Electric control

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Electric control

• Presented by

Andrew Mitch Sav Little Phil Justin
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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

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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

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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

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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

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• Flexible diaphragm
• Is a flexible material mounted within a housing
  that as the pressure changes the material flexs
  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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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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

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• 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

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• 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

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• 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

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• 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

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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
  its the hands of the system

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• 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

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• 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

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• 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

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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

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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

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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

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THATS ALL

• Further information can be found in the honeywell
  manual from the MRB