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120V from both legs

Discussion in 'Electrical Engineering' started by John Doe, Oct 4, 2004.

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  2. Dan Lanciani

    Dan Lanciani Guest

    |
    | | > In article <[email protected]>, (Don Kelly)
    | writes:
    | > | The main illogic (other than some numerical problems)
    | >
    | > Could you detail the numerical problems so I can fix them?
    | >
    | > | is that it is assumed
    | > | that the meter is somehow measuring power loss "upstream" of the meter.
    | >
    | > That's not the assumption; it's the result. :)
    | >
    | > | It
    | > | doesn't- It measures the power on the consumer side of the meter. This
    | will
    | > | include losses on the consumer's side - for which the consumer is
    | > | responsible - not those on the utility side.
    | >
    | > I think pretty much everyone else now agrees that the typical 4-terminal
    | > (i.e., "1.5 element") split-phase meter does indeed measure power losses
    | > in the neutral on the utility's side of the meter. If you do not agree
    | > could you explain exactly how the meter avoids this error?
    | >
    | > | I suggest that you check Charles Perry's reference
    | >
    | > Mr. Perry has posted a reference and provides one way of looking at
    | > the error as proportional to "1/2 of" the voltage difference. (I quote
    | > "1/2 of" since it doesn't actually add any information to a proportional
    | > relation, but I think we know what it means in this context.) I prefer
    | > to think of it in terms of the customer paying for 150% of the loss in
    | > the utility's neutral wiring, but that's just for the simple one-meter
    | > case.
    | >
    | > Dan Lanciani
    | > [email protected]*com
    |
    | Dan,
    |
    | You seem to having trouble with the handbook reference.

    No, I was trying to scrupulously avoid ambiguous terminology, but in the
    process I made it sound more confusing than I had intended. :) Normally
    when we say that A is proportional to B we imply some unstated multiplicative
    term (which could include constants and other variables) such that A = B * C.
    Since C could absorb any constant, saying that A is proportional to B means
    that A is proportional to B/2 or 2*B or 100*B for that matter. The danger in
    saying that A is proportional to B/2 is that it could be taken to mean that A
    is _equal_ to B/2, since otherwise why would you have included the meaningless
    constant. As I said, I believe we all know what the 1/2 means in this context,
    but I wanted to be clear.

    I apologize for the pedantry. I think I was worried that if I wasn't very,
    careful with my wording one of the hand-wavers would pick up on it as a
    reason to "debunk" me again...

    Dan Lanciani
    [email protected]*com
     
  3. And we should believe you? You still have not explained this "back emf"
    that is traveling from motors out into the power system. Back emf occurs in
    the motor as a result of the turning rotor. It creates a voltage (back emf)
    to oppose the voltage on the motor terminals. This reduces current, that
    otherwise would only be limited by internal resistance. The effect is
    reduced current entering the motor. If you externally turn the shaft of a
    motor at exactly the correct speed, you can balance the back emf and the
    terminal voltage and get zero current. If you externally turn the shaft
    faster, the back emf can exceed the terminal voltage and you get generated
    current. Of course, with an induction motor you have to have a source of
    vars since an induction motor, or generator, is always a sink for vars,
    never a source. So far the back emf does not really effect the utility,
    except the generator case. During system disturbances, motors can be
    sources of fault duty because of this. But under steady state conditions,
    the back emf doesn't travel anywhere.

    What does effect the utility is reactive current. Induction machines are
    inductive (of course) and thus require a good deal of reactive current to
    function. The utility would prefer a customer supply his own vars of
    course. The closer the vars are supplied to the load, the more efficient
    the power system operates. Still doesn't answer the question as to what you
    are referring to when you say "back emf". Perhaps you would like to
    explain? And since back emf is a voltage by definition, are you saying a
    voltage travels from the motor into the system?

    This might help you some:
    http://farside.ph.utexas.edu/teaching/302l/lectures/node76.html
    http://www.phys.unsw.edu.au/~jw/HSCmotors.html#back
    http://electron9.phys.utk.edu/phys136d/modules/m8/devices.htm

    Charles Perry P.E.
     
