In a job interview for the vacancy of assistant electrical engineer at a newspaper printing works, you are required to calculate the parallel capacitance required to bring a lagging load up to unity power factor from its current rather unsatisfactory 0.65. The power transmitted is: 5KVA The voltage is 440V (single phase) The frequency is 50Hz
Show your working.
Power transmitted 5KVA, I always took power to be power, if complex I might use the term apparent power or similar?
Current = 5kVA / 440V = 11.4A Real part = 11.4 x 0.65 = 7.4A Reactive part = sqrt ( 11.4^2 - 7.4^2 ) = 8.7A
X = 1/WC = 440V / 8.7A
C = 8.7 / (440 x 100 x pi) = 63 uF
In reality, probably use 56uF.
Good Lord!!! That was quick. Surely some mistake? I didn't expect to get an answer before tomorrow at the earliest so haven't actually worked it out myself yet. I'll have to do it this evening now (groan, moan, mumble, mumble):(
I could be wrong of course, I was last time with my first attempt!
I've got a sneaking suspicion you've missed something out, but I could be wrong. I'll report back later.... Whatever happens I think we've established you're the grande fromage here when it comes to electrical engineering. And Harry's at the other end of the spectrum. ;->
Urgh! I'm forced to use an unfamiliar calculator here and it's being a total *BITCH*. First thing I did was divide 5000 by 440 and the answer it gives is 125/11 !! WTF hell use is that? Bastard! Must be in some weird mode I guess. If anyone has a Casio fx-991ES Plus can they please tell me how to put it into 'normal'? Sigh. This is going to take a while..... :(
You have a computer. Use it.
Hur hur! Serves you right for posting all these stupid questions, Doom. Karma finally gotcha! :->
I wonder how many people are actually using their computer's calculating ability, and how many people are using google calculator?
Owain
Blimey, you're right, Fred! Sorry I momentarily doubted you. I make it
62.4uF to be preciseish (thank god for slide rules!) So the job is yours. ;-) For an extra bonus, inform the great unwashed why you plumped for 56uF in practice....
A number of reasons:
1 Cost. Smallest is bestest! 2 Standard value, though 47uF is probably a more used value 3 A power factor of 0.65 sounds rather an arbitrary value. Is this at max load, average? You can get dynamic correction if it's worthwhile. 4 56uF would give a reactive current of 7.7A, leaving a residual of 1A. That would leave a PF of 7.4 / sqrt(7.4^2 + 1.0^2) = 0.99. Actually 47uF would give a reactive current of 6.5A leaving a residual of 2.2A. That would leave a PF of 7.4 / sqrt(7.4^2 + 2.2^2) = 0.96 which I would have though was perfectly adequate?
At a guess, try SHIFT + MODE to enter setup then select Normal display which is possibly option 8.
Make sure you've set MODE + 1 for general calculations.
Something like that.
There you go, you continue to surprise me. What I was getting at was that a lot of engineers believe one should avoid shooting for unity because of resonance and the increased risk of instability and oscillation. Hence most exam questions seldom ask candidates for values to get unity; they usually specify 0.9 or thereabouts for this reason. Your experience may of course be different. BTW, *all* the figures were arbitrary; I plucked them out of my arse. ;-)
Night night!
Well done Fred!
440V single phase? How is that?
What else would you call it?
Well that would make sense, yes. Sadly this contraption doesn't work that way. We found the instructions for putting it back into factory default mode but it turns out that rendering regular integer operation results in vulgar fractions *is* the default! There's an [s-d] button you have to repeatedly click to cycle through *afterwards* to get your preferred answer format. Bonkers! Anyway it's all over now thank God.
I always use the calculator that comes with Linux. Well one of them anyway. I think there's about 4 or 5...
the E6 range is 1, 1.5, 2.2, 3.3, 4.7, 6.8 ....
So either a 47uF or a 68uF would be likely choices.
56 is part of the E12 series and 62 would be E24It's possible you could get a high voltage tight tolerance AC capacitor, but mostly in this game its all about 'near enough'
Push the S D button to swap between the answer formats...
(but yes I know what you mean, modern calculators just don't work like they should - fortunately my trusty FX601p is still going strong though
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