Single phase compressor for air-condition

In air-condition system,compressor is a main component to pump the refrigerant circulation in system.It like our heart to pump a blood all around human system.If this part breakdown or have a problem,all system is paralyze.

Normally for single phase air-condition system,the common type of electric motor is running capacitor motor and used scroll type compressor.The famous brand now in market such as Copeland,Daikin,Toshiba,Mitsubishi and others brands.

I want explain about single phase motor for air-condition or refrigerator compressor basic concept and how to check their winding condition and connection.

Wiring for single phase motor

1) Run capacitor


2) Start and run capacitor


How to check motor compressor connection?

For single phase compressor,normally it have 3 connection for winding.It is common ( C ) ,Starting winding ( S ) and Running winding ( R ).We can determine this 3 connection using a multimeter.We measure the ohm value of 3 connection to confirm the winding.

  • Measure from all of 3 connection and write down the ohm value,example :-

point 1 to point 2 = 3 ohm

point 2 to point 3 = 8 ohm

point 3 to point 1 = 6 ohm


  • Draw the triangle line and insert the ohm value,example :-


  • From this triangle,we can determine the connection of motor winding either common ( C ) , Starting ( S ) or Running ( R ).The formula is simple.

1) Common connection ( C ),ohm value is a highest.From this example it is 8 ohm.

2) Starting connection ( S ),ohm value is a second higher,it is 6 ohm

3) Running connection ( R ),ohm value is a lowest,it is 3 ohm.


How to check single phase motor compressor winding condition?

The methods to check motor condition is easy and simple.We must check the winding to winding short and  partially shorted winding.

1) winding to winding short

– Use a multimeter and select to ohm.Check all of 3 connection to earthing or compressor body( scratch out the paint from compressor body to get a continuity) .Write down the value from 3 of connection.

– For the good winding,the ohm value must infinity.It mean no connection between winding and earthing.

– For shorted winding,the ohm value is 0 ( zero ).it mean winding and earthing have a connection / shorted.


2) partially shorted winding

– Measure the ohm value for connection Common to Running and Common to Starting.

– Record the value, example :-

C to R = 0.5 ohm

C to S = 3 ohm

– The formula to determine a winding condition is good,the resistance of starting winding is equal 3 to 6 times of running winding

3 ohm / 0.5 ohm = 6 ( winding for this compressor is good )

– We must remember this formula : “The good winding ohm value must in range between equal 3 to 6 times.If value out off this range,we can assume the winding is shorted and defected.”


  1. air conditioner soft starter series including:
    1. Single-phase air-conditioner soft starter
    TL-ASSU220P3 applies to single-phase 1P/2P/3P air conditioner. It is used either for 50HZ or 60HZ.
    TL-ASSU220P5 applies to single-phase 4P/5P air conditioner. It is used either for 50HZ or 60HZ.
    TL-ASSU220P7 applies to single-phase 6P/7P air conditioner. It is used either for 50HZ or 60HZ.
    2. Three-phases air-conditioner soft starter
    TL-ASSU380 applies to those three-phases air-conditioner soft starter and electric motors which the power is less than 18.5KW. It is used either for 50HZ or 60HZ.

    The air-conditioning soft starter is an embedded air-conditioning controller, it regulates the start-up voltage and current of the compressor intelligently by utilizing advanced micro-control technology of SCM, and effectively reduces the instantaneous current and power of the compressor while starting-up, thus protects the compressor and prolongs its life. Simultaneously, it also depresses the shock while air-conditioning starting up in the power supply network.

    The soft starter is applied with embedded system technology. It has many advantages such as smaller size, lighter weight, an anti shock, longer life, higher level in reliability and stability, easier installation etc. Especially, it functions well in some harsh condition.

    The most important feature of our product is it can control the highest launching transient current of air conditioner to be about 2 times as great as its rated current,never exceed 3 times. It can effectively protect air conditioner compressor, extend its service time. Additionally, when it detects the compressor couldn’t start because of the imbalance of pressure,it will protect the compressor, then restart after 90 seconds.So then effectively prevents the compressor from overburning for failure start.

  2. kennard says

    If the sum of C to R (ohm)and C to S (ohm) is equal to S to R (ohm), then the compressor is good.

