The electric device of the electric valve is an indispensable equipment for the realization of the valve program control, automatic control and remote control, and its movement process can be controlled by the size of the stroke, torque or axial thrust. Since the working characteristics and utilization of the valve electric device depends on the type of valve, the device work specification and the position of the valve in the pipeline or equipment, the correct selection of the valve electric device is essential to prevent overload phenomenon (working torque is higher than the control torque).

Generally, the correct selection of valve electric device is based on the following:

Operating torque:

The operating torque is the main parameter of the valve electric device, and the output torque of the electric device should be 1.2 to 1.5 times of the valve operating torque.

Operational thrust:

There are two main structures of the valve electric device: one is not equipped with a thrust plate and directly output torque; the other is equipped with a thrust plate, and the output torque is converted to output thrust through the stem nut in the thrust plate.

Number of turns of output shaft:

The number of rotation turns 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. It should be calculated according to M = H/ZS (M is the total number of rotation turns that the electric device should meet, H is the opening height of the valve, S is the thread pitch of the valve stem, and Z is the thread head of the valve stem).

Stem diameter:

For multi-turn open-rod valves, if the diameter of the valve stem allowed by the electric device cannot pass through the valve 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 must be greater than the outer diameter of the stem of the open stem valve. For part-turn valves and multi-turn valves

The valve in the dark rod valve, although do not consider the stem diameter through the problem, but in the selection should also fully consider the stem diameter and keyway size, so that after assembly can work properly.
Output speed:

If the valve opening and closing speed is too fast, easy to produce water hammer phenomenon. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use. Valve electric device has its special requirements, that must be able to limit the torque or axial force. Usually the valve electric device uses a torque-limiting coupling. When the specification of the electric device is determined, its control torque is also determined. Generally run in a predetermined time, the motor will not be overloaded. However, the following situations may lead to overload: first, the power supply voltage is low, the required torque is not available, so that the motor stops rotating; The second is to set the torque limiting mechanism incorrectly to make it greater than the stopping torque, resulting in continuous excessive torque, so that the motor stops rotating; The third is intermittent use, which generates heat accumulation and exceeds the allowable temperature appreciation of the motor; four is due to some reason the torque limiting mechanism circuit failure, so that the torque is too large; five is the use of ambient temperature is too high, the relative heat capacity of the motor decreased.

In the past, the way to protect the motor is to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no reliable protection method for such variable load equipment as electric device. Therefore, it is necessary to adopt various combinations, which can be summed up in two ways: one is to judge the increase or decrease of the input current of the motor; the other is to judge the heating of the motor itself. These two methods, no matter which, should consider the time margin given by the motor heat capacity.

Usually, the basic protection method of overload is: the continuous operation of the motor or inching operation of overload protection, the use of thermostat; the protection of motor stall, the use of thermal relay; short-circuit accident, the use of fuse or overcurrent relay.