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Discussion in 'Electronic Design' started by Myauk, Dec 12, 2005.

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  1. Myauk

    Myauk Guest

    Is there any one who could explain to me the basic calculation scheme
    for the RLC components in Line Filtering?
    Is there any site explianing more details about the Power Line
    Filtering, I mean, for example,the output filtering for inverter
    designs?
    My problem is especially with inductors, how can I design and construct
    an inductor for line filtering?
    I found many links but most of them explain about signal filtering,
    antialiasing, and some DSPs but not the topic I am having problems
    with.
     
  2. That is a pretty tall order for a newsgroup post.

    In general, line filters are just passive filters designed with
    components that handle line voltage and load current. So any tutorial
    on passive low pass filters would be applicable, to some extent.

    A good place to see some typical filter configurations is the data
    sheets for commercial filters:
    http://www.cor.com/Powerline.asp

    A feature of line filters that you will not often see in signal filter
    designs is the common mode choke. This is a transformer with two
    similar windings, connected so that each winding carries the load
    current each way. But they are phased so that the line currents
    create canceling magnetic fields, so that a high permeability core can
    be used without saturating. This connection and construction allows
    the component to act as a high inductance between the line and load
    for signals that are in common to both lines, because those signals
    create magnetic fields that add in the core, instead of cancel. The
    symbol for a common mode choke is often a pair of coils with an arc
    with two arrow heads between the coils, representing the shared core flux.

    For example:
    http://www.cor.com/Series/PowerLine/B/
    Here is a data sheet for a common mode inductor:
    http://industrial.panasonic.com/www-data/pdf/AEX0000/AEX0000CE1.pdf
    http://www.jwmiller.com/pdf2/8100.pdf
     
  3. Chuck Olson

    Chuck Olson Guest

    A line filter is the same as any other signal LC filter, but with the
    requirements that it handle much more current and much higher voltage than
    typical signal filters. To design a filter, you first need to decide on the
    response shape. If you are doing this professionally, you should avail
    yourself of a filter design book - - the most useful of them being "Handbook
    of Filter Synthesis" by Anatol I. Zverev. Then you need to decide what
    frequencies to attenuate. Obviously the frequency to pass from your inverter
    is 60Hz, but a filter to eliminate all frequencies above the fundamental
    would certainly require rather large inductors and capacitors. I don't know
    much about how you create the 60Hz sine wave in your inverter, but if you
    can allow frequencies of 50KHz and below to pass through the filter
    unattenuated, then the component values will be much more reasonable in
    size. Lets say you need 60dB attenuation above 500 KHz, so the hash in your
    source can't interfere with AM broadcast band reception, and a 3-pole filter
    is all you have room for. Select a Butterworth response shape with cutoff at
    50KHz since that will reach 60dB at ten times the cutoff frequency. Now we
    have a bit of a problem - - filters normally have to be terminated with a
    specific resistive source and load to behave according to the typical
    response curve. It is also possible to terminate the filter at one end only,
    with the other end either a "zero impedance" voltage source or an open
    circuit. These are called extreme termination designs. The impedance of the
    source in a line filter may be assumed to be close to zero, let's say, and
    since the load voltage and current is known, the filter can be designed
    around that single resistive termination. The normalized component values
    for a 3-pole Butterworth singly terminated for a zero impedance source are
    (from Zverev) L1 = 1.500 henries for the source end, C2 = 1.3333 farad, and
    L3 = 0.5000 henry at the load end, where the load R = 1.000 ohm. To
    de-normalize the design to your desired cutoff frequency and load
    resistance, calculate a value for w = 2 x pi x Fc, where Fc is 50KHz in this
    example, so that w = 314,159.27. Each inductor is de-normalized by the
    formula L = L' x (Rt /w) where L' is the normalized value, and Rt is the
    actual termination resistance. Each capacitor is de-normalized by the
    formula C = C' x (1 / (w x Rt)) where C' is the normalized value. In this
    example, lets say your load pulls 2 amps at 120 volts AC, so the Rt is 60
    ohms. De-normalized values for 50KHz and 60 ohms are L1 = 286 microhenries,
    C2 = 0.0707 microfarads, and L3 = 95.5 microhenries. These may still be too
    large inductance values to be reached with open core ferrite inductors, so
    you may need to consider ferrite EI core or pot-core or toroid inductor
    design, or re-think the cutoff frequency or the number of poles in the
    filter. If you use more poles, you can reach 60dB attenuation with a higher
    specified cutoff frequency, and the components, though more numerous will be
    smaller. Or you might decide 40dB attenuation is sufficient, which similarly
    will allow you to raise the cutoff frequency and use smaller components.

    Good luck

    Chuck
     
  4. Myauk

    Myauk Guest

    Thank you very much for your precious hints!
    Now I could be able to learn and design filters for my UPS.
    My design include 3524 and the output wave form is modified sine wave
    We studied the Thai Design and improve it to fit in the local
    conditions of our country, Myanmar.
    I have studied the voltage regulation concepts, the inverter concepts,
    the control procedures, form the design and now I could be able to
    calculate and change the component values except the line filters.
    As far as I have studied, the output filter design should include the
    consideration of the type of load, and the estimate of RLC values of
    the load in addition to the consideration of output wave form produced
    without the filters.
    However I would like to calculate and estimate the required values for
    line filters practically.
    The design is intended to use for under 500VA with four FETs, push
    pull.
    We are also trying to design with PICs
    Right now my design has spikes when it has no load.
    When the load is connected the spike level is reduced to some extent.
    Do I need to make additional circuit for over 500VA designs?
     
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