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120V from both legs

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

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  1. daestrom

    daestrom Guest

    They have provided references to several university level texts, they have a
    particularly high level of education in the arena of electrical machinery
    and power systems. And their explanations are in align with many references
    found in university level texts and on-line web references. Your theory,
    however is *not* supported by the few web links you've given, is *not* found
    in any university text and does *not* fit the observable facts.

    You have claimed that your explanations are 'well known in the industry'.
    But people like Charles, Don and myself are members of that industry. And
    have each of us been in that industry for many years. We have separately
    worked in the field of power systems and large scale electrical machinery
    for quite some time, yet we haven't heard of it. You may claim we're
    ignorant, but if we haven't heard of it, so much for your assertion that 'it
    is well known in the industry.'
    Force = mass x acceleration is the physics of how a force interacts with
    any mass. It explains how a net force on a given mass will accelerate it.
    If a given mass is observed to be accelerating, the formula can be used to
    calculate what net force must be acting on it.

    But it is *NOT* the only definitive explanation of 'force'. It *has* been
    used as the definition of mass, which is also an interesting definition of

    Force = K * X
    where K is a constant for a linear spring and X is the spring displacement
    from its 'rest position' Hmmm, no 'mass' in this equation. Do compressed
    springs not exert a force?? And don't go claiming that a massive spring
    exerts more force than a small one. It can easily be demonstrated that the
    force exerted is not always dependent on the mass of the spring.

    Force = K * m1 *m2 / r^2
    where K is the universal gravitational constant, m1 and m2 are the masses of
    two bodies and r is the distance between the bodies. True, we have *two*
    masses here, but no acceleration term at all.

    Force = K * c1 *c2 / r^2
    where K is the faraday constant, c1 and c2 are the net charges of two bodies
    and r is the distance between the charges. No masses here. True, the
    fundamental unit of charge (so far in physics) is found on an electron (that
    has a mass). But the amount of force exerted by this is a function of the
    *charge*. The electrostatic force on two different amounts of masses can be
    the same if the *charge* on the two masses is the same.

    Force = K * c * E
    where E is the electric field strength acting on a unit charge. Again, the
    force is a function on the amount of *charge* on the object, not the mass of
    the object.

    Force = K * B X V
    where K is another constant, B is the magnetic field strength and V is the
    velocity of a unit charge moving through the field. Here we must be careful
    to use the 'cross product' of the two vectors.

    Interestingly, the international definition of the fundamental unit of
    Ampere uses 'force' in its definition....
    "The ampere is that constant current which, if maintained in two straight
    parallel conductors of infinite length, of negligible circular
    cross-section, and placed 1 meter apart in vacuum, would produce between
    these conductors a force equal to 2 x 10-7 newton per meter of length."

    This is the one electrical quantity that is in the SI base units. From this
    and the other base units, the derived SI units for electrical power,
    voltage, magnetic field and many others are derived.
    Actually, a 1 hp motor, or a 100 hp motor generate almost the same amount of
    'back emf' when running unloaded. They both generate 'back emf' that is
    *almost* equal to the line voltage applied ('almost' because they both have
    losses). And since 'back emf' is a voltage, *not* a current (as so many
    folks have tried repeatedly to explain to you), it is pretty much
    independent of motor size if the applied voltage is the same.

    And because the premise of your statement is flawed, (that larger motors
    generate more 'back emf' than smaller ones), the rest of your conclusion is
    flawed as well.
    No. A 1000 hp motor designed to run on a 4160V line, running unloaded, will
    be generating an internal voltage very close to 4160V. Similarly, a 1 hp
    motor designed to run on a 4160V line running unloaded will be generating an
    internal voltage very, very close to 4160V. If what you say were true, the
    1hp motor would be generating an internal voltage of only 4.160V. And if
    *that* were true, what limits the current flow from the 4160V to 4.160V in
    the 1 hp case? Winding resistance? The winding resistance of such a motor
    would be much less than 1 ohm, so Ohm's law would indicate the 1hp motor
    draws 1000 times the current of the 1000 hp motor when they both run
    unloaded. Isn't it obvious that that isn't true?
    You keep trying to separate the 'EMF' from 'voltage'. The term 'EMF' is an
    achronism for voltage.

    If you keep insisting that F=M*A is the only formula for a force and EMF is
    dependent on mass, then how do you explain the operation of a DC machine
    where the current is constant and there is no acceleration? By *your*
    notions, the force in a DC machine is zero, regardless of its mass since the
    current flow is constant and there is no acceleration. Let me guess,
    because the rotor is spinning, all the electrons on the rotor are undergoing
    acceleration??? That's a non-starter, it implies a large diameter rotor
    with the same mass rotating the same speed as a smaller diameter rotor
    generates a different amount of 'back emf'.
    Again, you keep coming up with some vague idea/definition of what 'emf' is,
    when the rest of the world use to use the term for voltage. The rest of the
    world agrees that 'emf' is an antiquated term for a voltage created by
    electromagnetics. Yet you keep insisting it is F=M*A.

