120V from both legs

[Steve Alexanderson]

| The main illogic (other than some numerical problems)

Could you detail the numerical problems so I can fix them?

| is that it is assumed
| that the meter is somehow measuring power loss "upstream" of the meter.

That's not the assumption; it's the result.

| It
| doesn't- It measures the power on the consumer side of the meter. This will
| include losses on the consumer's side - for which the consumer is
| responsible - not those on the utility side.

I think pretty much everyone else now agrees that the typical 4-terminal
(i.e., "1.5 element") split-phase meter does indeed measure power losses
in the neutral on the utility's side of the meter. If you do not agree
could you explain exactly how the meter avoids this error?

| I suggest that you check Charles Perry's reference

Mr. Perry has posted a reference and provides one way of looking at
the error as proportional to "1/2 of" the voltage difference. (I quote
"1/2 of" since it doesn't actually add any information to a proportional
relation, but I think we know what it means in this context.) I prefer
to think of it in terms of the customer paying for 150% of the loss in
the utility's neutral wiring, but that's just for the simple one-meter
case.

Dan Lanciani
ddl@danlan.*com

Dan,

You seem to having trouble with the handbook reference. In your example, the
difference is 121-118=3 V. Half is 1.5. 1.5*100%/120=1.25%

Also from your example the error was 150W out 11950 W measured, or 1.255%.

The "1/2 of" is needed here.

I re-read your example looking for errors and could find none.

Later,

Steve

 

[Dan Lanciani]

|
| | > In article <9KJ8d.640297$gE.469035@pd7tw3no>, [email protected] (Don Kelly)
| writes:
| > | The main illogic (other than some numerical problems)
| >
| > Could you detail the numerical problems so I can fix them?
| >
| > | is that it is assumed
| > | that the meter is somehow measuring power loss "upstream" of the meter.
| >
| > That's not the assumption; it's the result.
| >
| > | It
| > | doesn't- It measures the power on the consumer side of the meter. This
| will
| > | include losses on the consumer's side - for which the consumer is
| > | responsible - not those on the utility side.
| >
| > I think pretty much everyone else now agrees that the typical 4-terminal
| > (i.e., "1.5 element") split-phase meter does indeed measure power losses
| > in the neutral on the utility's side of the meter. If you do not agree
| > could you explain exactly how the meter avoids this error?
| >
| > | I suggest that you check Charles Perry's reference
| >
| > Mr. Perry has posted a reference and provides one way of looking at
| > the error as proportional to "1/2 of" the voltage difference. (I quote
| > "1/2 of" since it doesn't actually add any information to a proportional
| > relation, but I think we know what it means in this context.) I prefer
| > to think of it in terms of the customer paying for 150% of the loss in
| > the utility's neutral wiring, but that's just for the simple one-meter
| > case.
| >
| > Dan Lanciani
| > ddl@danlan.*com
|
| Dan,
|
| You seem to having trouble with the handbook reference.

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

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

Dan Lanciani
ddl@danlan.*com

 

[Charles Perry]

sorry, your mysterious PhD's 'references' dont hold any
water when it comes to making such ludicrious statements.

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

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

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

Charles Perry P.E.

 

[Don Kelly]

justified.

appearances can be decieving...along with assumptions.
Beyond the voltage reads at the motor terminals are wave
distortions in the AC current flow... harmonic
distortions...and these can and do produce significant
disturbance in the stator field of the motor and that creates
slip and as you know, things so south in that case fast.

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

 

[Don Kelly]

sorry, your mysterious PhD's 'references' dont hold any
water when it comes to making such ludicrious statements.

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

 

[Don Kelly]

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

Idon'tlikegreeneggsandspamIdon'[email protected]

 

[Charles Perry]

Here is one article of thousands... for articles on grid
effects add grid, degradation, harmonic distortions to the
seach...you will get thousands of hits... this is NOT a new
concept.

http://www.phys.unsw.edu.au/~jw/HSCmotors.html

Here is one from the University of South Wales. defining back
emf in the context of motors

http://www.phys.unsw.edu.au/~jw/HSCmotors.html

<snip>

You give me the same reference that I gave you?! And it does not say
anything about back emf effecting the grid.

correct. Otherwise described as 'back emf'

Hahahahahahahahahaha. You really should get a copy of IEEE STD 100. It
would help you with your terminology errors. Back emf and reactive current
are NOT the same.

http://electron9.phys.utk.edu/phys136d/modules/m8/devices.htm

References that do not address the issues of back emf and the
power grid distortion by such forces will of course not
address the issue at hand.

