How do harmonics damage the line/appliance?

Discussion in 'General Electronics Chat' started by alphacat, Oct 22, 2009.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Say that you switch an appliance and as a result harmonics are created in the LIVE and NEUTRAL wires and in the appliance itself (The harmonics are formed due to the nature of the appliance and how you switched it - at the peak of the sine or at its zero-cross).

    Why do these harmonics are bad?
    What damage do they cause?

    Thank you.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I haven't run into "harmonics" causing problems as such. However, many appliances nowadays have onboard electronics; even washers and dryers. My Mother destroyed the electronic controls in the washing machine twice by failing to pull out the knob (turning the machine off) before rotating it. The series of transients caused by power being suddenly applied and removed from the motor caused the control board to get "zapped".
     
  3. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    I beleive they do damage appliances and disturb the line, since there are all these harmonics filter that were invented just becuase of that.
     
  4. AdrianN

    Active Member

    Apr 27, 2009
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    I don't think that harmonics create damage per se, other than, at high powers, they can generate radiated emissions that affect other electronics around. The harmonics can also generate conductive emissions that can affect instruments and equipment connected at the same AC line. It is one of the reasons high-power devices implement power factor correction (PFC) circuits. A PFC role is to make the equipment connected to the AC line look like a simple resistor load. The PFC minimizes distortions and spikes in the AC current, keeping the harmonics low.
     
  5. wr8y

    Active Member

    Sep 16, 2008
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    I think THAT is the answer. Let's replace "harmonics" with "transients" in the OP and I think we are on to something.

    As a side note, does it surprise ANYONE here that we don't see more failures of electronic modules in cars than we do? I mean, take a scope and look at the garbage, spikes and surges showing up across the battery in your car when you start it! If the battery is weak, WOW - can you see some ugly stuff!!!!
     
  6. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
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    @AdriaN

    Thank you very much.
    Actually, I'm currently learning about transmission lines, which conduct high-frequency ac waves from the generator into an antenna which radiates the power outside the circuit into the air.

    In the household model of Live and Neutral wires, connected to each appliance in the house, how come the power is radiated outside if there's no antenna to transmit it?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Would you care to share a link?

    Any moving electric field creates a moving magnetic field, which in turn creates a moving electric filed. Dr. Maxwell established this in the 1880's, from which we got radio, and so on. The conductor is its own antenna.
     
  8. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
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    I mostly learn from watching lectures, but I also find this book helpful.
    http://www.ece.rutgers.edu/~orfanidi/ewa/

    Yes, I'm familiar with the two Maxwell laws you're talking about.
    From what I learned so far in the course, there must be a none resistive load that matches the transmission line's impedance (the better the matching, the larger the transmitted wave).
    So what is the load in our household system which the power is radiated from?

    [​IMG]


    * I must say that we havent reached resistive transmission lines in the course yet, so i might be missing something becaues of it.
     
  9. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    Radiated emissions are generated by transients. A transient is a fast change of current or voltage. It contains a large number of harmonics. The more steep the change the more harmonics. (See the Fourier Transform of a step signal).

    The AC line does not generate harmonics, because the 50/60Hz line signal has slow edges. It is the device connected to the AC line that can generate transients, based on the way it takes current from the AC line. These transients are reflected back into the line. Because they have a large number of harmonics, they can generate radiated emissions. Any piece of wire on the AC line can become an antenna and will do that.

    The same transients with their rich content of harmonics can generate conductive emissions back into the AC line. These emissions may reach other equipment connected to the same line and, depending on the amplitude of these transients, may affect that equipment.

    You cannot fight all that by considering the AC line a transmission line and try to properly terminate it. The AC line load is dynamic as people connect or disconnect devices all the time whether this line is residential, commercial or industrial. What one can do is to make sure all equipment connected to the AC line have PFC circuits or, if not, the generated radiated emissions and conductive emissions are low according to standards. Example, the CE [SIZE=-1]Generic Emission Standard EN 50081-2.[/SIZE]

    Also, when manufacturers design an instrument or any device they make sure this device is not affected by conductive emissions, RF emissions or electrostatic discharges. Example, the CE standard [SIZE=-1]EN 50082-1:1991 Generic Immunity Standard[/SIZE].

    So, to answer your question, any load in a household can generate radiations if it is not properly designed. Especially for the household market, where the manufacturers try to cut costs down to bare bones due to a fierce competition, chances of emissions are high. As an example, one notorious household aggressor is the CFL bulb.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    In the case of impedances, that of an AC distribution line is a bit low - perhaps on the order of .001 ohm. There is a fundamental difference in transmitting power to a load, and in transmitting power to a resonant load. Think, for instance, of a DC power supply. Nothing there to resonate with. For that matter, imagine structures resonant at 60 Hz.
     
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