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Phase angle controller Question

Discussion in 'Electrical Engineering' started by Bob, Jan 2, 2010.

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  1. Bob

    Bob Guest

    Is phase angle firing the method of the control when varying the power or
    is it actually changing the phase angle relationship value between voltage
    and current in a resistive load only?


    Bob
     
  2. Bob

    Bob Guest

    Also, is it possible to have phase angle shift
    in an ac circut without capacitance or inductance?
     
  3. daestrom

    daestrom Guest

    Phase angle firing is a term that usually refers to controlling the
    point in a cycle when a thyristor is turned on. If firing is very
    early, near 0 degrees, then the thyristor conducts for most of the
    half-cycle and current can flow to the load for most of the cycle. If
    firing is delayed to nearly 180 degrees, then current only flows to the
    load during the very late portion of the half-cycle.

    In a resistive only load, the current is in phase with the voltage when
    the thyristor is conducting, but effectively shut off during the first
    part of the half-cycle before the thyristor is fired.

    daestrom
     
  4. daestrom

    daestrom Guest

    Phase angle between what and what? If you mean between current and
    voltage, then in a purely resistive network the answer is no.

    daestrom
     
  5. Bob

    Bob Guest

    Voltage and current. Thanks!
     
  6. Bob

    Bob Guest

    Oh ok. that's what I am looking for. I think you answered my question. I am
    trying to settle
    a dispute. I have Phase angle fired controllers
    on a purely resisitve load only and they are Porprtional
    Zero cross triacs and they have 100% power factor
    acording to specs. That tells me there is no shift. I am being told my power
    meaurment is off becuse he thinnks the controller is changing the phase
    angle of current to voltage. I don't see how anything else could cause
    this. there are no caps or inductors in the circut.
    When I told him this he says hes not talking about
    Inductance and phase angle relationship between current voltage. So I have
    no clue what he means. To me that is the only thing that could cause a power
    reading error.
     
  7. The description of what you are using isn't clear enough to me to
    identify what it is. On one hand, you refer to phase angle firing
    which is like a standard light dimmer. On the other hand you make
    reference to Zero cross which might be referring to zero crossing
    switching, which is a different technique for heating loads, where
    on/off switching is always done at the zero crossing point (no phase
    control) and the proportional control is done by varying the ratio
    of on half-cycles to off half-cycles. You didn't say what the
    resistive load was either, so I can't guess from that.

    I have put together an example of a phase control dimmer waveform
    where you can change the level and see what happens...
    http://www.cucumber.demon.co.uk/PowerFactor/ it's the second example
    down the page, "Phase Control (Light Dimmer)".

    For a phase control dimmer, there is scope to easily mis-measure
    the power consumed, depending on the type/quality of the power
    metering device used. Your main electricity meter will get it right,
    but some of the devices you may have bought to measure power
    consumption might not. If you play with the Phase Control applet on
    the web page above, you will see that the power factor drops below
    1 when you start dimming, and the failure to take this into account
    with some measuring devices will lead to an over-estimation of the
    power consumed as you start dimming the light.
     
  8. Bob

    Bob Guest

    Thanks for the help.

    Here is what I am using.
    http://www.avatarinstruments.com/scr/scr.asp

    They say power factor is 100%.

    There is indeed a mis-reporting of power at low levels but for a different
    reason I think. I have an uneven heating load using these controllers on
    both sides of a 240 volt genset to the N phase The current sensors are
    taking average current, not peak. If they were peak sensors that would
    indeed show a gross over reporting at the load decreases beacuse the current
    is chopped.

    Here is where things take a bizzare turn. From 30% power on up the readings
    appear correct usning 1.111 X average correction.

    At about 25% power on down going lower let's say at around 10% Things get
    ugly, the atcual heating load using true rms voltage measurements at the
    load and known resistance it appears to be 250 watts when controls are at
    this low setting. I belive that. I should be using RMS sensors I know this
    but the error using average sensors is only 1.111 X avaerage.

    .. Well guess what happens when I hook up true RMS current meters? It reads
    Higher! It says there is 4 amps on L1 and so does my average sensor using
    1.111 correction. The current on L2 reads 6.5 but the average sensors
    says 3.4! SO the RMS reading is double on this side! In other words it
    agrees with one but not the other.

    .. This tells me that we have a total of 10.5 amps in the system. Well
    that's about 1250 watts. There is no way that load is on the engine. If you
    bypass the controllers with a known 1200 watt load you can clearly here the
    engine groan. So where is the other 1050 watts of power going or is not
    really there? I think it's either there and being stored in one side of the
    stator or it's apparent or reactive power becuase there is a phase
    imbalance.

    Do you see what I see? Here is what I think is going on. As the voltage is
    off longer there is more energy stored and there is an imbalance of load. It
    appears the stored enegery is being pushed back to the side with lower load
    on it and the system is now has inductance or reactive power in it. I think
    my rms current measurments prove it.
    It is a resisitive load but the generator is bascilly an electric motor and
    this is a closed system. Once you start turning off the voltage longer
    things go haywire.

