And Now MVaR in words of one sylable or less
And Now MVaR in words of one sylable or less
(OP)
Here's one for all of you clever electrical chappies out there.
Imagine trying to explain MVaRs to a new starter, ie. in the simplest way possible.
All analogies welcome (except maybe the horse and barge one).
Points may be awarded for creativity but deducted for over technicality.
This has always been one of those questions that,(along with what is entropy?)has been a source of consternation in power stations since Michael Faraday was a lad.
Go On, you know you want to............
JJ
Imagine trying to explain MVaRs to a new starter, ie. in the simplest way possible.
All analogies welcome (except maybe the horse and barge one).
Points may be awarded for creativity but deducted for over technicality.
This has always been one of those questions that,(along with what is entropy?)has been a source of consternation in power stations since Michael Faraday was a lad.
Go On, you know you want to............
JJ






RE: And Now MVaR in words of one sylable or less
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
10 Search this site for the Beer and froth analogy of power factor.
20 do: Take your student to the pub and order a beer for each of you.
30 Use the beer to explain the power factor analogy.
40 Drink the beer.
50 does the student understand power factor?: exit
60 loop to 20
Bill
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"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
55(?) Unable to stay upright on bar stool?: exit
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: And Now MVaR in words of one sylable or less
What do I get?
Keith Cress
kcress - http://www.flaminsystems.com
RE: And Now MVaR in words of one sylable or less
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: And Now MVaR in words of one sylable or less
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
"MVARs are like managers, they take a lot of room, increase losses, doesn't do useful work, but are still required to obtain the "REAL" work at the end of the line."
Please wait 'til someone else tells you a far better answer!
RE: And Now MVaR in words of one sylable or less
You two win the prize!
BTW. It is var nowadays. Not VAr or anything like that.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
Some companies will assess specifically for using the rails(vars) others won't.
Keith Cress
kcress - http://www.flaminsystems.com
RE: And Now MVaR in words of one sylable or less
We have many times in the past, tried to explain this concept. To para-phrase Lili von Stupt in Blazing Saddles:
"We're Tired!"
For a more serious treatment of the VARs concept, search this site. (I'm sorry Gunnar, I tried vars but I couldn't do it.)
Is this a European Union harmonization thing?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
JJayG:
This thread got the responses it deserved. Sorry.
burnt2x:
Excellent description, I must remember this.
Gunnar:
V and A I've learned and I'm used to write in capital letters as they are derived from Mssrs. Volta and Ampere. The r stands for reactive and is always written as a small letter in Europe. In the U.S. they have a different opinion, however. Would be nice to know whether there is a new standard regulating this.
Regards
Wolf
WWW.HYDROPOWER-CONSULT.COM
RE: And Now MVaR in words of one sylable or less
Yes, as always, the BIPM has set the rule. Reactive VA is a unit (has been so for something like 20 years) and shall be written "var". If we are at all going to have a unified and standardized technical language, we need to adhere to rules laid down by the top standard institute.
I know, it hurts, but instead of using VAr or VAR or VaR or, perhaps vAr or... I think it is meeaningful to use what the BIPM says. Privatized writing like the ones used above only confuse. And so do "simplified" explanations. But I liked the manager explanation.
We had a similar discussion a few weeks ago where the unit for temperature change was discussed. Same thing there. K is the unit to use while C or F are units for temperature. Most people seem to think that C and K are identical. They are not. BIPM agan.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
The issue is that the common expressions, such as kVAR or KVAR have been in use longer than the so called adoption of 'standards'. And hence the use of var in common writing will not be any more accepted than it has been in the last 20 yrs. Arbitrary and retrospective standards will always find the acceptance harder. Just like K vs. C or F.
I also start out writing var (since I learned of this not too long ago) in many cases and then usually decide to change it to kVAR to match with kVA in the reports.
It is no different than use of KVA for kVA. or KW for kW. All have been very common in writing and equipment data sheets/name plates with no real confusion or opposition.
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
The BIPM is run by the French isn't it? That explains everything...
http://w
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
If the subject doesn't 'get' this, I usually fall back to the beer/suds explanation.
RE: And Now MVaR in words of one sylable or less
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
Although, if the engineering standards keep up with nerdy ways, they will end up there.
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
We are off subject.
RE: And Now MVaR in words of one sylable or less
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
But what plays in Vegas stays in Vegas and what the french do to their own should stay in france.
No that's not a typo, de-capitalizing Ampere seems a little reactionary and the new rules say that reactionary entities lose their capitalization.
On a little more serious note; (emphasis on little)
Many years ago I learned a little bit of English grammar. One subject I dimly remember is abbreviation. I seem to remember that abbreviations shall be capitalized.
AVR: automatic voltage regulator, PMG: permanent magnet generator, CT: current transformer, PT: potential transformer, VAR: Volt Amps reactive, oops.
