120V from both legs

[daestrom]

If you take two half wits and multiply them you do not get
unity.... but 0.25....even if you add two half wits you dont
get unity even though the math could be seen as correct.....
Claiming PhD status just makes it worse. It also wouldnt be
the first time ive seen the less utterly brilliant working for
a power company.

You are asserting that a corporate postion implies competence?
I dont think you are actually...but thats how it comes off.

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.'

On the voltage/ force issue... those are similar but not
identical... that is the root of our little disagreement here.
...Force = Mass x Acceleration.., forever ... in physics..and
the study of electricity and its current is part of
physics....

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
inertia.

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.

Taking the mass out of it as you have in some of your examples
...say for instance a generator not connected to a circuit
indeed leaves voltage live and mass static...but its still
mass.. the electrons availabe to move.. the force remains
potential.

A large motor generatiing back emf .. would generate more of
that in terms of voltage and amperage than a smaller motor
obviously.... so this EMF term does have a variable other then
voltage...thats quite obvious in this example... or all EMF
would be the same in magnitude..and it isnt.

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.

A large motor, say 1000 hp is going to kick back 100.000 times
the net force (EMF), as a say a tiny 1/100 hp motor... that
should be implicity obvious. Is it obvious to you?

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?

Do I need to prove that... ?

the *voltage of the emf will be the same under the same
conditions of course... but the net EMF... with the force term
used, will of course be dramatically greater with the huge
motor...roughly 100,000 times greater with the larger motor.

That of course is because of the huge mass differences between
the two motors.

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'.

obviously then, not a straight Voltage =emf issue. emf
more closely relates to power when in motion..than merely the
voltage applied. Voltage remains constant regardless of
any motion or not...but the force varies... thus the term 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 think thats a fairly concise argument.

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.

I have used the term counter EMF in the past also... but do
not equate it to 'back EMF' as you do... back EMF evolved I
believe to more clearly describe emf coming back out onto the
grid, slightly out of phase with incoming current.. and at a
different amplide, somewhat as AM and FM radio waves modulate
amplitude or frequency.... yet the same signal being carried
on a single antena.

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.

As these emf influences leak back onto the power grid, they
create or constitute in themselves distortions in the electro
magnetic effects, reducing the efficiency of any connected
motors, designed of course to operate on clean power.

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).

I invite you then to review your materials on issues related
to some large industrial power users and their empact on the
quality of the power in the rest of the grid.. especially
customers farther from the generating source than the
industrial user in question.

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
discussion.

daestrom

 

[daestrom]

Dear Don, to avoid getting all pissy over this little
disparity in insight I snipped your no doubt superbly well
stated position and am posting this tidbit I found by accident
while responding in another thread re the vars issues.

This is about harmonic distortions and the broader power grid
from EC and M's ezine.... maybe you can forward it to Charlie
and his pal who have been trying to hose me with thier EE and
PE certs. Charlie had given up on me entirely.. he will find
this article enlightening, and from a top level source.



My remarks from another thread:

snipped

Beyond that there are harmonic distortions that any large
facility can feed back into the power grid, screwing things
up... the utility companies just hate that....motors and
transformers can both create unwanted harmonic effects.


Paragraph 2 of the following article addresses these harmonics
and power grid issues

http://ceenews.com/mag/electric_harmonic_measurements_ac/
"However, just providing the raw power isn't enough these
days. 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. Reflected back into the line, these harmonics tend to sap
usable power and cause overload in transformer neutrals,
circuit breakers, and motors. They can even damage other
sensitive equipment connected to the same source. Some
industry estimates predict that harmonics will eventually
consume up to 50% of the energy on the nationwide power
grid-clearly anunacceptable situation.

The solution, of course, is to suppress harmonics."

long article balance at URL above

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.

daestrom

 

[daestrom]

EMF of course is a very very generic term... all negative
harmonics, all phase power, *all current IS EMF.
Harmonics are EMF. a dead short is EMF... the energizer
bunny runs on EMF... Electro magnetic forces ... you see?

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???

These guys trying to split off a generic generic description
such as EMF from its more specific terms just ***boggle ones
mind. Then they deny there is any harmonic distortion of
significance feeding back into the grid?

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
are.

daestrom

 

[daestrom]

"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote in

Charlie and his PhD wavng friend are presenting fragments
that only mention some aspects of the power grid and metering
issues and are trying to assert that there are NO harmonic
feed back into the grid issues. They are utterly and totally
wrong...and now trying to trash me for simply stating an
oposite view.

