The Mysterious Three Phase Power

Discussion in 'Amateur Radio News' started by VK6FLAB, Aug 11, 2018.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    foundations-of-amateur-radio_300.jpg
    Foundations of Amateur Radio

    The mysterious three phase power ...

    There are times when you realise that you've always nodded your head when a particular topic came up and after doing that for long enough, you think you know what's going on.

    Turns out that, no, you didn't, but that the topic itself was interesting enough to learn from. In my case, Three Phase Power. I came upon this topic over the past month while I struggled with power interruptions, blinking lights, weirdness throughout my house. Turns out that it's been happening for a lot longer than I've lived here.

    After spending some time with the local power company, which I was told was filled with people who didn't care, turns out that they do, but they're busy people. After some back and forth, some logging, some finger pointing and head-scratching, the solution to my woes was to move me from the White Phase to the Blue Phase.

    I nodded and smiled and everything was well with the world.

    I know that there are three phases, Red, White and Blue. If you have overhead power in your street you'll likely notice four wires strung from pole to pole. One for each phase and one for neutral.

    Apparently there's a standard for which is neutral and the order, but there are too many exceptions for me to spell that all out here, so I'll move on.

    So, what's with these three phases?

    If you spin a magnet between two coils you have a generator. As the magnet spins, the magnetic field increases through each coil, then peaks, then reduces, and as the next magnetic pole comes along, the magnetic field reverses, increases, peaks, reduces, etc.

    If that sounds familiar, it's because I've just described a sine-wave. Every revolution of the magnet is a cycle and if you cycle, say 50 times, you get 50 cycles per second, or 50 Hz. For some countries it's not 50 Hz, but 60. Same thing, just faster.

    That single set of opposing coils and magnet is a single phase. If you add another set of coils, 120 degrees further along, you get the same phenomenon, completely independently from the first set of coils.

    That's the second phase. Rinse and repeat for the third phase.

    To get that power to the rest of the suburb, you need to run a single wire for each phase and a common neutral wire, giving you the four wires that you see on a power pole.

    Theoretically you could run with more phases, but you need to run more copper into the street, so power companies stopped at three.

    You can think of these as three completely independent circuits, but they all share the same neutral, so there are some subtle interactions, like if the neutral becomes disconnected, bad news happens, especially in a place like Western Australia where ground conductivity is very poor.

    In a normal home you'll get fed by one of those phases, in my case I changed over from the white phase to the blue phase. This means that each phase has a different set of users in the street. Roughly a third are using each phase.

    Looking at the actual voltage and current that comes through at high enough resolution and you'll begin to recognise it as an RF spectrum with harmonics, variations, interference and other artefacts that make power show up as a varying feast, rather than the rock-solid expectation of 240V, 50 Hz you see on the sticker.

    Three Phase Power, now you can nod along like I did and know how it actually works.

    I'm Onno VK6FLAB.

    To listen to the podcast, visit the website: http://podcasts.itmaze.com.au/foundations/ and scroll to the bottom for the latest episode. You can also use your podcast tool of choice and search for my callsign, VK6FLAB, or you can read the book, look for my callsign on your local Amazon store, or visit my author page: http://amazon.com/author/owh

    If you'd like to participate in discussion about the podcast or about amateur radio, you can visit the Facebook group: https://www.facebook.com/groups/foundations.itmaze

    Feel free to get in touch directly via email: onno@itmaze.com.au, or follow on twitter: @VK6FLAB (http://twitter.com/vk6flab/)

    If you'd like to join the weekly net for new and returning amateurs, check out the details at http://ftroop.vk6.net, the net runs every week on Saturday, from 00:00 to 01:00 UTC on Echolink, IRLP, AllStar Link and 2m FM via various repeaters.
     

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    K2CTR, AA9G, W6ERM and 2 others like this.
  2. SA6CKE

    SA6CKE Premium Subscriber QRZ Page

    There's as I see it no need to run more than three phases. If you have three phases and swap them you can just change the rotation. If you had more phases and swapped two of them you'd get a mess of things counter-acting each other.

    But three phase solutions are also useful since they allow for powering brushless asynchronous motors for a variety of devices without advanced electronics to control them.

