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kilowat
6

kilowat

kilowat

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

RE: kilowat

kW is kilowatts - a measure of real power.  1 hp = 0.746 kW.  

kVA is kilo Volt-amperes, which is just volts times amps, basically.  If the voltage sine wave is exactly in phase with the current sine wave, the power factor = 1.0 and kVA = kW.  Otherwise, the two are not the same.  

Try looking up "power factor" on Wikipedia or Google.  

RE: kilowat

Well to be more precise
P - Active power is measured in kW
Q - Reactive power is measured in kVAr
And
S - apparent power is mesured in kVA

S=sgrt(P^2 + Q^2)

Real power is S(underlined) =P+jQ = cos(f)S + jsin(f) in VA also. It is consisted of active and reactive part.
f- it is phase angle and its cosinus or sinus values are factors that  dpc mentioned. You have that definition in any book of basis of electrotehnics.

RE: kilowat

Other way of looking at it:

KVA= V*I/1000 for single phase circuit (multiply by 1.732 for 3 phase)

KW=KVA*power factor

RE: kilowat

Got enough to be confused?

If not, kW says how much work is done. In a resistor, all current is producing heat. kVA is simply volts times amps with the "k" meaning we are counting thousands ov VA. You can have 1 kVA and close to zero kW in a circuit. A high-quality capacitor or a low loss inductor can give you that. High current and high voltage, but no heat. Zero kW.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

2
Visualize a small cart on a railway track.
The track is oriented north and south.
It takes about 100 lbs. pull on a rope to start the cart moving. If you stand to the north of the cart and pull the rope with 100 lbs. of force the cart will start moving.
If you stand to the east or west of the cart, no amount of force on the rope will cause the cart to move north.
If you move to the north-east or north-west of the cart, it will take about 141 lbs. pull on the rope to start the cart moving.
The same effect is seen in electrical circuit when the voltage and current are not at the same angle.
In the example above, if the cart is moved 1 foot, the actual work done is 100 ft.lbs.
If you are to the north, the work that you do is 100 ft.lbs.
If you are to the north-east, the apparent work done is 141 ft.lbs., but despite the increased pull on the rope, when the cart moves one foot, the actual work done is 100 ft.lbs.
The 141 lbs pull from the north-east is analogous to KVA and the 100 lbs pull from the north is analogous to KW.
respectfully

RE: kilowat

... and the reason they are different (or "Why would I stand to the NE or NW when I could just stand to the North?") is because of the nature of the load you are referring to. As others have said, with a purely resistive load such as a heater element there is no difference (analogous to standing to the North on Waross' example). When your load is a motor or other inductive load, the natural properties of the motor construction cause a delay between the voltage and the current, know as "power factor" which essentially describes the angle by which your real power (kW) consumption is different from the kVA. Hence, back to the train track analogy, just because your load is a motor you are automatically relegated to the NE or NW of the track.

RE: kilowat

Hi jraef;
You have finished my explanation very well. Thank you.
lps
Bill

RE: kilowat

Pay no attention to this (but I had to): The NW position would be a funny motor, I think. Since it would represent a leading phase angle. As I said, do not pay any attention to this, not MUCH attention, anyhow.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

Synchronous motor?
 

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

RE: kilowat

Yes. Of course. Didn't think of 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: kilowat

And the reactive power is analogous to the side thrust of the wheels on the rails when the pulling force is not straight north. This is a real, measurable, force but there is no movement to the east or west and so no real work is done by this component of the total force.
respectfully

RE: kilowat

I am hoping against hope that someone will not bring that beer and foam in glass analogy here!!! Please don't!!

RE: kilowat

Yeah, that one isn't as good as the explanation Bill has put forward. I'll use the railcar analogy in future if it isn't copyrighted!
 

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

RE: kilowat

Why use analogies at all? Why not tell the plain truth?

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

gunnar:

Electical is mostly an abstract subject, analogy to "seen" things helps to non-electrical persons or new to electrical. Mechanical is much more easily understood. If it were so simple, the OP would not have asked to begin with.

RE: kilowat

As a ME, I wished my EE301 prof had used something I could see like that instead of getting on his imginary plane and leaving me behind. :)

what is the beer analogy?

RE: kilowat

byrid:

You will have to wait for someone else to "explain" that. I can not stand it. Something like foam in beer glass is KVAr. ..oops do not even start!

RE: kilowat

Folks,

Are we not engineers? Do we not know what a vector product is? I think we do know that. All engineering schools teach math - and vectors are part of that.

Then, why should we not accept the plain truth?

The plain truth is that voltage and current can be represented by two vectors, U and I (sorry can't put that vector bar on U and I).

All engineers know that the scalar product of two vectors U and I, with angle phi between them, is UxIxcos(phi)

Making people understand that should not be so difficult. Remember, these fora are for engineers.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

Hi Gunnar,

In my industry and no doubt in many others there are a lot of staff who are either not engineers or at least not electrical engineers. The technicians who operate the plant are one example: they do not all have electrical backgrounds - some have mechanical backgrounds from working in steam raising plants - but they need to have some understanding of how the generator they are controlling behaves. We don't need electrical experts in that role but we need to the ops techs to be able to have a reasonable technical discussion with the engineers. Analogies are a useful starting point.
 

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

RE: kilowat

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

RE: kilowat

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.

RE: kilowat

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  smile  ) 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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

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

RE: kilowat

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

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

RE: kilowat

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.


RE: kilowat

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

RE: kilowat

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.

RE: kilowat

But, we both like beer!

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

At last something we can all agree on!
 

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

RE: kilowat

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

RE: kilowat

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

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: kilowat

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.

RE: kilowat

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.

RE: kilowat

jghrist:

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

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