  3. commmon terminal can be taken from highest reading of resistance.ex.;term.1-2=18 ohms,2-3=12 ohms and 1-3=6 ohms
    take the highest reading as the base.term 1-2=18 ohms.terminal that is not indicated is the common terminal w/c means terminal 3 is the common. Then look for the second hignest and get the other terminal as the start and lastly the lowest is the run.

  4. hi sir
    i didnt understand what you mean by
    The formula to determine a winding condition is good,the resistance of starting winding is equal 3 to 6 times of running winding
    could you pleas explain than
    many thanks

  5. I have an American Standard heat pump . Once a week the unit will trip the circuit breaker . Single phase . The start capacitor and run capacitor are good as well as the rest of the controls . the breaker is new and properly sized and the connections are tight . The wire size is also correct and when the unit is running the amperage is well within specs . Can the compressor have a dead spot in the windings that will cause this ? I dont know what else to look for . Can you help ?

    • John Harmsworth says

      For a single phase unit that trips a breaker once a week, the probable cause is the unit short-cycling. This occurs when the unit satisfies the heating or cooling requirement and shuts off, then comes back on shortly afterward and before the discharge pressure has had a chance to equalize with the low side pressure. Some tims this occurs when the homeowner turns the stat up or down slightly shortly after the unit has shut down. Some units may require an anti-short cycle timer to reduce this occurence. If the unit works alright the rest of the time, it is highly unlikely there is anything wrong with the compressor

  6. @kennard: partial winding shorts cannot be detected by a DC resistance measurement. The sums will always be correct, regardless if part of a winding is shorted. What only gives a correct answer is an AC measurement.

    @John: maybe there is sometimes a short power outage, which stops the compressor. Because of the differential pressure present in the system immediately after a stopped engine the motor is unable to restart, causing the breaker to trip. Normally there is a so called Klixon (selfresetting thermal circuit breaker) to catch such cases. Or the compressor is started, run and restarted by a defective control circuit.

    PS pardon my bad english

  7. tunde bello says

    my chestfreezer is working but shocks when the scratched area is touched.whats the cause and solution.

  8. I know this website gives quality based posts and other data, is there any other website which provides such things in quality?

  9. I need chating with expert

  10. please kindly explain to me which component parts do I have to examine in other to solve problem of instantaneous power failure after installed a new compressor and it won’t start up to enable me charging it with refrigerant to it’s full load capacity. Unit make is Daikin air conditioner

  11. Aditya says

    What is the resistance between Compressor Terminal & Body of Compressor ?

    • This test must NOT be carried out with a Multi-meter.

      The Resistance should be 100 to 200 Meg ( Million) Ohms.
      Measured with an “Insulation Tester – Instrument ”
      The test Voltage coming from this Instrument should be at least 500 Volts DC.

  12. ive got vs24 adg reaading is 1-2 is 27.9 ohm…2-3 is 68.3 ohm 3-1 is 40.3 ohm…the resistance is to high from the normal…when i turn on i hear little click and turn off…i want to ask you guys if the compressor are damage?… or give some troubleshooting tips to repair my mini fridge..thank you

    • Klaus (Technician) says

      The click noise is generated from the Overload Bi-metal switching-operation.
      The Compressor motor stops, when the Overload activates.
      After a short while, the bi-metal within the overload will cool down.
      Then the overload resets, it switches the Compressor ON again,
      the Compressor hums and the Overload activates again.
      If there is a Capacitor connected to the Compressor-Motor, it could be a faulty capacitor.
      If no capacitor, the Compressor is faulty and un-serviceable.

  13. Netra Pandey says

    I have a compressor of air drier. it shows the reading 1-2: 2 ohm, 1-3: 4.5 ohm and 2-3: 6 Ohm. the problem is the compressor starts for a while then it stops resulting start capacitor overheat. also the 6 Amp circuit breaker at the supply also trip.
    What could be the solution for that?

  14. juantamad says

    this formula is very wrong

    from the start of triangle up to sum up.

    c-s =high ohms
    c-r=low ohms
    s-r=sum of high and low ohms

  15. Rasaq Salami says

    How can i connect the windings of a rotary ac compressor after rewinding, to run antilock wise

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