Electric actuators
Electric actuators are energized by electric power supply. They are available for many different voltages with different ingress protection grades as IP65, IP67, IP68, NEMA 4 and NEMA 6 and also with ATEX and NEMA 7 Approvals for use in Hazardous Zones.
The Electric Actuators can be connected to almost any digital Control System or Monitoring system such as AS-Interface (ASI-bus), Profibus, Foundation Fieldbus, Interbus, Modbus, Device Net, HART-protocol, etc, etc, by the use of suitable Communication accessories.
Electric Quarter Turn Actuators
Electric Quarter Turn Actuators are mainly used for automation of ball Valves, Butterfly Valves, Plug Valves and Dampers.
Electric Multi Turn Actuators
Electric Multi Turn Actuators are used for Gate Valves, Globe Valves, Plug Valves, Dampers and other applications where multi turn function is required, for example quarter turn valves with reducing gear boxes.
Fail Safe Electric Actuators
Most electric Actuators have no integrated fail safe function but can be equipped with battery packs for fail safe function. On the market are also some fail safe systems with spring return function available even if most of them are equipped with electro-mechanical release mechanisms which are not too reliable after longer use because of dirt, corrosion and aged lubricants. A more safe solution for this is the Electro Hydraulic Fail Safe Actuators which are using a similar spring return system as the pneumatic actuators where no release mechanism is required. These kinds of actuators are to be seen as electric actuators with spring return.
Duty Cycles
The On-Off Duty electric actuators are sensitive for high intensity cycling.
Therefore when modulating is required it is very important to choose an actuator with a maximum Modulating Duty intensity that corresponds to the requirements for the specific application.
Controlling and Signaling
Single Phase Electric Actuators are normally controlled by a 3-position switch function for Open-Stop-Close function.
Three Phase Electric Actuators must be controlled by Starter Relays since phase inverting is necessary to for changing the directions of a three phase motor.
Limit Switches
The Electric Actuators are often equipped with potential free limit switches for signalling to PLC controllers where the motor voltage of the actuator is not possible to use as feed back signal.
Local Control
For Local Control of the electric actuators a control unit is required, such units have a key selector switch for Local/Remote selection and a 3-position control knob for Open-Stop-Close control.
Positioner
For modulating applications with Electric Actuator a positioner is required. The positioners are available in most various designs, starting with the simplest for a 4-20 mA input signal only. If required there are optional feed-back signal transmitters available with or without relays. A basic rule is that the higher the resolution of the positioning and the feed back signal, the higher the price is.
The best positioners for electric quarter turn actuators can perform 450 points of resolution for a 90� turning angle. These positioners are equipped with dynamic braking control features and also adaptive control by continuously monitoring and compensating for actuator backlash, motor coast and load changes to eliminate positioner dead band. Moreover these positioners have built in advanced safety features as stall detection and duty cycle protection for the actuator to ensure that no overloading occurs.
http://www.articleco.com/Article/Valve-automation-with-an-Electric-Actuator/46106
