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  1. #1
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    High and low pressure switch in air-cond


    Hi Everyone;

    I quite new to this, i was wondering how the pressure switch work in air-cond?
    Is the pressure switch work like fuse when it sense the limit of low or high pressure in a an air-cond. It why high and low pressure is connected this way?1.JPG



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    Re: High and low pressure switch in air-cond

    Hi and welcome to the forum.

    Yes, your assumption is correct, in the event of either switch operating the compressor will be stopped to prevent possible damage to the machine.

    The low pressure will monitor the system for low pressure faults, such as refrigerant leakage or perhap a blockage. If the pressure gets too low then the compressor may overheat due to lack of suction gas cooling.

    The high pressure does a similar job. The pressure could rise to a too high level due to the condenser being dirty, or the condenser fan failing or even problems with the refrigerant mix if someone has been careless when working on the system. Also if the pressure stays too high the oil may be overheated when it will begin the break down and contaminate the refrigerant. This can lead to motor burn outs.
    Brian - Torquay, Devon, UK
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    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: High and low pressure switch in air-cond

    Hi, and congratulations on providing information on what you're asking!

    Nice to see someone asking for help that is helping everyone to help them for a change.

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    Re: High and low pressure switch in air-cond

    Thank for the detail. So can i connect the low and high pressure in series way to compressor? When compressor is "on" the pressure switch also "on"? Another question, does the compressor keep toggle between "on" and "off" while the toggle time is set by timer?

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    Re: High and low pressure switch in air-cond

    the pressure switch is wired in series with the anti cycle timer-when the timer is asking for the compressor to run the pressure switch will power the contactor coil(low voltage circuit)then the compressor runs-any problems with the fridge circuit and the contactor will de energise and the compressor will stop
    Quote Originally Posted by huntersouls View Post
    Thank for the detail. So can i connect the low and high pressure in series way to compressor? When compressor is "on" the pressure switch also "on"? Another question, does the compressor keep toggle between "on" and "off" while the toggle time is set by timer?

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    Re: High and low pressure switch in air-cond

    Thx install monkey, i got another problem how is the compressor in an air cond work? As i mention from my previous post , does the compressor keep toggle between "on" and "off" using anti cycle timer in a period of time which set by the timer?

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    Re: High and low pressure switch in air-cond

    Nope!
    Compressor operation is called by room temperature thermostat. Anti cycle timer is one of conditions which must be satisfied to allow compressor working called by thermostat.

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    Re: High and low pressure switch in air-cond

    Erm mean that compressor should not be always running and it only start run when thermostat called then it will run and stop run when it reach the limit time set on the anti cycle timer?
    Quote Originally Posted by nike123 View Post
    Nope!
    Compressor operation is called by room temperature thermostat. Anti cycle timer is one of conditions which must be satisfied to allow compressor working called by thermostat.

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    Re: High and low pressure switch in air-cond

    Not quite, the anti-recycle timer is often used to prevent the compressor starting too frequently.

    Every time a compressor starts the is heat added to the motor winding due to the starting load of the motor. After the compressor has started the motor is cooled by the incoming refrigerant vapour in the suction pipe; this cooling take a little while.

    If the compressor was stopped and started in quick succession then it is likelt that the motor will not get cooling properly and eventually overheat causing damage to the motor windings.

    Most modern air conditioning unit, for example, will delay the compressor start by 3 minutes. Larger compressors, in chillers etc, may well have a maximum allowance of six starts per hour.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: High and low pressure switch in air-cond

    Quote Originally Posted by Brian_UK View Post

    Most modern air conditioning unit, for example, will delay the compressor start by 3 minutes. Larger compressors, in chillers etc, may well have a maximum allowance of six starts per hour.
    And they could also have min running time to allow cooling of compressor winding by suction gas even if thermostat is calling for switching off of compressor.

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    Re: High and low pressure switch in air-cond

    Ok as you have explain, i dont quite understand how the compressor start running cause what i have see on the diagram it look like the timer is controlling the compressor to start and to off. For example i set my timer 5 minute and let the compressor run , does it mean that it will run 5 minute and then off it 5minute and then run back in 5minute?


    Quote Originally Posted by Brian_UK View Post
    Not quite, the anti-recycle timer is often used to prevent the compressor starting too frequently.

    Every time a compressor starts the is heat added to the motor winding due to the starting load of the motor. After the compressor has started the motor is cooled by the incoming refrigerant vapour in the suction pipe; this cooling take a little while.

    If the compressor was stopped and started in quick succession then it is likelt that the motor will not get cooling properly and eventually overheat causing damage to the motor windings.

    Most modern air conditioning unit, for example, will delay the compressor start by 3 minutes. Larger compressors, in chillers etc, may well have a maximum allowance of six starts per hour.

