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Szar

Supporting Member
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About Szar

  • Rank
    Commander of the Legion of the Unholy Pear

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  • Gender
    Male
  • Location
    Somewhere over the Rainbow

Previous Fields

  • Engineering Field
    Left, Occasionally Right, Sometimes Middle.
  • License
    EIT
  • Calculator
    TI
  • Discipline
    Electrical

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  1. Szar

    autotransformer question

    I'd like to add... in 11 years I have worked with Commercial, Industrial, and utility clients in the largest metropolis in the world... I have had ZERO jobs involving auto-transformers. These things are a purple unicorn used in specialty circumstances.
  2. Szar

    autotransformer question

    I would agree it sounds like a fundamentals thing. From what I can tell your looking to plug and chug the formula without the full understanding where the equation applies (or more important what the terms area). The "N" terms are not the voltage. Its the number of turns for that coil. A true Auto transformer, for which this equation was derived, has a series coil "se" and a common coil "c" and they are "wired" in series. Really its just an adjustable center tap and the the ratio between the the two develops the equation referenced. So while I get what your doing, its just a terminology thing I'm point out. While a single phase transformer can be connected like an Autotransformer to produce an electrically "equivalent" , its still physically a single phase transformer and is NOT a strait forward ratio between the primary and secondary. Draw out the single phase transformer circuits to visualize. The Single phase transformer wired as an auto-transformer has source voltage (480V) delivered to BOTH coils. The secondary coil (120V) actually acts against this supplied voltage to drop the voltage across the load to 360V. So the easy answer after all is said and done (and I just came to this myself since I never drew out the circuits in advance and only established what needed to be done with each to avoid math for this on the test...) one of the terms must be considered negative. This is not an additive configuration. The auto-transformer equation assumes the coils are in series and additive, where-as that is not the case here and they counteract each other. That is also why the "boost" version works. The equation assumes additive, boost is additive... so the result works.
  3. Szar

    autotransformer question

    I believe the answer key is correct. Single Phase transformers can be wired as Auto transformers in one of three (technically two) ways. In one way, the coils are additive (boost) and increase the supply voltage higher then what would otherwise be capable if configured normally while the other two (really one...) configurations have the coils wired "subtractively" (buck) and produce a lower and different voltage then would otherwise have been possible. Not really important, but it is: The two bucking configurations are really the same wired transformer, just having the supply connected to one side or other alters the circuit effect. If connected to the primary side, the rated current flows across the load. if connected to the secondary side, a reduced current flows across the load. The rated current of the coil cannot be exceeded when wiring a single phase transformer in auto configuration, so that is what limits the KVA output. The question is asking for maximum apparent power, so we can safely ignore the other secondary side "buck" configuration since it also tells us the supply is 480V and you would need 120V to connect it to the 120V coil. Due to how the coils are configured and wired, in this configuration with the supply on the primary side... 360V is developed across the load (the 120V induced voltage in the secondary subtracts from the 480V supply). As such, the transformer develops rated current in the secondary, 208A (= 25 kVA / 120) . As X1 and H1 are wired together, the secondary current produced is split across the primary coil and through the "source". (Primary coil current cannot be exceeded just like the secondary) The apparent power delivered to the load is 75 kVA (= 360V * 208A) If the source was 120V and connected the other way (rather then 480V supply we have here)" 360V is developed across the load (the 120V induced voltage in the secondary subtracts from the 480V supply). Rated current that follows on the primary side is only 52A. Since the primary coil current rated current cannot be exceeded, instead of 208A flowing across the load and only 52A @ 360V... the apparent power is limited to 18,720 VA. Notice... smaller then rated output of the transformer in single phase configuration. Only reason to do this is if you need the lower voltage and only have the secondary voltage available. It can be done, but you take a hit to delivered power.
  4. Szar

    autotransformer question

    I've seen this and will respond. But working the weekend and life issues at home... I'll try and respond before Monday.
  5. Szar

    Exam Day Strategy

    I think you'll find a binder per topic is a bit excessive. I had a single larger binder, tabbed, for each category on the syllabus I needed reference on. The material in this binder was not generic or extensive. It was specific, targeted to my weakness or to provide quick reminders for formula or common goofs I would make. I put like 75 hours into mine, but i drew figures in AutoCAD and made something I could probably turn around and sell to consumers with another 10-15 hours worth of formatting and checking work. Ultimately whatever organization method you do has to compliment and feel natural to you. That is probably the most important thing as time is very critical on the exam.
  6. Szar

    Thailand Soccer Team

    I think part of it is most people don't have any experience or ability to relate to this incident. Its Scuba Diving, in a dark cave, narrow passages, with with fast currents. As I understand it from you guys, that's difficult and a very niche experience. Personally speaking, my only comparison is snorkeling in a pool. I just though they they could strap a mask on them and pull them through the tunnel with a winch, so its probably best I deal with Electricity and not people as my job.
  7. Szar

    Thailand Soccer Team

    So with all said an done... was this just media over-hyped? I do not doubt it was potentially dangerous as a diver did die or that the boys would have survived if left alone, but the efforts actually required to save them were all pretty low tech, strait forward, and rapidly advanced.
  8. Szar

    Thailand Soccer Team

    A Clear map of the latest flooded areas.
  9. Szar

    Thailand Soccer Team

    Im glad they are being rescued but it kinda seems anticlimactic after all the hype and dangers. how "tesla" engineers were flying in and talking about boring down to save them and lack of oxygen (ironically due to all the people trying to save them using it up) and rising flood waters. In the end, some sumps and they ended up being able to walk most of the way with a few (albeot challenging) dives. (at least thats what i heard the rescue turned into)
  10. Szar

    Thailand Soccer Team

    This (your story) reminds me of one of Indiana Jones movie when he was younger in the caves.
  11. Szar

    Thailand Soccer Team

    Just a few tidbits from different news sources. "The trapped group consists of boys aged 11 to 16, and their 25-year-old football coach." (Hes basically a kid himself.) and from CNN reporting locals accounts: The Tham Luang Nang Non caves are known locally as off-limits, a dangerous place where parents warn their children not to go into, especially during monsoon season. "I couldn't believe this would happen to my students. In the classroom, I brought this subject up to teach students. You see, look at this event ... it happened because they were not being careful, their actions has caused a lot of worry and create all kinds of issues to all," he says. Not placing blame, but the news agencies don't seem to be reporting that this was a known no-go but they did it anyway type of thing.
  12. Szar

    Thailand Soccer Team

    Would not caving during monsoon season be a good lesson learned? My heart goes out to them, but your spelunking / caving in what seems like a fairly complex cave system with im guessing inadequate knowledge of the area and conditions. my view on the matter is you only go into the earth when your dead. i aint rushing it. (Claustrophobic too]
  13. Szar

    Thailand Soccer Team

    Simple: Knock the kids out, strap a mask to their face and duct tape it for good measure, lash the kids to their backs, (These are Navy Seals after all) and pull everyone out through the tunnel. We are engineers! We can science the shit out of this! (Although, this story sounds like a Darwin award in the making.)
  14. Szar

    Thailand Soccer Team

    You would think they would have some type of inflatable gerbil tunnel that could be snaked through the cave. I should invent one otherwise.
  15. Szar

    Transformer Question

    No problem. Hopefully it makes sense and helps! For what its worth, it took me a while to figure it out too. Phase angle is the last thing I care about and being that its a relative term anyway based on what you initially assign to zero... blah. In the real world no one cares about phase angle unless your doing studies... just get through the test and go back to blissful ignorance!
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