Although some may prefer to use the more recent name,I prefer the full descriptive name, Volt Amps reactive, and following the rules of English grammar abbreviate this to VAR.
Should a German, an Italian, or a Russian challenge my use of the rules of the English grammar I would be offended. Should a Francophone challenge my or demand that I change my habits, Oh well, that's just the French. Look at the reactionary treatment of the memory poor André-Marie Ampère!
You may say var. When I say VAR I am abbreviating Volt Amps reactive. grin Now let's head for the pub and work on the beer analogy. VAR - Very Agreeable Refreshment.
Bill
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"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
(You didn't get the unit multiplication thing. Think about it. It makes sense. But now for the beers!)
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
I also vote for beer.
RE: And Now MVaR in words of one sylable or less
..keep going with beer! I will be designated driver.
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
You guys must be really drunk.
BTW, I'm all for VAR, not some pansy var.
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
It's 0225hrs here. My litle girl has a cold and can't sleep, so I'm not allowed to sleep either. That whisky is looking more tempting by the minute!
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
Hope your little girl gets better soon. Sick children take a lot out of you. Been there, done that. My kids are all grown up and I just exchanged one set of problems for another. Now the older problem doesn't look not so intimidating anymore.
PS: In your frustration, don't give the whiskey to the daughter.
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
I like your description, pansy var. The governor of California would probably call them "Girlie vars."
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
RE: And Now MVaR in words of one sylable or less
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RE: And Now MVaR in words of one sylable or less
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
Since you asked for it, here is some information from a more authentic source, the IEC website.
Which clarifies the following:
The name var is not related to a person's name. It in fact stands for volt ampere reactive power and the unit is also var. (not that it is widely accepted or all have to agree to it).
Also there is an official conversion factor: 1 var= 1 V.A
(How original and useful! Conversion factor for an arbitrarily invented unit!)
So what is wrong with writing a name in Upper case? and units can still be VA , using the conversion factor!)
Then there is a Statement by J.H. Dellinger (U.S. Bureau of Standards) in 1916, which is even true today.
"In conclusion, this study has shown that the international system, based upon representing the electromagnetic system, is a convenient and satisfactory system of units for the purposes of electric and magnetic measurements. Proposed changes in some or all of the units do not appear to offer advantages such as to justify the confusion and inconvenience of changing the units as ordinarily used."
The links are:
http://www.iec.ch/zone/si/si_statements.htm
http://www.iec.ch/zone/si/si_elecmag.htm
By the way, IEC has also created a storm or a debate in IT world as well by creating some arbitrary units like 1KB=1000 bytes where as the conventional wisdom and fact is 1024. (or something like that).
Rafiq Bulsara
http://www.srengineersct.com
RE: And Now MVaR in words of one sylable or less
The beauty with SI units is that they can be used to check validity of equations, among other things. But, if you use an arbitrary addition to the "pure" SI units, then validity check will not work. And since W and VA are reserved for active and apparent power, there is a problem with reactive power. Using VAR or VAr will not work, simply because the "R" or "r" doesn't fit when doing "unit algebra". That is why the unit var was defined. The dimension of var is still VA.
I think that you should accept that scientific work needs more than arbitray conventions. But use VAr or vaR or VaR or whatever you feel like. But be prepared to meet one or two elevated eyebrow now and then.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
I am trying to follow your logic - I promise, I really am - but I am so far failing. Keep trying!
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
(Is it the "unit algebra" AKA "dimension analysis" that you don't understand? I shall google to find a few URLs for you, if that is the case).
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
S = P * P.F.
If P.F. is unitless, then S must have the same dimension as P.
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RE: And Now MVaR in words of one sylable or less
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: And Now MVaR in words of one sylable or less
P = S*P.F.
Same conclusion.
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RE: And Now MVaR in words of one sylable or less
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: And Now MVaR in words of one sylable or less
VARs and Slinky Analogy:
With two people on a long table, Holder A and Holder B, stretch a Slinky toy between them. Holder A (HA) is the utility generator, Holder B (HB) is the End User, representing the work being done. At one end, HA wiggles the slinky back and forth, making a series of sine waves. The energy exerted by HA is the Apparent Power (kVA) being exerted into the system. The relative motion of the wave peaks represents the AC current flow. The energy HB feels as the Slinky tries to move his hand is the work being performed (kW), the Real Power being delivered. But if you watch the slinky, the spring is being compressed and expanded by the action of delivering this "power"; that compression / expansion is the Reactive Power, the VARs, and is initially created by HA but then reflected back and forth between the two. It's just something that comes with the territory.
This is not as eloquent as I originally heard it, I know. If anyone has knowledge of the source of this, I'd love to see it again.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: And Now MVaR in words of one sylable or less
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RE: And Now MVaR in words of one sylable or less
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RE: And Now MVaR in words of one sylable or less
I like the observation in ePete's second-from-last post too.
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
Publican, another round if you please, and by all means have one yourself.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
1 - S, P, Q all have the same unit.