    So far, you've said that this 'back emf' is a function of 1)machine mass, 2)
    rotor speed, 3)hp rating. Have I left anything out, or is there some more
    variables you want to throw in?
    I don't. Study the equivalent circuit of a DC machine and you will find an
    internal *voltage* generated that is proportional to field strength and
    rotor speed. It is not described as a 'force' except in antiquated texts.
    Funny, I've seen the voltage at the terminals of many an AC motor on an
    oscilloscope and never seen such 'emf coming back out onto the grid'.
    (voltages measured line-line, line-neutral and current waveforms measured by
    the voltage drop they create through precision resistors) I have seen
    distortions in the supply voltage, but they only exist when a non-linear
    load is connected. The distortions in voltage are easily explained by the
    resistance/reactances of the supply and the non-linear currents caused by
    non-linear loads. Power supplies with much lower impedances tend to have
    much cleaner voltage waveforms than those with higher impedances.

    Utilities *do* get concerned when one customer has severely non-linear
    loads. Such a customer's severe high-harmonic *currents* cause high
    harmonic voltage disturbances due to the utility equipment's
    resistance/reactance. And if not controlled, the *voltage* disturbances
    will affect other customers, whom the utility is obligated to supply power
    to within the standards. Most utilities can compell such a customer to
    correct the problem themselves, or charge them for having the utility
    correct the problem so as not to affect other customers.

    But this just doesn't happen with induction motors. In fact, *large*
    induction motors, with many slotted windings draw almost perfect sinusoidal
    currents and thus cause little or no voltage distortions in the utility
    equipment's resistance/reactances.

    But *your* theories would dictate that *large* induction motors generate
    *more* of your mysterious 'emf' (it is clear now that your use of the term
    is unique and *not* in compliance with any industry practice). So *large*
    motors, because they have more mass, should generate more 'back emf' and
    distort the grid voltage more? Yet experimental data, seen everyday doesn't
    support that position. Large motors cause more of a voltage disturbance
    ,*when started*, but the disturbance quickly subsides as the motor reaches
    normal speed.
    Simple fact is, voltage distortions on grid supplies are caused by distorted
    *current* flow through utility equipment. The phenomenon can be fully
    analyzed using conventional circuit theory by replacing the ideal voltage
    source with a real one. Real world grid supply includes a variety of
    resistances and reactances, whose voltage drops are dependent on the
    currents flowing through them. I'm sure any university professor of power
    EE would be happy to explain it to you (as Charles and Don have already).

    Been there, done that. 'large industrial power users' come in all 'shapes
    and sizes'. Arc furnaces (not induction or resistive ones, *arc*) are one
    of the worst. They are a heavy load that is non-linear. High non-linear
    currents cause non-linear voltage drops in utility equipment's
    resistance/reactances. On the other hand, pumping stations whose major
    loads are induction motors are pretty benign. Rolling mills are a bit of a
    mix. Although the loads are induction motors that do not create a lot of
    harmonic distortions, the loads are starting/stopping/reversing all the
    time. Depending on their equipment, the constant starting/stopping of the
    many motors cause periodic dips/surges in supply voltage. But these are
    with a period of several seconds, not harmonics of the supply frequency. If
    the rollers are controlled by solid-state drive (as many modern ones are),
    the *drive system* can draw a lot of non-linear currents from the supply.
    But if the drive is an older motor-generator setup to DC roller motors, the
    harmonic distortions on the line are nil (but you still have the surges
    caused by varying load).

    Some customers have had problems when they installed versions of VVVF
    control on some of their machinery. Although the connected motors worked
    well because the VVVF drive has output capacitors, the VVVF drive's input
    stage (typically a phase controlled AC-DC converter) would draw highly
    non-linear currents from the supply. These currents would cause non-linear
    voltage drops in the utility supply equipment and thus the voltage supplied
    to other equipment is distorted. The severity of the problem is a
    combination of the severity of the distortions created by the phase
    controlled converter and the impedance of the supply. The 'fix' is to
    filter the input to the phase controlled converter such that the supply line
    'sees' less of the non-linear currents.

    To summarize, when a large customer draws a high amount of non-linear
    current from utility equipment, the resulting voltage drops created in that
    utility's equipment will also be distorted. And yes, that can affect other
    customers. But... 1) The distortion is easily analyzed by conventional
    circuit analysis of the load current and supply resistances and reactances.
    2) Induction motors are some of the *least* offensive loads in this regard
    (electronic phase-controlled and arc-type loads are the worst). 3) The mass
    of a motor has nothing to do with the amount of internal voltage developed.
    4) 'back emf' is an antiquated term used to describe the internal voltage
    developed in a spinning motor, nothing more (except in your very small
    little world of one).

