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  1. #1
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    Charging Split, Using Amps



    I was always taught to use all possible avenues when charging a system. Being a little stronger electrically, I like to use R.L.A. on the compressor figuring the motor is operating efficiently. This does not agree with the superheat reccommended. Is it best to ignore this or just take it into consideration? My superheat is okay now but I'm not satisfied with the currrent on the compressor. It's running approx 3 or 4 amps under what it is rated for. Any comments on this would be greatly appreciated.



  2. #2
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    Hmmmmm.......

    The motor may be running efficiently, but A/C systems seldom run at their design conditions, so your current draw can be anything. If your superheat is variable, I deduce that you have a typical capillary feed system. With this type of metering device you will only get the optimum superheats at the design running conditions.

    The only fool-proof way to charge a system, in my opinion, is by weight. The manufacturer will always provide a base charge on the data-plate. If it is a split system it will be for the standard length of pipe. This may vary but in the ISO standard for split systems, if I recall, is 7.5 Metres, The manufacturer will give additional weights per unit run of pipe over and above this up to their recommended maximum.

    I can remember Carrier provided a charging aid some years ago (in the 70?s) that used a pressure gauge in a little red plastic box with a temperature probe that you attached to the suction pipe.

    Anybody ever come across this thing?

    I never had much success with it because the UK ambient temperatures were always too cold. The idea was that you charged until predetermined pressures and temperatures coincided. A good idea but flawed insofar as they never said anything about the stabilization time. Even small systems take time to stabilize after you add gas. In the winter, you had to throw a sack over the condenser because the head pressures were down below 150 psi .
    We did a roaring trade in fan speed controllers to overcome this. Their next brainwave was a so ? called charging chart. They were never much use either. But, and this is the point, they always gave a charge weight on the plate and in the service book.

    An under charge will dangerously increase the motor temperature, not necessarily changing its current draw much and an over charge will wash the oil from the sump and probably damage the bearings. The point is you cannot determine this from superheat alone on a system that is varying its load.

    My advice is to charge by weight and if the current is down, you?re saving your customer some money.

    Good luck.
    ________
    XV535
    Last edited by Argus; 07-02-2011 at 08:45 AM.

  3. #3
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    I check a variety of temperatures while I'm charging a system. What I concern myself the least with is compressor amps.

  4. #4
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    The Prof is also disposed to follow Gary's approach here. If superheat, subcool, airflow, and delta Ts are in spec, the compressor current draw can be assumed to be normal. If one is diagnosing a system anomaly, and the compressor is suspect, then one should check compressor amp draw.

    But the Prof finds it interesting that the weakest subject area for the average hvac/r technician, in his opinion, is electrical. Your presence in this industry will help rectify this issue….
    Prof Sporlan

  5. #5
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    "But the Prof finds it interesting that the weakest subject area for the average hvac/r technician, in his opinion, is electrical. Your presence in this industry will help rectify this issue….

    This is a subject which really bugs me, how can hvac/r tech's be weak on electrical problems and still be considered competent engineers, surely a good 50% of service calls are down to electrical problems. I used to get fed up of engineers who would not be able to trace electrical faults or check motor windings or get completely flumexed by a star delta or part wind starter. Yet apprentices in the UK get very little electrical theory at college, when I started out there was no fridge college in my area so I completed an electrical course instead and learned fridge "on the job" and I feel I have benefitted from this.

    Needed to get that of my chest, rant over.

  6. #6
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    It seems every U.S. hvac/r trade school instructor the Prof has had the occasion to talk to has stated their students, in general, have most trouble with the subject of electricity. One provided a possible explanation: these students tend to be "touchy-feely" types, i.e., they better understand things that they can touch and feel, and electricity doesn't fall into this category...
    Prof Sporlan

  7. #7
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    Hey Prof

    You can certainly feel electricity if you touch it!!

  8. #8
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    I always felt sorry for the plumber installing a new heater and needed an electrician to finish the job wiring the thing. To specialize in a field one needs to know a lot about the other fields that surround it. Just my opinion! :-)

  9. #9
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    Hi, Gary
    I know I am going to kick myself for saying this, but I disagree with you about charging without am amp probe fitted. I would always monitor the motor amps during charging a system especially one which I had not worked at before. Matter what refrigerant the system requires for proper operation there is no point in adding refrigerant until the system is satisfied and them finding out that the compressor motor is overloaded. This overload can be a result of many different things, wrongly selected components, high ambients, incorrectly set controls,(a mop valve comes to mind)or simply a faulty compressor.
    I remember once working on a maintenance contract where loads of similiar split AC systems were fitted in various retail out-lets. The inspections were carried out without the use of gauges, all the units were clamp tested,amps noted along with various system temperatures and the prevailing ambient and room temperature. Refrigerant charge was them determined by the compressor amps and indication from the liquid line sight-glasses.
    Regards. Andy.
    If you can't fix it leave it that no one else will:rolleyes:

  10. #10
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    How about this. On a modern Japanese split system when we have weighed the charge in exactly or it is already precharged therefore adding no trim. How accurate is the running current to specified design current?