OR
2 - S, P, Q carry units with different names (VA, WATTS, VAR) which are all dimensionally equivalent:1watt=1va=1var. The different names are simply a convenience to help match traditional notation.
Either approach is logical to me. But picking on VAR without picking on VA not logical to me. And if you say VA is not a unit but a product, you should be equally happy to label Q in watts.
At any rate, life goes on and we all get the right answers, even the heathens like me who don't use the SI system exclusively.
Now it is time for a beer while I think about that slinky.
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RE: And Now MVaR in words of one sylable or less
Now I need a "real" tall beer (without suds please).
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: And Now MVaR in words of one sylable or less
I forget. Was there a point to all this? OP no doubt gave up on us.
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: And Now MVaR in words of one sylable or less
I agree. I don't think OP is ever coming back. We engineers have the uncanny knack of turning off people.
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
W and var reflect the same situation. W in same direction (angle 0 degrees) and var perpendicular (angle 90 degrees). But, as the OP implies, there is a lot of confusion about reactive power (I really do not see why) and thus the var was introduced as a separate unit so that the R (in VAR or vaR or whatever the preference is at the moment) shouldn't enter dimensional analysis.
The confusion about reactive power is probably mostly because of all the unnecessary simplified explanations. The produce nothing but confusion. The correct math is not hard to follow. And the results are easy to illustrate using an oscilloscope (or a simulation, or even pen and paper). So I would prefer not to see any simplified explanations at all. We are engineers - aren't we? Why, then, should we use electrician's thinking?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
I agree with you, for purposes of electrical engineers there is no benefit to give any analogy for power factor. The benefit comes for folks that aren't electrical engineers. I have to say I do benefit from analogies that convert mechanical problems (static springs or dynamic mass / spring /damper) into electrical problems (R / L / C) simply because it transforms the problem into something I am more familiar with. But transforming electrical into mechanical doesn't buy anything for me... maybe it would for a mechanical guy. Go ask the question in the mechanical engineering other topics forum and you might get "better" answers (answers more useful for people that haven't studied the power triangle to the point that it is intuitive.
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RE: And Now MVaR in words of one sylable or less
Gunnar:
Your post 5 Oct 09 12:45 made it clear, but the BIPM rule definitely hurts me. And the reaction of the audience shows that very few people are aware of this BIPM regulation.
I myself don't wish to return to horsepowers or lbf ft/s, but I'd like to stick to Volts x Amperes = Watts for active power. For expressing apparent power I'll use VA and for reactive power its VAr (r for reactive). I'm used to it and its easy to understand. I must admit, however, that P for active power, S for apparent power and Q for reactive power would be an attractive alternative.
I can gladly report that I'm used to C and K.
Regards
Wolf
WWW.HYDROPOWER-CONSULT.COM
RE: And Now MVaR in words of one sylable or less
A good analogy is vars are electrical transmission fluid needed to transfer energy across the air gap in generators, motors and transformers. Before the energy (watts can flow) we have to send amps to fill up the transmission fluid.
A great teaching tool is on this website(free)
ht
RE: And Now MVaR in words of one sylable or less
RE: And Now MVaR in words of one sylable or less
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
RE: And Now MVaR in words of one sylable or less
To quote myself;
I almost added, if a Swede challenges my use of the English language, tread cautiously, he may well be correct.
You are correct sir, as usual.
However, may I suggest an alternate point of view?
In a town lived a man and a Countess. The man despised the Countess and would only refer to her as a pig. (Please be patient, there is a point.)
Eventually the Countess had the man hauled into court. The judge explained to the man that he could not call the Countess a pig and he would be in serious trouble if he continued.
The man asked;
"If I am not allowed to call a Countess a pig, may I call a pig a Countess?"
"Yes you may." replied the judge.
The man then turned and said;
Good afternoon, Countess!"
That said,
I accept that I must call Volt Amps reactive vars.
But, may I call vars "Volt Amps reactive"?
If I am allowed to call vars "Volt Amps reactive" then by the accepted rules of English Grammar, the acronym VARs is acceptable.
I agree and accept that var is the proper name to use in a formula, and accept the responsibility for any loss or confusion that may result should I inadvertently capitalize any part of var.
However, may I use the grammatically correct acronym to replace the term Volt Amp reactive when writing conversationally?
Example:
You may speak about VARs all you wish, but if you are using the term in a mathematical formula the correct term is var.
I hope this suggestion is acceptable to most if not all of the parties to this discussion.
Yuma, I spent most of my life in Canada and have not heard the term.
Scotty, I hope your daughter has recovered from her cold.
On a lighter note;
Voltaire? What on earth has he to do with electricity, the Volt is named after Volta.
Voltaire had a knack for antagonizing those in authority, (possibly mostly those who were abusing their authority).
Despite his unpopularity banishments and exiles, he lived to a good old age and probably died of mostly natural causes. (Overexertion? An arduous journey of several days duration was the probable direct cause.)