    Keep in mind that we want the actuaual power to the shaft. If the power
    factor was alway's 100 then not an issue. I am no expert but it sure looks
    to me like we do have a big power factor problem. I think balancing the load
    would help. I guess my question is, just WTF is going on?

    I think the best way to deal with this is to correct the power factor if
    that is the problem, not try to measure it.

    Thanks.
     
  9. daestrom

    daestrom Guest


    Well this is 'mostly' right. When the triac is conducting, the current
    and voltage are in phase. If you go by the older idea of power factor,
    (pf=cos(theta)), then with 0 degree phase shift you would conclude that
    pf=100%

    When you're firing the triacs very early (nearly 0 degree firing delay),
    this all works out pretty much okay.

    But power factor is *really* defines as pf=W/VA. And measuring the
    current when you use this type of controller is a bit tricky. For
    example, when you fire the triacs at 90 degrees, there are some
    harmonics to the fundamental frequency current so a typical ammeter
    won't give accurate results.

    On top of this, is the issue of actually measuring the current. A
    typical D'Arsonval movement ammeter (an 'analog meter') responds to
    *average* current, not RMS. The meter face is re-calibrated by a factor
    of 1.1 so that it reads out RMS if the current happens to be sine wave.
    So determining the current is not so easy.

    A few meters are 'true RMS' and will read a different value than the
    typical D'Arsonval meter when used on a waveform like the current that
    flows in this case (90 degrees firing delay).
    He's wrong here. The triacs are not changing the phase relationship
    between V&I.
    The current measurement can have errors in it as I mentioned above, so
    that can introduce error in power measurement.

    If you had a meter that digitally sampled the current very fast and gave
    you the 'true RMS' reading, you won't have any error.

    Or a power meter that digitally samples the instantaneous voltage and
    multiplies by the instantaneous current, that will give an accurate
    reading as well.

    daestrom
     
  10. Bob

    Bob Guest

    Thought I would follow up. I installed a balanced load
    and it's like magic. Everything works fine even at late firing
    of the controllers. All measurnets are correct. One side still wants to
    drift a bit but I think the simple answer is a load balance capacitor. Any
    thoughts?
    I think this kinds proves my case that the controllers are making the system
    reactive.
     
  11. Get a good power electronics book. You will find that when using a phase
    angle fired controller, at any power level other than 100% on (0 degrees)
    you get BOTH a reduction is true power factor (W / (Vrms*Irms)) and
    displacement power factor (angle between voltage and current). Yes, Yes,
    the current is in phase with the voltage when during the portion of the half
    cycle after the firing BUT the fact that you are not getting a full half
    cycle of current causes a phase shift between the voltage and current at the
    fundamental frequency. This is explained in many power electronics books.
    When you do an FFT on the resulting waveforms (say at 50% power) you will
    see that the fundamental component of the current is shifted (lagging).

    What this means is that if you use this type of controller on a purely
    resistive load, at other than 100% power, your displacement power factor
    will be less than unity.

    It is quite easy to test with a light dimmer, an incandescent bulb, and a
    good PQ meter like the Fluke 43B. The Fluke will show both true power
    factor and displacement power factor. Changing the dimmer from full on
    through the range to zero on you will see the displacement power factor
    drop.

    If you had attended EPRI's Power Quality Assurance conference in New York
    last year you could have seen a paper presented on this very topic. It is
    not much of a problem when you are dimming a light bulb, BUT when you want
    to control a 4kV, 2 MW resistive heater....then it can have a negative
    impact on your bill (power factor penalties are not uncommon).

    Charles Perry P.E.
     
  12. Tom

    Tom Guest

    Charles that was most informative. Thank you.
    I agree the power factor should change off 100% on but it's not untill You
    get below 50 or 40% load.
    The total load is 6600 watts of heaters. With one controler or 2 in line
    There is no error in Power factor untill 40 or 50 % power reduction if there
    is then it must be very small
    This could be becuase the controllers are not standard light dimer. They
    cost 7 bucks and these cost 100 Each.
    Or perhaps the genset has some built in PF correction and we don't see the
    error?
    Anyway, thanks for help. At least we know just what is going on. Just
    guessing
    but based on readings it looks the PF is dropping from to about .08 at the
    low end of power on.
    ..
     
  13. Gerry Weatherford

    Gerry Weatherford

    2
    0
    Jan 27, 2010
    Phase angle shift

    I was searching for information regarding phase angle shift and found this forum, and liked what I have read. I am sure my question can answered from here. I am a Journeyman electrician, and former nuclear power plant operator on submarines. I am currently attend an HVAC school and I disagreed with a statement made tonight concerning the use of a "potential temperature coefficient" starting device. It was stated that when the PTC experiences LRA at start up it heats up and adds resistance to the circuit and thus "changes" the phase angle.
    I did not buy this, and argued that a resistor cannot change phase angle. But, I may be wrong. Could the added resistance change the total current in the circuit sufficiently to alter the inductive field flux and thus have an effect on the phase angle?
    My intuition tells me no, but I await the answer you have to this.
    Thanks
    Gerry
     
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