The device is powered by electricity, and its motion can be controlled by stroke, torque or axial thrust.Due to the operating characteristics and utilization of the valve electric device depends on the type of valve, the working specification of the device and the position of the valve in the pipeline or equipment.Therefore, master the correct choice of valve electric device;Preventing overload (when the working torque is higher than the control torque) becomes a crucial step.

　　The correct selection of valve electric device should be based on:

　　1.Operating torque: The operating torque is the primary parameter for selecting valve electric devices.The output torque of the electric device should be 1.2 ~ 1.5 times of the large torque of valve operation.

　　2.Operation thrust: the main engine structure of the valve electric device has two kinds, one is not equipped with thrust plate, at this time directly output torque;The other is provided with a thrust disc where the output torque is converted to the output thrust through the stem nut of the disc.

　　3.The number of rotation turns of output shaft: the number of rotation turns of 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, calculated according to M=H/ZS (where M is the total number of rotation turns that the electric device should meet;H is the opening height of the valve, mm;S is the pitch of stem drive thread, mm;Z is the number of stem threads.

　　4.Stem Diameter: For multi-turn open-stem valves, an electric valve cannot be assembled if the large stem diameter allowed by the electric device does not pass through the stem of the supplied valve.Therefore, the inner diameter of the hollow output shaft of the electric device is greater than the outer diameter of the stem of the stem of the stem valve.For partial rotary valves and multi-rotary valves in the dark rod valve, although do not need to consider the diameter of the stem through the problem, but in the selection should also fully consider the diameter of the stem and the keyway size, so that the assembly can work normally.

　　5.Output speed: the valve opening and closing speed is fast, easy to produce water hammer phenomenon.Therefore, the appropriate opening and closing speed should be selected according to different use conditions.

　　6.Installation and connection mode: the installation mode of the electric device includes vertical installation, horizontal installation and floor installation;The connection mode is: thrust plate;Stem through (open stem multi-turn valve);Dark rod multi-turn;No thrust disc;Stem does not pass;Part of the rotary electric device has a wide range of uses, is to achieve the valve program control, automatic control and remote control indispensable equipment, which is mainly used in the closed circuit valve.However, the special requirement of valve electric device can not be ignored - can limit the torque or axial force.Normally valve actuators are couplings that limit torque.

　　When the specification of the electric device is determined, the control torque is also determined.When it is running at a predetermined time, the motor is generally not overloaded.However, it can be overloaded if the following conditions occur:

　　1.Low power supply, not the required torque, so that the motor stops rotating.

　　2.Incorrectly set the torque limiting mechanism so that it is larger than the stopped torque, resulting in continuous excessive torque and stopping the motor.

　　3.Such as the intermittent use of the point, the heat generated savings, more than the allowable temperature appreciation of the motor.

　　4.For some reason the torque limiting mechanism circuit malfunction, so that the torque is too large.

　　5.The use of environmental temperature is too high, the relative decline of the heat capacity of the motor.

　　The above are some reasons for overload. For these reasons, the motor overheating phenomenon should be considered in advance, and measures should be taken to prevent overheating.

　　In the past, the way to protect the motor is 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, reliable protection method is not available.Therefore, a combination of various methods is adopted.However, because of the different load conditions of each electric device, it is difficult to put forward a unified method.But in most cases, common ground can be found.

　　Overload protection can be divided into two types:

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

　　2.The heat of the motor itself is judged.

　　The above two ways, regardless of which to consider the motor heat capacity given time margin.It is difficult to match the heat capacity characteristics of the motor in a single way.Therefore, we should choose the method that can reliably act according to the cause of overload -- combination and compound mode, in order to achieve comprehensive overload protection.

　　The basic methods of overloading protection are:

　　1.Thermostat is used for overload protection of continuous motor operation or point operation;

　　2.Thermal relay is adopted to protect the motor from blocking rotation.

　　3.Use fuses or overcurrent relays for short circuit accidents.