Although Voltaire died when Ampere was a young boy and ten years before the French revolution, Voltaire's ideas and attacks on the French system of government of the time was/is seen by some as part of reason for the French revolution, despite the ten years or so that elapsed between his death and the start of the revolution.
Ampere's father died in that revolution. Was there any family animosity, probably not, but it is possible.
Forgive me for a poor attempt at humor based on such a tenuous connection. It's the probably the result of a brain damaged by too many years of exposure to too many electro static and electro magnetic fields.
var has been around since about 1965? Yikes, I missed that one. Probably because I didn't stop drinking until the late 60's.
Despite no longer drinking myself, I'll be glad to buy you one of your choice, Gunnar and have a coffee with you.
When oh when will I learn never to doubt the master?
Bill
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"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
When I started to learn about electricity, from my father and uncles, to do with flashlights and bicycle generators and light switches, popular knowledge was that electricity flowed from positive to negative. Popular knowledge always lags behind scientific wisdom, witness the present discussion re: var, VARs.
As I grew older I learned about electrons and had to change my ideas. Electricity now travelled from negative to positive.
This was an advantage to dislectics everywhere. They could now use the right hand rule with their left hands.
Sometime later, word filtered down that electricity had been redefined from the movement of electrons to the movement of holes, or if you will, the movement of the absence of electrons, or the movement of nothing. Much ado about nothing?
I remember a question from one of my fellow instructors in the staff room back in the early 70's.
"Should we be teaching electron flow or conventional flow?"
There were a couple of non committal sentences and the subject was changed.
Question:
Was the BIPM the body responsible for disregarding electrons and redefining the flow of electricity as positive to negative?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
Thanks for the kind thoughts for our little 'un. I fear she is going to spend her first birthday quite ill, which is a shame. Fingers crossed that she perks up a little in the next couple of days.
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If we learn from our mistakes I'm getting a great education!
RE: And Now MVaR in words of one sylable or less
All I know is that it wasn't supposed to be an excuse for an argument on CaPitAl bLEediNg LetTers, how very very sad.
Why didn't you just tell me (as I suspected)that there isn't a simple answer and left it at that instead of going off on one?
Thanks to those who did take it in the spirit that it was written, and to those who merely used it as a vehicle to demonstrate their perceived intelectual superiority..........words fail me (But you don't quite spell it lIkE tHAt.
So, just to recap then:-
If I get two managers and tie them up in a vat of beer with a slinky on a railroad track...............
Never realised that my finger inadvertantly hitting/missing the shift key would cause such a fuss.
RE: And Now MVaR in words of one sylable or less
Sorry for that. No reflection on you whatsoever, we do this to each other.
How to explain power factor to a starter?
An analogy works by transferring knowledge from one field to another. Water pumping analogies for electrical effects only work if the learner is familiar with water pumping.
An analogy about analogies.
We were playing with a small motor bike once. We encouraged my sister-in-law to try it. I can't ride that she kept saying.
We kept assuring her that it was "Just like riding a bicycle."
Lynn finally tried it. Off she went, full throttle. Didn't make the first turn and plowed full throttle into a stand of saplings. She kept the throttle pinned and we could hear her screaming, the engine put-put-putting and the saplings breaking. Finally she stopped, in the center of the stand of saplings. When we finally got her back out in the clear, she was not happy with us.
"But, it's just the same as riding a bicycle."
"BUT I CAN"T RIDE A BICYCLE!!!"
Well, using an analogy from an area that the learner doesn't understand may be just as futile if less exciting.
Analogies work best one on one, or at least with a group that is familiar with the field referenced by the analogy.
An analogy based on the wheat fields of Kansas may be lost on a learner from the Everglades as will an analogy based on the Everglades may be lost on a learner who has never left Kansas.
Power factor and Volt amps reactive;
For the purists let's stick with displacement power factor.
When an alternating current is applied to a circuit, it will cause a current to flow. The current will be alternating. But, some circuit elements affect the flow of the current. Inductors inhibit the flow of current so that the current in an inductor, while still a sine wave, is phase shifted and reaches its peak value later than the voltage reaches its peak value. The peak current through a capacitor reaches it's peak before the voltage reaches its peak.
This effect may be described by a right triangle, The actual power consumed by the circuit is represented by the base, the Volts times Amps or VA is represented by the hypotenuse, and the altitude of the triangle is represented by an imaginary quantity called Volt-Amps-reactive. The angle will be the angle of the phase shift, or the degrees of phase shift described as one degree equals 1/360 of a complete cycle.
The right triangle and Pythagoras' theorem.
Your mission JJayG, should you choose to accept it, is to find out what fields your learner is familiar with where there is an effect that may be described by the right triangle and Pythagoras' theorem and make an analogy to transfer that knowledge to the electrical field.
One analogy that will certainly give your learner a lot of practice with the mathematics of power factor is the railway analogy.
