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  1. #1
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    Default Power Dips

    When we had our 8hp split system air con installed we had no problems with lights dipping - now 6 years later when the air con is on the lights dip when the compressor cuts in. Is this a normal 'aging' process of the contacts or symptom of another problem. Is it a manufacturer service call or an electrician call out?

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    G'day Wombat2 Compressors are exactly he same as motors the unit will Start up drawing a massive current peak and once it gets to run speed the current dips back to get the unit to kick over initially they use what are called Start Capacitors and overime these units can weaken and become almost ineffective and that when you start blowing Circuit Breakers or even Service fuses, the oher factor that may be playing havoc but in your case i doubt it is the A/C system might be overcharged with refrigerant this puts an extra load on the compressor on initial start up call the A/C Service agent and ask him to see if the unit is overcharged or the Capacitors might need changing

    Cheers
    Patty

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    Retired Marine Engineer 1K Club Member Ashore's Avatar
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    Wouldn't think its overcharged , has any work been carried out on the system lately?
    I think it is more likely the system is drawing more current at start-up due to patties suggestion re capacitors or bearing/s on their way out or you now use more load on your system , ie new fridge , plasma tv , computers , games consoles etc , have you added to the household load recently ?
    Ashore




    The trouble with life is there's no background music.

  4. #4
    wannabe woodworker
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    At 8hp is that a single or three phase unit? 3 phase wont have any capacitors at all and will just start DOL (direct on line)

    Could also be that there are more houses in the area and a greater load on the supply system droping the mains voltage

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    It is highly unlikely that a motor of this size (about 6kW) is a single phase motor. Even if it was single phase, any starting capacitors also absorb current (charge) upon startup & therefore will have always contributed to any startup "voltage drops". In single phase motors, capacitors are primarily used to create a displaced field (up to 90 degrees) in order to establish a rotational torque. The secondary role of these capacitors in a single phase motor is to reduce "running" vibration.

    As someone else has already stated, the a/c unit is most likely a 3 phase unit, in which capacitors are not used (unless for Power Factor Correction reasons - highly unlikely).

    I think that the problem the OP is experiencing is related to "distribution load". That is, in 6 years, the network (distribution load) has increased somewhat without the distribution transformer being increased. This will most definitely cause a voltage drop on the startup of a high current device. I do believe that "another termite" has also suggested this.

    Startup current for an 8 HP (6kW);
    Single phase - 25 Amps x 8 = 200 Amps.
    3 phase - 8.5 Amps x 8 = 68 Amps.
    Both figures ignore Power Factor.

    Who now wants to consider the Maximum Demand of the OPs' installation & the size of his "service" conductors?

  6. #6
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    Hang on a minute be careful some Air/conditioners companies use HP ratings to describe the heating and cooling capacity of the units and have corresponding Kilowatt ratings and BTU ratings (british thermal units)this has nothing to do with the Compressor/motor current drag so my question is where are you getting the 8 hp rating from?

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    Quote Originally Posted by patty View Post
    Hang on a minute be careful some Air/conditioners companies use HP ratings to describe the heating and cooling capacity of the units and have corresponding Kilowatt ratings and BTU ratings (british thermal units)this has nothing to do with the Compressor/motor current drag so my question is where are you getting the 8 hp rating from?
    That is the cooling rating - It is single phase being the largest I could get to cool the living area without going two or ducted

  8. #8
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    O.k well as Ashore and another termite have rightfully pointed out have you had any other major appliances installed? and as another termite also pointed out you may have another housing development on te same grid and this too has put increased load on the supply as you can see there are many contributing factors that must be taken into consideration...

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    No other major appliances installed - the house and suburb is 30+ years old - house down the road is the only place that is undergoing a major refurbishment at the moment and the only one in the area that I know of in the 6 years we have been here. I stabilized the digital TV about 2 years back by running a dedicated earth to the antenna shield and it worked well until about 6 months ago and now we are getting pixalation and sound drop outs again but not as bad. I checked the rating plate on the air con and it pulls 12.9 amps.

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    Wombat ok no new sub-divisons I am sure alot of your neighbours in the suburb have also had Air/con's installed lately as well see you really can go on and on with this question, come summer time and our area has been warned of serious blackout and electricity shortages due to the increasing households with a/c units either ducted or splits placing a massive strain on the already overloaded grid...