    And to return to the original point of this thread for a moment, neither
    voltage nor current distortions have any appreciable affect on the accuracy
    of the standard household kwh meter. Although a voltage imbalance between
    hot legs and the neutral can, as I recently learned from Dan Lanciani's

  2. daestrom

    daestrom Guest

    Here we see that the industry is talking about 'motor drives', not motors.
    Motor drives, with diodes and phase-controlled SCRs on their input stages
    are responsible for high amounts of non-linear currents drawn from the
    supply. Not the motors themselves. You have been arguing that motors
    themselves are the culprit and that is simply not the case (as we have
    repeatedly explained).

    These non-linear currents create corresponding non-linear voltage drops in
    the supply equipment's resistance and reactances. Since the output voltage
    of supply transformers and transmission lines includes these distorted
    voltage drops, the output waveform supplied to all connected loads is
    affected. None of this has anything to do with the 'back emf' generated
    internally in a motor. You've just proved that power supply harmonics are
    caused by something besides motors themselves. It has been an increasing
    problem because of the increased use of electronics (both in motor drive
    systems and non-motor related loads).

    Even a resistive heating element can be a problem, *IF* it is controlled by
    a phase-controlled proportional heating controller. No motor, no moving
    parts or appreciable magnetic fields, just a simple resistor element. But
    the controller for it, applying a phase-controlled voltage to the resistor
    results in distorted current waveforms that can affect the supply in the
    same way. Same problem, same cause, and no 'back emf'.

    This article is yet another reference you claim supports your position that
    'back emf' generated inside motors 'leaks out to the grid', when in fact it
    does nothing of the kind. It explains the problem of harmonics in supply
    voltage created by the type of load connected and that of electronics
    introducing a high amount of harmonic content on supply systems by virtue of
    their non-linear *current* waveforms.

  3. daestrom

    daestrom Guest

    Funny, LMAO!!!

    First EMF is a 'force' and all 'forces' are F=MA. Now it's a current, a
    harmonic, a circuit condition, a marketing program!!!

    Can't you tell which of your lies is which???
    YOU are the one using 'emf' in more and more generic terms. And no one has
    denied that harmonic distortions exist (another of your outrageous lies).
    What *real* engineers acknowledge is that harmonic distortions from
    solid-state motor drive systems is an issue. What we (the rest of humanity)
    claim is that *motors*, in and of themselves, do *not* generate significant
    harmonic distortions. But you're to obtuse to understand the difference
    between a *motor* and a solid-state *motor drive* system.

    Harmonics are becoming an increasing problem. But motor usage is *not* the
    cause. Solid-state devices that behave in non-linear fashion used to
    *drive* the motors (and other non-linear devices such as computer power
    supplies) are on the rise and *they* create harmonic distortions.

    The very article you reference for your position clearly states "...Motor
    drives present a nonlinear load to the power grid feeding them.
    Characteristically the diodes in an inverter's input stage create harmonic
    distortion as they switch on and off."

    See that sentence that starts with the words "Motor drives..."??? Not
    "Motors", "Motor drives" !!! Notice where they placed the letter 's'?? A
    "motor drive" is the solid-state controller that chops up the incoming
    voltage waveform to soft-start the motor. Or the VVVF controller that uses
    phase-control to convert AC to a variable voltage DC so that it can be
    inverted back to a variable frequency AC at a constant volts/hertz ratio.

    It is the increased usage of these solid state *motor drives* that has
    caused some concern. But the article you reference is seven years old now
    and new solid state motor drive controllers have design changes to limit the
    harmonics generated in them.

    The fact that you can't tell the difference 'boggles ones mind'. Of course
    this just shows how deplorable your English reading comprehension skills

  4. daestrom

    daestrom Guest

    No, that is *not* what they have been saying. Your reading comprehension
    skills are terrible. What we *all* have been saying is that induction
    motors do not create any appreciable harmonics. We have also said on more
    than one post that harmonics *are* caused by non-linear loads.

    Trying to win a debate by misquotes and lies is childish.
    You've quoted the same article in many of your posts now. Notice that it is
    seven years old? Has this harmonic distortions epidemic you keep ranting
    about reached the 50% level yet? Is seven years 'soon'?

