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Re: Balancing the Breaker Box

Discussion in 'Electrical Engineering' started by Bill, Nov 22, 2009.

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  1. daestrom

    daestrom Guest

    I disagree. Analog computers were around long before digital (the
    speedometer in a vintage car is an obvious one).

    The three coils create a torque on the disk proportional to real power.
    The arrangement of the coils and inductance of the voltage sensing
    coil are such that the power factor *is* implicit in the development of
    the torque. A load current that is 90 degrees out of phase with the
    applied voltage results in no torque developed. (which is why many
    claims that 'power factor correction' will save you big money, are bogus)

    If that were the only torque applied to the disk, it would spin rather
    rapidly because the mechanical friction is pretty small. And it would
    be hard to calibrate since the torque needed to drive all the gearing is
    somewhat variable. So another permanent magnet called a "drag magnet"
    is positioned next to the disk creates a counter-torque that is
    proportional to the disk's speed. Obviously the disk stops
    accelerating/decelerating when the torque produced by the sensing coils,
    friction and the drag magnet balance. The result is that the disk speed
    is proportional to the power in the sensing circuits.

    (As an aside, some criminals have tried to cheat the power company by
    positioning additional 'drag magnets' above/below the disk. This tends
    to increase the counter-torque developed when the disk is in motion,
    slowing the speed of the disk for a given power level. The law does
    provide sanctions for such meter tampering.)

    All the gears and wheels form an integrator (as in 'calculates the
    integral of the disk speed with respect to time'). Because the disk
    speed is 'revolutions / time', counting the revolutions is the calculus
    operation of integration with respect to time.

    It's a meter that calculates the energy usage by sensing voltage,
    current and the phase relationship between them (i.e. power factor) and
    integrating the results over time.

    daestrom
     
  2. krw

    krw Guest

    Certainly, but this doesn't change the fact that PF is not the
    measured quantity.


    Yes, others have clarified this better than I. I have no clue why
    he's still pursing this.
     
  3. daestrom

    daestrom Guest

    One could argue that such a speedometer 'calculates' the instantaneous
    speed by taking the derivative of distance. The turning of the speedo
    cable is a measure of distance. It drives the odometer directly. The
    magnet coupling develops a torque on the needle that is proportional to
    how fast the speedo-cable is turning (i.e. the derivative of distance
    traveled). The spring provides a counter-torque that converts the
    torque created in the coupling into a position (i.e. for a given
    coupling torque, the spring provides an exact counter-torque when the
    spring is stretched to one particular position). Just another example
    of an analog computer.

    The way a kWh meter works is the reverse of this. It develops a torque
    proportional to VA*pf. Vary the power factor and the torque developed
    in the disk varies. A drag magnet develops counter-torque proportional
    to disk speed. The result is disk *speed* is proportional to VA*pf.

    Your argument that the disk somehow measures power directly is false.
    The magnetic fields are created by a voltage sensing coil and two
    current sensing coils. Period. With these two magnetic fields, you
    don't have 'power', the best you have is VA. The meter shifts the phase
    of the voltage sensing field by 90 degrees by using a highly inductive
    coil. Then the three magnetic fields are arranged around the disk such
    that a torque is developed only when the fields sequence properly. That
    torque is maximized when the current is in phase with the applied
    voltage (one current coil's magnetic field leads the voltage coil's
    magnetic field by 90 degrees and the other current coil's magnetic field
    lags the voltage coil's magnetic field by 90 degrees).

    Some commercial meters can be modified to measure VAR (V*A*sin(angle)
    instead of V*A*cos(angle)). They too only have current and voltage
    sensing coils, but by careful arrangement their magnetic fields interact
    with different timing.

    <snip>
    daestrom
     
  4. daestrom

    daestrom Guest

    Nonsense. Ever see a 'power factor meter'?? I have, on old switchgear
    systems. These only measured the phase-based power factor (harmonic
    content wasn't a problem in these old systems). But they simply
    developed a position based on the phase difference between the applied
    voltage and current (one input polarized a moving vane in a magnetic
    field created by the other input). Because the position of the needle
    actually showed the phase angle, the meter face was marked in a cosine
    pattern (i.e. the distance between 0.9 lagging position and the 0.8
    lagging position was small, but the distance between 0.1 lagging and 0.2
    lagging was larger).

