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  1. #1

    3 phase power concerns

    I have just started an HVAC business in Columbia, and while I have only been a tech for 4 years, I have someone working with me who has been in the business for 25+ years. He has worked on a little bit of everything over those 25 years. We encountered a problem with a commercial building downtown. There is 3 phase power run to the building. All 3 legs of this 3 phase are run to the disconnects for the air handlers and the condensers, and yet the units themselves are single phase. My associate who has been in the field for 25 years says he has never seen anything wired this way. It is 3 phase wye, with one leg simply capped off at the disconnect. The owner says he has had nothing but problems with the units for years, and that he has replaced more compressors in this one building than in all of the rest that he owns combined. Again these are single phase, 208v, 60hz, compressor motors. I have been working for him for about one year and have found problem after problem that the last guy left behind. These compressors are popping like crazy, just after their 18 month warranty is up, and I don't have any good explanation for why they are doing this. We found fuses laying everywhere on the roof with the disconnects, and I guess they got tired of replacing fuses so non-fused disconnects were installed.
    Again, it has always been my practice to never marry the two. And I also know that there is a difference between single phase motors, and three phase motors. But is this an acceptable practice, or are these compressors dying because of the way they are wired?

  2. #2
    Join Date
    Jul 2000
    It is very common to pull 120 volt single phase or even 208 volt single phase off of a 208 3 phase panel. I would be willing to bet you have low voltage problems. 208 volt 3 phase can dip to 195 volts and still start and run. Below 200 volts single phase will cause you problems most of the time. If the panel loads are not balanced very well you could have some significant voltage dips when a lot of units are running.

    Some posible solutions are buck/ boost transformers to pick the voltage up to 240 for the units.

  3. #3
    that was our first thought but we couldn't find an engineer to explain to us exactly why the voltage would be dropping.

  4. #4
    Join Date
    Sep 2010
    ( My disclaimer, I may or may not make sense in this post ) I don't think that the configuration of the transformer would make a difference y or delta as long as the equipment is receiving the correct power, voltage and phase. It looks like they wired for three phase equipment but ended up installing single phase. I don't know if electrically there would be much of a difference in current draw between two pieces of equipment the same size but different phase requirement. The single phase equipment may require a circuit with higher ampacity and overloads with a higher rating. You could have an electrician verify that the the breaker, wire gauge and fuses are sized correctly for the electrical requirements of the units. If they were sized with the requirements of three phase equipment in mind this maybe the problem. But then again you would probably just pop fuses and trip breakers that would protect the units if this was the scenario. I would think that you would mention it if the equipment was in very poor shape as this could be a reason for compressors to be lost too. The type of compressor failures may hold a clue as to what is causing them to fail. IE.... Are they mostly electrical or mechanical failures? Are the systems cleaned properly after a burn? You could check for acid on a system with a replaced comp. to see if the comp. is again going to need replacement. Is it all the units losing comp's or just a few that have lost compressors repeatedly. If the majority of the systems are losing compressors I would look toward utility or building electric as the potential problem. If it is just a few units that are losing compressors repeatedly you may want to try to zero in on why that maybe. I am extremely tired right now, sorry if I repeated myself too much.
    Where is the manual? What does it say?

  5. #5
    Join Date
    May 2009
    Bardstown, KY
    They may have loaded the power supply heavey on two legs. Meaning if they ran 3-phase to everything and only used single phase, they may have loaded phase A-B (or A-C or B-C) more and that would cause a voltage drop in thos 2 legs potentially. Worked on a factory once that had added subpanels over the years all over the place and they eventually had to pay to have them rewired to even out the load across the 3 phases feeding the facility.

    You may need to spread the load around across all 3 phases more evenly.

  6. #6
    Join Date
    Mar 2009
    central illinois
    you may want to get phase protecters put on units. will not solve the problem.but may eliminate one. protecters have memory in them
    work to live not live to work.

  7. #7
    Join Date
    Jul 2007
    New Jersey
    #1...local553 is 100% correct...

    get an ICM450 on that supply power asap.

    #2...heres a great write up I found..check it out.
    “If I have seen further than others, it is by standing upon the shoulders of giants.” ~ Sir Isaac Newton

    I'm learning all the time, the tombstone will be my diploma!!!

    Did you ever stop to think, and forget to start again?