    Yes, their 'spin definitions' come from university texts and IEEE standards.
    Yours comes from some vague hand-waving and 'it is well known in the
    Lies. They have not denied issues about harmonics, merely that induction
    motors are *not* a significant source of harmonics. You can't even read.
    The shift of the predominant loads on the power grid from simple induction
    motors and resistive heating/lighting to more and more solid-state,
    non-linear devices (including computer power supplies, electronic-ballasted
    fluourescent lighting, and solid-state motor controls) is the cause for the
    increase in harmonic pollution concerns.
    Read the rest of your article and you can see in the third paragraph that
    the common approach to suppress the harmonics from a motor drive is to
    install filters between the power line and *THE INVERTER*. Because it is
    the motor drive's *inverter* that creates the harmonics. Not the induction
    motor connected to the unit, the *inverter* inside the motor drive. And in
    the final paragraph, "Harmonic distortion in ac industrial motor DRIVES
    [emphasis added] is a common, but correctable problem."

    You're ranting about harmonic distortion as if it will reach epidemic
    proportions and bring down the grid, yet your reference says it is "a
    common, but correctable problem." To paraphrase a lesson for all first-year
    law students, "Don't use a reference to support your position unless you
    know what it says."

  5. daestrom

    daestrom Guest

    Article is seven years old, yet the grid doesn't lose even 25% yet to
    harmonics. Seems some predictions are worse than others.
    Common induction motor, no.
    Transformer, not internally.
    Computer power supplies, definitely.
    Electronic ballasts, yes.
    Solid-state motor drives, yes.

    Harmonics are well known. But it is also 'well known' that induction motors
    and transformers are *not* significant creators.
    Problem is, you've extrapolated from non-linear devices in motor drives to
    *everything*. It is 'well known' that any non-linear device will create
    harmonics (even my dining room light dimmer). But you've just gone off the
    deep end claiming everything creates significant harmonics. Next thing you
    know, you'll claim a space heater with a simple on-off switch creates
    But most of us know enough to stop the extrapolation when it goes too far.
    One can say *water* is wet, but you are taking that as proof that the floor
    is wet without knowing if there is water on it.
    No, the problem isn't 'massive'. It is easily correctable (according to
    your own article's last paragraph). It has existed for at least seven years
    and yet the power grid stands. The problem *is* from VSD's (and there are a
    lot more than 'a few' now). It does come from many sources (and those
    sources are increasing), but it does *not* come from induction motors or

    Perhaps you are 'stunned' because the facts about motors not creating
    significant harmonics has shaken your personnel beliefs? If you can't tell
    that different types of electrical loads have different characteristics and
    *not* all of them create harmonics, then it isn't surprising that your
    'black/white' world is stunned to find out there are actual 'colors'.

    Produce a reference that explicitly states that an induction motor,
    operating on the line creates significant harmonics. You will have a long
    and fruitless search. The reason your search will be in vain is because
    induction motors do *not* create significant harmonics. You've extrapolated
    one problem into areas where such extrapolation is no longer valid.

    No, but you need to prove that all liquids are water before you can convince
    me they are all wet. That's the kind of ridiculous extrapolation you've
    done with this. Motor drives are a source of harmonics, so you've
    extrapolated that all sorts of things create harmonics. Since you don't
    understand the math and physics involved, you've extrapolated to areas/items
    that don't create significant harmonics.
    Again with the poor reading comprehension. Your article describes one
    source of harmonics, solid-state drives. There are, of course, many, many
    others. But that doesn't include induction motors. Computer power
    supplies, light dimmers, electronic ballasts, there are many sources of
    harmonics. But resistors, induction motors, transformers, and incandescent
    lights are *not* sources of harmonics.

    The article singled out solid-state motor drives as significant because the
    number of such drives is growing rapidly (it isn't just some 'rare few'
    anymore). And because of the amount of power that flows through them (often
    in the MW range) compared to other solid-state devices (often in the watt
    range), they in particular convert a lot of power into harmonics.

    And if you didn't know what we meant by 'non-linear loads', you should have
    just asked (or followed your own advice and used google). The term is used
    to describe loads whose conductance varies non-linearly with the applied
    voltage. A simple diode is a perfect example. Saturable reactors, SCR's
    and other 'gated' devices are others. Because of their varying conductance,
    the current waveform through them is not a sine wave even though the applied
    voltage is. *ALL* such non-linear devices will create harmonics in the
    current waveform when an AC voltage is applied to them. When you take the
    current waveform produced and convert it to the frequency domain you will
    find it rich in harmonics. IF the harmonic currents are large enough, they
    can cause significant distortion in the voltage source by acting through the
    voltage source's internal resistance/reactances.

    So you see, 'non-linear loads' are not some 'rare' or vague item. They are
    computers, televisions, compact fluourescent lights, solid-state drives,
    stereo systems, digital clocks, and almost any other electronic item (if it
    has an internal DC power supply, it probably produces some harmonics). Most
    such loads do not distort the supply voltage significantly because the
    amount of harmonic current is small and the internal resistance/reactances
    of the voltage source are also small. But if the harmonic currents are
    large (as the case with large motor drives), then even the small internal
    resistance/reactances of the voltage source will develop significant voltage
    distortions from the harmonic currents flowing through them.