    The position didn't vary with the magnitude of the voltage or current,
    only the phase relationship (of course if current was below a certain
    threshold, the meter wasn't reliable)

    daestrom
     
  5. krw

    krw Guest

    One could argue that the Earth is at the center of the universe too,
    and that the universe indeed exists inside the shell of the Earth. The
    math gets annoying and Occam doesn't like the idea, but it could be
    argued.
    Interesting. How do they deal with harmonics?
     
  6. krw

    krw Guest


    Exactly. They don't deal with harmonics, thus are really phase angle
    meters.
    So what? Without measuring harmonic content, they only measure phase
    angle not PF. ...unless you assume the world is a sine wave.
     
  7. krw

    krw Guest

    I know that you're the stupidest hack on the Usenet, and that is
    pretty damned stupid, Roy.
    Cheap shots for dumb asses.
    No, you are Roy the Retard.
     
  8. Guest

    Guest Guest

    ---------------------

    It appears that you may be confusing capacity and utilization. Substitute
    utilization for capacity and you'll be OK.
    You have a 100A, 240V system which is center-tapped so that you can get 100
    A at 120 V on each leg. If the load is balanced, the center-tapped or
    neutral connection has 0 current. The capacity of the system then is 100A
    at 120/240V which works out at 12KW per leg for a total capacity of 24KW.
    If you put all the load on one leg, then you are (current) limited to 12KW .
    The other leg is doing nothing. The capacity of the system is unchanged but
    is only half used.

    Here's a little chart: (120V per leg)
    Leg1 leg2
    total neutral utilization
    current 100A 0
    100A 100A 50%
    KVA 12 0
    12
    losses 1 units 0
    2 units 1unit

    current 50A 50A
    50A 0 50%
    KVA 6 6
    12
    losses 0.25units 0.25 units
    0.5units 0 units

    current 100A 100A
    100A 0 100%
    KVA 12 12
    24
    losses 1 unit 1
    unit 2 units

    current 75A
    25A ---
    50A 50%
    KVA 9
    3 12
    losses 0.56units 0.06
    units 0.88units 0.25units

    (unit taken as single wire I^r loss at 100A)

    I could quibble with you a bit with regard to the motors- it is the load
    that determines the torque requirement at any given speed. Yes, speed and
    voltage are related as are torque and current.-you have that right- but the
    intersection of the torque speed curves of motor and load, determines the
    operating point.
    Note that the watthour meter is a type of induction motor and thus is
    limited to less than synchronous speed. In fact, it would sepf destruct at
    that speed and is operating in a region where it is nearly at standstill so
    the torque is effectively constant at a given power, within the operating
    range. The drag torque is linearly speed dependent.

    However krw may have been referring to the fact that the inherent
    measurement doesn't deal with power factor as electromechanical wattmeters
    (dynamometer) and watthour meters inherently measure the instantaneous
    product of voltage and current and average this to get the true power
    without any consideration of power factor. So does a digital meter. They
    don't use power factor because of this-effectively they go back to
    fundamentals to make measurements. If we are using voltmeters and ammeters
    then we don't have this multiplication and averaging of the instantaneous
    values so have to account for phase. Power factor is one way to do this.
    We also use rms voltage and current- but these don't actually exist - what
    exists are sinusoidal (hopefully) voltages and currents. and their
    instantaneous product.
     
  9. Guest

    Guest Guest

    ---------------------

    It appears that you may be confusing capacity and utilization. Substitute
    utilization for capacity and you'll be OK.
    You have a 100A, 240V system which is center-tapped so that you can get 100
    A at 120 V on each leg. If the load is balanced, the center-tapped or
    neutral connection has 0 current. The capacity of the system then is 100A
    at 120/240V which works out at 12KW per leg for a total capacity of 24KW.
    If you put all the load on one leg, then you are (current) limited to 12KW .
    The other leg is doing nothing. The capacity of the system is unchanged but
    is only half used.

