The valve electric device is an indispensable device for realizing valve program control, automatic control and remote control, and its movement 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 device working specification and the position of the valve on the pipeline or equipment, the correct selection of the valve electric device will prevent overloading (working torque is higher than the control) Torque) is very important.
Generally, the basis for the correct selection of valve electric devices is as follows:
Operating torque: The operating torque is the most important parameter for selecting the electric device of the valve. The output torque of the electric device should be 1.2 to 1.5 times the maximum torque of the valve operation.
Thrust operation: There are two main structure of the valve electric device: one is to directly output torque without a thrust disk; the other is to configure a thrust disk, the output torque is converted into output thrust by the stem nut in the thrust disk.
Number of rotations of output shaft: The number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the thread pitch of the valve stem, and the number of thread heads. It must be calculated according to M = H / ZS (M is the total rotation that the electric device should meet The number of turns, H is the opening height of the valve, S is the thread pitch of the stem drive thread, and Z is the number of thread ends of the stem).
Stem Diameter: For multi-turn type open 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 open stem valve. For part-turn valves and hidden stem valves in multi-turn valves, although it is not necessary to consider the passage of the stem diameter, the stem diameter and the size of the keyway should also be fully considered when selecting, so that it can work normally after assembly.
Output speed: 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 use conditions.
Valve electric devices have their special requirements, that is, they must be able to limit torque or axial force. Valve electric devices usually use couplings that limit torque. After the specification of the electric device is determined, its control torque is also determined. Generally run within a predetermined time, the motor will not be overloaded. However, if the following conditions occur, it may cause overload: First, the power supply voltage is low and the required torque is not obtained, causing the motor to stop rotating; second, the torque limiting mechanism is erroneously set so that it is greater than the stopping torque. Causes excessive torque to be generated continuously, causing the motor to stop rotating; third, it is used intermittently, and the heat generated exceeds the allowable temperature rise of the motor; fourth, the torque limiting mechanism circuit fails for some reason, causing the torque to exceed Large; Fifth, the use of ambient temperature is too high, relatively reducing the thermal capacity of the motor.
In the past, the method of protecting the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no absolutely reliable protection method for variable load equipment such as electric devices. Therefore, a variety of combinations must be taken. There are two types of induction: one is to judge the increase and decrease of the input current of the motor; the other is to judge the heating of the motor itself. These two methods, regardless of the type, must consider the time margin given by the motor thermal capacity.
Generally, the basic protection method of overload is: the overload protection for continuous operation or jog operation of the motor, using a thermostat; the protection of the motor stalling, the use of thermal relays; the use of fuses or overcurrent relays for short circuit accidents. -Register Hong Kong company
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