An engine on one track is pulling a car on a parallel track. It takes a force of X units to move the car. The tracks are Y units apart. A rope Z units long is used to pull the car. What is the tension in the rope?
Solve for varying values of X, Y, and Z
Then have a beer and speculate on the ratio of beer to foam in relation to the volume of the mug.
Hope this helps.
Bill
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"Why not the best?"
Jimmy Carter
RE: And Now MVaR in words of one sylable or less
Beer is the Rosetta Stone of engineering.
RE: And Now MVaR in words of one sylable or less
The part people are missing is that Beer Mugs are very expensive and the price is proportional to volume.
So if your beer comes out of the tap with a healthy head of foam you have a bigger more expensive mug thna the people drinking out the tap that puts no foam on the beer.
To get the same beer you get they can buy a cheaper mug.
RE: And Now MVaR in words of one sylable or less
So let's use this analogy to try to develop analogy for power system in the m / k / c world.
I will start with a very simple mechanical system called 1m
System 1m:
Ground === vs(t) ===rigid bar===c_load ==== Ground
vs(t) = velocity source. Note if we apply a source displacement d(t)=d0* cos(w*t), this is equivalent to a velocity source v(t) = d./dt (d(t)) = w*d0*sin(w*t) = v0 * sin(w*t) where v0=d0*w is peak velocity.
c_load is a visous damper which removes energy from the system. Similar to a dashpot with viscuos fluid. It obeys c = f / v. f and v are the same as f and v of the source. A
There is no energy storage, so any energy input by the source vs is instantly converted to heat in the load c_load. f is in phase with v.
The average rate of energy transfer from source (vs) to the load (c_load) is:
Pm1 = 0.5 * v0 * f0 = 0.5 * v0^2*c
note vs as above, and fs is source force (which happens to be same as load force in this simple system.
and analogous electrical system will be called system 1e
System 1e:
Ground === Vs(t) ===ideal wire ===R_load ==== Ground
where Vs(t) = V0*sin(w(t), Is=I0*sin(w(t) and the ideal wire had neither resistance nor inductance.
Again there is no energy stored in the system, so any energy input by the source Vs is instantly converted to heat in the load R_load. I is in phase with V.
The average rate of energy transfer from source (Vs) to the load (R_load) is:
P1e = 0.5 * V0 * I0 = 0.5 * V0^2 / R
Now let us examine a second system Pm2
System 2m:
Ground === vs(t) ===k===c_load ==== Ground
where we have added spring k instead of bar.
The spring is an energy storage device and we now can have energy storage in the sytem within the spring. The energy that comes out of the source does not instantly appear at the load. The load can also pull energy out of the spring at times when the source isn't adding energy. The force is no longer in phase with the voltage.
The average rate of energy transfer from source (vs) to the load (c_load) can be shown to be:
Pm2 = 0.5 * v0 * f0 * cos(theta) where theta is the angle by which fs lags vs.
* I'm not going to dwell on the proof of this, but will come back later to disuss why it's important. *
We can also see that due to the presence of the new cos(theta) term resulting from energy storage within the system, we now need a higher magnitude f0 to transfer the same average power from the same source.
The electrical analogy to System 2m is System 2e
System 2e:
Ground === Vs(t) ===L ===R_load ==== Ground
Where we have replaced the "ideal" wire with one that has some inductance L.
We now have energy storage in the system (within the magnetic fields of L). The power leaving the source no longer instantaneously enters the load. The source current is no longer in phase with the source voltage.
The average rate of energy transfer from source (Vs) to the load (R_load) is:
P1e = 0.5 * V0 * I0 * cos(theta) where cos(theta) is power factor.
*** Most EE's are probably saying by now – this is worthless.... you haven't made the system easier you've made it harder. And I agree 100%. But turn it around and assume you were given the above mechanical system 2m and asked to analyse it! It would take a little bit of work I imagine and results would not be intuitive. But transfer it to electrical system 2e and it's a piece of cake.
Presumably, the situation is reversed for an ME. He looks at system P2e and sees a tricky system, but he looks at system P2m and sees something recognizeable. I think that is the same point waross was making.
By the way, turning this sytem from a mechanical system to a fluid system is fairly easy. Velocity is replaced with volume flow rate. Force remains force. Ideal wires connecting components are replaced with rigid pipe filled with incompressible fluid. Spring is replaced with elastic pipe or vessel containing incompressible fluid. Damper can still be a dashpot since that is sort of a fluid device.
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RE: And Now MVaR in words of one sylable or less
"P1e = 0.5 * V0 * I0 * cos(theta) where cos(theta) is power factor."
should have been
"P2e = 0.5 * V0 * I0 * cos(theta) where cos(theta) is power factor."
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RE: And Now MVaR in words of one sylable or less
If I understand your analogy correctly, beer is damp and foam is springy. I think I'll need another one to fully comprehend the situation.
RE: And Now MVaR in words of one sylable or less
Just to keep the "fire" on!
