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Thread: buzzzing circuit breaker

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    buzzzing circuit breaker

    i installed a 20 QF1 C1 cbi circuit breaker for some aircons.amp...while commisioning the aircons i noticed a buzzing sound coming from one of the circuit breakers...

    there are 3 x daikin invertor 9000 BTU aircons connected to 1 x 20 amp C1 circuit breaker...which in theory should be no problem.

    while discussing the sale to the customer the aircon rep indicates that they only draw between 4 -6 amps...low current energy saving etc...you know sales talk...so trusting the rep as he sells these units on a daily basis...bad idea i should have done my homework like i normally do...the units get installed and i read the full load current on the side of the condenser unit...full load current 8 amps per unit...x 3 = 24 amps...now suddenly my installation is under rated...working on 6 amps full load current is all 3 were running at the same time for a short period of time wouldnt have been a problem...but running at 24 amps even for a short period of time now becomes and issue.

    so thinking it was just a faulty circuit breaker i collected a new one and replaced it...wow great service the customer thought...but unfortunately the new circuit breaker made an even louder buzzing sound...now the customer is starting to doubt my abilities...so i remove the new circuit breaker and replace it with an older type C2 white circuit breaker...turn the aircons on to 16 degrees and run all 3 together...no buzzing sound...so i decided to collect my fluke 435 and monitor the power while the units are running...now things really start going pear shaped...the reason for the buzzing sound is the extremelt distorted current sine wave...with a crest factor of over 3.

    i also decided to clip my current clamp which can detect 50/60 hz frequencies or you can switch it over and it can read higher frequencies a quick way of checking current harmonics.
    i noted a 1 amp difference in the current...current thd is over 40 %

    i ran the units for a couple of hours and downloaded the info...if anyone wants to see screen shots.

    i also ran one unit by it self to see what would happen still made a buzzing sound...so the bottom line is whether i installed 3 seperate circuit or 1 circuit i would have still had the problem with the circuit breaker buzzing...my problem now is eventually the white lever C2 breaker will fail because of the startup

    and anyone buying invertor aircons beware they dont have filters and feed back into the electrical system...causing all sorts of problems to other equipment on your premises.

  2. #2
    Diamond Member Justloadit's Avatar
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    What you may need there is the motor circuit type breaker. the one with the orange lever. It is a dual type breaker, which works on thermal and magnetic in the same unit. So starting up, it will allow a 20% over current for about 20 seconds before tripping. If there is a short, it trips instantaneously.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
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    i installed a 20 QF1 C1 cbi circuit breaker for some aircons....

    a 20amp QF1...C1 is a motor starting orange lever curve 1 circuit breaker designed for motor starting...it is that circuit breaker which is buzzing...i have had to remove the orange lever breaker and now replaced it with a white lever curve 2 circuit breaker.

    it seems the new design QF series have a spring around the coil and i could be the spring creating the noise.

    my concern is if there is a large number of these invertor units installed in a building could they create a problem with the electrical installation and affect other components connected to the same electrical grid...including the transfomer

    if a new electrical instaltion is not correctly designed and a large number of invertor aircons...computors and electronic light fittings are installed and not fitted with adequate filters it could become a problem

    and now that i have done the research and found out how an invertor aircon works i can understand why i having a problem and you should not need a motor starting breaker because the units are controlled by a varible frequency drive ( the problem)...so there is no initial startup.
    Last edited by murdock; 07-Mar-11 at 07:33 PM.

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    now i am starting to wonder how many people are having problems with earth leakage units tripping randomly from these invertor units...considering they are normalling installed and just plugged into the socket closest to the unit.

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    Diamond Member Justloadit's Avatar
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    Hy Murdock,
    Interesting info you bring up about the inverter aircons. The drives are supposed to be designed in such a manner that they draw almost Power Factor of 1 for efficient use of the incoming power. If they are doing high pulse current draw, then you will experience what you have described.

    They should not inject noise on the supply lines, or else they will contravene the basic by laws. Not sure where to find this though, as it's part of EMI
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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