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Electric control. Presented by. Andrew Mitch Sav Little Phil Justin. Safety requirements. Correct electrical testing Correct electrical isolation procedures as a lot of the control gear is 240 V
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Electric control Presented by Andrew Mitch Sav Little Phil Justin
Safety requirements • Correct electrical testing • Correct electrical isolation procedures as a lot of the control gear is 240 V • Care needs to be taken if the system is 24 v as circuits are easy to damage and costly to repair • Avoid contact with elements with in the system as they may have harmful contents being controlled
Principles • Uses signals sent or switch between electromechanical devices to control valves dampers pumps just to name a few • There are many applications where electric control can be used • Can easily be integrated in to pneumatic or electronic control • Is made up of three main components • First the disturbance sensing device • Second the controller • Thirdly the final control element
Disturbance sensing device (sensor) • Device that is with in the occupied space which senses various elements of the space providing information to a controller • Is the eyes of the system • There are many different types of DSDs dependant on the application desired
Disturbance sensing devices • Float switch • The float mechanism works by either by making or a contact or by driving a valve arm to open or close a valve needle and seat • The most common form of float mechanism is in your toilet systern
Flexible diaphragm Is a flexible material mounted within a housing that as the pressure changes the material flex’s driving an attached arm either backwards or forwards • Pressure Bell • Sensitive to small changes in pressure Consists of a bell shaped cap floating on oil that coves a stem with a hollow in the middle that allows pressure to build up under the bell causing it to move • Often used in pairs in a scale shape arrangement
Bimetal element Two dissimilar metals are bonded together and because of their two different coefficients of expansion they bend or straiten on change of temperature most common use of bimetal strips is in thermostats • Rod and tube type element A high expansion metal tube with a low expansion metal rod inside and bonded at one end when the temperature around tube changes the high expansion metal changes length Often used in insertion and immersion temp controls
bellows There are three types of bellows Fluid inside fluid outside and sealed • Fluid inside bellows The fluid inside bellows have a linkage fixed on one end and the other end is connected to a fluid source as the fluid source changes pressure the bellows move • Fluid outside bellows The fluid outside are mounted within a housing which gets filled with fluid and as the pressure changes in the fluid it pushes on the bellows causing them to move the linkage is connected to the inside of the bellows
Sealed bellows Theses are liquid or gas filled bellows much like the liquid inside bellows except that the sensing substance is sealed in and uses a remote bulb to activate the bellows • Remote bulb Is a bulb connected to sealed bellows or a diaphragm by a capillary tube which when the bulb chances temp it changes the pressure of the liquid or vapour within the bulb and capillary causing drive device to move
Fast response bulb A fast response bulb is a capillary tube wound in the shape of a bulb due to the greater surface area of the coils the bulb I able to respond up to 7 times faster than a normal remote bulb • Averaging element It is a cross between a remote bulb and fast response bulb it has the capillary the same as a fast response only the bore is larger giving it the same volume as a normal remote bulb
The averaging element is distributed across the cross section of the duct evenly for a more reliable average of duct temperature • Nylon ribbon / human hair Nylon and human hair absorb moisture from the air around Human hair or nylon is stretched between two points on a pivot which as the hair or nylon absorbs moisture from the air gets longer it effects the position of the pivot arm
Wooden element • One end of a piece of timber is anchored so it can move and the other is free to move the free end is connected to a linkage that as the timber absorbs moisture the linkage is move as the timber expands this type of sensor is very slow acting • Seldom used these days
Mercury switches • Mercury switches are a glass tube filled with mercury fitted to a rotary plate driven by linkages the class tube also has contact pins fitted and the tube rotates on the plate the mercury moves making or breaking the contact pins
Controller • The controller receives information from the disturbance sensing device then decides what to do with the information once it makes a decision it then switches the final controller to do a give task • The controller is the brains of the system
Forms of control • On off control • On off control is as the name suggests the controller simply turns the point on or off as demand requires • Multi position on off control • Is where there is more than one on or off call per mode of control I.e. one for heat 3 for cool allowing to run more than one stage of cooling
Floating control • Similar to on off control except that the controller has two sets of contacts within it as temperature changes it can rotate to heat or cool dependant on what is required at the time this then can drive final control device open or closed dependant on what is required • Proportional control • Proportional control is similar to floating control except for instead of using a mercury switch it uses a potentiometer to control movement of the valve dependent on which side the current is flowing the greatest
Snap acting switches Has a snap acting arm that snaps from one contact to another contact when driven by a plunger • Mercury plunger relays Work similar to a solenoid valve it has a glass tube filled with mercury a insulated contact runs up the center and one in the base of the glass tube at the top of the center contact there is a ceramic cup also filled with mercury around this contact there is a ferrous tube. this is all surrounded by a electric coil that when energized drive the plunger down making contact between the two contacts causing a circuit to form
Electromagnetic switches • Are simple relays where one voltage is used to operate a coil opening or closing contacts which may be carrying a greater current or load • Potentiometers • Potentiometers are fine wire wrapped around a core with a wiper contact that slides along the wire wrapped around the core dependant on the position of the wiper the resistance changes within the core wire thus controlling the amount of current that flows through • These are used to control modulating valves
Final control • Final control refers to the final element that is actually connected to the valve linkage or damper linkage to actually drive the given item • It is the part of the system that does the work it’s the hands of the system
Actuators and linkages • There are two main forms of final control devices valves and dampers valves to control the flow of liquid and dampers to control the flow of air these are driven by actuators connected to drive motors • An actuator is the piece of equipment that starts, stops and varies the operation of the final control element the linkage is the flexible jointed parts that connect the actuator to the final control element
Valves • Valves control the flow of liquid or vapour by opening or closing a plunger the plunger is connected to a stem which is driven by an actuator • Valves come in different types 2 way or 3 way valves 2 way are a open close type • three way change the flow direction allowing parts of the system to be bypassed for example hot or cold water coils in AHUs
Dampers • Damper are blade shaped louvers that as they rotate can control the amount of air that flows across them they proportion the amount of air that flows across them by opening or closing to particular degrees of rotation the dampers are driven by actuators and motors
Communication between elements • The three elements communicate with each other via electrical wiring this allows the communication to occur instantly and the distance between parts can of great distances
Commissioning requirements • Check al connections are tight and secure • Check all connections are correct polarity • Check all components correctly installed and in the right place • Check correct labeling • Check wiring for signs of damage • manually give components signals to ensure all components working to design • Turn system to auto to ensure all components work together as per design • Check for leaks on all valves and dampers
Service requirements • Check al connections are tight and secure • Check all installations are still correctly installed • Check correct labeling • Check wiring for signs of damage • manually give components signals to ensure all components working to design • Turn system to auto to ensure all components work together as per design • Check for leaks on all valves and dampers • Lubricate all moving part that require lube
THAT’S ALL Further information can be found in the honeywell manual from the MRB
