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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

    Yes, yes, I'm familiar with two phase motors and the 'shift' of the
    flux. But since the disk is not ferromagnetic, that alone doesn't
    create a torque (i.e. it's not a reluctance motor). So we have to have
    an eddy current induced in the disk (a 'rotor current') and to my mind
    it's the interaction of that eddy current with the magnetic field that
    produces a torque. Much like the current in the squirrel cage of a
    conventional single-phase motor.

    A disk that was composed of numerous 'pie slices' insulated from one
    another would not work. Right?

    R/X near 1 would certainly work, but an R/X near infinity might not
    (i.e. there has to be some inductance). With the eddy currents exactly
    in phase with one winding's flux, the currents would decay before the
    other winding's flux increased very much. I see your point about R/X
    near unity, after all as you said class D motors have some of the best
    starting torques and that is how they achieve this.
    Well without getting into 'gawdoffal' calculations, it just seems that
    if the eddy currents in the disk lag behind the flux that produces them
    (i.e. the disk has some inductance) then the torque pulses would seem to
    lag behind the instantaneous power 'pulses' that occur at twice line
    frequency.

    So I wonder how close the disk is operating near 'synchronous speed'.
    Where I think the synchronous speed in this case is function of
    mechanical shift between current and voltage coils (i.e. the equivalent
    'number of poles' arranged around the rotor). But also the phase shift
    between current in the two coils (which of course depends on connected
    load and R/X of potential coil). That is, the time shift between the
    flux of one pole and the other.

    If the maximum power to be registered is anywhere near a fraction of
    this 'sync speed', then you effectively have a significant variation in
    slip of the rotor. Of course some motors have a pretty flat torque
    curve at the lower end of speed when starting and for a given applied
    voltage/current, that's what we want here. So that torque produced is
    *not* a function of %slip but only the measured power. Or variations in
    %slip are tiny because the 'sync speed' is so much higher than expected
    speed of the rotor.

    (ouch, my head's starting to hurt ;-) )
    All of the references I've found on the matter seem to say that the
    potential coil does have a low R/X so that magnetic flux is nearly 90
    lagging from applied voltage. If it's something 'in between', wouldn't
    that give you some calibration issues? For example, if R/X is 1 in the
    voltage coil, then a connected load with a current lagging by 45 degrees
    would be exactly in-phase with the voltage coil's current. No torque
    produced but the customer is getting power (at 0.707 pf)

    daestrom
     
  2. Guest

    Guest Guest

    ----------
    I'm not even considering a ferromagnetic disk - and a good example of an
    unbalanced two phase motor is an drag cup control motor. A beer can stuck on
    a nail in a rotating field will run as a motor. All that is needed is a
    conductor and a component of the stator field that "shifts". (A paper clip
    will also rotate- done both)
    ---
    Not necessarily but I would expect poor performance. If the slices were
    connected only at the center and periphery then it may actually work better
    than an unsliced disk. Messy to analyze.
    --
    Using conventional steady state induction motor models, the slip at which
    maximum torque occurs is determined by s =R/X The magnitude of this
    maximum is independent of R/X. If R/X is infinite - infinite slip is the
    result and the torque at any finite speed is 0.
    However, consider a coil carrying a steady state sinusoidal current and
    producing a flux in phase with the current. This flux induces a disk voltage
    (d(flux)/dt which will lag the coil flux by 90 degrees. Now this voltage
    will produce a disk current lagging the voltage by 90 degrees. This disk
    current produces a flux in phase with the disk current -*Lagging the
    original current by 90 degrees*-and the disk flux will produce a voltage
    lagging by 90 degrees or 180 degrees out of phase with the original current.
    In other words, when a current flows, it will see a back emf and power is
    delivered to the disk and dissipated as I^2R loss in the disk. What you
    have with this single coil is a transformer and the inductances that we
    normally look at, which are the leakage inductances, do not enter into the
    transformer action.
    Any considerations of flux decay are transient conditions but since
    transient conditions are really not of importance in a KWH meter, this is a
    red herring.
    -------------
    So what? If you consider the flux due to each winding then you will have a
    product depending on flux sub I and flux sub V with the same relative phase
    angle as the original I and V and the same average over a cycle (even using
    the same starting point for the average).
    OK- you've got it. Look at 2 phase drag cup control motors (and your Navy
    experience may have introduced you to them) These operate near standstill
    where ideally torque is independent of slip.
    -----------
    The R/X ratio is not simply that of the voltage coil itself. In fact a low
    value is what one will expect. When one considers the effect of an R/X ratio
    in a motor , the rotor R and the total X is considered. The disk has an
    effect that is included. In fact the resistance and the leakage inductance
    of the coil would ideally be 0- as in an ideal transformer. This also
    means that the effect of the current winding and a hell of a lot of
    geometrical considerations are important. I used the R/X ratio because, in
    conventional steady state analysis, it can be shown that this is an
    approximate maximum torque condition. Ideally the coil would have 0
    resistance, 0 leakage inductance leaving only mutual inductance because
    these do introduce a voltage drop in the winding which is a factor to be
    considered. In this situation the R/X ratio is that of the disk itself.
    When I said "you don't want it to have flux in phase with the current" I
    should have said "you don't want it to have voltage coil flux in phase with
    the current coil flux"