  4. Don Kelly

    Don Kelly Guest

    ----------
    Excuse me. On what do you base this?. If the voltage at the terminals has
    negligable harmonics the only source of harmonics in the current are due to
    saturation effects if the motor is operated at a voltage above the rated
    range. In the case of a motor where there is an air gap, the harmonic
    content of the exciting current is small. Power factor correction tends to
    raise voltage -so that, if (?) this is a problem, the problem is worsened.
    There are harmonics in the field produced-even with a sinusoidal current-
    these exist with or without pf correction and are due to winding
    distribution and slot distributions in the motor - without rebuilding the
    motor there is nothing that can be done to "correct" this. Typically the
    7th spatial harmonic is cited asthe dominant one and this will produce a
    local peak in the speed torque curve at about 1/7th the synchronous speed
    but will have essentially "sweet toot" effect on the operation at normal
    speed. This is nothing new, nor is it generally significant ( I had a
    textbook from the 40's-50's which dealt with this quite nicely). External
    capacitors or tuned filters will not deal with this.
    By the way- slip is essential for the production of torque in an induction
    motor so the statement that slip makes things go south fast does not appear
    to be based on any understanding of induction motors.
    Now - with electronic drives- then there is a harmonic problem- and this
    can affect other electronic drives. I have a copy of a former student's PhD.
    thesis dealing with this problem.
     
  5. Don Kelly

    Don Kelly Guest

    ----------
    I am not one Charles' "PhD references" but do hold a PhD (EE) from
    University of Illinois, Champagne Urbana and spent 35 years teaching in the
    areas of power systems and machines (as well as having spent some time in a
    utility environment) and fully agree with Charles. From what he has been
    saying and what he has said before on other topics, it is clear that he
    knows and understands what he is talking about. I cannot say the same for
    you. I have been assuming and hoping that it is a communication problem.
     
  6. Don Kelly

    Don Kelly Guest

    I have re-read your various comments and have also looked at the "numerical
    error". Please accept my apology for a stupid, off the cuff, analysis. I had
    taken the sum of voltages as 118+120 - which is incorrect.
    In any case - I should have picked up on the fact that the result is not
    1180 watts.
    You are correct and I am red faced..
    Thank you
    --
    Don Kelly

    remove the urine to answer

     
  7. <snip>

    You give me the same reference that I gave you?! And it does not say
    anything about back emf effecting the grid.
    Hahahahahahahahahaha. You really should get a copy of IEEE STD 100. It
    would help you with your terminology errors. Back emf and reactive current
    are NOT the same.
    Odd, you give the same reference as support for your arguement and then say
    when I give the same reference that it is irrelevant.

    I think you should get an engineering education and then return to this
    group. You evidently are suffering from knowing a problem needs addressing
    (reactive current) but never learning the correct terminology used in the
    field. Just as Don Kelley, and I, said, you are having a problem
    communicating because you are "speaking the wrong language". Documents like
    IEEE STD 100 (it is a dictionary of electrical terms) exist so that people
    can discuss technical subjects using a common terminology. Of course in
    this case, any power systems text book would correct your error in
    terminology.

    Charles Perry P.E.
     
  8. Back EMF is a voltage.

    Back EMF is a voltage. It opposes the terminal voltage, but it doesn't go
    anywhere.
    Wrong answer. By steady state I mean the power system is supplying power to
    the motor which is turning a load as opposed to the power system having a
    fault and the motor acting as a generator supplying fault current.

    Wrong, back emf is a voltage.
    Now you are just being annoying. Back EMF, or counter EMF, as defined by
    IEEE STD 100:
    "The effective electromotive force within the system that opposes the
    passage of current in a specified direction."

    Now we look up EMF:
    "Electromotive Force. See Voltage"

    What was that? Oh no, it really is a voltage. Just as anyone familiar with
    Faradays Law would know.

    For your reference, this was out of IEEE STD 100-1988 but I doubt the
    definition has changed since then.