    It is the increased usage of such devices that has been cause for concern.
    But again, as your own reference points out, the problem is easily

  6. Don Kelly

    Don Kelly Guest

    Phil - here are some quotes and added comments
    10-6-04 8:20PM
    Kelly wrote:
    "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."
    Comment: note sinusoidal voltage at the motor terminals-not non-sinusoidal.
    "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."
    Added comment -the thesis deals with the topic of analysis of the harmonic
    load flow for a power system with harmonic sources--i.e - finding out what
    is actually going on when there are harmonic sources at various points on a
    "The subject that I was responding to was power factor correction and some
    the claims that you have made with respect to the reason for this and claims
    as to its benefits."
    Comment: this delineates my targeted response.

    Did you actually read the above?
    10-7-04 3:29PM Kelly wrote:
    "Please tell me the basis for your claim of harmonic
    distortions- you have evaded that question"

    10-8-04 8:44 PM A correspondent wrote:
    EMF = electro magnetic force, or electro-motive force? Has something
    Kelly answered:
    "That was my typo- I thought that I had caught and corrected it befor
    sending -Yes - it does mean "electromotive force" Please accept my

    Comment: rechecking through the net- emf and back emf are given in the sense
    that I calim- standard terms.

    After all this you FINALLY came out with what YOU define as emf and back
    emf. Thank you for that.

    The fact that your definition is yours alone and is contrary to the
    definition used in all the literature doesn't appear to bother you. You have
    simply ignored the evidence with respect to these terms.
    In addition, the definition that YOU use is already pre-empted for
    mechanical forces of electromagnetic origin -in power systems and machines
    it generally is restricted to the magnetic part of the Lorentz force (which
    has no mass or acceleration term). There are deeper meanings in the physical
    sense but in terms of conditions in a power system- it boils down to this.
    It is true that there is now a use of the term "EMF" (F for field- shades
    of Maxwell) which the non- technical use to lump electric and magnetic
    field boogiemen together but that is obviously not what you meant (Consider
    that a complement).

    However, you have FINALLY made it clear that what you are dealing with is
    electronic motor drives.
    I have never denied the harmonics generated (see quotes) in these motor
    drives and neither has Charles who referred to IEEE standards on harmonic
    content. . The source of the harmonics is the fact that these drives do not
    produce sinusoidal voltages and currents.
    Your "technical" reference is not really a technical reference-I would
    classify it as an "info-mercial" where firstly facts are presented (along
    with scare tactics "some say that .. 50%..." which are like "some say the
    sky is falling"- giving rise to the questions: "who are the some?" and "what
    is the basis for this?") and in later paragraphs- lo and behold- a
    particular company's harmonic filters are presented as the solution. The
    combination is suspect. If this is a top quality reference - God help us.
    This article was in 1997 - there are IEEE papers and working groups dealing
    with this since the 70's. The thesis that I referred to was in 1996 and the
    author had, at the time, and still has, a very successful Industrial power
    system consulting company so he is not in the least unaware of the problems.
    The info-mercial provides no new information. The concept of harmonic
    filtering for rectifier/inverter usage in power systems has been around over
    50 years. So what's new?

    If you had said, in the first place, that electronic motor drives present
    harmonic problems affecting both the grid and the motor, and that corrective
    measures are needed, then you would have had no adverse comments as that is
    true. You didn't.
    What you did say is another matter and that is the problem. .
    'Nuff said

    Don Kelly

    remove the urine to answer

  7. daestrom

    daestrom Guest


    I said, "..., yet the grid doesn't lose even 25% yet to harmonics." I did
    *NOT* say it was anywhere "close to 25%".

    Another example of your complete inability to read and comprehend the
    English language.
    NO!!! I did *not* say it was close to 25%, I do *not* have to tell Charles
    anything, and the only 'progress' being made is you are proving more and
    more with each post what an ignorant, uneducated fool you really are. You
    can't comprehend what you read.

  8. Don Kelly

    Don Kelly Guest

    I do realise the difference between VA and VARs. I don't know where you find
    it vastly more complex as it isn't.
    Again, you are still sticking to your personal definition of emf - which is
    not used in the rest of the world.
    I did not state that I wish to use EMF as electromagnetic field. Please
    learn to read.
    To disabuse you, Charles is not the desperate one. He knows his business.
    If you want to say that, in the case of unbalanced systems supplying
    non-linear loads, there are harmonic problems- fine (see below) - but using
    terms such as emf with your own definition doesn't help. Back emf is well
    defined in a way other than what you define it as. Please note that
    utilities are well aware of the problems with electronic drives for about 30
    years. If, on the other hand, as you imply above, unbalance can cause
    problems-fine- what else is new? Have you heard of negative and zero
    sequence components- techniques for analysis of unbalanced systems have been
    around since about 1914?.

