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Help with KVA / KW
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Re: Help with KVA / KW
Power in watts is V*I*PF. The VA rating is simply V*I. Of course we would use KW, and KVA in this case. I believe that the VA or KVA ratings are used because they reflect both the iron losses (voltage) and copper losses (current) in transformers, AC motors, and AC generators.
You must know the power factor to compute power in AC circuits unless you are dealing with a purely resistive load in which case PF = 1 or 100%. PF is defined as the cosine of the phase difference between voltage and current. Current may lead or lag the voltage depending on whether the load is capacitive or inductive.
I would think there is a portable instrument for measuring power. Such an instrument would require a pair of leads for voltage and a clamp on probe for current. You would want to measure the voltage anyway for a better calculation.
I first encountered PF as a snot nosed kid working as a clerk for a company that audited utility bills. The electric company charged a penalty for low PF. No one there really understood PF, just that a low PF cost money.
[ December 04, 2004, 08:10 AM: Message edited by: rattus ]
Power in watts is V*I*PF. The VA rating is simply V*I. Of course we would use KW, and KVA in this case. I believe that the VA or KVA ratings are used because they reflect both the iron losses (voltage) and copper losses (current) in transformers, AC motors, and AC generators.
You must know the power factor to compute power in AC circuits unless you are dealing with a purely resistive load in which case PF = 1 or 100%. PF is defined as the cosine of the phase difference between voltage and current. Current may lead or lag the voltage depending on whether the load is capacitive or inductive.
I would think there is a portable instrument for measuring power. Such an instrument would require a pair of leads for voltage and a clamp on probe for current. You would want to measure the voltage anyway for a better calculation.
I first encountered PF as a snot nosed kid working as a clerk for a company that audited utility bills. The electric company charged a penalty for low PF. No one there really understood PF, just that a low PF cost money.
[ December 04, 2004, 08:10 AM: Message edited by: rattus ]
Ed MacLaren
Senior Member
Re: Help with KVA / KW
With only the line currents given and not knowing whether the loads are wye or delta connected, one would have to average the line currents -
325 + 321 + 330 = 976/3 = 325.33 amps
VA = V x I x 1.73
VA = 480 x 325.33 x 1.73
VA = 270,154 or 270.154 KVA (load)
Approximately 229.845 KVA remaining
Ed
[ December 04, 2004, 08:36 AM: Message edited by: Ed MacLaren ]
With only the line currents given and not knowing whether the loads are wye or delta connected, one would have to average the line currents -
325 + 321 + 330 = 976/3 = 325.33 amps
VA = V x I x 1.73
VA = 480 x 325.33 x 1.73
VA = 270,154 or 270.154 KVA (load)
Approximately 229.845 KVA remaining
Ed
[ December 04, 2004, 08:36 AM: Message edited by: Ed MacLaren ]
- Location
- Massachusetts
Re: Help with KVA / KW
Thank you rattus and Ed.
Thank you rattus and Ed.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Help with KVA / KW
There are three things wrong with the question, not just the one you have all already mentioned. The first is that they give you no information related to power factor. The second is that they did not say (and we are left to assume) that the currents are RMS values, as opposed to peak values. The third is that they did not meter the voltage. Here again, we are left to assume a value of 480 volts.
Please note that they did give the KW and KVA ratings of the generator. My guess is that they wanted us to use those figures, calculate a power factor of 0.8 (corresponding to a phase angle of 36.87 degrees), and use that power factor in solving their problem. That would, of course, be utter nonsense. The fact that the generator is capable of 400 KW at that specific power factor does not imply that this particular load is running at that same power factor. It?s just a bad question.
I agree with Ed?s approach and his result, based on the two assumptions I cite above. But I think the actual measured voltage is likely to be lower than 480. So the actual ?measured? power is likely to be less than 270 KVA, and that the amount of KVA remaining is likely to be more than 230 KVA.
There are three things wrong with the question, not just the one you have all already mentioned. The first is that they give you no information related to power factor. The second is that they did not say (and we are left to assume) that the currents are RMS values, as opposed to peak values. The third is that they did not meter the voltage. Here again, we are left to assume a value of 480 volts.
Please note that they did give the KW and KVA ratings of the generator. My guess is that they wanted us to use those figures, calculate a power factor of 0.8 (corresponding to a phase angle of 36.87 degrees), and use that power factor in solving their problem. That would, of course, be utter nonsense. The fact that the generator is capable of 400 KW at that specific power factor does not imply that this particular load is running at that same power factor. It?s just a bad question.
I agree with Ed?s approach and his result, based on the two assumptions I cite above. But I think the actual measured voltage is likely to be lower than 480. So the actual ?measured? power is likely to be less than 270 KVA, and that the amount of KVA remaining is likely to be more than 230 KVA.
Re: Help with KVA / KW
Not sure if this applies to this project but something to consider when your measurements were taking also, fire pump load & elevator laods. Typically elevators are sequenced to run one at a time. Sometimes these are not running when measurements are taking on the actual generator and their load needs to be accounted for.
Not sure if this applies to this project but something to consider when your measurements were taking also, fire pump load & elevator laods. Typically elevators are sequenced to run one at a time. Sometimes these are not running when measurements are taking on the actual generator and their load needs to be accounted for.
- Location
- Massachusetts
Re: Help with KVA / KW
Hi Charlie I am glad you posted to this thread.
I have a question about this;
Ed's answer was very close to the posters answer so your assumptions about what the poster was looking for must be correct.
Hi Charlie I am glad you posted to this thread.
I have a question about this;
I am not disputing you, I am just curious why you believe the voltage would not be 480?
Ed's answer was very close to the posters answer so your assumptions about what the poster was looking for must be correct.
- Location
- Massachusetts
Re: Help with KVA / KW
That is also a good point but I believe this is purely an imaginary generator.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Help with KVA / KW
I suppose I was just being overly critical. (Mea culpa, mea culpa, mea maxima culpa.) I sometimes get that way when people who are not training in the art of conducting training start asking technical questions in an apparent effort to ?stump the lesser qualified.?
On the other hand, I applaud anyone who asks a question in an effort to learn, and who is not afraid to display their need to learn. That is the principal reason I enjoy participating in this Forum. That and the preponderance of wit.
Because we don?t know where he took the measurements. If it was not at the generator?s terminals, then there would be some voltage drop from the point of measurement back to the point at which the voltage regulator takes its measurements and makes its adjustments. We also don?t know what type of voltage regulator was installed, and whether it was set for ?droop? or ?isochronous.?
I suppose I was just being overly critical.
(Mea culpa, mea culpa, mea maxima culpa.) I sometimes get that way when people who are not training in the art of conducting training start asking technical questions in an apparent effort to ?stump the lesser qualified.? On the other hand, I applaud anyone who asks a question in an effort to learn, and who is not afraid to display their need to learn. That is the principal reason I enjoy participating in this Forum. That and the preponderance of wit.
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