    Other references that will help you understand exactly what emf is:

    "Electromagnetics", Third Edition, J. D. Krauss, McGraw Hill, 1984, pages
    124-128.

    "A Programmed Review for Electrical Engineering", Second Edition, J. H.
    Bentley, K. M Hess, Van Nostrand Reinhold Company, 1984, page 91. (nice
    equation for emf, answer is in volts...imagine that)

    "Electric Machinery", Fourth Edition, A. E. Fitzgerald, C. Kingsley, S. D.
    Umans, McGraw Hill, 1983, pages 151-152. (Very nice explanation. They even
    use the term "speed voltage" to try to give an visual of what EMF is).

    "Electric Machinery", Fifth Edition, A. E. Fitzgerald, C. Kingsley, S. D.
    Umans, McGraw Hill, 1990, page 10. ( very nice illustration and discussion
    of emf..again as in induced voltage).

    "Standard Handbook for Electric Engineers", 12th Edition, D. G. Fink, H. W.
    Beaty, McGraw Hill, 1987, page 8.19. (nice formula for calculating emf.
    Sadly, for you at least, the answer is in volts).

    "Electrical Machinery", F. A. Annett, McGraw Hill, 1938, pages 119-137. (
    an old book but a great explanation of how motors work. It is particularly
    good for people without engineering degrees since it uses some very nice
    analogies...you should like this one.)

    These are just some of the books that I have in my office. I could come up
    with dozens more if I walked down the hall to our library.
    I think you would be well served to read the above mentioned texts.
    Even your own reference calls it a voltage. Sad really.

    You have shot your credibility with your posts. You have used incorrect
    terminology in nearly every post in this thread. When this was pointed out
    to you, you attacked those who knew what they were talking about. I suggest
    you reference a few text books before posting again. This may save some
    embarrassment on your part.

    Charles Perry P.E.
     
  9. I give up. I should know better than to try to teach electrical engineering
    to an electrician. Some just won't listen.

    Charles Perry P.E.
     
  10. Don Kelly

    Don Kelly Guest

    ----------------
    I am quite aware of what a motor does. You keep referring to a motor that is
    slipping- all induction motors slip - they must if they are to work. Most
    motors are induction motors. No slip- no mechanical output. Basic.
    Also please note that any motor must produce some back emf- in fact this is
    necessary- a consequence of conservation of energy- emf proportional to
    speed and current proportional to torque. No emf- then torque*speed = mech
    power=0. Again basic.

    If an induction motor is disconnected from the system, it will generate a
    back emf for a very short while. Adding pf correction at the terminals
    increases the time that this occurs. This can lead to problems if the motor
    is reconnected while this voltage exists- the problem will be with excess
    torque on the machine itself.

    If you are referring to a synchronous machine and this may be where your
    confusion is coming from- there will be a field controlled generated voltage
    (i.e. your back emf) and this is normal and necessary in operation.

    If a synchronous machine slips- there is a problem with system stability
    (in fact -the lower the emf generated the more likely is the chance of
    instability). That is a problem that will not be corrected by power factor
    correction (in fact a synchronous machine doesn't need external power factor
    correction) as it is a fairly complex dynamic situation involving the masses
    of the (more than one) synchronous machines coupled through impedances of
    transmission lines.

    The concept of EMF "escaping" into the system appears to be your own
    invention. You have given noting to back up this idea. The only reference
    that you have given so far is quite alright but sketchy and certainly
    doesn't support your contentions.

    I would suggest that you review <your> data base and add some energy
    conversion theory and concepts to it as you have, so far, shown a distinct
    weakness in this area, just as I would show a distinct weakness in
    electronic or RF technology.
     
