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Control/Dimming So you think you know about power?

Discussion in 'Lighting and Electrics' started by derekleffew, May 31, 2008.

  1. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Check out this article, written by [user]STEVETERRY[/user], published in the Spring 2008 issue of ESTA's quarterly publication, Protocol.
     
  2. JD

    JD Well-Known Member

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    Interesting article. It sounds like this was an in-house distribution transformer, so there was access to it for removing/disabling the filter. The problem I see is if utilities start using blocking filters in their 13k site transformers. (You know how they are about non-liner draws!) Imagine calling them up and trying to get them to take it out! You'll be lucky if you get them to send anyone out, and that will probably be a guy with a light bulb who say, "Yup.. It Lights.. You got power. Bye!"
     
  3. David Ashton

    David Ashton Active Member

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    I certainly know the problems, however I have never understood why the neutral current is described as harmonics, in a practical sense the neutral on a dimmer has a fundamental frequency of 150hz [180hz US] with harmonics running off that, if you put a frequency meter on any dimmers neutral this is what you read.In any case we should change the name, there's nothing harmonious about them.
     
  4. JD

    JD Well-Known Member

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    True (kind of **), but dimmers do produce one heck of a lot of harmonic distortion.* (except pure sine)

    To simplify the scenario, lets look at a single phase leg. It's primary frequency is 60 cps. A square wave is described as "a sine wave with every possible harmonic added." To this extent, the square edge of the thrysistor (Thrysistor = SCR, Triac, SSR, etc.) turn on in a dimmer has the same characteristics of a waveform with a much higher frequency. In fact, these harmonics extend all the way up into RF frequencies. (Thus the need for a filter choke.) What Steve demonstrated is that the harmonic filter blocks conduction at frequencies above 60 cps. The result is that the turn on current pulse ends up being blocked causing the voltage to collapse. If it collapses later in the waveform the zero voltage cross detectors in the dimmer will read that as the start of a new waveform, thus turning on the thrysistor too early in the next cycle.

    Now, on to that 180 cps neutral. The reason you would read 180 cps on the neutral of a three phase circuit is that the current peaks six times per full cycle instead of twice**. In looking at Steve's graph, it does appear that the harmonic filter has peak action at 180 cps, as the recovery slope is about 1/3 of the line frequency slope. Although I am sure the electric company is real happy about this (they wish everything was resistive) I am not sure any of us should be too happy. Most large three phase motors are Delta, so the neutral is not a factor. The harmonic filter appears to be an attempt to address neutral over-current by getting rid of the neutral! Our particular industry however ends up getting the shaft. (About the last thing any of us want is a funky neutral.) The worst case scenario gets handed off to any system that runs a three phase chopper. (Sometimes known as a dimmer system.) This is especially true if the "chop" is occurring in the last half of the AC waveform, with the greatest current pulse occurring if the chop is set at the AC peak. (50%)

    Some clarification is needed in that this will only effect phase chopped dimmers. Pure sine dimmers would work just fine. Reverse phase dimmers, which turn on at ZVC may also work fine, although there may be a voltage overshoot at the thrysistor turn off point. (Someone will have to test that as I don't have any!)

    So, what are the advantages? Delta motors will run fine, Non-dimmed lighting loads will run fine, Resistance loads will run fine, and neutral over-current is limited. I am sure that was what the manufacturer was thinking. It's just that they forgot (or chose to ignore) us!

    Here's the "rub": In an attempt to "green" the world, there is a war against low power-factor devices. (These are devices where the current peak and voltage peak are not liner.) I am not sure if this was the motivation for this transformer to be equipped with a harmonic filter, but I am sure that this will not be the only device we have to worry about!

    * "The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the Fundamental frequency." Total harmonic distortion - Wikipedia, the free encyclopedia

    ** EDIT: One thing I forgot to add- The 180 cps neutral is somewhat of a hallmark of bad power factor or neutral over-current. It indicates that all three legs are peaking in current that is returned separately through the neutral at different times. If all three legs were perfectly balanced and resistive in load, then the common neutral would have no current flowing back to the transformer because the load would essentially change to Delta, with each leg finding its return through the opposing phase leg loads. In standard architectural wiring, if one phase was out of balance, the neutral would have a 60 cps return. When you add dimmers, which alter the phase angle of peak current when they are not at 100%, that concept goes in the trash, thus 180 cps neutral, and the core problem of the thread.
     
    Last edited: Jun 1, 2008
  5. STEVETERRY

    STEVETERRY Well-Known Member

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    Unlikely that the utility would do this, since the triplen harmonics we deal with are blocked by and delta-wye transformer and thus stay off the grid.

