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| Subject - What do you think? | |
| LIVEWIRE |
I have "metered" the load on a 480/3V,400KW,500KVA generator. The meter was a typical hand held clamp-on Fluke. The meter readings were Ph A:325A, Ph B:321A, and Ph C:330A. Assuming peak capacity is the nameplate rating, how much capacity is left on the engine-generator in KVA?, in KW? |
| iwire | Approximately 125 KW remaining capacity. I do not think we can give the answer in KVA with the info provided. Can we? |
| JimmyDee | quote: We would have to know if the load was an inductive load or restive load or if it were a blend, and the power factor to know the # of KW left. The generator is allowing for a 80% PF with its 400/500 ratio. Going with the highest leg, 125 KW remaining is correct. V x A x 1.732 (sq rt of 3) = 275KW. It is also very important to not allow the PF to drop below 80%. All kinds of weird things can happen to the field currents and the magnetic field it produces. Been known to overload the field current because of low PF. Jim |
| LIVEWIRE | Help me out now.... If KVA=VxAx1.73/1000 and KW=VxAx1.73xPF/1000 Then how can we know KW without knowing PF? |
| JimmyDee | Direct reading with a KW meter. It can be calculated, but don't ask me how. Jim |
| Dave Nix | Hi Guys, Maybe this will help! 
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| LIVEWIRE | Anyone else? |
| JimmyDee | quote: As I've already said direct reading is one answer and unless you have a pure resistive load, (in this case, KVA = kw which would be a unity power factor) you would have to know the power factor or the parts of the formula that power factor is figured from like the phase angle, etc. Jim |
| LIVEWIRE | Now we're getting somewhere... So, can we all agree that an amp reading from your clamp-on meter only helps you find KVA of load? and that we won't inadvertantly use the term "KW"? - it confuses those who look up to us. My take on the original question is that available KVA is: 500,000VA-(325A+321A+330A)277V=229.65KVA and that available KW cannot be determined with data listed. We know that a "watt meter" is able to determine PF/KW because it monitors both voltage and current and derives the phase angle between the two. This phase angle determines the PF. So.. if we know what the KVA load/rating is on a generator...why is a KW rating listed? |
| Russell120 | Sorry, but I have been told that you cannot make accurate three phase readings without a three phase anemeter-power meter. The engineer who explained this then went on to tell me why. He smiled a lot at me and nodded his head a lot: as if he were speaking to a somewhat "slow" child. He was correct in his estimation. I smiled and nodded back...and didn't understand a word he said. I think it had something to do with not accurately capturing some aspect of the 120 degree phase relationship. I also recall reading something to this effect in an ECM Magazine artical a few years back. |
| LIVEWIRE | Ya, and it's probably this engineer's cousin that sells those light fixture lenses that can actually intensify the lumens given off by the lamp.........at least that's a salesman's story I was told once. The guy had a trunk full! I would assume that accurate 3 phase KVA readings can be taken by using a clamp-on ammeter on each of the three phases, as we don't care about the phase angle between the voltage and current. For accurate 3 phase KW readings, the KW meter must monitor the voltage and current of each of the phases, as the PF can be different on the different phases due to varying single phase loads. What else might this engineer have been tring to get at? |
| iwire | LIVEWIRE Seeing as I blew your question I was curious to how other people would answer it.I posted your question here http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=3;t=001163 you may want to check it out. Some questions have been asked, power factor of course but also true voltage and where the readings true RMS values. When I posted the question I knew it would get some more responses. This is not to say there are not great members here it is just that the other site has more traffic. |
| LIVEWIRE | Reading one of the lengthier responses on the other site reminds me of my college days and a test I took in a very high level math course. The following "bonus question" was offered: If a squirrel can put three ears of corn in a stump per day, how many days does it take to put 21 ears of corn in the stump? This left students asking "Is the squirrel union or non-union"? and "Does the squirrel work on Sundays or not"? and on and on....... Many of the students simply thought there was not enough information to answer the question and that they were being fooled, somehow. As for KVA vs KW - It is important to realize when there is not enough information to answer a question. Recognizing this shows you understand the issue. As for "RMS vs peak" amperage values - you'd have to know something about the readings of a typical clamp-on Fluke (mainly that a clamp-on ammeter is not an oscilloscope). How about it tech/apprentice students? When is the last time you saw someone roll an oscilloscope up to a generator. As far as the 480V debate - it's kind of the squirrel and stump thing. As far as "conducting training" - I have mentored many and plan to continue. As far as "apparent attempt to stump the lesser qualified" - the answer to the squirrel and stump question is "7" (if you know what you're doing). |
| Russell120 | But is that 7 KW or 7 Kva? |
| JimmyDee | I've taken it upon myself to delete a couple of posts on this thread that, even though not crossing the line as a personal attack, could very well escalate into that. Lets keep the content technical in nature and not personal. Thanks, Jim |
| LIVEWIRE | Amen |