    Here's a little chart: (120V per leg)
    Leg1 leg2
    total neutral utilization
    current 100A 0
    100A 100A 50%
    KVA 12 0
    12
    losses 1 units 0
    2 units 1unit

    current 50A 50A
    50A 0 50%
    KVA 6 6
    12
    losses 0.25units 0.25 units
    0.5units 0 units

    current 100A 100A
    100A 0 100%
    KVA 12 12
    24
    losses 1 unit 1
    unit 2 units

    current 75A
    25A ---
    50A 50%
    KVA 9
    3 12
    losses 0.56units 0.06
    units 0.88units 0.25units

    (unit taken as single wire I^r loss at 100A)

    I could quibble with you a bit with regard to the motors- it is the load
    that determines the torque requirement at any given speed. Yes, speed and
    voltage are related as are torque and current.-you have that right- but the
    intersection of the torque speed curves of motor and load, determines the
    operating point.
    Note that the watthour meter is a type of induction motor and thus is
    limited to less than synchronous speed. In fact, it would sepf destruct at
    that speed and is operating in a region where it is nearly at standstill so
    the torque is effectively constant at a given power, within the operating
    range. The drag torque is linearly speed dependent.

    However krw may have been referring to the fact that the inherent
    measurement doesn't deal with power factor as electromechanical wattmeters
    (dynamometer) and watthour meters inherently measure the instantaneous
    product of voltage and current and average this to get the true power
    without any consideration of power factor. So does a digital meter. They
    don't use power factor because of this-effectively they go back to
    fundamentals to make measurements. If we are using voltmeters and ammeters
    then we don't have this multiplication and averaging of the instantaneous
    values so have to account for phase. Power factor is one way to do this.
    We also use rms voltage and current- but these don't actually exist - what
    exists are sinusoidal (hopefully) voltages and currents. and their
    instantaneous product.
     
  10. Guest

    Guest Guest

    I made no reference to the current rating, I just picked a power out of the
    air. It might have been clearer to you if I had said 12KW. as I wasn't
    considering overload conditions.
    [/QUOTE]
     
  11. Guest

    Guest Guest

    -----------------
    I have to disagree here, the instantaneous torque is proportional to
    instantaneous va or power. Inertia averages this to get the average power
    over each cycle (or longer). VA*pf implies that it measures the product rms
    voltage and current and applies a bugger (oops power) factor. The meter
    "sees" none of these because they are not physically there. the equivalence
    is there in that the time varying instantaneous values can be represented by
    their rms frequency domain equivalents in steady state. Analysis of the
    meter uses the rms approach as you have done (it avoids a nasty mess of
    non-linear differential equations) but the meter doesn't.
     
  12. daestrom

    daestrom Guest

    In the 30's and 40's, when these were often used, the world of
    electrical loads was a sine wave :)

    daestrom
     
  13. krw

    krw Guest

    I know. I used to have such meters (a mechanically resonant Hz meter
    too). My father was a power engineer and *collected* stuff (several
    tons of such stuff in the attic when he passed).
     
  14. krw

    krw Guest

    30%, 40%, maybe more of the VA is in harmonic content, particularly
    with electronics. Switching power supplies are often *very* bad.
    They deal with it perfectly. We keep telling you that they measure
    *POWER*.
    You can calculate PF from VA and power, sure, but that's not the same
    as measuring PF. Your logic is backwards.
     
  15. daestrom

    daestrom Guest

    But wouldn't you say that the torque developed is a function of the
    phase angle between current and voltage? If the current lags 90 degrees
    from the applied voltage, then the magnetic fields of the current coil
    and voltage coil are almost exactly in-phase (owing to the high
    inductance in the potential coil). With the two magnetic fields pulsing
    'in-phase', there is no torque either forward or reverse developed.

    The simple fact that power flow in the opposite direction develops
    torque in the opposite direction shows that phase-angle between current
    and applied voltage is 'built-in' to the device.