I suggeset instead of looking in the wrong places you look at
references that address these issues.

Odd, you give the same reference as support for your arguement and then say
when I give the same reference that it is irrelevant.

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

Charles Perry P.E.

 

[Charles Perry]

"the term Back EMF' is also self describing.. a back flow
of electro motive force though the feeders from the motor.
your complete unawareness of this very commonly faced and
studied situation defines your level of insight in the
electrical engineering field.

Back EMF is a voltage.

Correct. But missing the next step...this back emf goes back
into the power grid ..and that increases with phase angle
distortions etc. You think there is some magic door at the
motor leads that stop these effects? read up bud.

Back EMF is a voltage. It opposes the terminal voltage, but it doesn't go
anywhere.

But under steady
state conditions,

ah now you are getting a little brigher ...you have covered
your ass, albeit a little late and issued an exception...in
your last sentence... 'under steady state conditions'... most
industrial motors operate in anything but steady state
conditions..with line voltage constantly varying... and low
voltage an absolutely cronic problem in hot weather in most
parts of the country... so much for 'steady state conditions'

Wrong answer. By steady state I mean the power system is supplying power to
the motor which is turning a load as opposed to the power system having a
fault and the motor acting as a generator supplying fault current.

correct. Otherwise described as 'back emf'

Wrong, back emf is a voltage.

And since back emf is a voltage by definition, are
you saying a


look... really you must get a grip here...voltage alone as
you surely must know is not particularly relevant unless there
is amperage involved..so the back EMF is NOT just voltage as
you state...it is as its own term says Electro Motive
Force...and that is comprised of both voltage, and amperage
and in an AC system, wave function. Force is an energy
function... the number of electrons and the voltage applied
defines the net 'force'... thus the term EMF.

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

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

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

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

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

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

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

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

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

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

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

These are just some of the books that I have in my office. I could come up
with dozens more if I walked down the hall to our library.

You should know enough to address EMF issues in its proper
context but obviously you do not. Thats revealing. I suggest
you avail yourself of the nearly countless books and articles
on these very very common issues. Its not rocket science...

I think you would be well served to read the above mentioned texts.

http://www.phys.unsw.edu.au/~jw/HSCmotors.html#back

Even your own reference calls it a voltage. Sad really.

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

Charles Perry P.E.

 

[Charles Perry]

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

Charles Perry P.E.

 

[Don Kelly]

communication problem.

If you are not aware the back emf from a motor, especially
one thats slipping..generates emf that escapes the motor back
into the grid... you have some serious holes in your data base

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

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

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

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

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

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

 

[Don Kelly]

What? you said *voltage *above the rated range cause
harmonic distortions?...thats rare to the point of being
almost nill....

---------
If so then the claim that you have made for harmonic distortion has no
basis in fact. Please tell me the basis for your claim of harmonic
distortions- you have evaded that question
-----------.

Voltage **below the motor rating creates slip, and is utterly
and pervasively common..along with phase
imbalances....especially in high torque applications with the
voltage falling 10% or more below the rating..and YOU excluded
this?

---------
Dealing with induction motors - there will be slip at any load and any
voltage. Slip is needed for energy conversion in these machines which are
the dominant industrial motors. Certainly, at any given torque, the slip
will be higher at lower voltage (or, conversely, the torque will vary
approximately with the square of the voltage) at any given slip- at rated
voltage a slip of 3% may occur- at 90% voltage the slip will be about 4% -
difference is 18rpm for a 4 pole 60Hz motor). I did not deny that. It also
follows that at starting the available torque at 90% rated voltage will be
81% of the normal starting torque.

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

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

 

[Don Kelly]

more bogus argument Charlie... without amperage there is
nothing that flows to create the *force implicit in the
term.... force charley is Mass x Accleration... you are tying
to leave the mass out of the equation...and describe EMF as
voltage only.

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

Please note that no-one is saying that back emf doesn't affect current. In
fact it better do so if a motor is to operate at any speed other than 0 or
"fly apart".

it doesnt get any farther south than that bud

You cant possibly be that ignorant Charlie... and if its
spin,,, you are spinning yourself in with such ludicrous
positions.



you have now contradicted your earlier remarks that
back EMF goes into the grid if the power to the motor
generating it is unbalanced (and it virtually allways is
Charlie)... have you ever seen for instance a dead nuts
perfect read of 240 volts to ground on a three phase
service..for an entire day with no deviation? Of course
not... our nations power grid is not remotely that stable.

