Though the pneumatic actuator will probably continue to be the workhorse throughout the process industry for the automation of rotary valves (ball, plug, and butterfly valves), there are many applications where an electric actuator may be considered. For the process control engineer a good understanding of the basic function and features of electric actuators and their options and versatility will lead to a better technical and practical evaluation of their usefulness.

Why use electric actuators? Although electric actuators may be used anywhere a power source (electricity) is available, there are many applications where they are particularly well suited. For instance, in many remote installations it may be impractical to run an air supply and to maintain it. Air lines that freeze up may clog and render the equipment inoperable to damage more delicate instruments. If only a few actuators are to be installed in an area, electric actuators offer a simple means of automation for these smaller systems.

Computer compatible. Perhaps one of the most important reasons for the trend toward using electric actuators has been the decreasing cost of using computers as system controllers and the ease and economy with which the actuators can be interfaced to such systems. This trend can be expected to accelerate with the advent of new microprocessors, or smart controllers, based on the versatile, low-cost microprocessor chips. Because of the increased speed and decision- and control-making capability that the computer adds to a process system, there is less need for final control elements which have high control capability such as characterized globe and plug valves. As a result, the simpler and less expensive electric actuators and ball and butterfly valves have become more acceptable and are proving more than adequate for many applications.

How to evaluate costs. Generally, it is more economical to install and maintain electric valve actuators than pneumatic ones. A pneumatic system includes not only the actuators but compressors, piping, filters, air lubrication systems, and dryers. After installations, its many components cost more to maintain and usually require the coordinated service of both the plant's electrical and plumbing departments. Where electronic sensors are used with pneumatically controlled systems, the cost of current-to-pneumatic (I-P) converters must be added if positioning of the actuators is required.

Electric actuators eliminate the need for air, an expensive source of energy, and do not require energy when not in motion. There is no need to be concerned with compressor noise, housing to shield against noise, air venting, or other operating restrictions associated with pneumatic systems.

How electric actuators work

An electric actuator is basically a geared motor. The motor is the primary torque-generating component. Motors are available for a variety of supply voltages including standard single-phase 110 V ac, three-phase, and dc voltages. To prevent heat damage due to excessive current draw in a stalled condition, or due to overwork, electric actuator motors usually include a thermal overload sensor embedded in the winding of the stator. The sensor is installed in series with the power source and opens the circuit when the motor is overheated and then closes the circuit once it has cooled to a safe operating temperature.

Control Accessary Options

Probably the most important reason for the widespread use of electric actuators is their control circuit versatility. As an electric device, electric actuators naturally lend themselves for use as an enclosure for a variety of control and feedback devices. Furthermore, the switch and cams may be set and wired for almost any contact development for process and valve control.

A few examples of the control circuit versatility of electric actuators are three-position control and interposing relay. Three-position control is especially useful for the automation of multiported (three- or four-way) valves. An interposing relay is powered by a simple on-off switch. This circuit is similar to the type used in energizing single- acting solenoid valves in pneumatic actuator installations.

One advantage of an electric actuator is its ability to manually "jog" the valve position when it is used in filling operations. By installing a feedback potentiometer an operator can monitor the actuator's exact position and stop it at any point between open and closed with a manual control switch.

Gearing. Electric actuators utilize a gear train (a series of interconnected gears) to enhance the motor torque and to regulate the output speed of the actuator. Some gear styles are inherently self-locking. This is particularly important in the automation of butterfly valves or when an electric actuator is used in modulating control applications. In these situations seat and disc contact or fluid velocity act upon the closure element of the valve and cause a reverse force that can reverse the motor and cam shaft. This causes a reenergization of the motor through the limit switch when the cam position is changed. This undesirable cycling will continue to occur, unless a motor brake is installed, and usually leads to an overheated motor. Spur gears are sometimes used in rotary electric actuators but are not self-locking. They require the addition of an electromechanical motor brake for these applications.

A few of the self-locking gear styles include the worm and wheel and some configurations of planetary gears. A basic worm gear system operates as follows. A motor applies a force through the primary worm gear to the worm wheel. This in turn rotates the secondary worm gear which applies a force to the larger radius of the secondary worm wheel to increase the torque.