    Any thoughts?

  11. #11
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    Compressor manufactuers (Copeland for instance) publish compressor performance data that reference "current" in relation to discharge and suction pressures, which also correspond to refrigerating capacity. In that sense it would be relative.

    If you study one of the "charts" you will see that current fluctuates "normally" according to conditions, and such fluctuation is directly related to the amount of work being done (refrigerating effect). Without the performance data, I do not think that you would be actually determining much.

    The question to ponder, is under what conditions the RLA rating is derived.
    Last edited by herefishy; 09-07-2002 at 06:30 PM.

  12. #12
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    Hi, I supose if you wanted to determine system performance by compressor Amps, good commisioning data would be a help. With known conditions a comparison could be made during subsequent visits.
    Regards. Andy.
    If you can't fix it leave it that no one else will:rolleyes:

  13. #13
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    Measuring compressor current draw while charging is good for determining if the compressor motor is overloaded, misapplied or failing. I have never seen any equipment manufacturer's literature that suggests charging to a specific current draw. Anyone else?

  14. #14
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    There is only one way to charge a fixed orifice or cap tube system other than weighing in the labled amount. You need to know the evaporator load, which is determined by the indoor wet bulb temp. Then a chart (I have one from Carrier) tells you what the superheat should be at a given condensing temperature. Adjust the charge to give that superheat and that's it. Conditions vary widely and the evaporator can operate from the freezing point to the floodback point. Under normal operating conditions you should fall someplace in between. If the system is operated with a cold outdoor ambient temp you may have a problem with freeze ups. This occurs when a fixed orifice is used on a commercial application, such as a store or restaurant, that may require cooling even when it is relatively cold outdoors. In such cases a TEV or condenser fan control should be used. To address your question about Amperage, be sure you are using a true RMS meter. The actual amperage will vary with the load. The only concerns are that you don't exceed the rated load and that the compressor is pumping properly. Compressor efficiency can be determined by comparing the amperage with the performance charts from the manufacturer. Sometimes it is difficult to diagnose a compressor that is not pumping efficiently.

  15. #15
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    From my experience, the RLA value (Rated - not Runnng) load amps is going to be around 20 to 30% higher than the amperage you should be reading. This is mostly Copeland experience. For example, a Copeland 7.5 hp discus compressor might have a 41 RLA, but should only be drawing 31 amps at a 55 deg F suction and 130 deg F condensing temperature with R22. The RLA is mostly a safe value for the electricians and manufacturers to size breakers and wire - not a value to use for diagnosing refrigeration problems.

  16. #16
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    The RLA or run load amps is what the compressor is rated for in the application it was installed for. The FLA or full load amps the manufacturer rarely prints out. When I charge a refrigeration unit, I use a weighed charge as a base line charge, the amps will be high in most cases till the box comes down to temperature. I always monitor the amp draw during charge. If the unit is without a CPR you may have to frontseat the suction valve and crack it open slightly till the box comes down in temp for several hours. I have a unit like this that I left overnight with my gauges and it was only at 25F the next morning, I opened it up the rest of the way and the amp draw was high for several minutes but lowered once the box came down to temp (much faster). Air conditioning is a little different. If its underamping, the coils are clean and it's not the hottest day it was designed for.
    Mike
    My wife says I don't listen to her....or something like that....

  17. #17
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    Red face Re: Charging Split, Using Amps

    Old thread but hope someone can respond.

    1. Charging a mini split AC, the safe method as suggested is manufacturer's labelled weight. So can anyone confirm that superheat values (or sub-cooling too) are optimal when charging by manufacturer's labelled weight.

    2. If rated current is being drawn and superheat suggests more charge required, does this mean AC system is not designed well (assuming no parts failure).

    3. My 24000btu split is labelled at 1740grams 25ft(7.5meters) and additional charge of 20grams per meter of additional pipe up-till 20meters.

    My actual requirements are filled by around 3.5 meters.

    My question is, in my case is 1740grams of r22 an overcharge for my 3.5 meters, if yes should i adjust the refrigerant to match 3.5meters (how many grams for 3.5meters) or should i use a pipe of 7.5 meters. If this might help local weather temperatures are between 40-50 °C during day.

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