Valve electric device is one of the devices used to operate the valve and connected to the valve. The device is driven by electricity, and its movement process can be controlled by the stroke, torque or axial thrust. Due to the working characteristics and utilization of the valve electric device, 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, grasping the correct choice of valve electric device; considering the prevention of overload (working torque higher than control torque) has become a vital link.
 
The correct choice of valve electric device should be based on:
1. Operating torque: Operating torque is the main parameter for selecting valve electric devices. The output torque of the electric device should be 1.2 to 1.5 times the high torque of the valve operation.
2. Operating thrust: There are two main structures of the valve electric device. One is not equipped with a thrust disk, and the torque is output directly at this time; the other is configured with a thrust disk, and the output torque is passed through the stem nut in the thrust disk. Converted to output thrust.
3． The number of rotations of the 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 pitch of the valve stem, and the number of thread heads. Total number of rotations; H is the opening height of the valve, mm; S is the pitch of the valve stem transmission thread, mm; Z is the number of valve stem thread heads.)
4． Stem diameter: For multi-turn type bright stem valves, if the large 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 is larger than the outer diameter of the stem of the bright stem valve. For the part of the rotary valve and the multi-turn valve of the dark stem valve, although the passage of the stem diameter need not be considered, but when selecting and matching, the stem diameter and the size of the keyway should be fully considered to ensure normal operation after assembly.
5． Output speed: The valve opens and closes quickly and is prone to water hammer. Therefore, the appropriate opening and closing speed should be selected according to different use conditions.
6. Installation and connection methods: The installation methods of the electric device include vertical installation, horizontal installation, and floor installation; the connection methods are: thrust disc; valve stem passes (open stem multi-turn valve); dark stem multi-turn; no thrust disc; valve stem does not Passed; part of the rotary electric device has a wide range of uses. It is an indispensable device for valve program control, automatic control and remote control. It is mainly used for closed-circuit valves. But the special requirements of the valve electric device cannot be ignored-it can limit the 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. When it runs for a predetermined time, the motor is generally not overloaded. However, it can be overloaded if:
1. The power supply voltage is low, the required torque cannot be obtained, and the motor stops rotating.
2. The torque limiting mechanism was erroneously adjusted to make it larger than the torque that was stopped, resulting in continuous excessive torque and stopping the motor.
3． Use intermittently like jog, the heat generated is accumulated and exceeds the allowable temperature rise of the motor.
4． For some reason, the circuit of the torque limiting mechanism is malfunctioning, causing the torque to be excessive.
5． If the ambient temperature is too high, the heat capacity of the motor will be relatively reduced.
 
The above are some of the reasons for overload. Motor overheating caused by these reasons should be considered in advance, and measures should be taken to prevent overheating.
 
In the past, the methods to protect the motor were to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods also have their own advantages and disadvantages. There is no reliable protection method for variable load equipment such as electric devices. Therefore, a combination of various methods is adopted. However, due to the different load conditions of each electric device, it is difficult to propose a unified approach. But to summarize the majority of cases, you can find common ground from them.
 
The overload protection methods adopted can be summarized into two types:
1. Judge the increase or decrease of the motor input current;
2. Judge the motor itself.
 Regardless of the above two methods, the time margin given by the thermal capacity of the motor must be considered. It is difficult to make it consistent with the thermal capacity characteristics of the motor in a single way. Therefore, we should choose a method that can reliably operate according to the cause of overload—combination and compounding method to achieve comprehensive overload protection.
 
The basic protection method for overload is:
1. The thermostat is used for the overload protection of the motor continuous operation or jog operation;
2. Thermal relay is used to protect the motor from stalling;
3． For short-circuit accidents, use fuses or overcurrent relays.