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.

How does this reference grab you in that case...it states
that harmonic distortions fed back into the grid are estimated
to reach 50% levels soon and threaten the power supply.

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'?

Sorry..these two have then tried to spin defenitions of emf
and harmonics ... not impressive at all. Sorry. Harmonics
are compreised of Electo Magnetic Forces//

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
industry'.

amazing these two guys. What will be next from them... will
they deny what they have typed earlier or just say they have
killfiled me and dont speak to idiots.

None of that will fly. We have here two guys long on record
asserting no harmonic feed backs of any consequence to the
grid... denying the industry wide notice of this massive and
fast growing problem from many sources as I have stated...
including motors and power supplies for computers.. but not
given as examples in this article...so now Charlie is trying
to asert that this massive 50% threat only comes from a few
electronic motor drives because thats the example they used?

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.

*****
This is about harmonic distortions and the broader power grid
from EC and M's ezine....

Beyond that there are harmonic distortions that any large
facility can feed back into the power grid, screwing things
up... the utility companies just hate that....motors and
transformers can both create unwanted harmonic effects.

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."

daestrom

 

[daestrom]

Yes indeed this article uses only ONE example.. a relatively
uncommon one at that...but states that the problem of harmonic
feed back threatens ***half the power on the grid...!

Article is seven years old, yet the grid doesn't lose even 25% yet to
harmonics. Seems some predictions are worse than others.

The fact is that there are dozens of ways you can get harmonic
feed back.... (all of that is EMF by the way.....its 'back'
emf when it goes out onto the grid...*including from a common
induction motor, computer power supplies and many different
types of transformers...and its is utterly well known and
established and published as well.

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.

No single article is going to give all possible examples...
this one though states the problem as massive even though it
only gives a single example... the gentle reader is required
to extrapolate such things.

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
harmonics.

One can say water is wet... without then requiring someone to
go out and prove every spot in the ocean and water well to be
wet....

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.

There is a massive harmonic feed back problem that threatens
to consume half the power on the grid by some industry
estimates. That problem is not coming from a few
VSD's...but from dozens of other sources ive been mentioning
specifically..and that are commonly known and published about
in the industry..

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
transformers.

I am simply stunned that an industry PE and PhD are taking the
ludicrous lines of argument they are in the face of this very
obvious and utterly pervasive and well known situation with
electrical power, its distribution and use.

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.

Do I need to prove enery square inch of the ocean is wet to
you, before you admitits comprised of water?

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.

Thats the ludicrous level some are arguing this point
on...first by saying there was no such thing as harmonic feed
back to grid... then now by claiming 'well its only this one
source'...then trying to say its rare non linear or whatever.

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
correctable.

daestrom

 

[Don Kelly]

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
system.
.......
"The subject that I was responding to was power factor correction and some
of
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

being assumed changed in this pedantic discussion?
Do any of you use of know about Maxwell's equations with respect to
all this? Just curious.

........
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
apologies."

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.
Goodie.
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
[email protected]
remove the urine to answer

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.

you mentioned that rarely, then when you did you asserted
that it was NOT significant problem to the grid.... sorry.
it IS a very significant problem to the grid.

Furthermore Charlie and his PhD waving friend tried for a
time to assert that there were no back emf issues from motors
onto the grid...........bzzzzzt that was wrong too. Then all
of you have chosen to become abusive in response.

not impressive. But it is telling.

Trying to win a debate by misquotes and lies is childish.

How does this reference grab you in that case...it states
that harmonic distortions fed back into the grid are estimated
to reach 50% levels soon and threaten the power supply.

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'?

Sorry..these two have then tried to spin defenitions of emf
and harmonics ... not impressive at all. Sorry. Harmonics
are compreised of Electo Magnetic Forces//

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
industry'.

amazing these two guys. What will be next from them... will
they deny what they have typed earlier or just say they have
killfiled me and dont speak to idiots.

None of that will fly. We have here two guys long on record
asserting no harmonic feed backs of any consequence to the
grid... denying the industry wide notice of this massive and
fast growing problem from many sources as I have stated...
including motors and power supplies for computers.. but not
given as examples in this article...so now Charlie is trying
to asert that this massive 50% threat only comes from a few
electronic motor drives because thats the example they

used?

