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Correctly select the valve electric device to prevent overload. The 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 motion process can be controlled by stroke, torque or axial thrust. Because 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. Therefore, master the correct choice of valve electric device ； It is crucial to consider preventing overload (working torque higher than control torque) from occurring. The correct selection of valve electric device should be based on: 1． Operating torque: Operating torque is the most important parameter in selecting a valve electric device. The output torque of the electric device should be 1.2 to 1.5 times the maximum operating torque of the valve. 2． operating thrust: There are two types of host structures for valve electric devices. One is without a thrust plate, in which case the torque is directly output. ； The other is equipped with a thrust plate. At this time, the output torque is converted into output thrust through the valve stem nut in the thrust plate. 3. 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 valve stem pitch, and the number of thread heads. It is calculated according to M=H/ZS (in the formula: M is the total number of rotations that the electric device should satisfy ； H is the opening height of the valve, mm ； S is the pitch of the valve stem driving thread, mm ； Z is the number of valve stem thread heads. ) 4. Stem diameter: For multi-turn rising stem valves, if the maximum valve stem diameter allowed by the electric device cannot pass through the valve 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 valve stem of the rising stem valve. For partial-turn valves and concealed-stem valves in multi-turn valves, although there is no need to consider the passage of the valve stem diameter, the valve stem diameter and keyway size should also be fully considered when selecting and matching, so that they can work normally 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 usage conditions. 6． Installation and connection methods: The installation methods of electric devices include vertical installation, horizontal installation and floor installation. ； The connection method is: The valve stem with thrust disc passes through (open stem multi-turn valve). The concealed stem multi-turn non-thrust disc valve stem does not pass through the partial rotation electric device. It is an indispensable device for realizing valve program control, automatic control and remote control. It is mainly used on closed-circuit valves. But the special requirements of the valve electric device cannot be ignored - it must be able to limit the torque or axial force. Usually valve electric devices use couplings that limit torque. After the specifications of the electric device are determined, its control torque is also determined. Motors are generally not overloaded when running for a predetermined period of time. However, it can be overloaded if the following conditions occur:: 1． The power supply voltage is low and the required torque cannot be obtained, causing the motor to stop rotating. 2． The torque limiting mechanism is incorrectly set so that it is greater than the stopping torque, causing excessive torque to be continuously generated and causing the motor to stop rotating. 3. When used intermittently like jogging, the heat generated accumulates and exceeds the allowable temperature rise of the motor. 4. For some reason, the circuit of the torque limiting mechanism malfunctions, resulting in excessive torque. 5. The operating environment temperature is too high, which will relatively reduce the heat capacity of the motor. The above are some 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 motors were to use fuses, overcurrent relays, thermal relays, thermostats, etc. However, these methods have their own pros and cons. For equipment with variable loads such as electric devices, 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 method. But by generalizing most situations, we can also find common ground. The overload protection methods adopted can be summarized into two types:: 1. Judge the increase or decrease of motor input current ； 2. Determine whether the motor itself is heating up. Regardless of the above two methods, 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 using a single method. Therefore, we should choose a method that can reliably act according to the cause of overload - a combination method to achieve comprehensive overload protection. The motor of Rotork electric device has a thermostat embedded in the winding that is consistent with the insulation level of the motor. 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-limiting characteristics are determined by the heat capacity characteristics of the motor, so this is a reliable method. The basic protection method for overload is: 1． Thermostat is used for overload protection of the motor in continuous operation or inching operation. ； 2． Thermal relay is used to protect the motor from stalling 3. Use fuses or overcurrent relays for short circuit accidents. The correct selection of valve electric devices and the prevention of overload are closely related. Attention should be paid.