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Information on three phase systems

  1. May 21, 2014 #1
    There is a lecture called fundamentals of electric and electronics and there is a new topic in that lecture I have to learn but I have little information on it. Topic is three phase systems or three phase power calculations or something like that. There are big letter such as R,S which confuses me and systems called star or triangular.Can somebody guide me please how to learn that topic. there are physics books called physics 2 which all of them explains the electrical forces,fields RLCs but they do not cover that three phase topic. Can you give me book names about it for examples does that topic belongs to circuit theory?
     
    Last edited: May 21, 2014
  2. jcsd
  3. May 21, 2014 #2

    berkeman

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    Start with the Wikipedia description of Y-Delta Transforms, and then continue with the "See Also" reading links at the end of the article:

    http://en.wikipedia.org/wiki/Y-delta_transform

    .
     
  4. May 21, 2014 #3
    I have to learn this subject very fast but it is far to my study area so I can not understand even simple explanations. I need to ask this: " to return the current from the appliance back to the load centre" this is about the neutral wire.

    Best Regards.
     
  5. May 21, 2014 #4

    berkeman

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    You should be able to answer your question by reading the main part of the Wikipedia link that I gave you.
     
  6. May 21, 2014 #5

    psparky

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    In laymans terms, Look down the highway and powerlines when you are driving. You will see the wires in groups of 3.....also known as three phase power.

    A generator naturally creates 3 phase power. (3 wires). Your residential home is typically single phase (2 wires). These two wires are "grabbed" from two of the three phase lines......say line 1, line 2, and line 3. You can grab from any two lines to make your single phase power. Send it to a transformer and you get 240 volts into the house (USA). They then "center tap" that transformer, effectively cutting that transformer coil in half and cutting the voltage down to 120 volts via a nuetral (white wire) in your breaker box.

    In bigger commercial buildings and industry, they use three phase almost exclusively. Generators natually create 3 phase power. Three phase motors naturally "take" this power.

    Think of it like this for three phase.
    Picture three torque arms around a shaft like a propeller. Not think of three forces acting on these torque arms turning that shaft all together. Quite a lovely deal.

    Now picture single phase.
    Think of 1 torque arm attached to a shaft. Now picture one force acting on this. Not quite as lovely, perhaps a bit wobbly, etc. Single phase is great for your blender, not so great for a 200HP constant running motor.

    Why do we use three phase power? Because we can. But in residential, they just feel it's not cost efficient. In some other companies they do use three phase power in the house.

    In your typical 480 system, line to line voltage is 480. Line to neutral voltage is 277. (square root of three is the factor)

    In your typical 208 system, line to line voltage is 208. Line to neutral voltage is 120. (again, square root of three factor)

    Ok...that's the introduction. Google three phase power, there are tons of articles that explains the vectors and math behind it.
     
  7. May 21, 2014 #6

    jim hardy

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    A mechanical, you say?

    Think three cylinder radial engine.

    We represent voltage as a vector that rotates, and call it a phasor.
    That is the beginning of understanding three phase.
    Here's a decent link.
    http://www.electronics-tutorials.ws/accircuits/phasors.html

    It's easier than what you've already done in mechanical engineering, just the terms are unfamiliar.

    If you're like so many people, intimidated by this mysterious "charge" you cannot see moving to and fro,

    see if this guy's down to earth explanations are any help.
    http://amasci.com/ele-edu.html

    eighth article down, "why three prongs", relates to your question above regarding neutral.

    Begin 3 phase at the beginning and it'll fall into place quickly.

    EE's have it easier than ME's in many ways. Our units are already metric and the charge that transports our energy has no mass. So our flow equations aren't complicated by Bernoulli.

    Good luck!
     
  8. May 22, 2014 #7

    Thank you but can you advise me a book covering instructive examples? I like learning through
    solved problems.
     
  9. May 22, 2014 #8

    psparky

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    http://www.ece.umn.edu/users/riaz/animations/alternator.html

    This link shows an excellent illustration of three phase power. It can't be any more clear than this.