Gunnar is correct. The unit of reactive power is "var", not "VAR" nor "VAr". by SI convention. This was decided a long time ago. So, if we want to use correctly the SI units, we may use "var" for reactive power.
Also, we should write "ampère" not "Ampère" for current unit (the symbol is A). The same for the voltage: "volt" not "Volt" (the symbol is "V"). Also, "kV" (not "KV") and "kvar" (not "Kvar" nor "KVAR"). This is all by convention (according to SI).
I think that all this confusion is because we, as engineers, have the tendency to simplify the model of everything and. sometimes, we forget to go back to the basics.
Also, we always keep trying to visualize a mathematical entity as being physical entity. Most of the time, this is not possible.
In reality, there exist fundamentally the electric and the magnetic filed, which are associated to voltage (V) and current (A), respectively. The electrical engineering "quantities" S, P and Q are derived mathematically from these two physical entities (electric and the magnetic field, so V and A) and they have different units, VA, W and var, respectively. So we can not just add P and Q to get S. Mathematically, S is the square root of the sum of P squared plus Q squared.
All these quantities (S, P and Q) were defined based on linear system and sinusoidal waves. If the voltage and/or current waveform are distorted, another electrical quantity D (distorted power) has been defined (I do not remember its unit). So, S will be now (?) the square root of the sum of P squared plus Q squared plus D squared. This quantities is only maths, not physical.
To finalize, I would like to stress and remind you all that the apparent (VA), real (watt), reactive (var) and (now) distorted (?) power are only mathematical models created to simplify (?) power system analysis. So, S, P, Q and D are defined as being components of the instantaneous power wave, These components do not exist as separate entities but they might be conveniently considered for purpose of engineering analysis.
Herivelto Bronzeado
RE: And Now MVaR in words of one sylable or less
Now, Herivelto, that D for distortion "power" is also something that needs discussion. Especially the way it is added perpendicular to P and Q. The D^2 in your expression for S.
That has been done for decades and I think that it is something one shouldn't do. It may very well be one of the "engineering simplifications" that you mention in your post.
The reason why I think it is wrong to do so is that "D" does not lend itself to a geometrical construction with fixed angles (because the frequencies involved are different). But, on the other hand, signal components with different frequencies have always been added as root mean square - or RMS. So, there may be a good reason for doing so. But, on still another hand, the RMS addition is used for components in noise where there is no relation between the different components and their frequencies.
Can anyone show that the geometrical addition of P, Q and D is correct? I am, of course, thinking of you, Pete.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
OK, there is one "K" - used for 1024 in KB as opposed to "k" as in kB (1000 bytes).
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
Muthu
www.edison.co.in
RE: And Now MVaR in words of one sylable or less
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: And Now MVaR in words of one sylable or less
At any rate, I took the bait and here is my analysis of the relationships S, P, Q, D
First of all, D is sometimes defined as D is defined as sqrt(Strue^2 – P^2 – Q^2).
In that case, the conclusion that Strue = SRSS (P, Q, D) is trivial.
(SRSS = square root of sum of squares)
So I will instead use an alternate definition of D (more below)
Start with definition of apparent power:
Strue = Vrms * Irms [equation 1 – definition]
With harmonics present (and the waveforms periodic in w0)
I(t) = Sum Im(t) for m = 1 to m_max
where Im(t) = sqrt(2)*Im_rms_ * sin(m*w0*t + theta_m)
V(t) = Sum In(t) for n = 1to n_max
where Vn(t) = sqrt(2)*Vn_rms * sin(m*w0*t + theta_n)
Strue = Irms * Vrms = SRSS(Im_rms) * SRSS (Vn_rms) [equation 2]
If we multiply the terms of equation 1, we get a square root of products of the form Im_rms^2 * Vn_rms^2. But this represents SRSS combinations of the terms Im_rms and V_n_rms.
So we can write:
Strue = SRSS (In_rms*Vm*rms) where all n and m combinations are included [eq 3]
Now we define the "SRSS combination property"
SRSS Combination Property: SRSS(A,SRSS(B,C)) = SRSS(A, B, C).
This can be verified by simple algebra and is an important property which will be used again and again.
Now let us look at the pile of terms Im*Vn within equation 3 and sort them into buckets D, P, Q:
If m < > N, then the term goes into the D bucket and combined using SRSS:
D = SRSS (Imrms Vnrms) for m < > n
(By the way, directly above is promised alternate definition of D)
If m = n, then we need to break down Im further into two parts: Imrms_p=Im_rms*cos(theta_mm) and Im_rms_q = Im_rms*sin(theta_mm) where theta_mm angle between voltage and current for this harmonic. Then we use these two parts to fill the buckets P or Q accordingly
P = SRSS (Im_rms_p^2*Vnrms^2) where we include all m = n
Q = SRSS (Im_rms_q^2*Vnrms^2) where again we include all m = n
Define S1 =Sapparent power of fundamental components
S1 = SRSS(P,Q) should be obvious since each we have broken Im_rms into two pieces (Im_rms_p and Im_rms_q) whose SRSS combination is Im_rms based on sin^1+cos^2=1
Now at the end of all this, every term has ended up in one and only one bucket.
