Air Conditioning

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Air Conditioning

• Starting Relays

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Starting Relays

• Purpose
• Remove the start winding and/or start capacitor
  from the circuit
• Compressor start relays are mounted outside the
  motor
• Centrifugal switch could not be used in hermetic
  motor

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Types of Relays

• These are the three most common
• The HP of the motor will determine what type will
  be used
• Service man may replace not
  re-design.

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• Three types
• Current Coil Relay
• Solid State Relay or PTC
• Potential Relay

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Current Coil Relay

• Works off amperage to the run winding.
• On compressor start up
• Motor draws high current
• Induces a strong magnetic field in the relays
  magnetic coil through the run winding
• This magnetic field lifts a plunger with
• contacts and a counter tension spring
• Contacts to start winding are made
• Start winding is energized

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• Motor now starts because start winding
• creates a strong magnetic field
• Motor reaches 75 running capability
• Current drops down
• Relay coil is in series with run winding
• Not enough current remains to keep coil
  magnetized
• Spring pushes plunger down
• START WINDING IS DE-ENERGIZED

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• Used mainly on fractional horsepower compressors
  for small refrigeration units, not A/C

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Solid State Relay

• Also works off current
• Has materials that changes its resistance
• PTC-Positive Temperature Coefficient
• Temp goes up/ resistance goes up
• Wired in series with start winding only

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• Uses up voltage (loads in series)
• Example
• Voltage drop across relay would be 110v
• Voltage drop across start winding 10v
• REMEMBER series law (EtE1E2)

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• Solid State relays have a greater range
• Example 1 SS relay may be used for
• 1/16 to 3/4 Hp motor
• 3/4 to 2 HP motor
• If a current coil was used
  different relays would be needed.

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Types of Solid State relays

• Two wire is relay only
• Five wire is start capacitor, relay and overload
• Testing
• When Ohmming them out, a good relay will have a
  (low) resistance reading
• Must cool down before testing

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Potential Relay

• Operates from voltage created by start winding
• Also known as
• Back Electromotive Force, or Back EMF
• Operation
• Contacts are N.C. and allow full voltage
  to start winding

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• Terminals that are used and how to wire potential
  relay (this is the ONLY way)
• Line to start capacitor
• Start capacitor to TERM 1
• TERM 2 to start winding
• TERM 5 to common terminal
• All other terminals are blanks

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Checking the Potential relay with Ohmmeter

• Term 1 to Term 2
• Continuity
• Term 1 to Term 5
• Ohm Reading
• Ex. 2k ohms
• The same for Term 2 to Term 5

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