Unit 39 Controls

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Unit 39 - Controls
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39.1. Controls for Air Conditioning

• a. Must correctly control the system components
• 1. Indoor fan
• 2. Outdoor fan
• 3. Compressor
• b. Indoor fan must be on when compressor operates
• c. Outdoor fan must be on when compressor
  operates (except for fan cycling head pressure
  control)
• d. Indoor fan may operate continuously
• e. Operating controls and safety control
• 1. Thermostat - operating control
• 2.High-pressure switch - safety control

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39.2 Prime Movers - Compressors and Fans

• a. Consume the most power
• b. Operated at high voltage (usually 230V)
• c. Control circuits are usually 24 v

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39.3 Low-Voltage Controls

• a. Low voltage provided by a transformer (usually
  40 VA)
• b. Maximum amperage for a 24 volt circuit is 1.7
  amps
• c. 40 VA / 24 V 1.7 A
• d. Controls the operation of devices that, in
  turn, control the system compressor and fans
• e. Example when a low voltage contactor coil is
  energized, contacts close, energizing the system
  compressor

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39.4 Some History of Residual Central Air
Conditioning

• a. Became popular in the late 1950s
• b. Pricing became competitive
• c. Central air conditioning is desirable in all
  new construction
• 1. Selling feature
• 2. Resale value
• d. Original residential systems were water-cooled

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39.5 Economics of Equipment Design

• a. First air cooled systems
• 1. Heavy duty and bulky
• 2. Hard to handle
• 3. Slow compressors (1800 rpm)
• 4. Belt driven open compressors
• 5. R-12 systems
• b. Newer systems
• 1. R-22
• 2. Copper and aluminum coils
• 3. More compact
• 4. Faster compressors (3600 rpm)
• 5. More efficient

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39.6 Operating Controls For Older Air-Cooled
Systems

• a. Thermostat
• 1. Controls space temperature
• 2. Power-passing device
• d. Fan relay
• 1. Starts and stops the indoor fan
• 2. Power-consuming device
• c. Compressor contactor
• 1. Starts and stops the compressor and outdoor
  fan
• 2. Power-consuming device
• 3. When the coil is energized, its contacts close
  to energize the compressor and outdoor fan
• d. Compressor starting and running circuits
• 1. Capacitor start, capacitor run motors
• 2. High starting torque

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39.7 Safety Controls for Older Air-Cooled Systems

• a. High pressure control
• 1. Stops the compressor
• 2. Senses excessive high-pressure
• b. Low pressure control
• 1. Stops the compressor
• 2. Prevents system from operating in a vacuum
• 3. Senses a low-charge situation
• c. Compressor overload protection
• 1. Protected the common and run circuit
• 2. Heat sensitive bi-metal device
• d. Internal motor protection
• 1. Senses motor temperature
• 2. Can open the power or control circuit
• 3. Located inside the compressor
• e. Short-cycle protection
• 1. Prevents the compressor for re-starting soon
  after shutting down
• 2. Gives system pressures a chance to equalize

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39.8 Operating Controls for Modern Equipment

• a. Room Thermostat
• 1. Physically smaller
• 2. Can be electronic with thermistor sensors
• 3. Can be programmable
• b. Fan relays are smaller than older ones
• c. Compressor contactor
• 1. May have one, two, three or more sets of
  contacts
• 2. Some feed power to the compressor in the off
  cycle to keep the crankcase warm
• d. Starting circuit
• 1. Fewer components
• 2. Run capacitor
• 3. PTC

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39.9 Safety Controls for Modern Equipment

• a. Motor temperature control
• 1. Shuts compressor off during the following
  situations
• 1. Low refrigerant charge
• 2. Dirty condenser
• 3. Over current situations
• 4. Short cycling
• 2. Pressure relief valves
• b. Loss of charge protection
• 1. Low pressure cut-out
• 2. Located in the liquid line
• 3. Senses a complete loss of charge
• 4. Will not prevent atmosphere from entering
  through the low side

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39.10 The Working Control Package

• a. Should provide simple and effective system
  control
• b. Technicians should be familiar with all
  controls
• c. Some manufacturers use solid state boards

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39.11 Electronic Controls and Air Conditioning
Equipment

• a. Can monitor high voltage conditions
• 1. Higher voltages will cause lower currents to
  flow
• 2. Overloads will not trip on low current
• b. Can cut out the compressor during low voltage
  conditions
• c. Check manufacturer's control manual when
  checking solid state circuit boards
• d. Some solid state boards have a self-diagnostic
  feature