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And Now MVaR in words of one sylable or less 6

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JJayG

JJayG

Electrical
Jul 22, 2007
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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
 
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OK, I still like the beer / foan analogy better, bt this one is at least different. I heard this (or read it) once, I cannot find any other decent reference to it, but when I came across it the first time it made perfect sense. Now all I can do is regurgitate it as best as my failing memory can recall, here goes...

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.


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Thanks for your agreement (?). My point was if we don't allow VAR as a unit distinct from VA, then we also shouldn't allow VA as a unit distinct from watts. Of course V*A is a valid product of units, but that product has the name watts.

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Gunnar - yes, although I've spent a bit of time looking myself and maybe understand what you were saying. Or maybe not!

I like the observation in ePete's second-from-last post too.


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Some people seem to have no greater use than a slinky, but they can still bring a smile to your face when you see them falling down the stairs.
Publican, another round if you please, and by all means have one yourself.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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To summarize my opinion. There are two logical approaches:

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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electripete - I understand your point and I think I agree. Based on tradition, I would go with your option 2. In this case the confusion is created by mixing vector and scalar quantities. I think we could refer to Q as watts. In school I was taught that we call it VA because they didn't want to call it imaginary watts. VA seemed to work, but is perhaps not the most descriptive choice that could have been made.

Now I need a "real" tall beer (without suds please).


Alan
----
"It’s always fun to do the impossible." - Walt Disney
 
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Muthu,
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
 
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What about torque (Nm) and work (Nm)? Same units, but torque is so obvious (force perpendicular to distance) and work is also obvious (force in same direction as distance) so the confusion never starts.

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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Yes, I agree with you Gunnar, that if it's ok to say torque in N-m vs Joules, then it should be ok to say apparent power in VA vs watts. That supports your original post and undermines my objections.

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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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
 
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I'm a little late in chiming in. I think my designer authored the comparison of vars as managers. We measure the company's "power factor" by the ratio of engineers and designers to total employees on the phone list.

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)

 
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Not a familiar term in my part of the world. I'm UK-based but I've done a fair bit of work with European and US OEMs and haven't heard it in dealings with them either.


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