Valve electric actuator is an indispensable device to complete valve program control, automatic control and remote control. Its motion process can be controlled by stroke, torque or axial thrust.
How to choose a valve electric actuator (valve electric equipment) is very important to avoid overload (the operating torque is higher than the control torque).
In general, how to choose a valve electric actuator depends on the following points:
1. Operating torque: Operating torque is the most important parameter for selecting valve electric equipment. The output torque of electric actuators should be 1.2 to 1.5 times the maximum torque of valve operation.
2. There are two types of host structure for operating the thrust valve electric equipment: one is to output torque directly without a thrust disk; the other is equipped with a thrust disk, and the output torque is converted to output thrust through the valve stem nut in the thrust disk.
3. Rolling number of output shaft: The number of rolling number of the output shaft of the valve electric equipment is related to the nominal diameter of the valve, the pitch of the valve stem, and the number of screw heads. It should be calculated according to M = H / ZS (M is the electrical equipment should be satisfied The total number of rolls, H is the valve opening height, S is the thread pitch of the valve stem transmission thread, and Z is the number of stem thread heads).
4. Stem diameter For multi-turn open-stem valves, assuming that the maximum stem diameter allowed by electric equipment cannot pass through the stem of the valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric equipment must be larger than the outer diameter of the stem of the bright stem valve. For the part of the rotary valve and the multi-turn valve of the dark-stem valve, although the passage of the diameter of the stem does not need to be considered, the diameter of the stem and the size of the keyway should be fully considered when selecting and matching, so that normal operation after assembly.
5. Output speed: If the opening and closing speed of the valve is too fast, water hammer will easily occur. Therefore, the appropriate opening and closing speed should be selected according to different operating conditions.
6. Valve electric actuator has its special requirements, that is, it must be able to limit torque or axial force. Generally, the valve electric equipment uses a coupling that constrains torque. After the specifications of the electric equipment are confirmed, the control torque is also confirmed. Generally work within the pre-confirmed time, the motor will not be overloaded.
However, overloading may occur if:
First, the power supply voltage is low and the required torque cannot be obtained, causing the motor to intermittently roll;
The second is to arbitrarily set the torque constraint arrangement so that it is larger than the intermittent torque, causing excessive torque to occur in succession, causing the motor to intermittently roll;
The third is intermittent use, the onset of heat savings exceeds the allowable temperature increase of the motor;
Fourth, for some reason, the torque constraint arranges the circuit to malfunction, which makes the torque too large;
Fifth, the temperature of the application is too high, which relatively reduces the thermal capacity of the motor.
In the past, the methods for protecting the motor were to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their advantages and disadvantages. There is no absolutely reliable protection method for load-changing equipment such as electric equipment. Therefore, it is necessary to adopt various combination methods. There are two types of induction: one is to judge the increase or decrease of the input current of the motor; the other is to judge the self-heating of the motor. These two methods, regardless of the type, must consider the time margin given by the motor thermal capacity.
Generally, the foundation protection method for overload is:
1. For the overload protection of the motor in continuous operation or jog operation, use a thermostat;
2. For the protection of motor stall, select thermal relay;
3. For short-circuit accidents, use fuses or overcurrent relays.
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