In fact, the electric equipment in the electric valve can realize the program control of regulating valve, remote control and automatic control are indispensable equipment, and the movement process is controlled by the stroke and thrust. Therefore, the correct selection of electric device for regulating valve can prevent load phenomenon.
Generally, the basis for the correct selection of electric device for regulating valve is as follows:
Operating torque: the operating torque is the most important parameter for selecting the electric device of regulating valve, and the output torque of the electric device shall be 1.2-1.5 times of the large operating torque of the regulating valve.
Operating thrust: there are two main structures for regulating the valve electric device: one is to output torque directly without thrust disc; the other is to configure thrust disc, and the output torque is converted into output thrust through stem nut in thrust disc.
Rotation number of output shaft: the number of rotation turns of the output shaft of the electric device of the regulating valve is related to the nominal diameter of the regulating valve, the screw pitch of the valve stem and the number of thread heads, which shall be calculated according to M = H / ZS (M is the total number of rotations that the electric device should meet, h is the opening height of the regulating valve, s is the screw pitch of the valve stem transmission, and Z is the number of thread heads of the valve rod).
Stem diameter: for multi turn rising stem regulating valve, if the large stem diameter allowed by the electric device cannot pass through the valve stem of the regulating valve, it cannot be assembled into an electric regulating valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be larger than the outer diameter of the stem of the rising stem regulating valve. Although it is not necessary to consider the passage of stem diameter for the Non rising stem regulating valve in partial rotation regulating valve and multi rotation regulating valve, the valve stem diameter and the size of keyway should be fully considered in the selection, so that the valve can work normally after assembly.
Output speed: if the opening and closing speed of the regulating valve is too fast, it is easy to produce water hammer phenomenon. Therefore, the appropriate opening and closing speed should be selected according to different service conditions.
The electric device of regulating valve has its special requirements, that is, it must be able to limit the torque or axial force. Usually, the torque limiting coupling is used in the electric device of regulating valve. When the specification of the electric device is determined, the control torque is also determined. Generally, the motor will run within a predetermined time, and the motor will not overload. But if the following conditions may lead to overload: first, the power supply voltage is low, can not get the required torque, so that the motor stops rotating; second, the wrong adjustment The constant torque limiting mechanism makes it greater than the stop torque, resulting in continuous excessive torque, which makes the motor stop rotating; the third is that the heat accumulation generated by intermittent use exceeds the allowable temperature appreciation of the motor; the fourth is that the torque is too large due to the circuit fault of the torque limiting mechanism for some reason; the fifth is that the thermal capacity of the motor is relatively reduced due to the excessive temperature of the operating environment.
In the past, fuse, over-current relay, thermal relay and thermostat were used to protect the motor, but each of these methods has its own advantages and disadvantages. There is no absolutely reliable protection method for the electric device, which is a variable load equipment. Therefore, it is necessary to adopt various combination methods, which can be summed up in two ways: one is to judge the increase or decrease of the motor input current; the other is to judge the heating condition of the motor itself. In either case, the time margin given by the thermal capacity of the motor should be considered.
Generally, the basic methods of overload protection are: thermostat is used for overload protection of motor continuous operation or inching operation; thermal relay is used for motor stalling protection; fuse or over-current relay is used for short-circuit accident.
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