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    Re: High and low pressure switch in air-cond

    Quote Originally Posted by huntersouls View Post
    Ok as you have explain, i dont quite understand how the compressor start running cause what i have see on the diagram it look like the timer is controlling the compressor to start and to off. For example i set my timer 5 minute and let the compressor run , does it mean that it will run 5 minute and then off it 5minute and then run back in 5minute?
    No.

    When the thermostat switches the circuit ON via terminal Y the timer starts running for 5 minutes. After 5 minutes the compressor will be switched ON. The compressor will continue to run until the thermostat switches OFF.
    Last edited by Brian_UK; 08-01-2012 at 11:46 PM. Reason: Wrong terminal number used.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: High and low pressure switch in air-cond

    Quote Originally Posted by Brian_UK View Post
    No.

    When the thermostat switches the circuit ON via terminal C the timer starts running for 5 minutes. After 5 minutes the compressor will be switched ON. The compressor will continue to run until the thermostat switches OFF.
    Then the short cycle timer locks out the control circuit should there be any attempt by the thermostat to start the compressor again within 5 minutes. After 5 minutes, timer then closes the control circuit and compressor is free to start when required, so long as pressure switches are also closed.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: High and low pressure switch in air-cond

    Thanks Mike, good addition to cover my ommission.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

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    Re: High and low pressure switch in air-cond

    Ok,let summary back again. It mean that the anti short cycle timer is to delay the compressor start when the thermostat is on. The compressor will continue on till the thermostat is off and in the same time it also block compressor on back within 5 minute when thermostat is on? I hope i get it right

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    Re: High and low pressure switch in air-cond

    How do i block the attempt by the thermostat to start the compressor again, cause in the diagram if the thermostat is off and on back it will attempt start the compressor within 5 minute.


    Quote Originally Posted by mikeref View Post
    Then the short cycle timer locks out the control circuit should there be any attempt by the thermostat to start the compressor again within 5 minutes. After 5 minutes, timer then closes the control circuit and compressor is free to start when required, so long as pressure switches are also closed.

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    Re: High and low pressure switch in air-cond

    Thx guys i solve it already. I need some advice on phase protector.phase proctetor.JPG
    This the phase protector i using. I need it to be connect in this way.
    example.JPG

    The phase protector have 6 pin (RST and ABC) , i not quite sure how it work. Can anyone explain it to me?

  18. #18
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    Re: High and low pressure switch in air-cond

    OK, so the thermostat and anti-restart problem is resolved.

    The phase protector works by sensing whether the three phases are connected correctly, i.e. U,V,W.

    If any two phases are swapped then the protector will keep the control circuit open.

    The reason for the reverse phase protection is to protect rotation sensitive machines from running the wrong way.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

  19. #19
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    Re: High and low pressure switch in air-cond

    Looks like R S T go to the phases and A B C are for the control circuit to turn the compressor contactor off in the event of a fault with the supply.
    Not sure if A B C are live in and out and a alarm contact or live in and out and a neutral like the kirwan phase protectors as your diagram is not clear.
    Just trace the wires on the diagram and see where they go. Or phone the manufacturers tech dept.
    Mostly found in Oxfordshire, UK :)

  20. #20
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    Re: High and low pressure switch in air-cond

    its a common ,no,nc contact-stick 415v on and test continuity on a to b or c to work out ur control circuit,also disconnect the 415v to prove the contact breaks also-carefully and safely

  21. #21
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    Re: High and low pressure switch in air-cond

    Does it mean i need give external power to it on the common pin?


    Quote Originally Posted by install monkey View Post
    its a common ,no,nc contact-stick 415v on and test continuity on a to b or c to work out ur control circuit,also disconnect the 415v to prove the contact breaks also-carefully and safely

  22. #22
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    Re: High and low pressure switch in air-cond

    Yes, power supply to pin C, switch contacts B & C
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

  23. #23
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    Re: High and low pressure switch in air-cond

    your low voltage control circuit will loop through the normally closed contact(when 415v is present to the relay
    Quote Originally Posted by huntersouls View Post
    Does it mean i need give external power to it on the common pin?

  24. #24
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    Re: High and low pressure switch in air-cond

    if you wiring phase rotation relay as per wiring diagram the only power going to relay is the 3 phases, the nuetral is being switched,saying that if phase relay is open then you will have control power to relay but nuetral when closed.

  25. #25
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    Re: High and low pressure switch in air-cond

    switching neutrals is frowned upon in the uk,unless its an ice machine like the phillips ignis k20
    Quote Originally Posted by jason09 View Post
    if you wiring phase rotation relay as per wiring diagram the only power going to relay is the 3 phases, the nuetral is being switched,saying that if phase relay is open then you will have control power to relay but nuetral when closed.

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