    Yes, of course you're right that the inertia *averages* out the torque,
    but it's not 'instantaneous va', it's 'instantaneous power'. But
    'average power' *is* rms-volt * rms-current * power-factor in a
    sine-wave only system (i.e. no harmonic content)

    daestrom
     
  16. Guest

    Guest Guest

    ---------------
    I have no problem with this. The meter doesn't measure phase angle.
    Note that the induction disc motor is essentially a form of single phase
    motor. The disc wont start but if already rotating there will be a torque
    bias in that direction. A single phase motor depends on this bias which is
    greatest near synchronous speed. However, in the case of the induction disc
    there will be very little unbalance torque -if any as the torque speed curve
    is essentially (and desirably) constant torque in the operating region
    (just about standstill) There will be a pulsating torque.
    Yes it is, in the same way it is built into a conventional electromechanical
    (dynamometer) wattmeter. The torque at any instant depends on the product
    of instantaneous voltage and instantaneous current. On this basis, it simply
    averages the instantaneous torque. If the voltage and current are 90 out of
    phase, the instantaneous voltages and currents result in a double frequency
    power with 0 average

    Mathematically we can say -for sinusoids:
    p(t)=Vmcos(wt)*Imsin(wt+phi) =(VmIm/2){cos(phi) +a second harmonic power
    term with 0 average]
    The average over a period is (VmIm/2)*cos phi +0 =Vrms*Irms* cos(phi)

    Physically the meter simply produces a torque which is proportional to the
    instantaneous power and inertially averages it. Specifically, it doesn't
    measure phase angles, rms voltages or find pf -that is my point.
    If the voltage and current coils have high R/X values then the meter will
    be able to handle distorted waves with reasonable accuracy (and without
    doing a fourier analysis). A digital KWH meter will simply do the same
    v(t)*I (t) and averaging as a mechanical meter but with a few bells and
    whistles can be made to measure KVAH (pf*H is possible but meaningless) as
    well but such measurements aren't required or necessary for determination
    of energy.
    instantaneous power = v(t)i(t) This happens to be the same as instantaneous
    va but I should not have used that term. The concepts of VA and VAR's are
    related to phasor analysis which is a mathematical model which gives us the
    pertinent information without the labor of solving a mess of differential
    equations. At your 120V outlet- there is no actual 120V source as can be
    seen if you examine the voltage with an oscilloscope.

    Certainly, use of rms volts(magnitude)*rms current magnitude *power factor
    will give the same average power. That's part of the reason we use this
    model- it works.
    Also, this "model" allows us a reasonable chance of solving not-so-simple
    circuits. Think of what the situation would be in solving load flows for
    large systems using differential equations! Phasor analysis essentially
    replaces these differential equations by algebraic equations.
    Another convenient model is the use of symmetrical components for fault
    studies.
    Another is the use of forward/backward fields to model a single phase
    machine as two opposing machines.
    In these cases there are direct relationships between the model quantities
    and the actual quantities present.

    You know all this. The point of this long winded diatribe is that, too
    often, people think that the model is the actual thing.
     
  17. krw

    krw Guest

    Neither. It's a harmonic load. ;-) Uncorrected it has a very crappy
    current waveform.
    A fully loaded motor should have a PF pretty close to unity. What any
    particular VFD does to it is anyone's guess. I suggest you buy a
    "Kill-A-Watt" or "PowerAngel", or some such. These things are about
    $25 and will report V, A, VA, W, PF, F, kWH, and H (did I leave
    anything out?). You can then play with various loads to get a feel
    for what they're about.
     
  18. krw

    krw Guest

    Roy, you are the one why has been spraying graffiti all over this
    newsgroup for the past year.
    You're a liar, Roy.
    You've been visiting the gay web sites again, Roy. Mommy doesn't like
    that.
    You are LIAR.
     
  19. daestrom

    daestrom Guest

    (you're too kind :)

    I understand that the 'true' measure of power is instantaneous V *
    instantaneous I and that that can simplify to simpler terms in certain
    specific situations (such as DC or sine waveforms).

    I guess I just can't 'wrap my head around' the meter responding to
    instantaneous V*I when the magnetic field from the potential coil is
    delayed nearly 90 degrees from V.

