Correctly choose the valve electric device to prevent the occurrence of overload

Correctly choose the valve electric device to prevent the occurrence of overload

The valve electric device is one of the devices used to operate the valve and connect to the valve. The device is driven by electricity, and its movement process can be controlled by stroke, torque or axial thrust. Because the valve electric device should have working characteristics and utilization rate depends on the type of valve, the working specification of the device and the position of the valve on the pipeline or equipment. Therefore, master the correct choice of valve electric device; consider preventing the occurrence of overload (working torque is higher than control torque) has become a vital part

The correct selection of valve electric device should be based on

Operating torque: Operating torque is the most important parameter for selecting valve electric devices. The output torque of the electric device should be the maximum torque of the valve operation

Operating thrust: There are two main structures of the valve electric device. One is without a thrust plate, and the torque is directly output at this time; the other is equipped with a thrust plate, and the output torque is passed through the stem nut in the thrust plate. Converted to output thrust

Number of turns of the output shaft: The number of turns of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the stem, and the number of threads.

Stem diameter: For multi-turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the stem of the matched valve, it cannot be assembled into an electric 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 valve. For part-turn valves and dark-stem valves in multi-turn valves, although the passage of the stem diameter does not need to be considered, the stem diameter and the size of the keyway should be fully considered when selecting and matching, so that it can work normally after assembly.

Output speed: The valve opens and closes quickly, and it is easy to produce water hammer. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use

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 passing (open-stem multi-turn valve) dark stem multi-turning, no thrust disc valve stem does not pass Partial rotation

The use of electric devices is very wide. It is an indispensable device to realize valve program control, automatic control and remote control. It is mainly used in closed-circuit valves. However, the special requirements of the valve electric device cannot be ignored, and the torque or axial force must be limited. Usually the valve electric device adopts a torque-limiting coupling.

After the specification of the electric device is determined, its control torque is also determined. When it runs within a predetermined time, the motor is generally not overloaded. However, it can be overloaded if the following conditions occur: The power supply voltage is low, and the required torque cannot be obtained, causing the motor to stop rotating. The torque limit mechanism is set by mistake to make it larger than the stopping torque, which causes the continuous generation of excessive torque, which causes the motor to stop rotating. If it is used intermittently as inching, the heat generated accumulates and exceeds the allowable temperature rise of the motor. For some reason, the circuit of the torque limiter mechanism failed, which caused the torque to be too large. The use of the environment temperature is too high, relatively reducing the thermal capacity of the motor. The above are some of the reasons for overload. The motor overheating caused by these reasons should be considered in advance and measures should be taken to prevent overheating.

In the past, the way to protect the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods also have their own advantages and disadvantages. For electric devices such as variable load equipment, there is no absolutely reliable protection method. . Therefore, a combination of various methods must be adopted. However, due to the different load conditions of each electric device, it is difficult to propose a unified approach. But generalizing most situations, you can also find common ground.

The overload protection methods adopted by can be summarized into two types:

1. Judge the increase or decrease of the input current of the motor; 2. Judge the heating of the motor itself.

For the above two methods, no matter which kind, the time margin given by the motor's thermal capacity must be considered. It is difficult to make it consistent with the thermal capacity characteristics of the motor in a single way. Therefore, it is necessary to choose a combination of composite methods that can reliably act according to the cause of the overload to achieve a comprehensive overload protection effect.

Rotork electric device motor, because it has a thermostat with the same insulation grade as the motor embedded in the winding, when the rated temperature is reached, the motor control circuit will be cut off. The heat capacity of the thermostat itself is small, and its time-limited characteristics are determined by the heat capacity characteristics of the motor, so this is a reliable method.

The basic protection method of overload is:

A thermostat is used for overload protection of the motor in continuous operation or inching operation;

The thermal relay is used to protect the motor from blocking

Use fuses or overcurrent relays for short-circuit accidents.

The correct selection of the valve electric device and the prevention of overload are closely related. Should be taken seriously.