What is an Electric Actuator?
An electric actuator or to be more precise an electric valve actuator, is a machine that converts electrical energy into rotary torque which is used to drive a valve.
Typically an electrical actuator consists of an electric motor or motors, which produce low torque at high speed. Connected to a gearbox, the speed is slowed and as it does, the torque increases. At the electric actuator’s output drive, the final low speed produces high torque so that the required rotary torque is available at an acceptable speed.
Electrical circuits or electronic circuitry is used to control the starting and stopping of the motor when the valve is at the required position.
HOW DOES AN ELECTRIC ACTUATOR WORK?
There are two main methods of achieving an electric opening and closing of a valve, one relates to a quarter-turn valve only, the other to either a part-turn or multi-turn electric actuator.
The type of electric actuator that only works on a quarter-turn valve – and generally only with an electric ball valve – is a uni-directional electric valve actuator. A uni-directional electric actuator always rotates in the same direction with the motor being stopped at 90 degree intervals, typically by cams fixed to the output shaft striking electrical micro-switches which cut the power to the motor, and await an electrical command to run to the next position. This type of electrical actuator function reduces the cost of the electronic controls as the motor always rotates in the same direction. Despite this uni-directional electric actuators are not a popular design and are now relatively rare.
By far the most common type of electric actuator is the reversible electric valve actuator. This type of actuator drives a valve from open to close in one direction – usually clockwise, then reverses to the open position. The direction of rotation of the motor reverses to achieve this operation.
In on-off reversible electric valve actuators the motor runs and via the gearbox to which it is connected, rotates the output drive until either cams fixed to the output shaft strike electric micro-switches, or magnetic sensors detect the desired position and stop the motor. The motor awaits the next electrical command, at which point the motor runs in the opposite direction to reverse the output shaft, again until either cams or magnets stop it.
WHAT DIFFERENT TYPES OF ELECTRIC ACTUATOR ARE AVAILABLE?
Relating to valves, there are initially two main categories of electric actuators, those for domestic valves and those for industrial valves.
Domestic valve actuators do not require a housing that protects the actuator from ingress of water from the weather as they are installed inside homes or buildings and never exposed to weather. Their construction is correspondingly light and combined with very large unit volumes, domestic electric valve actuators are extremely low in cost compared with their industrial counterparts.
Electric valve actuators are installed in industrial applications are more robust in their construction and their housings are typically weatherproof or waterproof to varying degrees.
Within these two categories there are two main types of electric valve actuators, rotary part-turn, and rotary multi-turn. A third variant, the linear electric actuator is also made, but is not generally used in valve actuation.
Part-turn electric actuators are used in valves that have a control element that rotates around a shaft, typically ball valves, butterfly valves, plug valves and dampers, and the operation is commonly 0-90 degrees, or quarter-turn. Multi-turn electric actuators are used where the control element is driven by a shaft that rises and falls, (or the control element rises and falls) typically in gate, sluice, globe and diaphragm valves, and a the electric actuator may turn several times to achieve the desired rise and fall.
HOW DOES AN ELECTRIC ACTUATOR REVERSE?
An electric valve actuator reverses by driving the electric motor in the opposite direction. The motor is typically controlled by electronics that detect when the valve has reached its desired position from a signal generated inside the valve actuator either by a micro-switch triggered by a cam affixed to the electric actuator’s output shaft, or the shaft’s angular position detected by a magnet sensor. This received signal stops the motor where it then awaits the next command, on receipt of which the electronics run the motor in the opposite direction, thereby reversing the direction of rotation of the output shaft.
HOW IS AN ELECTRIC ACTUATOR CONTROLLED?
An electric valve actuator is controlled by electrical signals. There are a wide range of functions that can be achieved by an electric actuator and the applied electrical signals determine how the valve actuator reacts.
Most commonly, electrical actuators drive a valve fully open, or fully closed, and in most cases, this is achieved by setting a switching live electrical connection using an SPDT (single pole double throw) switch or relay to either the open electrical terminal, or the closed electrical terminal. An alternative method is to have either the open electrical terminal permanently connected, the using an SPST (single pole single throw) switch to connect to the closed electrical terminal to close the actuator. This latter system could alternatively have a permanent closed connection and switching the open terminal on or off.
Where proportional control is required, in which the electric actuator’s stopped position is controlled by and proportional to an input control signal, a permanent electrical power supply is maintained (as opposed to being switched in an on-off electric actuator) and the motor run or stopped by the change in the input signal as the actuator subsequently achieves the proportional movement required by the input signal.