Valve electric devices have special requirements that must be able to limit torque or axial forces. Usually the valve electric device uses a torque limiting coupling. When the valve electric device specification is determined, its control torque is also determined. Generally running in a predetermined time, the motor will not be overloaded. However, if the following conditions occur, the overload may be caused: first, the power supply voltage is low, the required torque is not obtained, and the motor stops rotating; second, the torque limiting mechanism is set incorrectly to be greater than the stopped torque. Cause continuous excessive torque generation, so that the motor stops rotating; Third, intermittent use, the generated heat accumulation exceeds the allowable temperature rise value of the motor; Fourth, for some reason, the torque limiting mechanism circuit fails, causing the torque to pass Large; five is the use of ambient temperature is too high, relatively reducing the thermal capacity of the motor.
In the past, the protection of the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no absolute and reliable protection for variable-load equipment such as valve electric devices. Therefore, various combinations must be adopted, which can be summarized in two ways: one is to judge the increase and decrease of the input current of the motor; the other is to judge the heat of the motor itself. In either case, the time margin given by the thermal capacity of the motor must be considered.
Generally, the basic protection method of overload is: using a thermostat for overload protection of continuous or jog operation of the motor; using a thermal relay for the protection of the motor stall; and using a fuse or an overcurrent relay for a short circuit accident.
The valve electric device is an indispensable device for realizing valve program control, automatic control and remote control. The movement process can be controlled by the stroke, torque or axial thrust. Since the working characteristics and utilization rate of the valve electric device depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment, the correct selection of the valve electric device prevents overloading (the working torque is higher than the control). Torque) is crucial.
In general, 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.
Operational Thrust The main structure of the valve electric device is two types: one is that the thrust plate is not arranged to directly output the torque; the other is to configure the thrust plate, and the output torque is converted into the output thrust through the stem nut in the thrust plate.
Output shaft rotation number The number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem and the number of threads. It should be calculated according to M=H/ZS (M is the total rotation circle that the electric device should meet). Number, H is the valve opening height, S is the stem drive thread pitch, and Z is the stem thread number).
Output speed If the opening and closing speed of the valve is too fast, water hammer is likely to occur. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use.
Stem Diameter For multi-turn type open rod valves, if the maximum stem diameter allowed by the electric device cannot pass through the valve stem of the valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow shaft of the electric device must be larger than the outer diameter of the stem of the open rod valve. For the partial rotary valve and the dark rod valve in the multi-turn valve, although the problem of the diameter of the stem is not considered, the diameter of the stem and the size of the keyway should be fully considered in the selection, so that the assembly can work normally.