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.

*****
This is about harmonic distortions and the broader power grid
from EC and M's ezine....

Beyond that there are harmonic distortions that any large
facility can feed back into the power grid, screwing things
up... the utility companies just hate that....motors and
transformers can both create unwanted harmonic effects.

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."

daestrom

 

[daestrom]

Ah Daestrom! Its great to see you noticing the harmonics
problem on the grid !!... I knew you had some integrity...it
showed in spots of your previous arguments....close to 25% is
it already?

LIAR!!!!!!!!

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.

Now thats an issue!

Tell Charlie...and his friend waving the PhD... that will be
'exciting'.

Progress.

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.

daestrom

 

[Don Kelly]

Dere Don.. I might let this dead cow rot in place...but
probably not.. I will just hang loose for a while until the
carcass festers in the sun and the combination of more flies
and more references show up.

That always works best.

Your response here was spun but not as badly as previous
response from Charlie...resulting in his inevitable self
destruction (as follows)

Charles Perry (unless some forged a post)
"
You are an idiot who cannot read. I have said, and still
maintain that back
emf from induction motors is NOT a concern for utilities.
Harmonics are a
concern, but are in NO WAY related to emf. Harmonics from
induction motors
are almost nonexistent and not a concern. The harmonics from
motor drives
are a concern, but they are in no way related to emf."

Amazing... isnt it... I was stunned into utter speechlessness.
I take NO satisfaction that Charlie has blown his dick off
that remark... I would have much preffered to have been
enlightened and enjoyed a beer with the man.



Similar distortions abound from this group.. parsing off that
which should not be parsed off ...or isolated from the larger
very much integrated and integral scene....

No wonder there is such a shortage of clues abounding on this
topic... all of these issues we have mentioned, voltage,
amperage, vars, emf as back emf or counter emf (the type that
doesnt flow back out of the device).. resistance,
capacitance...and various induced lags..are not separable in
the faintest even though we must separate them in order to
remain functional at base levels...

all of the time we talk about VA when we actually mean vars
because the vars number is vaslty more complicated to discuss
and is variable by a wide range of influences..so we keep it
simple..

---------
I do realise the difference between VA and VARs. I don't know where you find
it vastly more complex as it isn't.
-----

and that over simplification it seems has tended to blind
some on this issue of motor generating capacity and issues
related to unbalance line feeds.. (voltage and ampacity as the
voltage is reduced, and phase distortions primarily)

Then we get all this spin ensuing.

To clear up one point however, you had credited me with being
correct in my assertions because certain VSD's DO produce
negative harmonic feed back onto the grid... well you might
wish to retreact some of that as I still mean to state that
negative EMF feed back onto the grid comes from many many
sources *including the average run of the mill fan motors
found in any whore house, gin mill or factory.

----------
Again, you are still sticking to your personal definition of emf - which is
not used in the rest of the world.

and I mean to state that even if you wish to state that EMF
means Electro magnetic FIELD... (which is doesnt as the term
is pervasively used) still directly implies a field
strength... and that of course coming *from a three phase
electric motor will be *balanced...oposing that is the most
commonly seen *unbalanced power supply.

------------
I did not state that I wish to use EMF as electromagnetic field. Please
learn to read.

More on that in a minute


And I do agree fully that *theoretically there IS no feed
back...and truly truly you can easily, as I can, produce an
absolute plethora of evidence and testing to support that very
notion....

(so now of course you have braced yersef knowing full that
the old asshole here is about to set the hook...and I am)

the hook is (and Charlie in a last desperate move to save his
ass aluded to it)...in the case of ****Unbalanced power to the
motor*** .

------
To disabuse you, Charles is not the desperate one. He knows his business.
------------

If the voltage is not balanced one phase to the next..and if
the amperage is NOT the same in each leg going into the
motor...it will generate back emf that will NOT be neutralized
by the incoming power..and that will move directly ,,,,without
passing go.... back to the transformers (where it will screw
things up modestly) .. and then back out ont the grid
dirtying up the grid power and screwing up electronics across
the proximate grid..prompting the customers to piss and
moan...and then directly causing the power company executives
to lie...then go out for drinks... fattening up thier already
huge blubbery asses.