  11. Don Kelly

    Don Kelly Guest

    ---------
    If so then the claim that you have made for harmonic distortion has no
    basis in fact. Please tell me the basis for your claim of harmonic
    distortions- you have evaded that question
    -----------.
    ---------
    Dealing with induction motors - there will be slip at any load and any
    voltage. Slip is needed for energy conversion in these machines which are
    the dominant industrial motors. Certainly, at any given torque, the slip
    will be higher at lower voltage (or, conversely, the torque will vary
    approximately with the square of the voltage) at any given slip- at rated
    voltage a slip of 3% may occur- at 90% voltage the slip will be about 4% -
    difference is 18rpm for a 4 pole 60Hz motor). I did not deny that. It also
    follows that at starting the available torque at 90% rated voltage will be
    81% of the normal starting torque.

    I suggest that you re-read what I said -not what you wish I said, . Pf
    correction can reduce slip ONLY if it improves the voltage at the motor
    terminals.

    I note that you have not produced a counter argument (or definite references
    to back up your position) to my point that the efficiency and other
    performance factors of induction motors depends on the terminal
    voltage(including its frequency) and the mechanical load on the motor- you
    don't correct the pf of the motor itself- you correct the overall pf of the
    "motor Plus capacitors". The difference is fundamental.
    -------------------------
     
  12. Don Kelly

    Don Kelly Guest

    --------
    Are you saying that back emf is a mechanical force? That is what mass times
    acceleration implies. Note also that a spring produces a force proportional
    to position and there is no acceleration involved. Also frictional forces
    depend on speed -not acceleration speed. Your "definition" of force is
    strictly one of several mechanical cases.
    Sorry- by all definitions- emf is a voltage only- the problem is that the
    name "electromagnetic force" was coined analogously over a century ago and
    this nomenclature leads to misconcepts such as you indicate. The analogy was
    that as a force can cause motion, a voltage can cause a current flow (note:
    "can" doesn't mean that it will).
    In a motor there will be a generated voltage opposing the applied voltage
    and this got the unfortunate name "back" emf. In fact, in analysis of an
    induction motor, it is not actually a useful concept. In a synchronous or DC
    motor it is just the voltage that is generated internally at a given speed.
    As with any voltage- it exists only between two points-it doesn't propagate
    or flow.

    Please note that no-one is saying that back emf doesn't affect current. In
    fact it better do so if a motor is to operate at any speed other than 0 or
    "fly apart".
    ----------
    Normal fluctuation of voltage (or frequency) on a power system is not
    considered any form of instability. If you are referring to power systems-
    "stability" has a particular meaning. Voltage instability has another
    meaning and this is not related to fluctuations at the 240V level. Unbalance
    does affect motor performance - that is true. Since loads on a system,
    particularly at the low voltage distribution level (i.e 240V) are generally
    not balanced the voltages seen at a particular point will be unbalanced.
    Neither harmonic or pf correction will deal with this. Again- emf =voltage
    doesn't propagate.
    ----------------
    -------------
    EMF or electromotive force according to the Oxford dictionary- "a potential
    difference between two points which tends to give rise to a current"
    (<tends> doesn't mean that it will actually do so-that depends on the
    circuit)
    "Back emf" is an emf opposing some applied voltage.I also checked with
    several university level texts.
    I note that texts on electric machines don't even bother with the term
    "back emf"or even the term "emf". A beginning circuits text mentions "so
    called" emf.
    The term is an anachronism and is disappearing because it gives rise to the
    nonsense that you have given above. You may give whatever interpretation to
    the words that you want but I would recommend that you use the terminology
    that is in standard use. I would also recommend a basic high school physics
    book and then some of the texts that Charles has suggested.

    If you are depending on such simplistic sources as the one (repeated below)
    as you gave Charles, you really have problems. It is a hand waving approach
    which really doesn't cut it. This appears to be aimed at prospective grade
    school teachers. It certainly is not what I would expect from an engineering
    faculty. http://www.phys.unsw.edu.au/~jw/HSCmotors.html

    If you want to rest your case- fine- but please learn some of the
    fundamentals of what you are resting it on.
     
  13. You need to read my posts. I never said anything about unbalance voltage.
    Faults, yes.

    Poor idiot can't even read.

    Your real hangup is with the term "force". The trouble is you have to use
    the whole term for emf, and that defines a voltage only. I suggest you read
    up on Faradays Law. You will find that the emf is generated on any wire
    moving through a field, even a wire with no connections on the end. How is
    this possible? Easy, the emf is a voltage, you are generating a voltage.
    No current need be involved. This is a freshman EE experiment.

