kilowat
kilowat
(OP)
what is the difference in kw (kilowat) and kva?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: kilowat
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
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
KVA= V*I/1000 for single phase circuit (multiply by 1.732 for 3 phase)
KW=KVA*power factor
RE: kilowat
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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
RE: kilowat
You have finished my explanation very well. Thank you.
lps
Bill
RE: kilowat
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: kilowat
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
respectfully
RE: kilowat
RE: kilowat
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: kilowat
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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
what is the beer analogy?
RE: kilowat
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
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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.
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: kilowat
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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
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
Why, then, should we use trains and beer to explain simple electric facts?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
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
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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
RE: kilowat
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: kilowat
A cold glass of milk please!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: kilowat
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kilowat
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: kilowat
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
RE: kilowat
Thank you. I wanted to mention the controls subject courses. But did not want to drag on this thread.