Valve electric actuators are essential components for achieving programmable, automated, and remote control of valves. They regulate the valve's movement by controlling stroke, torque, or axial thrust. The performance and application of these devices depend on the type of valve, operational specifications, and the valve’s position in the pipeline or system. Correct selection is crucial to avoid overloading, which occurs when the operating torque exceeds the control torque. This can lead to mechanical failure or reduced service life.
The most critical factor in selecting a valve electric actuator is the operating torque. The actuator’s output torque should be 1.2 to 1.5 times the maximum torque required by the valve. Another important consideration is the thrust, which depends on the actuator’s design—some provide direct torque output, while others use a thrust plate to convert torque into linear force.
The number of revolutions the actuator’s output shaft makes is also vital. It depends on the valve’s nominal diameter, stem pitch, and thread count, calculated as M = H / (Z × S), where M is the total revolutions needed, H is the valve lift, S is the pitch, and Z is the number of threads.
For multi-turn rodless valves, the actuator’s hollow output shaft must have an inner diameter larger than the stem’s outer diameter to ensure proper assembly. For rotary or multi-turn valves with a non-rod design, stem diameter and keyway size still need to be considered to ensure smooth operation.
The output speed of the actuator affects the valve’s opening and closing rate. Too fast a speed may cause water hammer effects, so it's important to choose an appropriate speed based on the application. Actuators must also include torque or axial force limiting mechanisms, typically through torque-limiting couplings. Once the actuator’s specifications are set, the control torque is fixed. Under normal conditions, the motor will not overload within the designated time. However, several factors can lead to overloading: low voltage, incorrect torque settings, intermittent use causing overheating, circuit failures in the torque limit mechanism, or high ambient temperatures reducing the motor’s thermal capacity.
Traditionally, motor protection has involved fuses, overcurrent relays, thermal relays, and thermostats, each with its own pros and cons. No single method is entirely reliable for variable-load applications. Therefore, a combination of approaches is recommended. One method involves monitoring motor current to detect changes, while another focuses on the motor’s temperature. Regardless of the method, a time margin should always be considered based on the motor’s thermal capacity.
Common overload protection methods include using thermostats for continuous or inching operations, thermal relays for locked motors, and fuses or overcurrent relays for short-circuit protection. A well-designed protection system ensures the longevity and safe operation of the valve electric actuator.
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