    Look at the 5 links on the bottom of this web page under "Related Discussions".

    This will be one of your best sources to enlighten you into the fascinating world of three phase power.
     
    Last edited: May 22, 2014
  10. May 22, 2014 #9

    jim hardy

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    Wow Sparky i really like the visualizations on that site.

    My textbook is forty years old. I can't point our mechanical friend to newer texts because i havent used any.

    This "all about circuits" site is popular, and this page looks like an okay start at going from visualizations to pencil and paper calculations.
    It presumes familiarity with polar notation, which should be covered a few pages earlier if he needs a refresher.
    http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

    might help, i dont know. When i learned three phase we started by manipulating the phasors. It is VERY important to be rigorous in naming them

    for example in our text VAB meant Voltage from A to B with phasor's tail at A and its head at B.
    But that was 1966 or so. We used sliderules, too .
     
  11. May 22, 2014 #10
    What will a mechanical engineer do if he learns this unrelated subject for him?
     
  12. May 22, 2014 #11

    jim hardy

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    You'll find yourself useful on the job as an interdiscipline translator. My Wheatstone bridge is your four bar mechanism.
     
  13. May 27, 2014 #12

    psparky

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    If the mech engineer work with electrical motors such as HVAC motors or any electrical motor, knowing the difference between 3 phase and single phase, proper motor voltages will help him not only design better, but be able to communicate better with the electrical engineers.

    It would do a mech engineer well to know the difference between watts, KVA and KVAR. When mech engineers dont know these terms (when determining horsepower or heat...) , they come off as "not knowing" which then turns into a motor, heating device, cooling device, etc.... with the wrong specs. For example, there is a 1,000 KVA transformer sitting in a room and a mech engineer needs to size the cooling unit. His question often is..."what's KVA....is that the same as watts? NO!

    You can search physics forum for "power triangle"....."power factor"
     
    Last edited: May 27, 2014
  14. May 27, 2014 #13

    jim hardy

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    We all need to cross-train . I always enjoyed interacting with our mechanicals and nukes.


    Somebody far removed from the plant designed a machinery support for a 60 ton airconditioning unit, unaware that a "ton" in HVAC world is heat flow not mass. Needless to say it was robust.
     
  15. May 27, 2014 #14
    Actually psparky, if you pay attention to power lines in the country where three-phase is not always needed or available, they normally have only two power conductors, a line and a neutral. Your typical residential pole-top transformer is fed by a single fuse, the primary side is line-to-ground and not line-to-line. The center tap, or midpoint ground, is on the secondary side, the derived system has two ungrounded lines but still only one phase; this is why it is misleading to term such lines "phase a" and "phase b", for example, as one would in a three phase environment. The waveform of opposite ends of the coil are naturally oposite polarity however they cross zero at the same time. What you describe, using two lines for the primary, more closely relates to "two phase" systems which have fallen out of use with the advent of a three phase standard for polyphase systems.

    http://www.esubnet.com/fragment-electricity-primer.html

    This illustrates three phase versus single phase nicely in another way, using graphs to show the mechanical advantage of using three phase to power a motor. It helped me to see the differences between AC and DC in graph form when I was new to the subject, if you're familiar with a battery just envision a straight horizontal line plotted on the graphs at a given value and you'll see how a three phase source fits DC's continuous nature more closely than single phase.

    As a mechanical engineer it is quite likely you will someday work on design for something which utilizes an electric motor, transformer, coil, or possibly all three. In this it would certainly be useful to understand the subtle differences between nominal 120, 208 and 240 volt circuits and where you are likely to encounter which.

    [edit] The use of two phase conductors to feed the primary of a single phase transformer is probably more analogous to an open delta configuration than what was historically termed "two phase". My mistake. And yes it probably does happen, however I observe the line-ground connection is far more commonly used and to my mind would be a more desirable application for a few reasons.
     
    Last edited: May 27, 2014
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