    A couple of references for you:

    Grainger,L.G. , Spencer,R.C. "Residual Harmonics in Voltage Unbalanced
    Power Systems" IEEE Transactions on Industry Applications-30,No.5 Sept/Oct
    1994., p1398

    Dewinter, F.A., Grainger, L.G. "A Practical Approach to Solving Large Drive
    Harmonic Problems at the Design Stage", ibid- 26,5,Nov/Dec 1990, p 1095

    There is a long list of related material in IEEE publications dating back
    for at least 20-25 years. .

    The subject is well known and the terminology that you use is not
    applied -in fact the people who know the area would react as I have.
    Depending on the balance there can be neutral current and possible triplen
    harmonics -so what's new?>
    If the input is unbalanced, there will be negative and zero sequence
    currents and negative and zero sequence voltages fed back - so what comes
    back is not necessarily "clean" Is the power "clean" - who knows- the
    statement is meaningless.
    Do you know the difference between unbalanced voltages and currents and
    harmonic voltages and currents?
    Just a question.
    Bull. -again you are using the "defined" concept of "back emf" mixed in with
    your own interpretation. >
    Are you claiming that you are God?
    You are insulting the people in the utilities who have to deal with the
    problems. I find that engineers and technicians with utilities are extremely
    interested in finding the truth and the solutions to problems.
    Did you ever think that they might just know a bit more than you do about
    A factor that has been with us for the past 100 years. Not exactly a new
    Unbalance- true-a cause of problems. Back emf- again, you are using your
    personal definition. I cannot comment on your experience but I really
    suggest that you use accepted terms for what you mean- it helps in
    Ah, but you are looking at the present where "Grocery store" economics
    overrule sound engineering practice. I hate to admit it but I have to agree
    on this point.
    Oops "negative feedback" is generally a calming thing- do you want "positive
    feedback"? Exponentially worsening- maybe, maybe not- depends strongly on
    the system and attenuation of harmonics in a system.
    Again, watch your use of terms which are established as this leads to
    confusion and misunderstanding.
  9. You should really take a course in reading comprehension! I said, and I
    still say, that back emf from induction motors does not affect the grid.
    Period. It is 100% correct. You are the only person in the entire world
    using your definition of emf. I said over and over that vars were a
    problem, best corrected at the load of course. I also said that harmonics
    are a problem, but not from induction motors because induction motors
    produce such low values of harmonics. Adjustable speed drives and other
    power electronics devices are the prevailing sources of harmonic currents.
    They are not emf, current, not emf, current...perhaps if it repeated often
    enough you will understand. I also did not say anything about unbalance,
    you did. I said under fault conditions, a motor can act as a generator and
    thus feed fault current. Pay attention. Get a dictionary. Have someone
    read posts to you if necessary.

    By the way, I checked several college level physics books today. All of
    them define emf as a voltage. And yes, emf can exist with no current.
    Farady was able to prove this many, many years before you were born.

    You can disagree with my if you like, but do NOT lie about what I say. It
    shows a very low level of intelligence on your part.

    Charles Perry P.E.
  10. You are a fool. That is not what I posted at all. You really cannot
    understand what you read. Sad.

    Charles Perry P.E.
  11. operator jay

    operator jay Guest

    Hello there. Here's a case for 'back voltage'

    Picture a 120V 'supply' source connected to a 20 ohm resistor, which
    is then connected to ground. 6A flow.

    Now add a variable voltage source to that circuit. The 120V source is
    still connected to the 20 ohm resistor, which is then series-connected
    to the new variable voltage source, then to ground. With the variable
    source at 0 volts, we have the same result as above, 6A flow from the
    source into the resistor, then the variable voltage source, and
    finally to ground.

    Now set the variable voltage source at 20V such that it 'opposes' the
    120V source. There are 100 V across the 20 ohm resistor. 5A flow
    from 'supply', through the resistor, through the variable voltage
    source, and to ground.

    Set the variable voltage source at 40V. 4A flow.

    Set the variable source at 60V. 3A flow.

    Set the variable source at 80V. 2A flow.

    Set the variable source at 100V. 1A flows.

    Set the variable source at 120V. 0A flow.

    Set the variable source at 140V. 1A flows from the variable source,
    INTO the 120V supply.

    Set the variable source at 160V. 2A flows from the variable source,
    into the 120V supply.