----------
Normal fluctuation of voltage (or frequency) on a power system is not
considered any form of instability. If you are referring to power systems-
"stability" has a particular meaning. Voltage instability has another
meaning and this is not related to fluctuations at the 240V level. Unbalance
does affect motor performance - that is true. Since loads on a system,
particularly at the low voltage distribution level (i.e 240V) are generally
not balanced the voltages seen at a particular point will be unbalanced.
Neither harmonic or pf correction will deal with this. Again- emf =voltage
doesn't propagate.
----------------

current.


Yer wobbling real bad with that bit of spin there Charlie.
Real loopy argument there pal, it doesnt parse well... its is
in contraction to your long time assertion that back emf
doenst feed back onto the grid...that is if one can even parse
such a masacre of the english language.







Again yes back EMF has voltage as *one of its aspects...
but by leaving out the amperage component you attempt to deny
flow, and the FORCE implicit the term,.. voltage alone is not
force... voltage alone charlie is just a potential... when
^force comes into the equation, and the third word in the
anagram EMF is force... then you have actual current flow...
back EMF is *current flow charlie...and current flow affects
the grid obviously

-------------
EMF or electromotive force according to the Oxford dictionary- "a potential
difference between two points which tends to give rise to a current"
(<tends> doesn't mean that it will actually do so-that depends on the
circuit)
"Back emf" is an emf opposing some applied voltage.I also checked with
several university level texts.
I note that texts on electric machines don't even bother with the term
"back emf"or even the term "emf". A beginning circuits text mentions "so
called" emf.
The term is an anachronism and is disappearing because it gives rise to the
nonsense that you have given above. You may give whatever interpretation to
the words that you want but I would recommend that you use the terminology
that is in standard use. I would also recommend a basic high school physics
book and then some of the texts that Charles has suggested.

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

If you want to rest your case- fine- but please learn some of the
fundamentals of what you are resting it on.

 

[Charles Perry]

You are not lookinga the unbalance situation when back EMF is
not neutralized in the motor...but progagates to the source
feed and grid, as yer friend Charlie finally admitted to...in
the case of unbalanced voltage or other harmonic disturbances
across the lines..

You need to read my posts. I never said anything about unbalance voltage.
Faults, yes.

Poor idiot can't even read.

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

Charles Perry P.E.

 

[Charles Perry]

"Charles Perry" <[email protected]> wrote in message

Hey!!! I resemble that remark!!!

What do you have against electricians?

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

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

So, I have had some interesting encounters with electricians that thought
they knew more than they did.

I've got a news flash for you. This clown (Phil)is a moron, or is it
maroon :-]! Comparing him to an electrician, even a substandard one,
is an insult!

At this point I am surprised Phil can remember to breath by himself.

Charles Perry P.E.

 

[daestrom]

I am saying it is Electro Magnetic Force... if you cant
fathom that look up each word in the term... and dont skip
*force.. and the definition of force... In the physical
sciences, including electro magnetics,,, force is defined as
*MASS x Acceleration ... its very simple.

the MASS in the term is the mass of the electrons when you
are talking about Electro Magnetic FORCE...



You are not at all up to speed this extremely rudimentary
aspect of physics...and yes it applies broadly to electricity
as well...where ever force is involved there is MASS as far as
modern science understands the issue to date.

No MASS no force.

Voltage without something to move (electrons) is a sheer
potential... only with electrons involved and moving does it
become FORCE.. EMF.

Sorry... thats real basic.

Lacking those insights you end up with a very very thin grasp
of electricity... then fail to understand these sorts of
issues, confusing a sheer *potential.. (voltage) with power,
of which electro magnetic force is the integral derivation

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

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

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

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

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

daestrom
<snip>

 

[Don Kelly]

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

Utter rubbish snipped.
plonk

 

[Don Kelly]

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.

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

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

 

[Don Kelly]

Not to take sides or cause conflict, but would you (or Mr. Scott) please
explain the dynamics of back emf as you see it. That is, its cause and effect
on motor operation and the same on the energy source supplying the motor?
Also is it a voltage, current, or force?

Northstar

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

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

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

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

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

I hope this is of help.

 

[daestrom]

As I requested from Mr. Kelly or Scott, would you please explain the
dynamics
of back emf as you see it. That is, its cause and effect on motor
operation
and the same on the energy source supplying the motor? Also is it a
voltage,
current, or force? I have seen conflicting explanations here and there,
and
I figure you guys likely know the score. TIA

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

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

daestrom

 

[Charles Perry]

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

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

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

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


You have really shown your hand this time.

Charles Perry P.E.