Electric actuators

Electric actuators are motor driven devices that utilize an electrical input signal to generate a motor shaft rotation. This rotation is, in turn, translated by the unit’s linkage into a linear motion,which drives the valve stem and plug assembly for flow modulation. In case of electric signal failure, these actuators can be specified to fail in the stem-out, stem-in, or last position. Commonly used motors for electric actuators include steppers and servos. 

steppers

A step motor uses gears with increments in the range of 5,000 to 10,000 at 90 degree rotation for accurate positioning at lower speeds. The disadvantage is that steppers may lose synchronization with the controller when employed in an open loop without an encoder or if they are undersized for an application. 

servos

Servos, by definition, are closed loop and provide superior performance at high speeds, but at a higher cost. High precision screws and anti-backlash mechanics provide accuracies to ten-thousandths of an inch. Standard precisions with standard components range from a few hundredths to a few thousandths of an inch. Brush DC motors and AC motors are sometimes used with limit switches when positioning accuracy is less critical. The motor is connected to a gear or thread that creates thrust to move the valve. To protect the valve the torque sensing mechanism of the actuator turns off the electric motor when a safe torque level is exceeded. Position switches are utilized to indicate the open and closed position of the valve. Typically a declutching mechanism and hand wheel are included so that the valve can be operated manually should a power failure occur. 

Advantages

• Provide precise control and positioning in comparison to pneumatic actuators. 
• Response time is essentially instantaneous. 
• High degree of stability. 
• Help adapt machines to flexible processes. 
• Low operating cost. Controllers and drivers low voltage circuitry consume power to a far lesser degree.

Disadvantages

• The primary disadvantage of an electric actuator is that, should a power failure occur,  the valve remains in the last position and the fail-safe position cannot be obtained easily unless there is a convenient source of stored electrical energy. 
• Higher cost than pneumatic actuators. The total cost ranges from $800 to $3,000 and up. High component costs often deter the use of electric actuators because savings in operating costs compared to pneumatics are often not  adequately considered or are outright ignored. 
• The actuator needs to be in an environment that is rendered safe. Generally not recommended for flammable atmospheres.