    Charles Perry P.E.
     
  14. Nothing really. Most electricians know what they are doing and know the
    limits of what they can do. Just as I know there is no way I can bend
    conduit, so I leave that up to the electricians.

    I have run into some that were quite scary with their almost supersitious
    beliefs in oddball electrical practices. I have been called in to
    troubleshoot in plants where the electrician has installed all kinds of
    "sprecial" grounds to help the control system work better. Talk about fun;
    trying to convince him that he was doing it all wrong. And then the
    electrician who just simply would NOT believe that the meters on all houses
    measure kWh with the same accuracy (within the accuracy range of course).
    He swore that newer homes got "faster" meters. Good grief. And believe it
    or not, I have run into several electricians who believe all of the "free
    energy from the vacuum" crap. They were convinced utilities were holding
    back the progress in that area and that anyone who didn't agree was part of
    the conspiracy!

    So, I have had some interesting encounters with electricians that thought
    they knew more than they did.
    At this point I am surprised Phil can remember to breath by himself.

    Charles Perry P.E.
     
  15. daestrom

    daestrom Guest

    This is rich!!! You pick the two smartest guys in the electrical
    engineering group (Don & Charles), that have probably over 75 years
    experience between them designing electrical machinery and power systems,
    and call them idiots. They (and I) have studied and worked with utility
    grid systems for years and are members of the industry that you're trying to
    claim has known about "back emf affecting power grids".

    Then you hang your whole argument on an antiquated term (back emf, although
    we used to call it 'counter emf' in my 'neck of the woods') as 'proof' of
    your arguments.

    And just for the record, you can have a force without motion (stand on a
    scale to weigh yourself and don't move). Similarly, the force on a charged
    particle in an electric field is independent of its mass (try explaining
    *that* with F=M*A ). And the force on the electrons in a conductor moving
    in a magnetic field is independent of the electron mass (scalar product of
    field strength and velocity of charged particle).

    The 'forces' on the charges in a conductor moving through a field do *not*
    necessarily cause motion. In fact, in a motor, the forces on the charges in
    the moving conductor actually impede their movement. Without these forces
    impeding motion, the applied voltage (from an external source) would cause
    much more current.

    A simple proof of this is a simple generator with no load connected. You
    have electrons in the conductor moving normal to a magnetic field. A simple
    way of looking at it is all the electrons have a 'force' exerted on them and
    this develops the terminal voltage. But the electrons don't move because
    there is no external circuit. In beginning texts, the movement of the
    conductor in the magnetic field generates an 'emf'. No current flow, just a
    voltage/emf.

    daestrom
    <snip>
     
  16. Don Kelly

    Don Kelly Guest

    You remind me of the Irish saying "It's no use being ignorant if you don't
    show it."

    Utter rubbish snipped.
    plonk
     
  17. Don Kelly

    Don Kelly Guest

    _-------------

    That was my typo- I thought that I had caught and corrected it befor
    sending -Yes - it does mean "electromotive force" Please accept my
    apologies.
    Yes, I do know Maxwell's equations with respect to this (more convenient to
    use the integral form when dealing with magnetic circuits as in machines).
     
  18. Don Kelly

    Don Kelly Guest

    Basically EMF or "Electromotive force" is a term which appeared in the late
    half of the 1800's to describe a potential difference (voltage) as the
    "force" causing a current to flow (N.B. Electrons weren't known at the time)
    It is an anachronistic term which has led to many misconceptions.

    "Back EMF" is a term that originated with early DC machines to describe the
    speed dependent voltage that was generated by the motor stator conductors
    moving in a magnetic field. This generated voltage opposes the applied
    voltage (it damn well better says "conservation of energy" ). In terms of
    power the mechanical power developed is given by Pmech=Torque*angular
    velocity which is the same as current*(back emf). At no load, ignoring
    losses, the speed of the motor is such that the back emf =the applied
    voltage and the current in is 0 --ditto with the power.