    I would liken back emf to the variable voltage source in the above
    simple example. Say the current into a dc machine goes from 6A at
    startup down to 1A at steady-state running speed. This would
    correspond - as far as the above example is concerned - with the back
    emf going from 0V at rest up to 100V in steady state. If the dc
    machine were driven by a prime mover, such that its speed increased
    further, the emf could rise to the point that no current flows into
    the machine. If the prime mover drives the armature even faster, the
    machine would enter generating mode and will drive current (supply
    power) into the 'supply'.

    This is how I think of back emf. The back emf 'subtracts from' the
    supply voltage insofar as it decreases the votlage across the
    machine's resistance (the resistor in the above). In my example,
    there would always be 120V measured at the 'terminals' as you
    correctly pointed out would be required for a satisfactory model. The
    back emf voltage would be produced by the conductor (armature) 'cuting
    the flux' of the magnetic field. This should be a real, measurable
    voltage. It would be present even if the motor terminals were opened
    (no current flowing). In this way it seems to me that calling it a
    'back' voltage is in keeping with the physical reality of the machine.

    An induction machine is a little more complicated. But the term 'back
    emf' does get used used in that situation as well. I believe any emf
    induced in the rotor circuit will have current associated with it as
    the rotor is 'shorted' through the rotor resistance. The rotor
    voltages and currents will 'reflect' back to the stator through the
    transformer action that occurs. The rotor voltage reflected to the
    stator can be thought of as back emf.

    A synchronous machine is often modelled very simply as an impedance in
    series with a voltage source, like the resistor and variable voltage
    source in my numerical example, except with an inductance rater than a
    resistance. The 'back emf' here is the excitation voltage caused by
    the rotating field of a revolving dc rotor inducing voltages in the

    However one can measure that current
    I'm not certain what circuit configuration you would be suggesting. I
    note that a Norton equivalent of the circuit I described above would
    give accurate terminal quantities, and would have a current source to
    model a 'back current'. Maybe this is what you are proposing? Would
    your circuit consist of only E=120 feeding R=10 (connected then to
    ground) and a current source Ib parallel to R and injecting current
    into the circuit? If so, this is equivalent (at the terminals). An
    'Ib' could be calculated at any instant of my numeraical example above
    Ib = (voltage of variable source) / (20 ohms).

  12. Don Kelly

    Don Kelly Guest

    Any circuit model that we use must fit the observed behaviour of the device.
    The series Ra and Eg model does so better than the model that you propose.
    However, don't throw your model away yet- see below.

    I will do the DC motor with separate (or permanent magnet) excitation fields
    for simplicity.
    First of all we can see that if the motor shaft is rotated from an external
    voltage source, there will be a voltage developed at its terminals.(re:
    Faraday) this voltage is proportional to speed and the field flux and is
    give by Eg=K(flux)w where w is the speed. This can easily be measured. The
    windings of the generator will have some small resistance Ra.
    If a load R is connected across the terminals then there will be a single
    loop circuit such that Eg=Ra*I +R*I =Ra*I +V
    Now, instead of a load resistor, consider a second voltage source V. If V is
    less than Eg the current will flow from Eg to V and the power flow will be
    from the mechanical side to the electrical side. The machine is still a
    If V=Eg there will be no current and no power flow.
    If V>Eg then the current will reverse and the direction of power flow will
    also reverse. The machine is now called a motor. .
    In the motor case the " back emf" is simply the generated voltage.

    The motor circuit will look like this

    + I-> + | V=R*I +Eg or
    V Eg

    At start: the speed w is 0 and then Eg=K(flux)w =0. The current will be V/Ra
    and will be maximum.
    The torque is given by T=K(flux)*I (same K(flux) as above in MKS units and
    speed in radians/sec)
    The torque will accelerate the motor and as speed increases, so will Eg.
    Eventually the motor will reach a constant speed which would result in 0
    current, 0 torque and Eg=V ----IF-there were no losses in the mechanical
    side. In practice there will be a no-load speed which will correspond to Eg
    being slightly less than V such that only enough current flows to balance
    the losses. Now a mechanical load is applied to the motor shaft. This will
    cause the motor to slow down a bit and the result is that Eg decreases a bit
    so the current will rise, producing a higher torque. The motor will settle
    down at a slightly lower speed such that the torque produced balances the
    torque required by the load. If the mechanical load decreases, the reverse
    occurs as the motor will speed up, reducing the current and torque.
    Looking at power: the input power will be Pin =V*I =(I^2)*R +Eg*I
    However Eg*I =Tdev*w =mech power developed =mech loss +mechanical power out.

    There are variations on this depending on how the machine and these will
    affect the torque speed characteristic.