----------
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.
--------

A simple matter of actuality... and you can test this easily
with a scope on a well functioning 3 phase motor *under
load*... then attach a heavy 120 volt to ground load from *one
of the legs..enough to drop the voltage to ground on that leg
as it is attached to the motor...say by 20% ....see what
changes that creates on yer scope/



I routinely find large motors drawing 10 or 20% less amperage
on one leg than the others...most commonly almost without
exception... all three legs are different... by 5 to
20%....unbalanced supply you see..even as the voltage at times
much more closely balanced...phase distortion and other
harmonics factors coming directly off the grid affecting
current flow through the associated motor windings.

-----------
Depending on the balance there can be neutral current and possible triplen
harmonics -so what's new?>

...... the *back emf generated through the motor though....
is *not* a direct function of input imbalance, it tends to
come back as clean power, in balance one leg to the
next...because the rotor spins at the same speed
regardless..and the windings of the motor all generate
accordingly. You see?

--------
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.

...so the back emf in that case (the most prevelant operating
condition) is not opposed and glides right past the supply
current like a fat boy on a wide glide harley past Moms whore
house (just past the old mustang ranch on the right going
north)

--
Bull. -again you are using the "defined" concept of "back emf" mixed in with
your own interpretation. >

Yers in absolutely flawless certitude, never in error in his
entire life on any
issue and one of Mona's biggest fans. (thats in Elko...across
from the commercial hotel.)

----
Are you claiming that you are God?
-----

The actual situation is this...if the utility companies admit
in any way that their supply grids are out of wack with
distortions, low voltage and variable impedence (due to
variable harmonic distortions one leg to the next) they open
themselves to massive liability law suits... for instance
compressor burn outs on HVAC equipment in the summer, very
much affected by low and unbalanced voltage... not to mention
electonics destroyed by spikes, surges etc. So we get all
this brainwash...often from the utility companies own staff
who are lied to extensively on these issues.

----------
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
it?
--------------

What causes all this baloney when they start with clean 3phase
power? In a few words, single phase services.. .. these do
not always balance out.. so you get line to line variables,
variable voltage and impedences... thats why the end user
sees such a wide disparity in amperage draw across the three
legs of is motors.

---------
A factor that has been with us for the past 100 years. Not exactly a new
problem.
-------------

In 40 years I dont think Ive seen more than a handful of
motors running with the same amperage on all three
legs...tighter lets say than 1% most as Ive said vary 5 to
20% per leg...ive seen some pulsing on one leg...(not a dirty
contactor problem either). ------------

the resulting back emf into the grid then exacerbates the
problem.
-----

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
communication.
----------

The nations power grid is an utter mess...some areas are a
complete disaster...and with more use and no upgrades its
getting worse.

----------
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.
---------------------

Trying to assert we have clean power is error number one.
Then comes the assertions you see lately on the ng.

The theoretical rules for clean power cease to apply when the
power is dirty... that generates is own negative feed back....
exponentially worsening the problem for users farther away
from the local source.

--------
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.

 

[Charles Perry]

Not much is new...Charlie just tried to assert that there were
no such problems related to feed back from motors etc by end
users...he is incorrect is all. Then he later tried to spin it
with exceptions such as with unbalanced power...the grid is
pervasively unbalance... so you get pervasive feed back and
vars related problems..

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.

 

[Charles Perry]

oh please you typed earlier that adding pf correction
capacitors to three phase motors was a waste of money...a
black box con or whatever.. you blew that horn long and hard
from many different angles...now you are back peddling.

You are a fool. That is not what I posted at all. You really cannot
understand what you read. Sad.

Charles Perry P.E.

 

[operator jay]

Thank you. Would you please comment on the following view? One hears it
said back emf subtracts from applied voltage, but if applied voltage to the
motor is constant from start-up to steady-state then there cannot in reality
be a back emf voltage. Only in theory.

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
stator.

However one can measure that current

reduces from start-up to steady-state, so the difference must be back
*current* generated by the motor. Accordingly back current Ib would be

Ib = E/R - E/Z

Say E=120 volts, R=10 ohms, Z= 50 ohms then back current = 9.6 amps.

My the same rational then one could calculate back power.

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
as:
Ib = (voltage of variable source) / (20 ohms).

j

 

[Don Kelly]

Thank you. Would you please comment on the following view? One hears it
said back emf subtracts from applied voltage, but if applied voltage to the
motor is constant from start-up to steady-state then there cannot in reality
be a back emf voltage. Only in theory. However one can measure that current
reduces from start-up to steady-state, so the difference must be back
*current* generated by the motor. Accordingly back current Ib would be

Ib = E/R - E/Z

Say E=120 volts, R=10 ohms, Z= 50 ohms then back current = 9.6 amps.

