kilowat 6

[mikespurewater]

what is the difference in kw (kilowat) and kva?

 

[Skogsgurra]

Yes, I am fully aware of that. And I use analogies a lot.

My point here is that we have a policy for the site saying that it is for engineers and engineering questions. Given that policy, we should not need to explain basic concepts in a non-engineering way. And, if we do it, we should do it in a way that does not obscure the concept.

Bill's analogy, where he uses a rope to represent VA, the force composant parallel to the track to represent W and the force perpendicular to the track to represent var is an excellent one.

But, does anyone that doesn't know much about AC understand at all why you introduce angles in the equation. "What angle?" he is likely to ask. To that question, the railway analogy gives no answer.

I know, I have taken a rather "puristic" stand here. I do that to remind ourselves that we should follow the site's policies - not only when it comes to "student postings".



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Gunnar:

I would say, the analogy presented by Bill was an "engineering" example. I have used barge and tug boats example in similar vain. While this site is for engineers not all of them are electrical.

 

[Skogsgurra]

No, but all of them have had math and should be able to understand simple vector math. We are not into Div and Curl or complex matrices here. Just a scalar product.

What you do when you use analogies is to explain the math in a way that says nothing about the underlying electric phenomenon.

I have tried both methods in classes. After showing how a rotating vector projects a sine, and after introducing the phase angle, it is an easy matter for everyone to grasp the VA, W, var subjects. Using ropes, trains, boats, beer glasses and what have you will never answer the question fully. It is simply not engineerish.

I understand some things better than others, electricity a lot better than trains, for example. But I do not expect someone to explain the behaviour of a train (or a glass of beer ) using electric terminology. I expect to have the actual system explained to me.

Why, then, should we use trains and beer to explain simple electric facts?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Gunnar:

I would beg to differ on this basic concept. In my opinion if math is taught with real life examples and analogy, more students will grasp math (and electrical concepts)much more easily and also find it interesting. Math and formulas without perspective could become very boring or abstract.

It is also helpful for electrical engineers to relate to some mechanical analogies, as it was evident in one the posts here relating to centrifugal pumps. One of our illustrious collegues failed to relate fluid pressure or pump head to voltage, fluid flow to a current and friction/restricion to resistnace. (If you ask him again he would say yes I knew it but his analogy was not correct).

Power=K*Head*flow for a pump is very analogus and mathematically equivalent to Power=K*V*I in electrical.


Rafiq

 

[Skogsgurra]

We have different opinions - that's how it is in life

But do not get me wrong. I use analogies where needed. But I do not think that it is necessary, or good, here.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JBinCA]

Interesting segue on analogies and explanations.

I differ in oppinion with you as well, Gunnar.

There are plenty of engineers who have learned to plug values into equations but never really learned what the math actually means. This is more evident in those new in their field.

As far as I'm concerned, the more analogies and explanations the better. I think a fuller understanding comes from looking at a concept or problem from multiple angles (angles of perspective, not phase angles!). One of the main reasons I read and use this forum is that you will explain something differnetly than waross, who will eplain it differently than DPC, who will explain it differently than Jraef and rbulsara.

And, of course, the analogy can be used on its own to explain something abstract in a sufficient manner to answer a high level question such as the OP. I don't think Mikespurewater is going to be designing a VAR regulator or WHDM based upon the answer he receives to his question.

 

[Skogsgurra]

But, if he were to?

I am going to visit a utility company around June 26-27th. They seem to have a problem with exactly a var regulator for a 6 MW synchronous motor for a heat pump. It probably (I haven't been there yet) has not been installed correctly since it is not possible to run it in capacitive mode. And, if one tries, the control loop changes sign and the excitation goes all the way up to maximum excitation. Which takes some time (very long I time on this regulator). The operators notice this after some time and turn the setpoint down to inductive. They have to make the setpoint very inductive to overcome the false loop polarity. Then, whoops, comes excitation down and the motor trips. Which means restarting, sometimes a few times/hour. If there ever was anything like a built-in cooling time, it must have been by-passed many years ago.

Result: this machine has been to the repair shop every second year for quite a time. That is an expensive and totally unnecessary excersise.

Reason: The people that did set this up and commissioned it obviously did not have enough knowledge when it comes to W, VA and var. They seem not to have been able to distinguish between inductive and capacitive phase angle. And this is probably the root cause for the problem. I have asked the maintenance people to do a check on VTs, CTs, var regulator, the meters and all the circuitry involved.

They do. And I get reports that I cannot trust. Incomplete, with contradictions, and this analogy that seems to be preferred over the simple vector diagram with U and I and phi clearly labeled. A complete mess. And it has, so far, cost at least three rewinds, long stand-stills and very low availability. With customers complaining.

So, I will have my view on this simple matter. And I will try to inform people, who need to know, in a way that I find best. And that does not involve railways or beer glasses.

Who knows? The OP may well be one of the operators at the plant I am going to visit. Even if he isn't going to design the var regulator, he needs precise and complete knowledge to connect, maintain and troubleshoot it.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

I'm with you Gunnar, it just isn't that difficult a concept to get a handle on. And I never did like the beer analogy.

 

[Skogsgurra]

But, we both like beer!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

At last something we can all agree on!


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Whoa wait a minute! I don't like the beer analogy or beer.

A cold glass of milk please!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Smoked! You are always that guy that sticks out...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Just as long as it isn't my neck...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Zogzog]

While this site is for engineerers I dont think the guy who asked the original question "What is the difference between kW and kVA?" is an electrical engineer, just a wild guess.

Analogies work well for introducing concepts to new students and that seems to be the case here. If you knew nothing about how a nuclear reactor works and I threw the nuetron life cycle at you your head would explode, first you need to learn the basic concepts, like monkeys and ping pong balls.......but we wont go there.

 

[jghrist]

It is certainly not unheard of to use an electrical analogy for a mechanical system. I took a whole course in controls that did just that. I took another course on analog computers (I'm really dating myself now) where we wired up the electrical analogy and saw the results of things like bouncing balls on a CRT.

 

[rbulsara]

jghrist:

Thank you. I wanted to mention the controls subject courses. But did not want to drag on this thread.