is driven to rotate forward or reverse, the output shaft of the actuator can achieve upward or downward action. During the movement, observe the valve position opening meter on the electric operator, When reaching the required control valve opening, immediately release the change-over switch.
1.3 Automatic adjustment mode of electric actuator
When the switch of the electric operator is placed in the "Automatic" position, it is in the automatic adjustment state. The control process is as follows:
When the input signal I λ 1=0 (or 4mA DC), the feedback current I of the position transmitter is inversely=0 (or 4mA DC). At this time, the servo amplifier has no output voltage, the motor stops running, and the output shaft of the actuator is stabilized at the preselected zero position;
When the input signal I λ 1>0 (or 4mA DC), the magnetic potential of this input signal and the position feedback current of the system itself is comprehensively compared in the pre stage magnetic amplifier of the servo amplifier. Because the two signals are unequal in size and opposite in polarity, there will be error magnetic potential, so that the servo amplifier has enough output power to drive the motor, The output shaft of the actuator moves in the direction of reducing the error magnetic potential until the input signal and the position feedback signal are equal. At this time, the output shaft is stable at the position corresponding to the input signal.
2. The electric actuator consists of motor, reducer and position transmitter
2.1 Motor
The motor is a switching power supply that receives the output of servo amplifier or electric operator, converts electrical energy into mechanical energy, and then drives the actuator.
2.2 Reducer
There are manual parts, output shaft and mechanical stop block on the reducer. The reducer converts the high speed and small torque of the motor into the output power of low speed and large torque to drive the valve mechanism. There are two brake pads on the base, which can limit the angle of the output shaft within 90 ° to ensure that the adjustment mechanism and relevant connecting rods are not damaged.
2.3 Position transmitter
The position transmitter is composed of power transformer, differential transformer, printed circuit board and other components.
When the output shaft of the reducer moves, the cam rotates accordingly, which means that the iron core connecting rod of the differential transformer pressed on the cam slope produces axial displacement, changes the position of the iron core in the coil of the differential transformer, so that the voltage at the corresponding position output by the differential transformer is converted into a standard DC current signal (4~20mA). The angular displacement of the reducer output shaft is linear with the output current of the position transmitter.
3. Principle of servo amplifier
In the automatic control system, these actuators need external servo amplifiers for on-off control. The command signal of the electric actuator and the valve position feedback signal are compared and amplified in the servo amplifier, and then the 220V switch signal is sent through the silicon controlled rectifier. The servo amplifier is composed of an electronic circuit board composed of electrical components, including a front-end magnetic amplifier circuit board, a trigger circuit board, and a thyristor AC switch circuit board. The comparison and amplification of the command signal and the valve position feedback signal of the electric actuator are realized by the operation of these electronic circuit boards.
The servo amplifier has two modes to choose from:
One is the control panel of the actuator itself with servo amplifier function, which is compact in structure, does not occupy the space behind the instrument panel, and is relatively simple in installation and commissioning (i.e. electronic integration).
The other is a separately placed position locator, which is installed behind the instrument panel. This is a more traditional application method, and it is easy to repair and replace (i.e., split proportional adjustment type).
Servo amplifier is the control unit instrument of electric actuator, which can be divided into wall mounted and rack mounted. The preamplifier includes the functions of signal isolation, comparison, polarity discrimination, fault diagnosis, etc. There are two AC switches, power transformers and DC regulated power supplies on the main circuit board. The front panel is installed on the main circuit board. It receives the standard signal (4-20mA) of the regulating instrument and the feedback signal of the actuator, outputs 220V AC to drive the servo motor to rotate forward and reverse, and continuously adjusts the valve position opening. Realize automatic adjustment of various process parameters.
4. Conclusion
In daily maintenance, we found that the failure of electric actuator is mostly caused by servo amplifier failure. The servo amplifier is composed of electronic circuit board composed of electrical components. The quality of electronic components and the reliability of the circuit board determine the reliability of the servo amplifier. The anti-interference ability of conventional servo amplifier is generally not high, and the influence of external environment such as voltage fluctuation, high temperature, high dust, vibration, etc. is easy to cause servo amplifier failure. In view of the poor reliability of the servo amplifier, it is recommended to use DCS to realize the control function of the servo amplifier. Because DCS has flexible configuration methods, rich software functions and high reliability, it can automatically judge the status of the actuator. When the solid-state relay breaks down or the valve position feedback signal is wrong, it can keep the electric actuator in its original position to prevent the expansion of the accident. And the reliability has been greatly improved.
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