    The only way I can make sense of it is if the eddy currents in the disk
    are highly inductive (much like they are in a conventional single-phase
    induction motor near stalled conditions). If the eddy currents lag the
    air-gap flux by nearly 90 electrical degrees, I can see a torque
    developed proportional to real power.

    But the lag in eddy current would mean the torque pulse also is delayed.
    That would mean that a 'peak' of instantaneous power (peak V and I for
    a resistive load) creates a 'peak' of torque a quarter of a cycle later
    when the instantaneous power has actually dipped to zero. And that
    momentary zero instantaneous power that occurs when V and I are
    zero-crossing is not 'sensed' by the disk until another quarter cycle
    when the eddy currents in the disk are zero-crossing and developed
    torque is momentarily zero.

    I know that for revenue purposes, the meter's response to harmonics
    caused by non-linear loads is acceptable enough, but with both the
    inductance of the potential coil and the disk, I can't help but wonder
    just how accurate they can be with large loads of this type. Seems the
    currents induced in the disk by a non-sine current through the current
    coil would not be a perfect match and thus not perfectly accurate.

    daestrom
     
  20. Guest

    Guest Guest

    ---
    The voltage coil and current coils are not co-axial. If the voltage coil is
    displaced from the current coil, then when the current is maximum, the flux
    of this coil is maximum but that of the voltage coil is 0. When the
    voltage coil produces maximum flux , the current coil flux is 0. So , as
    seen from outside, there is a shift of flux .
    Ideally, if the voltage coil is 90 degrees apart in space from the current
    coil, you would get a "2 phase" motor with only a rotating field . Since
    the displacement is not 90 degrees, you get a weaker rotating field as well
    as a pulsating field. Another example is a shaded pole motor as used in
    small fans.
    I think you have part of it but a conventional single phase motor without a
    starting winding will, at start, have 0 average torque but a healthy
    pulsating torque. If you have a second winding which is ideally 90
    degrees electrically (physically 90 degrees for a two pole machine) AND the
    flux due to the second winding is delayed 90 degrees in time, then you have
    a situation where you have a true rotating field with no pulsating torque-
    as in a 2 phase motor, 3 phase motor or a single phase motor with a
    dominant capacitor on the second winding.
    A single phase meter lies somewhere in between -think of the voltage coil as
    a starting winding which is always in the circuit. It may not be in
    physical quadrature (e.g the shaded pole doesn't shift flux by 90 degrees)
    The torque will have pulsations as well as a steady component but the
    pulsations have a 0 average.
    Should the disk R/X be low? Since the "start winding" is always in service
    the physical position of this winding with respect to the other winding
    provides the needed bias. It seems, on a first consideration, that a R/X
    ratio near 1 in the disk may be desirable in that the torque will be nearly
    constant and maximized near standstill- just as in the case of a class D
    induction motor. --
    The disk doesn't see the voltage and current directly but does respond to
    the total of currents in the v and I windings interacting with the currents
    in the disk. You can then consider a model with two stator coils and two
    rotor coils and wade through a bloody "gawdoffal" mess of math (task given
    to grad students) to come up with an expression for torque dependent upon
    both the position of the coils and the currents in them to find an
    instantaneous torque expression. Then you can consider steady state
    (sinusoid) at some given speed ( in any motor the torque has nothing to do
    with Ldi/dt but is affected by I*dL/dx* dx/dt or the speed voltage) and come
    up with a model using rms voltages and currents and the phase between them.
    The fact that this model typically agrees with simpler approaches used in
    typical steady state handwaving analysis of induction machines is
    comforting.
    Krause "Analysis of Electric Machinery" deals with some of this (at near
    graduate level) and an old book by White and Wilson "electromagnetic energy
    conversion" is another- this is definitely a graduate level text. Many
    other texts simply jump into the steady state models- a bit more practical
    but losing some concepts.
    --
    The voltage coil may not have a low R/X ratio. However, you don't want it
    producing flux in phase with the current -
    High harmonic content is still a problem because you do want the torque
    independent of winding impedance- variations which comes back to the
    desirability of an R/X that is on the high side.
     
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