Need help understanding wiring diagram

Discussion in 'General Electronics Chat' started by kevin.khan, Feb 9, 2015.

  1. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
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    0
    Hi there,

    I was given the task to understand and explain the circuit depicted in the attached document (p. 16 which depicts the wiring diagram). The schematic is the wiring diagram for a freezer and a temperature controller for the freezer. However there are several items in the schematic that I would like some help trying to understand. For starters can someone explain what is the Contactor Box depicted in the wiring diagram (what is this, my guess was that it might be a transformer or relay).

    Please let me know if more information is required.
     
    Last edited: Feb 9, 2015
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    784
    They usually use an induction motor with separate start and run windings.

    In the stalled condition the run winding draws high current, its common practice to feed this current via a solenoid coil - that high current pulls in a contactor and connects the start winding, once the motor gets going the run winding current lowers and the contactor drops out.

    Some cheap & nasty (LEC) domestic fridges used a PTC thermistor in series with the start winding, the thermistor is heated by the current through it and rises to a high resistance - not the most reliable, motor burnouts were fairly common.

    AFAIK LEC abandoned that method and took to using Danfoss compressors.
     
  3. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
    23
    0
    Wow. Thanks so much for all this information.

    I don't have any specific part numbers for the contactor or the compressor, but based on the wiring diagram that I provided would you be able to point out which terminals on the contactor box is used for the start and run winding.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Not from the drawing provided - you need to closely examine the appliance and sketch out where the wires go, and what they do when they get there.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    a contactor is the industrial term for a high current motor starting relay. usually has the current limitting built on it too.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,564
    2,379
    The OEM part number for the contactor is shown in the list.
    The compressor motor and fan is fed directly off the bottom end of the contactor, just two conductors to both.
    The compressor takes either 6.4 or 4.6 amps dependant on model.
    Due to the high cycle times of a refrigerator unit the contactor is often what is known as 'Special Purpose' and has extra feature such as arc shields and special contacts etc.
    Max.
     
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