    If a mechanical load is applied to the motor, it slows down and the back or
    generated emf drops. Then the supply voltage exceeds the back emf and a
    current is produced so that torque (depending on current) is developed and
    power is transferred to the mechanical side. Back emf is simply a speed
    voltage that is generated. Note that there is no difference between a motor
    or a generator except in the way that the power flows. Try to drive a DC
    motor to a higher speed will result in an increase in the generated voltage
    and, if it exceeds the supply voltage- a reversal of power flow so the
    machine will generate.

    In an AC synchronous machine- this back emf can be varied in magnitude and
    the phase of this voltage with respect to the supply determines the power
    transfer. A synchronous machine can only run, in steady state, at a speed
    determined by the frequency. If the load on a motor is increased, the "back
    emf" drops back in phase and power flows. If the back emf is greater
    (magnitude) than the applied voltage- the motor is overexcited and will be
    capacitive. If underexcited, it will appear inductive. However, here, the
    term "back emf" is rarely , if ever used- excitation voltage may be used as
    normal operation is at one speed so the field is the controlling variable.

    Most motors are AC induction motors. These get their excitation from the
    supply. It is somewhat harder to identify the "back emf" and it is generally
    not worth while bothering to do so(In writing the general set of
    differential equations for an induction motor there are speed voltage terms
    but the usual steady state model is that of a transformer-hey- it works). An
    induction motor can be used as a generator if it gets excitation
    (magnetisation) from the system or from some other source such as capacitor
    banks.
    Note that -in any machine- if the back emf is 0- then there is no power
    transfer taking place-In practice this is at 0 speed and current will
    generally be highest in this condition. Lots of torque but no power.

    I hope this is of help.
     
  19. daestrom

    daestrom Guest

    I fully agree with Charles and Don's explanations. 'back emf' is a voltage
    developed in a motor by virtue of the rotor conductors moving in a magnetic
    field. It opposes the applied voltage and limits current flow (but not
    create a current flow of its own, except in the rare situation of a
    transient fault on the supply). In simple DC machines, it is easy to
    understand, but it also exists in AC machinery.

    Its only effects on the power source is to oppose the applied voltage and
    reduce the current flow. *IF* the applied voltage is rich in harmonics, the
    'back emf' can be affected because said harmonics can distort the magnetic
    fields. As such, significant currents at the harmonic frequencies can thus
    flow from the source of the harmonics. These currents can cause excessive
    heating. But the currents come from the source of the harmonics, not
    necessarily the utility source.

    daestrom
     
  20. Hahahaha. Notice it says motor drives?! Motor drives are power electronic
    devices between the motor and the utility. Of course drives produce
    harmonics. The convert the AC input to a DC voltage and then invert the
    power back to a pulse width modulated voltage that approximates AC (at least
    most AC drives are PWM). The front end of the drive can be as simple as a
    diode bridge, or use SCRs, or even IGBTs. You can load the front end of the
    drive with a resistor on the DC bus and guess what? You get harmonics on
    the AC system. Most AC drives are 6 pulse devices and you get primarily 5th
    and 7th harmonics. It is not uncommon for very large (thousands of
    horsepower) drives to be 12pulse. This requires a phase shifting
    transformer to basically create 6 phase AC that is then rectified. The
    characteristic harmonics for a 12 pulse are 11th and 12th.

    This is a good overview of motor drives. It has a small section on
    harmonics that they call "power line pollution". It will help you quite a
    bit. I am surprised in all of your years of experience you have never seen
    a motor drive or understand how they differ from a directly connected motor.
    I guess you learn something new every day.
    http://www.emerson-ept.com/eptroot/public/schools/adjsddrv.pdf

    Here is good one about the harmonics from a drive. Watch the wrap in the
    link. You may have to cut and paste it into your browser:
    http://domino.automation.rockwell.c...s.pdf/$file/Getting_a_Handle_on_Harmonics.pdf


    You have really shown your hand this time.

    Charles Perry P.E.
     
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