My the same rational then one could calculate back power.

Northstar

--------
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
generator.
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

__________Ra_____________
+ I-> + | V=R*I +Eg or
I=(V-Eg)/Ra
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.

 

[operator jay]

I believe one needs to account for phase (add power factor to your equation).

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.

j

 

[Don Kelly]

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.

j

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.

 

[Don Kelly]

Much jumble snipped.

Again thank you, but unreadable. I believe you said basically


________ Ra______
| |
| |
Vin Eg
| |
|_________________|

Vin = voltage applied Ra = armature resistance Eg = back emf

Then Vin = I Ra + Eg

Northstar

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-
sorry.

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.

 

[Don Kelly]

Of interest is the relation of the load voltages (I Ra + eg) to the
electrical power factor (cosine angle between current and input voltage).
Any comment?

Northstar

------------
Yep:
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
inductive.
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.

 

[Don Kelly]

Thank you. So what is the electrical power factor in the form of R/Z
for an ac motor? TIA

----------
The question makes no sense. An AC motor is not an impedance load. R/Z isn't
meaningful.
As the load changes, the power factor changes. Power factor can be expressed
in terms of R/X for a passive resistance/reactance device but stick to the
original definition in terms of current and voltage for motors.

Suppose that at a given load, a 240V motor draws 10A at 0.8 pf lag. Under
these conditions, and ONLY under these conditions, it may <look> as if it
has an R/Z of 0.8 and the motor will <appear> to have a Z =24 ohms at angle
36.87 degrees or a resistance of 19.2 ohms in series with an inductance of
14.4 ohms. In fact the motor may have an actual R =0.1 ohms and an actual
reactance of 1 ohm in series with a voltage source.
Knowing this <apparent impedance> doesn't give any useful information as,
when load or voltage changes, the <apparent impedance> calculated from the
current and voltage will change. In other words if I know that at 240V,10A
0.8 pf the <apparent impedance> =24 ohms, that knowledge is absolutely
useless for finding the current at 230V. Ohms law (E=IZ for Z constant)
isn't worth a $3 bill. Sure it would be possible to calculate the apparent
impedance for different conditions but this would require calculation or
measurement of the V and I under a wide range of conditions, followed by a
lot of mathematical analysis. Why bother- it serves no useful purpose?
Motors, generators and batteries are active devices, involving energy
conversion and cannot be properly modelled by an impedance.

 

[Don Kelly]

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

--
Don Kelly
[email protected]
remove the urine to answer

 

[Don Kelly]

What is the original definition?

----------
Look at DC where power =V*I
Now look at AC where it became obvious (about 100 years ago) that power is
not V*I . The term V*I was then called apparent power and the real power was
realised to be V*I *cos(angle of I with respect to V). Essentially then,
power factor was defined as cos (angle of I with respect to V) or real power
(watts)/apparent power(volt-amps). Reactive (vars) then was defined as
V*I*(angle...).
In the case of a passive load Z, where one can say Z=V/I the angle
associated with Z is the inverse of the angle between current and voltage.
V=100V @ 0 degrees. I =10A at -30 degrees.
S(apparent power) =1000Volt amps. P(real power) =1000*cos (-30) =866Watts
pf=0.866 lag( I lags V)
If the load is passive then one can say Z=100/10 =10 ohms and R =8.66 ohms,
X=10*sin(30) =5 ohms.
-----------

Sorry but I disagree. What about shaker tables and speaker motors?

----------
Good question: The difference is in how the motor is loaded and what the
operating conditions are. Speakers and industrial (or washing machine)
motors operate in different regimes with different performance criteria.

Let's look at speakers: We have an electrical network coupled to a
mechanical network and the main variable is frequency .
Assuming linearity (as analysis does in this case) the relationships are-
for steady state at any given frequency:
E=ZcI+Eg where Zc is the coil impedance R+jwL) at radian frequency w=2*pi*f
Eg=CV where C is essentially flux of the magnet times the effective coil
length and Vis the coil velocity.
The mechanical force is F=C*I
The mechanical load consists of damping D, mass M, and spring compliance K -
all constant- independent of frequency force or velocity (again linearity
assumed)
Then, mechanically the load consists of F=Dv +j(M*w-1/K*w)V =Zmech*V
From the electrical side Eg = C*V=C*F/Zmech =C*C*I/Zmech
Then E={Zc +(C^2)/Zmech]* I
and the apparent impedance is Zc +(C^2)/Zmech
(You may refer to Beranek- "Acoustics" -if you bow 3 times beforehand.)