  8. #8
    Join Date
    Sep 2008
    Cedar Rapids, IA
    Quote Originally Posted by handyandyshvac View Post
    We encountered a problem with a commercial building downtown. There is 3 phase power run to the building. All 3 legs of this 3 phase are run to the disconnects for the air handlers and the condensers, and yet the units themselves are single phase. My associate who has been in the field for 25 years says he has never seen anything wired this way. It is 3 phase wye, with one leg simply capped off at the disconnect.
    Your single phase units are optimized for a 230 volt supply. They will work on the 208 volt 3 phase wye system, but you are at the bottom of their voltage range. From your description I am going to assume that these units are in a large building and may be far from the electrical service entrance.

    Contributing factors.

    Power supplied can be 10% low at times at the service entrance.

    Length of wire, size of wire, and unequal loading on the 3 phase legs can cause additional voltage drops.

    I would put low voltage monitors that shut down on voltages under 195 or so on each compressor. Also stagger the delay on start so they cannot all start at once.

    Put hard start kits on all of the compressors. They really need the kick when they are starting at low voltages.

  9. #9
    Join Date
    Dec 2010
    Port Orchard, Wa.
    Since the compressors are single phase, they don't have as much torque on startup as a three phase compressor will. As a result of the low voltage (bottom of acceptable range), the smaller wires (disconnects set up for three phase, but single phase equipment installed - likely the feeders to the disconnects are a size smaller than they should be for single phase), distance from the building switchgear and other factors, the voltage is dipping further during compressor startup (inrush). As a result, the compressors stall on startup someof the time. The compressor then sits there overheating until the internal thermal O/L pops. Once the compressor cools down enough, the internal O/L closes and the compressor again attempts to start. Sometimes it does, sometimes it doesn't. Doesn't take a whole lot of these cycles to burn a winding in the compressor. It was also likely taking out a fuse along the way - which is why the disconnects were changed out.
    You should install a hard start kit in the units. This will give the single phase compressors enough torque to start every time. They are pretty cheap too. You should keep a couple of sets of hard start kits on your truck. If you work ruraly, this is pretty common as well.
    You should check the feeder wire sizing and make sure that it is not undersized. You can look up the required feeder size in the table of the NEC.
    If you end up changing out an entire condensing unit, replace it with a three phase unit.
    Since there are a bunch of burnouts, you likely have an acid issue in all of the units. Check for acid and use a liberal dose of acid away. Using type KK liquid line drier and installing a suction line drier may also help. Be aware that if you run a KK, you will need to come back and change it out in a couple of months to a regular drier. A replaceable core drier with isolations and an evacuation port will help in this.

  10. #10
    Join Date
    Feb 2004
    New Mexico
    Repeat failures means the problem wasn't discovered. The extra wire doesn't matter as single phase is run from 3 phase panels all the time.
    Electrical supply imbalance is a good place to start.
    Pull the panel covers and look for any sign of high temp at terminals, wire size, correct breaker. If the breakers are sized ok there is no reason to fuse the disconnect.
    Because the owner has replaced a bunch of compressors, look for something all the units share. Probably elec supply problem. By all the blown fuses it looks like the equipment has starting problems. Wire size, length of run, power company problem.
    "What Fools these mortals be"....Puck

  11. #11
    Join Date
    Nov 2010
    Andover Kansas
    Typically around mid day in large business areas when the electical demand is high many businesses suffer voltage drops. A drop in voltage means the amp draw will go up and if it goes above what the RLA or FLA is on the compressor then they will not last long running this way. Often on 3 phase power there is one leg that is commonly referred to as a "Wild leg" and will show a higher voltage to ground then the other two.

    If it were me I would go out around noon and while all units were running check with my volt meter at each unit voltage across the two legs of power coming into the unit and check each leg by itself to ground and write these volages down, then check amp draw on the compressors and the entire unit and see what the RLA or FLA is on the data plate for those compressors.

    If you do this, I would be curious to know what your readings are.

    The statement below is my signature and just my overall feeling towards our industry and does not necessarily pertain to you nor this thread.

    There really isn't a legitimate excuse for not doing the job correctly!

  12. #12
    Join Date
    Feb 2010
    You are losing compressors and replacing with the same?
    Heres a stupid question. Why not replace the compressors with 3 phase and modify the power and control as needed?

    Maybe I missed something.

  13. #13
    Join Date
    Aug 2006
    Line backers and a 48hr voltage monitor and I think you will find your problem(s).

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