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.”


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    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.
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    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.

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  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. ahmed says:

    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 ?

  6. tcfkat says:

    @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.

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