Book - School of Science and Technology

628 Automatic controls and building management systemsassociated flow rates would be sized at the peak design load, but would operate for mostof the time at some partial load. The control device, therefore, has to provide stablecontrol over the full range of operating conditions.The movement of a valve or damper is determined by an actuator, which is thecomponent that responds to the signal from the controller. The actuator characteristicswhich are of importance are torque (the ability to cause movement of the control device)and stroke period which is the period of movement between the limiting positions (open toclosed and vice versa). Selection of the actuator type will depend upon the choice ofcontrol system.Electric motorsFor control mechanisms, these range between 5±50 VA and operate via reduction gearingto give a high torque/low speed characteristic: they normally operate using a single phasesupply at 240 V or lower (normally 24 V) to suit the system. Two-position motors are ofthe unidirectional spring-return, or unidirectional three-wire signal type, and are usedwhen the speed of movement produced by a solenoid is too fast.Modulating control requires a reversible motor that can be held at any positionthrough the movement: either reversible induction or shaded-pole motors may be used.A typical arrangement driving a control valve is illustrated in Figure 22.7. The motor maybe provided with cam operated auxiliary switches to open or close at any set position ofthe movement. An auxiliary potentiometer may also be driven to provide a signal toanother control device.Some lower cost thermic actuators work on the basis of the expansion of an electricallyheated wax capsule which then moves the valve spindle. Their slow response is appropriatefor most building controls.For the operation of dampers, the power required may be considerable (as discussedearlier under `system types') and the motor must be chosen to suit. Sometimes the damperwill require to be sectionalised, each section being worked by one motor with linkage.Again, such damper motors may be on/off, i.e. open/closed, or they may modulate to givesettings in any intermediate position.Position indicatorActuatorMotor drive shaftActuator crankMountingbracketLinkageSpindleFixing screwsFigure 22.7 Electrical control valve