The terms where m=n have ended up in the P and Q bucket where S1 = SRSS(P,Q).
The terms were m<>n have ended up in the D bucket.
By the combination property of SRSS
S = SRSS(everything in the bucket) [equation 3]
S = SRSS(D, S1)
again by the combination property we can split S1 into P and Q
S = SRSS(D, P, Q) [equation 4]
Sounds very complicated, but simple with the combination property. Each term gets added directly into S in eqation 3. We SRSS combine individual clumps of terms in equation 4. By the SRSS combination property we expect the same results either way.
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RE: And Now MVaR in words of one sylable or less
First of all, D is sometimes defined as D is defined as sqrt(Strue^2 – P^2 – Q^2).
In that case, the conclusion that Strue = SRSS (P, Q, D) is trivial.
(SRSS = square root of sum of squares)
So I will instead use an alternate definition of D (more below)
Start with definition of apparent power:
Strue = Vrms * Irms [equation 1 – definition]
With harmonics present (and the waveforms periodic in w0)
I(t) = Sum Im(t) for m = 1 to m_max
where Im(t) = sqrt(2)*Im_rms_ * sin(m*w0*t + theta_m)
V(t) = Sum In(t) for n = 1to n_max
where Vn(t) = sqrt(2)*Vn_rms * sin(m*w0*t + theta_n)
Strue = Irms * Vrms = SRSS(Im_rms) * SRSS (Vn_rms) [equation 2]
If we multiply the terms of equation 1, we get a square root of products of the form Im_rms^2 * Vn_rms^2. But this represents SRSS combinations of the terms Im_rms and V_n_rms.
So we can write:
Strue = SRSS (In_rms*Vm*rms) where all n and m combinations are included [eq 3]
Now we define the "SRSS combination property"
SRSS Combination Property: SRSS(A,SRSS(B,C)) = SRSS(A, B, C).
This can be verified by simple algebra and is an important property which will be used again and again.
Now let us look at the pile of terms Im*Vn within equation 3 and sort them into buckets D, P, Q:
If m < > N, then the term goes into the D bucket and combined using SRSS:
D = SRSS (Imrms Vnrms) for m < > n
(By the way, directly above is promised alternate definition of D)
If m = n, then we need to break down Im further into two parts: Imrms_p=Im_rms*cos(theta_mm) and Im_rms_q = Im_rms*sin(theta_mm) where theta_mm angle between voltage and current for this harmonic. Then we use these two parts to fill the buckets P or Q accordingly
P = SRSS (Im_rms_p*Vnrms) where we include all m = n
Q = SRSS (Im_rms_q*Vnrms) where again we include all m = n
Define S1 =Sapparent power of fundamental components
S1 = SRSS(P,Q) should be obvious since each we have broken Im_rms into two pieces (Im_rms_p and Im_rms_q) whose SRSS combination is Im_rms based on sin^1+cos^2=1
Now at the end of all this, every term has ended up in one and only one bucket.
The terms where m=n have ended up in the P and Q bucket where S1 = SRSS(P,Q).
The terms were m<>n have ended up in the D bucket.
By the combination property of SRSS
Strue = SRSS(everything in the bucket) [equation 3]
by the combination property (inverted), we can break that into terms where m<>n (show up in D and terms where m=1 (show up in S1):
Strue = SRSS(D, S1)
again by the inverse combination property we can split S1 into P and Q
Strue = SRSS(D, P, Q) [equation 4]
Sounds very complicated, but simple with the combination property. Each term gets added directly into S in eqation 3. We SRSS combine individual clumps of terms in equation 4. By the SRSS combination property we expect the same results either way.
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RE: And Now MVaR in words of one sylable or less
was not correct.
Should have been:
Q = SRSS (Im_rms_q*Vnrms) where m=n=1
The remaining terms (Im_rms_q*Vnrms where m=n <>1) should be thrown into the D bucket. We still have everything accounted for once and only once. Btw voltage THD typically low so these terms generally not very big anyway.
JJayG - sorry for the tangent.
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RE: And Now MVaR in words of one sylable or less
As you correctly note, voltage THD is typically low so the distortion power is small. It brings into question how to account for the reactive component of each of the harmonics. For each harmonic frequency, you could say:
Sh = Vh·Ih·[cos(ah) - j·sin(ah)]
where ah is the angle between Vh and Ih.
Because the system impedance is usually highly reactive (even more so at higher frequencies), a is usually close to 90° and the real component of the distortion power is low.