  11. #11
    wannabe woodworker
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    Quote Originally Posted by patty View Post
    Wombat ok no new sub-divisons I am sure alot of your neighbours in the suburb have also had Air/con's installed lately as well

    I agree i live in an older area a few new homes have gone in but not many but every man and his dog has been getting at least 1 and in some cases two (When they are single phase) units installed... in fact the guy across the road from me has a 3hp (lounge) a 2hp(rumpus) and a 1.5hp(master bed) all on a single phase supply. Am now just waiting for the "air conditioners dont work call". I dont think a max demand calc was done there.... should be interesting...
    P.S. I had nothing to do with the install and warned against it for a three phase supply system.... but i was told this was cheaper.... not for long....

  12. #12
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    Wombat
    I would get an electrian in to check the Neutral contacts on the neutral bar and the mains box.
    Allso get him to put a tong tester on the supply (he will know what I'm on about) and test current being drawn.
    Failing that report it to the supply authority and get then to check there end.
    They should do an "NST" etc.

  13. #13
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    In my opinion & so far, the accumulated information says "excess load on the distribution transformer" plus a possible (but not likely) poor neutral connection. Broken or poor neutrals generally manifest as a higher than usual voltage being presented "somewhere".

    It could also be a rather extreme problem that relates to harmonics, since most people now have computers, electronic lighting ballasts & "inverter" air conditioners. High levels of harmonics (voltage related harmonics are the worst type) can & will cause voltage problems. But I'd rather go with my first guess...excessively loaded distribution transformer.

    BTW, you obviously have an a/c that is electrically rated at about 3kW. So for now, we can forget about capacitors as being part of the problem.

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    Single phase - 25 Amps x 8 = 200 Amps Elkangorito how do you come up with 200 amps on a single phase supply? I can understand where you are geting the 25 amps from but why are you multipling it by 8 for? his is how i see it if the compressor was rated at 8hp for instance 750watt is roughly = 1hp i actually think it is 746 watts but anyway 750X8 = as you stated 6000w or 6kwif you divide this by 240 volts that will give you the current drag on the motor= 25 amps which is what you have also done but why did why did you multiply it by 8 for again are you using the 8 as 8 times the the current draw to give a rough estimate of start current just curious

  15. #15
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    Quote Originally Posted by patty View Post
    why did you multiply it by 8 for again are you using the 8 as 8 times the the current draw to give a rough estimate of start current just curious
    He, as you have said used it to estimate the start current.

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    Quote Originally Posted by Wombat2 View Post
    I stabilized the digital TV about 2 years back by running a dedicated earth to the antenna shield and it worked well until about 6 months ago and now we are getting pixalation and sound drop outs again but not as bad..
    Wombat,
    Where are you in Brissy?? I have started getting major pixelation on Channel 7 and some on Channel 9. Im thinking it is something locally causing the interference. Its been dodgy for the past week or so.

    Im in Bald Hills.

    Sorry Ive got no different suggestions on your power issue!

    Cheers
    Steve

  17. #17
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    Quote Originally Posted by patty View Post
    Single phase - 25 Amps x 8 = 200 Amps Elkangorito how do you come up with 200 amps on a single phase supply? I can understand where you are geting the 25 amps from but why are you multipling it by 8 for? his is how i see it if the compressor was rated at 8hp for instance 750watt is roughly = 1hp i actually think it is 746 watts but anyway 750X8 = as you stated 6000w or 6kwif you divide this by 240 volts that will give you the current drag on the motor= 25 amps which is what you have also done but why did why did you multiply it by 8 for again are you using the 8 as 8 times the the current draw to give a rough estimate of start current just curious
    Quote Originally Posted by another termite View Post
    He, as you have said used it to estimate the start current.
    "another termite" is on the ball.

    Now that we know the OPs' a/c unit is about 3kW (electrical), 12.9 x 8 = about 103 Amps (worst case scenario). Such an inrush current, added to the existing domestic electrical load, can definitely cause voltage drops if;

    1] electrical connections have become more resistive with age.
    2] the distribution transformer is nearing its' full load capacity.
    3] electrical motors do exhibit an increased current usage with age, although this is often negligible.

    A late edit:
    4] the consumer mains are too small (due to increased demand over the years).

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