    I have a feeling of going in circles because what is needed is an organized
    step by step analysis which is hard to do without equations and diagrams.
    Anyhow, you have presented intriguing questions .

    Much of what I knew about these meters in particular has long faded so I am
    looking at basic induction motor theory. I do recall that there are
    corrective adjustments in the meter such as magnetic shunts, which can be
    used in calibration. Obviously they work.
     
  3. Dave Foreman

    Dave Foreman Guest

    Don
    Do you have any more information on the second book by White and
    Wilson? I used AddAll and found quite a few copies of the book by
    Krause at very reasonable prices but did not find any thing on the
    second book.
    Thanks
    Dave Foreman
     
  4. Guest

    Guest Guest

    Oops. Correction:

    It is White & Woodson (of MIT), "Electromechanical Energy
    Conversion"Wiley 1959, Lib. of Congress No. 59-5874
    It is the original book that covers machines from a generalized basis and is
    quite mathematical , by page 30 it has covered electromagnetic force
    production basics and is starting on the Hamiltonian. The developments go
    on to general basic models which can be used, depending on the restraints
    applied, to represent any machine. Years ago, Westinghouse capitalized on
    this to make a generalized lab machine set where, depending on connections
    could be a DC machine, a 2 phase induction machine, a synchronous machine,
    etc-- unfortunately is was atypical and somewhat inferior machine in any
    configuration but did illustrate the theory.
    Amazon lists one copy at $165 -it's not worth that! Krause,, while sticking
    to more conventional machines is probably a better bet.
     
  5. Dave Foreman

    Dave Foreman Guest


    Thanks Don
    Dave Foreman
     
  6. daestrom

    daestrom Guest

    I was thinking of completely insulated, so there was no 'complete
    circuit' in the rotor.

    Thanks for the discussion, it helped refresh my mind how how these
    buggers work. Of course the new digital ones are a whole different story.

    daestrom
     
  7. Guest

    Guest Guest

    If no rotor currents cannot flow (or are restricted as in the situation you
    mention- which is analogous to the concept of laminations to minimize eddy
    current- then there will be little or no torque.

    I have since run across a site which indicates that the flux from the
    voltage coil is adjusted , apparently by a shading coil, so that it is , at
    unity pf, 90 degrees out of phase with the current coil flux. Beyond this
    the site really didn't get down to specifics other than considering the
    meter as a type of 2 phase meter where torque at standstill (or at near
    standstill) will be essentially directly proportional to current at a given
    voltage. One could take a 2 phase control motor, add gears and come up with
    a watthour meter. It is interesting that the "Ferranti effect" which is
    related to induction machines, was known before Tesla came up with the
    polyphase induction motor.

    You have the term "bugger" correctly applied but in polite society it is
    the "Bougerre Factor".
     

  8. This "PROTEUS" retard is the WebTV idiot that has claimed for months to
    be a computer program, and not a person at all. Then, the dope goes and
    lets his emotions run away with him every time he posts.

    Not only does this idiot need help, but merely asking him to seek it
    isn't enough. He needs to be reported to someone that will actually
    check him out and commit his aberrant ass to an institution.

    I think that "Proteus IIV" is a danger to himself and others. He
    should be placed into a mental institution or jail as is the current
    practice.

    Go away, ProTardeus, there is no help for your illness here.
     
  9. Guest

    Guest Guest

     
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