After all this - what is the difference?

The apparent impedance of the speaker is frequency dependent but not
dependent on voltage or current - given a speaker in a particular situation
(i.e. enclosure or room), it's mechanical load is strictly (within limits)
dependent on the force (current) and velocity (back emf) as well as the
frequency. With the supply voltage magnitude fixed, the current is dependent
on frequency only. It looks like a somewhat buggered up transformer as does
the coupling between the speaker cone and the acoustic load. Since the
frequency is variable, a plot of Z vs frequency is meaningful (in terms of
the load on the source) and considerations of resonances are important.
Impedance matching may be a concern where it isn't for an industrial motor.

The speaker load may be represented by an impedance in normal operating
conditions.
This is rarely a useful approach for a general motor load.

The objective, in any situation is to use a model which reflects, as well as
possible, the reality of the situation.
The basic relationships are essentially the same but the mechanical load
calls the shots- The motor , in either case, must meet the load
requirements.

In the case of the speaker, the mechanical load looks, within limits, like a
constant R,L, C circuit under variable frequency conditions. In the case of
a fan or pump, the mechanical load doesn't look like a constant R,L,C
circuit and is usually under constant frequency conditions (variable
frequency complicates things somewhat).

Example: A speaker with an input of 10V may have an output of 10 watts. At 5
V the current is also halved so the output is roughly 2.5 watts. This is
consistent with an impedance model.
A motor at 10V may have an output of 10 watts but at 5 V the output may be 5
watts or 2.5watts or 1 watt, depending on the load's speed torque
characteristic.The impedance model doesn't represent this correctly
unless one goes to a non-linear model which multiplies the analytical effort
by a hell of a lot.

(Example: 100=10*I, find I vs 100=(0.8*I +0.2*I^3 ) - find I- which is more
work?)

It is possible to use an impedance model- I don't deny that. However, in
most motor applications, it doesn't save work or improve understanding. In
the case of an induction motor, the load may be modelled by a slip
dependent resistance load on a transformer. It works. However determining
the operating point when the motor is driving a given load becomes a
graphical analysis or successive approximations- where do the motor and load
torque speed curves cross?

Part of the difference, outside of this, is that "Load" in utility or
industrial terms is consdered to be power (including volt-amps as apparent
power etc) where in the electronic industry, it is an impedance. This leads
to a lot of confusion.

 

[Don Kelly]

-----------much snipped.------------

Thank you for the excellent analysis. Very interesting. The only
thing I see needed to complete the analysis is the relationship
or relationships for mechanical power. Again, thanks.

Northstar

Take the DC case for simplicity:
Pin =V*I =R*(I^2)+Eg*I
R*(I^2) is loss in the winding resistance. Eg*I is the power transferred
from the electrical side to the mechanical side. As Eg=K*(flux)*w and
Torque T=K*(flux)*I
where K is a bugger factor of the individual machine and flux is the
magnetic flux per pole from a field winding or a permanent magnet. w is the
mechanical speed ( radians/ sec that makes the K the same in the MKS system)
Then Eg*I =Tw =the mechanical power developed (watts)
Pout is then Tw-mechanical losses.

AC Synchronous motor: Essentially the same relationship but phasor
relationships must be included.
The power converted will be Tw = Eg*I* cos(angle between Eg and I) w will
be the same at all loads and is ws (see below).

Induction motor: In the usual steady state model, the mechanical load is
represented by a resistance Rr/s where Rr is the rotor resistance and s is
(ws-wm)/ws
where ws is the synchronous speed determined by the frequency (same as in
synchronous motor) -say 1200rpm for a 4 pole 60 Hz motor and wm is the
actual speed -say 1150 rpm. Then s =50/1200=0.0417 or 4.17%
The torque is then (poles*phases/(2*pi*frequency)*(Ir^2)Rr/s
The speed is ws(1-s) and from these the Twm can be found. (mechanical
losses can be deducted from this)
The current Ir can be found from the circuit model.