Compressor Locked Rotor Diagnostic

Each year, the compressor manufacturer’s receive numerous field returned compressors that apparently have failed. A significant number of these, believe it or not, are still in good working order.  A high percentage of these good compressors are single phase compressors condemned for locked rotor.

Single phase compressors have 3 windings, COMMON, START, AND RUN. All of these play an important role is getting the compressor to start and pump.  Aside from possible refrigerant issues like stuck closed TXV’s or other problems, it is important to understand how to properly check out a compressor that doesn’t seem to want to “get up and go!”

Many times these compressors are returned because the technician measured locked rotor current (amps) through the compressor COMMON lead and condemns the compressor based on that measurement alone. There is another current measurement that should be made.

I was always taught that the current (amps) be measured in the START AND RUN leads and not just the COMMON lead. Almost all single phase compressors have permanent capacitor motors inside the can. This means the compressor will not start without a run capacitor (A) in series with the start winding (B).  (Refer to figure 1 below to follow the description).  The start lead current (amps) measurement, give a good indication of the condition of the capacitor.  Compressor winding check

With the contactor (C) engaged, there should be some amount of current flow through the start winding (B) of the compressor.  If there is a START KIT (D) in the circuit, the current will be momentarily high.  As the compressor comes up to speed, the START RELAY (CSR-1) CONTACTS will remove the start capacitor from the circuit. The start leg current should then drop to a lesser value.  If instead, the current drops to zero amps, we can suspect an open  run capacitor (A). 

If there is no start kit in the circuit, the current surge through the start winding will be considerably less and will remain at a low value while running. Without a start kit, the system pressures need to be equalized or close to it.  If the compressor is cycling against a high pressure difference, the start kit may be required. `

If there is current flow through the RUN winding, but none through the Start winding, either the motor start winding is open or there is a problem with the starting circuit or wiring or connections. Check for an open start winding, open capacitor, weak capacitor (see posts on compressor or capacitor diagnostics). Remember, on most single phase compressors, the internal overload is in the common winding.  An open between common and start or common and run is an indication of an open overload.  An open reading between run and start is a bad compressor.

The last measurement which should be made is a voltage check. First, check the voltage without a load to see that it is within the range specified on the units nameplate.  Then, close the contactor and measure the voltage on the LOAD side. The voltage under load should not drop more than 10%. If it does, check for loose or corroded or dirty electrical connections.  Also check for pitted or dirty contact on the contactor.

Compressor winding check 2

If all the checks mentioned in the post and in my other posts prove OK and the compressor is still drawing LOCKED ROTOR AMPS — now you can replace the compressor.

Last note — keep in mind that if the compressor has not failed electrically, it is having a mechanical problem.  The most common cause for compressors seizing is liquid refrigerant in the compressor. If you replace the compressor and don’t correct this problem and make sure there is no liquid in the compressor, you will be replacing another compressor down the road.




About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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11 Responses to Compressor Locked Rotor Diagnostic

  1. abcd says:

    good job. the more you spread knowledge, more you gain

  2. Chip McClain says:

    I love your blog. Every time I read it. I learn something… Awesome. Keep up the great job…

  3. Janu Thomas says:

    Very interesting and helpful,Thanks a lot

  4. I have a question – i had a contractor tell me my Locked Rotar Amps are running in the 150s. The Max LRA listed on my machine is 148. I was told my machine is working harder. My evaporator coil is leaking and needs to be replaced.
    It is a 2005 unit, and I am trying to decide whether to replace the whole thing.

    • First of all, all compressors will momentarily draw locked rotor amps when starting. This is normal as long as once it is running the amp draw is at or below RLA (rated load amps). So there may not be an issue with the compressor.
      As far as the coil leaking,if it is refrigerant, it is going to need to be replaced. This can be done separate from the whole unit. Being a 2005 unit it is probably an R-22 refrigerant system. Again, if the compressor is not bad, the decision will be up to you. Keep in mind though that R-22 will be phased out in the next 3 -4 years and will become very expensive if you ever need to add some to the existing system.
      By replacing the whole system at this time, you should see some operating cost savings since Energy Standards for minimum efficiency are higher now than they were in 2005.

  5. Nick says:

    I have a similar question, back in march I started getting a locked rotor error. The AC guy swapped the controller Board in the outside unit and said it was the wrong one. He installed a hard start unit as well. He never touched the Start / Run cap as I had changed that and it was new (for exactly the same that was in there). It’s now august and the unit stopped again. The board was showing an 04 code which was for a locked rotor. The AC guy came back, said the Start/Run cap was the wrong one and changed it and fiddled with the controller and told my wife it was probably because we didn’t have a communicating controller. How can that be? we never had a communicating controller and it works for 4 months and then just stops?
    It stopped again after he left so I turned it off for a while and then it started again.
    Armana Outside Unit…..very confused here

    • I am not familiar with the Amana a diagnostic control in your unit. If the flash coe 04 is for locked rotor, then your service tech should be looking to see why you are getting locked rotor. Flash codes are just a “diagnostic” to lead someone in a certain direction. From there, they need to find out why it is happening and what may be causing it. Besides capacitors and start kits, you could have a bad contactor, loose wire connections, low voltage, etc. There are a lot of reasons beyond capacitors that could cause problems. Your tech needs to perform a thorough diagnostic to see why you are getting the flash code.

  6. Nick says:

    OK, it just seemed odd to me that on 2 different Boards it gave the same code and somehow it’s because the wall controller is a non communicating type….and they are not cheap

  7. Nick says:

    Thanks for the help

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