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Thread: Conduit Buzzing Compressor startup

  1. #1
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    Jul 2008
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    Conduit Buzzing Compressor startup

    I need some advice.....

    I moved into house with new AC. When the ac / scroll compressor kicks on, I hear a buzz noise on the initial start. The unit cools correctly and the charge on the AC has been recently checked and confirmed. Breaker has never tripped.

    I thought I heard it by panel as well. FOr safety sake, I had the installation company send out their electrician to take a look inside. Pulled the breaker to check for corrosion/burn. Took amperage readings at start up and normal operating. Put same breaker back and made sure connections were tight. He said everything was OK.

    The run of wiring is 50 foot at most from panel to outside unit. Its run with 12 awg on a 20 amp double throw breaker. The same wiring the old unit used.

    Within the ratings on name plate I think:

    ------------------
    LRA: 68 amps
    RLA: 14.1
    Minimum Ciruit Amp. 18.7
    Maximum Breaker 30 amp
    -------------------


    The conclusion the Electrician suggested that it is the conductor inside conduit vibrating/buzzing at the high startup amp of the unit. He said that it was normal. And if it was really irksome, perhaps pulling 10 awg with a 30 amp breaker may stop it. But that was unnecesary money spent and that the buzzing caused no harm in his opinion.

    My research also suggests maybe a hard start kit would help. But its my understanding that a kit like that only lessens the duration and magnitude of conduit buzzing and diming lights/voltage drop and does not eliminate it.


    Should I be concerned further with this situation or can I go on enjoying the AC this summer? Is this indeed "normal"? For anyone reading, do you ever notice this type of thing when you switch your AC unit on?

  2. #2
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    I would consider that it is not unusually and I would not spend money on it.

  3. #3
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    You have to remember that motors draw hundreds of amps for a fraction of a second upon startup. This is a basic physical fact about motors that cannot be changed (unless you use an electronic motor drive, but that is irrelevant to this discussion). The inrush current can be decreased by the use of the hard start kit as you discovered from your research, but never eliminated.

  4. #4
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    especially with modern high efficiency motors, you can get some really serious currant draws for a cycle or two. its just the way it is. bigger wire won't change the amount of current, it may even make it slightly higher. and the current draw is what is causing the magnetic field that is causing the wire to shift that is causing the noise.
    Bob

  5. #5
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    Quote Originally Posted by petersonra
    bigger wire won't change the amount of current, it may even make it slightly higher.
    I agree and think that people tend to overlook that sometimes.

  6. #6
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    I had a pump motor once that would make the conductors in the conduit sing like a kazoo for a couple seconds on startup. They megged just fine I just jerked them up a bit tighter on the one end and it changed the buzz on startup to just a plain hum. Nothing like "tuning" your conductors.

  7. #7
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    Quote Originally Posted by mdshunk
    Nothing like "tuning" your conductors.
    What do you use for a tuning fork in that situation?

  8. #8
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    Quote Originally Posted by peter d
    What do you use for a tuning fork in that situation?
    Maybe, if you want that perfect C-sharp. :grin:

  9. #9
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    Quote Originally Posted by mdshunk
    Maybe, if you want that perfect C-sharp. :grin:

    Having heard buzzing conductors on a few occasions they have a nice mid register to them.

  10. #10
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    Conduit Buzzing Compressor startup

    The line and neutral conductors in any circuit generate magnetic forces that try to physically push them apart (its physics). The higher the current, the stronger the force. Loose conductors in conduits will usually end up against the inside wall of the conduit and vibrate. The best cure is to twist the line and neutral together so they're relatively immobilized (or use Romex).

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