    Here's another article that talks about various harmonic mitigating techniques

    ST
     

    Attached Files:

  6. JD

    JD Well-Known Member

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    The article was a good read, I applaud your continued good work.

    I suspect in a few years this will be more of a "legacy" issue, as sine-wave dimmers come down in price. That being said, short of legislation, the lifespan of a dimmer insures we will have the current systems around for many many years.

    From a design standpoint of view, the IGBT vs SCR/Triac/SSR does not justify the current cost difference. It is more of a software issue as you need to produce a series of on/off times as compared to simply an "on" for phase chopping, or an "off" for reverse phase dimmers. sine-wave dimmers also do not require the large filter chokes that are currently used, although a smaller choke is still required as to keep the FCC happy!

    Cost, I feel will be governed more by market conditions then parts requirements.
     
    Last edited by a moderator: Jun 4, 2008
  7. Darthrob13

    Darthrob13 Active Member

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    Keep in mind that much of the cost difference is from the tiny, speciallized computers that are required to run these modules.

    No they aren't very powerful, no they aren't special, but no...there aren't many being made.
     
  8. STEVETERRY

    STEVETERRY Well-Known Member

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    Guys--

    The cost in Sine Wave dimmers is in:

    1. Power devices
    2. Component count (100X more parts than SCR dimmers)
    3. Required cooling to much lower temperatures than SCR dimmers
    4. Magnetics for input and output filters for conducted emissions
    5. Topology and interconnects required by high frequencies
    6. Low volume

    It's chicken-and-egg--the volume cannot rise due to the high cost and the high cost cannot come down due to the low volume.

    ST
     
  9. cutlunch

    cutlunch Active Member

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    IGBT dimmers are a lot more complicated because of all the protection circuitry needed to protect the IGBT.
    This includes over-current sensing so the IGBT's don't go into what is known as a runaway state where they destroy themselves.

    If you study power electronics you will see how much more complicated an IGBT control circuit is then those needed for TRIAC's / SCR's.

    This design work is why it's taken so long to get them to market. Because who would buy a Sinewave dimmer that is less reliable then an SCR/TRIAC based dimmer. Also we have survived with this sort of dimmer for over 40 years.

    Gafftaper if you are reading this, your new dimmers are they TRIAC/ SCR or Sinewave based?
     
  10. JD

    JD Well-Known Member

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    We saw the same thing with switch-mode power supplies that are now pennies on the dollar of their price 10 years ago. They handle very large power loads and operate at 200khz up. In fact, a lot of that technology is shared with SWD. Darth makes a good point as design price must be paid and if not many are produced, the cost per unit is high. Still, when China jumps in, we will see a major price correction as much of the unit can be blown onto one LSI chip. It's just a matter of waiting for someone to pop the bucks for that, and we are only a nitch market.

    Sign wave dimmers are a better answer then chopped phase dimmers, and technical progress tends to be logarithmic. I remember servicing the first CD players back in the early 80s. They were over $1000, and I was told at the time that they would never get below $1k.

    1, 3, & 6 will be the primary price holdouts.

    EDIT: Here's a neat little page on an 80kw (Actually 80kVA) three phase IGBT sign wave inverter built by Siemens. http://references.transportation.siemens.com/refdb/showReference.do?r=487&div=7&l=en (For the locomotive industry.) That industry has started to use sign wave IGBT systems for traction drive as well, with units (not Siemens) rated up to 6400kVA but Europe is way ahead of us. (Google IGBT locomotive drive for more) The pure sign locomotive drive systems have become popular for the same reason they are advantageous to us. The big advantage is that technology is being developed in another industry which should have a positive trickle-down effect for us.
     
    Last edited: Jun 4, 2008
  11. David Ashton

    David Ashton Active Member

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    It may be of interest that Lite-puter have patented a sine wave dimmer.
     
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  12. JD

    JD Well-Known Member

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    WOW! I just looked at it Lite-Puter get Sine Wave Patent

    Looks like we have be Shanghaied by Shanghai !

    The patent looks so generalized I can't believe they got it!

    This may actually be a very bad thing as it is a US patent and may inhibit any US companies from developing it. The text is so general that they could put a claim against anyone who builds or has built a sine wave dimmer since 2006:

    Quote from site:
    DEAR ALL:
    Lite-Puter get United States Patent: Sine Wave Light-Adjusting Apparatus on February 2, 2008.
    Unite State Patent Certificate No. :7329996.
    The period of validity: From May 4, 2006 to July 26, 2026
     
    Last edited: Jun 5, 2008
  13. STEVETERRY

    STEVETERRY Well-Known Member

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    The patent is well known to me, and is of questionable validity, to put it mildly. I would not worry about it having an effect on development.

    ST
     

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