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Need advice on THD

Discussion in 'Electronic Design' started by Matt, Jun 6, 2007.

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  1. Matt

    Matt Guest

    Hello,

    I have designed a power supply using Vicor parts
    (www.vicorpower.com). I am trying to meet a part of MIL-STD-1399(NAVY)
    300A (military power standard) that states:

    "no individual harmonic line current from the second harmonic through
    20kHz exceeds a magnitude of 100/n percent of the unit's rated full
    load fundamental current."

    Our system only brings in 2.5A and runs 120V 60Hz. The EMI filter we
    have out in front of the AC/DC vicor module is the JMK filter
    HH-1355-6.3 that really starts filtering from 10kHz-20Meg which helps
    get rid of the conducted emissions. I am having problems in the
    4kHz-8kHz range of my current THD however and am wondering how I
    should go about fixing it. A simple filter out in front of the EMI
    filter would cause a slip in the power factor of the unit which is
    also regulated by MIL-STD-1399(NAVY) 300A. Are there EMI filters that
    can kill the lower frequencies (4-8kHz) as well as the high (20MHz)?
    Have you ever used one that was designed for Power Factor Correction
    AC/DCs such as the VI-HAM from vicor?

    Thank you in advance for any advice,
    Matt
     
  2. Eeyore

    Eeyore Guest

    Al though you mention no numbers, it seems like you need active power factor
    correction.

    Graham
     
  3. Phil Allison

    Phil Allison Guest

    "Matt"

    ** So you have about 1% current around 1/00 th harmonic of 60 Hz ??

    Most folk just add a series inductor in the AC line to improve that
    situation.

    A few mH ought to do it.

    Just make sure it can handle 4 amps peak without saturation.



    ....... Phil
     
  4. Matt

    Matt Guest

    Phill, thanks for the reply. If I include this 4mH inductor, wouldnt
    that cause my power factor to shift? I will give it a try and let yall
    know if it does ;)
     
  5. Matt

    Matt Guest

    Also, just thought of this, would a common mode be more useful than a
    single inline inductor?
     
  6. Eeyore

    Eeyore Guest

    It's *LINE CURRENT* this is about ! The actual load !

    A common mode filter won't do anything to affect power factor / THD. It's there
    for RF !

    Graham
     
  7. Phil Allison

    Phil Allison Guest

    "Matt" <>

    ** Groper Alert !!


    ** Got a fucking clue what power factor even is ?

    Surprise me - post the definition relevant to your case.

    Bet anything you cannot.



    ....... Phil
     
  8. Phil Allison

    Phil Allison Guest

    "Matt" <


    ** Groper Alert !!!


    ** Got any idea what a "common mode " inductor is ?

    Got a fire extinguisher on your hip ?

    In case of unexpected anal conflagragrations ?




    ......... Phil
     
  9. Matt

    Matt Guest

    Isn't power factor a measure of how the load draws real power? Just a
    ratio of real and complex. I thought any reactive load would change
    the power factor.
     
  10. Phil Allison

    Phil Allison Guest

    "Matt"

    ** Is your arse on fire ??

    Had a look lately ?

    Think I smell smoke.



    ........ Phil
     
  11. MooseFET

    MooseFET Guest

    Common mode inductors block common mode current. They do very little
    for the power factor.

    A LC EMI filter in front of the bridge will suppress high frequency
    currents but not do anything much to the low numbered harmonics. It
    does increase the impedance in the path back to the power line a
    little but thats aout it.

    An inductor on the output of the bridge will spread the current pulse
    out. Any frequency that is high enough to have the current pulse be a
    significant fraction of its cycle will be reduced. When the current
    int he inductor doesn't quite go to zero, the 3rd harmonic will be
    down about 7dB.

    The inductor also shifts the phase of the input current later.
     
  12. Eeyore

    Eeyore Guest

    How does a common mode voltage affect the power drawn then ?

    And futhermore, THD isn't itself the classic 'power factor' either. It's the
    harmonics they're worried about, not the phase angle of the load.

    Graham
     
  13. Eeyore

    Eeyore Guest

    Indeed it will.

    Another 'trick' worth investigating with line frequency supplies, is using a
    smaller size reservoir / filter cap than you might otherwise have done, should
    circumstances allow. This also lengthens the conduction period.

    Graham
     
  14. The Phantom

    The Phantom Guest

    Is the Vicor a PFC converter?

    Can you post the failing spectrum analyzer picture over on
    alt.binaries.schematics.electronic so we can see how bad it is?
     
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