RE: And Now MVaR in words of one sylable or less
To summarize the partioning of Im_rms * Vn_rms into buckets:
For m<>n, the term goes into D
For m=n terms, we need to split the current into in-phase and quadrature components which results in two terms: Vn_rms *In_rms_p (goes into the P bucket) and Vn_rms *In_rms_q (goes into the Q bucket)
An interesting question, why bother splitting Q from D.... after all neither one contributes average real power and both add in the same way (SRSS) toward Strue.
The answer is apparently that D is not "convserved" like P and Q are. This is based on the excerpt below from "Electrical Power Systems Quality" by McGraw Hill (which also gives the definition of D as product of ImVn where m <> n).
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RE: And Now MVaR in words of one sylable or less
If one analyzes current harmonics in much the same way one separates fundamental components into positive, negative and zero sequence, you will find that these currents do result in real power being dissipated somewhere in the system. Typically in I^2R conductor losses. And this real power at higher frequencies does come from somewhere. Usually, the non-linear loads that produce the current harmonics also produce the power at those frequencies and feed it back into the system. But there are cases in which generators contribute to harmonic power flows. And this will result in torque harmonics, or vibration.
So just hand waving and saying that these components are not conserved could lead some to ignore their effects on other parts of the power system.
RE: And Now MVaR in words of one sylable or less
Now let's look at the "power flow" aspect.
Let's say I have two frequencies: f1 and f50 = 50*F1 (never mind that even harmonics are uncommon... it's just an example)
There are 2 P terms: I1p * V1 and I50p*V50
There are 2 Q terms: I1q * V1 and I50q*V50
There are 2 D terms: I1*V50 and I50 * V1
Consider the above to be at the input to a shunt impedance Zs = R + j*w*L.
Now I want to calculate the output quantities.... So I need to subtract the P, Q, and (?) D consumed in Zs.
I can calculate the real power losses in Zs and the reactive (Q) power consumed in Zs. The output S will be reduced by the amount of real and reactive power consumed.
But how am I going to calculate the D "consumed" in Zs so I can determine the output? For example the term I1*V50 ... which frequency to use in computing D consumed in this shunt element?
The answer is it is nonsensical to talk about D consumed. D is an artificial quantity whose primary purpose is to enable us to predict D in presence of harmonic content.
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RE: And Now MVaR in words of one sylable or less
D is an artificial quantity whose primary purpose is to enable us to predict S in presence of harmonic content.
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RE: And Now MVaR in words of one sylable or less
RE: And Now MVaR in words of one sylable or less
S = Irms * Vrms
where
Irms = SRSS(I1p, I1q, I50p, I50q)
where p and q are components in-phase and in quadrature with respective voltage harmonic
Vrms = SRSS(V1, V50, V50)
Now plug the expression for Irms and Vrms into the expression for S:
S = sqrt<(V1*I1p)^2+(V1*I1q)^2+(V50*I50p)^2+(V1*I50q)^2 + (V1*I50p)^2+(V1*I50q)^2+(V50*I1p)^2+(V1*I50q)^2)
Now separate those terms into buckets:
S = SRSS(P, Q, D)
where
P = SRSS(V1*I1p, V50*I50p)
Q = SRSS(V1*I1q, V50*I50q)
D = SRSS(V1*I50p,V1*I50q,V50*I1,V1*I50q)
recombine the p and q components within D
D = SRSS(V1*I50,V1*I50)
So, the only purpose of D is to faciliate computing S. I agree there is no other useful purpose of combining voltage and current of different frequencies. That was the point of McGraw Hill's quote above - D is not real in any sense other than the leftover terms in S which are not captured in P and Q.
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RE: And Now MVaR in words of one sylable or less
should have been
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RE: And Now MVaR in words of one sylable or less
Thanks especially to Waross (Bill) for his patient explanation as well as his Fortran program ( who'd have thought a 50 year old Formula Translator language would still be up and running, yeah OK limping a bit, today?
Thanks also to Burnt2x who has definitely won the prize for the answer to give on an interview.............
RE: And Now MVaR in words of one sylable or less
What was wrong with the analogy I posted 10 Oct 09 17:19? In fact the analogy is mathematically exact as I have shown. But to explain it to your non-technical buddy, you don't need the math, just the mechanical analogy which is something within the experience of most people.
First you just need an example of a viscous damper. That could be moving a potato masher up and down in a bowl of molasses. Or perhaps the hydraulic piston on a screen door (particularly in the closing direction, may have a check valved bypasss to allow rapid opening, but ignore that). Or could just be moving
Now in scenario 1m, you push the piston using a rigid extension bar. The damper is simply a mechanical heater – converting mechanical energy into heat. The rigid extension bar has no effect on your efforts – your push/pull force/distance is the same as if the bar were absent. (the extension bar is the lossless, inductanceless line).
Finally in scenario 2m, you add a spring in series. Immediately you should realize you are going to have to push a little farther to accomplish the same heating since you have to compress and decompress the spring. The product of force and velocity (apparent power) is higher in scenario 2m than scenario 1m to achieve the same heating. As a result the pusher will probably need to drink more beer.
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