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phase angle convention

Discussion in 'Electronic Design' started by John Larkin, Nov 11, 2007.

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  1. John Larkin

    John Larkin Guest

    Suppose we have a 3-phase, Y-connected power system. Call the phases
    A, B, and C. Assume A peaks at T=0, B peaks next (120 degrees, 5.6
    msec later) and C peaks last. Call the phase angle of A zero.

    So, what's the convention for the sign of the phase of B? Is is +120
    degrees, or -120?

    Similarly, if the current in an inductor lags the applied voltage, is
    the current +90 or -90 degrees relative to the voltage?

    I used to know this stuff.

    John
     
  2. Jim Thompson

    Jim Thompson Guest

    Does it matter what you call it ?:) Or are you having to write a
    specification ?:)

    ...Jim Thompson
     
  3. Jamie

    Jamie Guest

    I'll most likely be flamed for this.
    it's W,X,Y,Z W = Ground X,Y and Z = phase 1,2 and 3
    Phase 2 (B) would be +120 since it don't take place until
    the time displacement of 120 degree's later.

    As far as the inductor. It's know to be -90 only because the
    effects that take place were due to something that happen 90 degrees
    before..

    Ok, Guys. flame my ass [email protected]
     
  4. John Larkin

    John Larkin Guest

    I'm (still) doing the firmware for a dds-based waveform generator. If
    the user specifies a phase angle for a waveform, I'm adding a value to
    the memory address pointer that aims into the 2048-point sinewave
    lookup table. So if he says to set channel 3 to +45 degrees, I'm
    adding 256 to the ch 3 address pointer. That makes the peak of the
    resulting sine wave come out 1/8 of a cycle earlier in time.

    I like that, but is it what electrical guys expect? By my convention,
    to generate a textbook 3-phase set on channels 0, 1, and 2, I'd have
    to send the serial commands

    0Phase 0
    1Phase -120
    2Phase -240

    which might be weird to power folk.

    I did take two semisters of Electrical Machinery, but it's been a
    while. Class average on exams was, as I recall, about 15. I ran in the
    mid-40's, which was an A.


    John
     
  5. Guest

    Seems to me that the user should legitimately be able to enter either
    +45 or -315 and get the same results:

    PhaseAngle = GetUserInput();
    offset = (PhaseAngle/360) * 2048;
    if(offset < 0) offset = offset + 2048;

    Or something like that...
     
  6. John Larkin

    John Larkin Guest

    Certainly; everything is mod 360, and -270 is the same as +90. But I
    still want to get the sign right.

    Actually, I get the user input (say, 22.5) as a signed 32-bit integer
    that's scaled in millidegrees (22500), multiply by 1024, divide by
    5625, and the low 16 bits becomes the phase rotator integer which is
    poked into the fpga, the high 11 bits of which are the actual memory
    address adder. All in assembly. The scaling takes two instructions on
    the 68332. The "rational" math avoids rounding errors which you'd
    probably get with floats.

    John
     
  7. krw

    krw Guest

    I would say phase B lags phase A, so the sign of the phase B should
    be negative, WRT A.
    Again '-' for the same reason.
    Maybe I did too. ;-)
     
  8. Sarason

    Sarason Guest

    Check out a HP/Agilent box as the electronics industry tends to follow
    them for their conventions. A HP8904 Multifunction Synthesizer would be
    a good place to start.

    Andrew
     
  9. For Va to peak at t = 0 ( w = 21.6E3 degrees per sec):

    va = V * cos( wt )

    If Vb peaks 5.6 ms later:

    vb = V * cos( wt - 120 )

    wt - 120 = ( 21.6E3 * 5.6E-3 ) - 120 = 0
    -90 degrees. Using the same logic as above, if the voltage peaks at t =
    0, then
    v = Vp * cos( wt )

    If the current peaks 4.167 msec later (90 degrees at 60 Hertz), then

    i = ( Vp / Zl ) * cos( wt - 90 )
     
  10. Joerg

    Joerg Guest

    Shouldn't be. Power folks also consider 360 degrees full circle. Now
    when they introduced a 400 degree convention I bailed ;-)
     
  11. Phil Hobbs

    Phil Hobbs Guest

    Congratulations, you're a physicist. ;)

    The sign of the phase depends on your Fourier transform sign convention,
    and physicists and engineers use opposite ones. We'd all like to have a
    positive frequency be exp(i omega t) and exp(i k dot x). We also all
    like to consider a general plane wave as going in the positive x
    direction.

    Unfortunately you can't have all those things at once--a positive-going
    wave is either (1) exp(i(k dot x - omega t)) or (2) exp(j(omega t - k
    dot x)). Historically, EEs have cared more about temporal behaviour, so
    they choose convention (2) and physicists more about spatial behaviour,
    so they choose convention (1).

    Applying a time delay means that the measured phase corresponds to an
    *earlier* time, i.e. t' = t-tau. The phase is just the imaginary part
    of the exponential, so if (as an EE would say) a positive frequency is
    exp(j omega t), the phase is increasing with time. Thus that phase lag
    gives a *negative* phase shift, corresponding to an earlier time. So
    the AC leg that comes to a peak 1/3 cycle later is -120 degrees.

    (And yes, this also applies to phase lags that don't introduce delay,
    e.g. RC integrators.)

    Cheers,

    Phil Hobbs
     
  12. Don't worry. That convention is now gon.
     
  13. Joerg

    Joerg Guest

    I still remember many EE exams where my brain went like this: I know
    it's wrong but I've got to do it anyhow so that I pass. Like impedance
    matching of a large transmitter final amp which, if it was ever done,
    would result in flying glass and a 5-alarm blaze.
     
  14. JosephKK

    JosephKK Guest

    John Larkin posted to
    sci.electronics.design:
    I am not really a power type. But as i understand the conventions
    phases a, b, and c in your definition are called 0, 120, and 240
    degrees. As stated, these will represent lag values. For normal
    power engineers just use 3-phase, with phase rotation a, b, c which
    will indicate the order of positive peaks. For other persons you may
    want a different interface (read modal interface). Some would call
    this a suboptimal solution. My response to them is show me a better
    one.
     
  15. JosephKK

    JosephKK Guest

    John Larkin posted to
    sci.electronics.design:
    I presume that you are familiar with with binary angle measurement
    system (BAMS) where the (fractional) msb represents 180 degrees. It
    makes a whole lot of angle and transcendental function computation
    much easier. Establishing the equivalence of -90 and +270 is
    trivial in this mode.
     
  16. JosephKK

    JosephKK Guest

    Joerg posted to
    sci.electronics.design:
    It should not have had that effect in a correctly designed unit.
    Quite the opposite in fact.
     
  17. John Larkin

    John Larkin Guest

    That's exactly what my final 16-bit binary is.... 0 to 65535 == 0 to
    359.99 degrees. The msb is 180.

    My users program the thing in serial ascii, in degrees, so I convert.

    John
     
  18. John Larkin

    John Larkin Guest

    I'm getting the general impression that a positive angle represents
    lag to the power people. But they rarely use signed angles... they say
    "45 degrees lagging (or leading)." That makes math funny, but maybe
    they don't do much math.

    John
     
  19. JosephKK

    JosephKK Guest

    John Larkin posted to
    sci.electronics.design:
    Thanks, i thought i understood the subcontext in the thread.
     
  20. JosephKK

    JosephKK Guest

    John Larkin posted to
    sci.electronics.design:
    Unfortunately true, they would rather use tables.
     
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