    Now look at the AC analog of this motor/generator and substitute Za =Ra +jXa
    for the series resistance. Ignoring Ra which is relatively small, the power
    developed becomes (after looking at I=(V-Eg)/Za as a start)
    P=[(V*Eg)/Xa] sin (delta) where delta is the phase angle of Eg with respect
    to V. The effect of the magnitude of Eg is less than that of delta and Eg
    may be more or less than V for both motoring and generating. If Eg lags
    behind V then it is a motor, if Eg leads V, it is a generator. In detail it
    gets a bit messier but that's the basic gist of it.

    The major motor in use is the induction motor: General analysis of this (as
    can be doen with the other motors) is on the basis of coupled current
    carrying windings. I won't go into detail but in this case, for steady state
    operation the model becomes that of a transformer where the load is a
    ficticious resistance dependent on the rotor resistance and the slip
    (difference in speed between the motor's rotating field and the rotor of the
    motor. At 0 slip, there is no mechanical power developed and the only
    current is the magnetising current.
    Now here is where your model comes in:
    -----Rs+jXs------o-------Rr/s +jXr-----| Stator impedance Rs +jXs
    I---> | Ir---> | Rotor
    impedance Rr/s +jXr hwere s is the slip
    V Zm | Zm is the
    magnetising impedance branch
    Torque proportional to (Ir^2)Rr/s Pmech =(120*pi*frequency/poles)*
    phases*ws* (Ir^2)Rr(1-s)/s

    I do have a program that looks at the behaviour of DC motors of different
    kinds as well as that of synchronous and induction motors. It also shows the
    effects of non-linearity on magnetising currents and interactions between
    synchronous machines on a 2 machine system (including governor droop) If
    you want it, let me know. It is written in Turbo Basic and compiled to an
    exe DOS based executable. (Full screen under windows is best). However, it
    doesn't delve into the theory per se- just the behaviour of the machines in
    steady state operation.
  13. operator jay

    operator jay Guest

    I hope I gave a suitable description. It was definitely a long ways
    short of being an analysis. Power factor is, as you say, important
    for any of the ac machines.

  14. Don Kelly

    Don Kelly Guest

    Actually, your description was very good. There is a real back emf- the
    voltage produced by windings moving in a magnetic field. In an AC machine,
    this voltage may or may not be in phase with the applied voltage (and
    generally is not in phase). However, the "power factor" of a motor only
    relates to the voltage and current at the terminals- not the relative phase
    of two voltages. The magnetising current of an induction motor is the main
    culprit there.
  15. Don Kelly

    Don Kelly Guest

    Damn- I am having problems- equation is correct for a DC machine as you have
    written it.
    Also it appears that I got carried away with too much detail at once-

    Just let us say that the resistance and the back emf are in series. (Both DC
    and AC synchronous machines) The polarity of the back emf Eg is the same as
    that of the source voltage.

    Also, to get down to basics- Back emf is the voltage expressed by Faraday's
    Law and the direction given by Lenz' law. These apply to coils,
    transformers or motors- wherever there is a changing flux linking a winding.

    There is no equivalent in terms of current.

    The commonly used term "induced current" is, in fact wrong and misleading.
    Induced voltages do exist and these can cause current to flow or modify
    existing current.
  16. Don Kelly

    Don Kelly Guest

    a) DC machine -phase angle is not of consequence and pf is not a useful
    concept (always 1). The only reason that Vs and Eg can be different is due
    to the IRa drop.
    Note that the "load" voltage is actually Eg -not IR+Eg as Eg is the
    internal speed voltage. The voltage that you have given is the applied
    voltage at the terminals.

    b) in an AC machine the leakage reactance is much larger than the resistance
    so the supply voltage is
    Vs =IZa +Eg as phasors. Again, the only reason Vs and Eg can be different is
    due to the IZa drop.

    c) Rewrite: I =(Vs-Eg)/Za
    so the phase angle of the current depends on both the relative phase of Vs
    and Eg as well as the angle associated with Za. This applies to the
    synchronous machine.
    In the case of a synchronous machine, the magnitude of Eg is usually
    controllable (by controlling the DC field current) and the power factor can
    be swung from lagging to leading by doing so. A synchronous motor which is
    overexcited (Eg>Vs) will be capacitive. If underexcited (Eg<Vs) it will be
    I can give an example using phasors if you wish or send phasor diagrams
    directly if your address above is correct.

    d)In an induction motor, the main cause of poor power factor is the exciting
    current which is nearly constant at all loads and lags the voltage by a
    large angle. As the motor is loaded, the load current will be somewhat
    inductive but the overall pf will improve. The model is more complex than
    the V=IZ+E model.
  17. Don Kelly

    Don Kelly Guest

  18. Don Kelly

    Don Kelly Guest

    I have replied to this under a new subject heading: motor models

    Don Kelly

    remove the urine to answer
  19. Don Kelly

    Don Kelly Guest

  20. Don Kelly

    Don Kelly Guest

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