Valve actuators are one of the devices used to operate the valve and connect it to the valve. The device is powered by electricity and its movement can be controlled by stroke, torque or axial thrust. Due to the proper operating characteristics and utilization of the valve actuator, it depends on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment. Therefore, to grasp the correct choice of valve electric device; consider the occurrence of overloading (working torque higher than the control torque) has become a crucial part. The correct choice of valve electric device should be based on: 1. Operating torque: Operating torque is to choose the most important parameters of 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. 2. Operation thrust: There are two kinds of main structure of the valve electric device, one is do not configure the thrust plate, this moment output torque; the other is configured with a thrust plate, then the output torque through the thrust plate in the stem nut Converted to output thrust. 3. Output shaft rotation: Valve motor output shaft rotation number and the valve nominal diameter, stem pitch, the number of threads related, according to M = H / ZS calculation (where: M for the electric device should meet The total number of revolutions; H is the valve opening height, mm; S for the stem thread pitch, mm; Z for the stem thread number.) 4. Stem Diameter: For multi-turn open-stem valves, the valve can not be assembled as a motor-operated valve if the maximum stem diameter allowed by the actuator does not pass through the stem of the associated 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 considered the problem of the passage of the stem diameter, but also should fully consider the stem diameter and keyway size, so that the normal work after assembly. 5. Output speed: the valve opening and closing fast, easy to produce water hammer phenomenon. Therefore, should be based on different conditions of use, select the appropriate Kai, closing speed. 6. Installation, connection: the installation of electric devices are mounted vertically, horizontal installation, floor installation; connection: a thrust plate; stem through (Ming rod multi-turn valve); dark rod multi-turn; no thrust plate; stem Not through; part of the rotary electric device is used for a wide range of purposes, is to achieve program-controlled valves, automatic control and remote control indispensable equipment, which is mainly used in closed-circuit valves. However, the special requirements of valve actuators can not be ignored - torque or axial forces must be limited. Valve actuators are usually torque-limiting couplings. When the specifications of the electric device is confirmed, the control torque is also determined. When it is operating within a predetermined time, the motor is generally not overloaded. However, it can be overloaded if: 1. Low supply voltage, can not get the required torque, the motor stops rotating. 2. The torque limit mechanism was incorrectly set so as to be larger than the stop torque, resulting in continuous generation of excessive torque to stop the rotation of the motor. 3. As intermittent use of jogging, the heat generated savings, exceeding the allowable temperature rise of the motor value. 4. Torque limit mechanism for some reason circuit failure, the torque is too large. 5. Ambient temperature is too high, relative to the motor heat capacity decreased. The above are some reasons for overload, motor overheating for these reasons should be pre-considered, and take measures to prevent overheating. In the past, the way to protect the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc. However, these methods all have their advantages and disadvantages. Absolutely reliable protection methods are not available for such variable-load devices as electromechanical devices . Therefore, we must adopt a combination of methods. However, due to the different loading conditions of each electric device, it is difficult to propose a unified approach. But in most cases, you can also find common ground. To take the overload protection, summarized in two ways: 1. To determine the increase or decrease of the motor input current; 2. The motor itself to determine the fever. The above two methods, regardless of the kind to consider the motor thermal capacity given time margin. If a single way to make it consistent with the thermal characteristics of the motor is difficult. Therefore, we should choose a reliable method of action based on the cause of overload - a combination of complex ways to achieve a comprehensive overload protection. Rotork electric motor, because it is buried in the winding and the insulation level of the motor thermostat, when the rated temperature, the motor control circuit will be cut off. The heat capacity of the thermostat itself is small, and its time-limited nature is governed by the thermal capacity of the motor, so this is a reliable method. The basic method of overload protection is: 1. On the motor continuous operation or jog operation overload protection using thermostat; 2. The protection of motor stalls using thermal relay 3. Use short circuit fuse or overcurrent relay. The correct choice of valve electric devices and overload prevention is related, should be taken seriously.
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