generally, the basis for the correct selection of valve electric device is as follows:   

 

   operating torque operating torque is the most important parameter to select the valve electric device. The output torque of the electric device should be 1.2 ~ 1.5 times of the maximum operating torque of the valve.

 

   there are two main engine structures for operating thrust valve electric device: one is not equipped with thrust disc, which directly outputs torque; the other is equipped with thrust disc, which converts output torque into output thrust through stem nut in thrust disc.   

 

   number of turns of output shaft the number of turns of output shaft of valve electric device is related to the nominal diameter, stem pitch and thread number of valve, which shall be calculated according to M = H / ZS (M is the total number of turns of electric device, h is the opening height of valve, s is the stem transmission thread pitch, Z is the number of stem thread).

 

   stem diameter for multi rotary rising stem valves, if the large stem diameter allowed by the electric device cannot pass through the stem of the valve, the electric valve cannot be assembled. Therefore, the inner diameter of the hollow output shaft of the electric device must be greater than the outer diameter of the stem of the rising stem valve. For the part turn valve and the Non rising stem valve in the multi turn valve, although the passage of stem diameter is not considered, the size of stem diameter and keyway should also be fully considered in the selection, so that the valve can work normally after assembly.   

 

   if the opening and closing speed of output speed valve is too fast, it is easy to produce water hammer phenomenon. Therefore, the proper opening and closing speed should be selected according to different service conditions.   

 

   valve electric device has its special requirements, that is, it must be able to limit the torque or axial force. Usually, the valve electric device adopts the coupling device of limiting torque. When the electric device specification is determined, its control torque is also determined. Generally, the motor will not be overloaded when it runs within a predetermined time. However, the following situations may lead to overload: first, the power supply voltage is low and the required torque can not be obtained, which makes the motor stop rotating; second, the torque limiting mechanism is mistakenly set to make it greater than the stop torque, which causes continuous excessive torque and makes the motor stop rotating; third, the heat accumulation generated by intermittent use exceeds the allowable temperature appreciation of the motor; third, the motor is not fully loaded; The fourth reason is that the circuit of torque limiting mechanism breaks down for some reason, which makes the torque too large; the fifth reason is that the temperature of working environment is too high, which makes the heat capacity of motor decrease relatively.

 

   in the past, fuse, over-current relay, thermal relay and thermostat were used to protect the motor, but these methods had their own advantages and disadvantages. There is no protection method for the electric device, which is a variable load equipment. Therefore, we must take a variety of combinations, summed up in two ways: one is to judge the increase or decrease of motor input current; the other is to judge the motor itself heating. Either way, the time margin given by the thermal capacity of the motor should be considered.

 

   generally, the basic protection methods of overload are: thermostat is used for overload protection of motor continuous operation or inching operation; thermal relay is used for protection of motor locked rotor; fuse or over-current relay is used for short circuit accident.