    And I don't think that three-phase power is mysterious at all, it's a very useful and even beautiful solution.
     
  3. NM7L

    NM7L Ham Member QRZ Page

    3-phase voltage graph = ice cream cones!
     
  4. N1OOQ

    N1OOQ Ham Member QRZ Page

    No neutral conductor here that I'm aware of. 3 phases and there's also a skinny little ground wire most of the time. But I'm pretty sure the return is the actual earth ground.
     
  5. N2MDA

    N2MDA Ham Member QRZ Page

    One of the benefits of three phase power is the ability to control. For example a synchro motor can operate in revolutions or in portions of a revolution. Synchro motors were used in Fuel Quantity indicators in aircraft. Along with advanced electronics, the synchro motor proved to be a very accurate and reliable device. Other devices in an aircraft that were powered by three phase current was the Vertical Gyro. A simply beautiful piece of equipment, that could give you resolution of under 1 degree. I had the privilege of working on this type of equipment for many years and even now it's still impressive.
    Also, let's not forget the man who invented Three Phase Power was a Genius among men, Nicola Tesla. One of the greatest minds of our time.
     
    AC5RH likes this.
  6. N7JI

    N7JI Ham Member QRZ Page

    Power is fun, but not intuitive.

    Three phase with a neutral (usually grounded) is called WYE (like the letter "Y").

    A transformer with a WYE connection looks like a letter "Y" with each phase connected to the neutral. Three single transformers all commoned together.
    In the US, the WYE variant of 120 VAC is called 208/120 VAC. It's 208 VAC because the line-to-line voltage, should you ever want to use it, is 208 VAC.
    More often than not, industry uses 480/277 VAC, which is a WYE. Power companies distribute power from 765 kV (or higher) and transform it down to something you can use.

    The transformers up on the pole typically do not RECEIVE power in a WYE format. It's usually sent with only a ground wire, and fed into a DELTA transformer.
    A DELTA is an equilateral triangle in which there are also three transformers - one from the A to B phase, one from B to C, and one from C to A.
    You say, "where is the return line????"

    Ideally, there is NONE. It comes from vector math. If currents are equal on all three phases, each 120 shifted around a circle, and you add the vectors together...
    They add up to zero, so the net current is zero although each phase is performing work. Messy, right?

    The equilateral triangle (120 degree phase shifts between phases) makes three phase power work.

    A transformer can be DELTA-DELTA, WYE-WYE, or even (very commonly) DELTA-WYE, where the primary (high voltage) side is DELTA and the secondary (low voltage) side is WYE.

    Scott N7JI
     
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  7. K2WH

    K2WH I'm Up, I'm Awake What more do you want ? Volunteer Moderator QRZ Page

    Ah, yes, all true but do you know what a wild or red leg is ?
     
    KC1DR likes this.
  8. W0ELH

    W0ELH Premium Subscriber QRZ Page

    Two things: Could some learned individual give a good explanation of VARZ and 2, that skinny # 10 wire running down the pole and connected to a ground rod can sometimes come loose from the connector on the rod.
    DO NOT play with this as it could be the only ground connection for a Y or Delta system. If you become part of the circuit you will be in a world of hurt!
    Remember, if the line is not grounded its not dead.
    73 and stay safe, W0ELH
     
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  9. WD5JFR

    WD5JFR Ham Member QRZ Page

    Before automobiles had alternators they had DC generators to charge the battery. With a DC generator higher engine RPM (speed of car) the faster the the charge and it took much longer. With a alternator instead of a DC generator we have a 3 phase AC alternator with DC output and much more power to charge a battery quick and at lower speed (idle.) Another advantage with an alternator is no commutator to wear out brushes but instead slip rings without grooves. But alternators didn't become an advantage until the necessary rectifiers became available at low cost with high reliability. I haven't looked at an alternator lately but in the old days you could usually see three pressed in silicon rectifier studs and on some you could change the slip ring brushes without removing the alternator.
    Hank WD5JFR
     
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  10. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I vaguely remember learning a proof during circuits class that three phase power is the most efficient of all polyphase power systems. In other words, there is no benefit from more phases.

    (That sure seems like a long time ago...and it was!)
     

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