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Intensity vs. Current for ETC Sensor?

Discussion in 'Lighting and Electrics' started by mnfreelancer, Oct 15, 2008.

  1. mnfreelancer

    mnfreelancer Active Member

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    I am training some people who are less familiar with lighting control at work and pointing out that the relationship between dimmer intensity (%) and the current running on the feeders is not one of a linear nature. Someone asked the obvious question I would have asked if I were in their shoes; what is the specific nature of the relationship and where are the cutoff points? So, can someone point me to a reliable source of either a graph that shows the % dimmer intensity (on a per-dimmer basis or averaged across an entire rack) and current drawn on the feeder; or real-word (or theoretical) data to draw my own conclusions? I would like to create a more empirical graph of % intensity vs. % current based on the rack at 100% intensity.

    (before someone points it out, yes I considered that in a sense I want to compare intensity to intensity as current is represented as "I" meaning intensity...hopefully I was clear in what I'm really after in the above post.)
     
  2. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    See page 49 of the attached document for CEM classic.

    OR
    page 67 of this document for the CEM+: http://www.etcconnect.com/docs/docs_downloads/manuals/Sensor__CEM__v2.2__User-Config_Manual_revA.pdf.

    edit: upon re-reading your post, the above may not be exactly what you want, but I don't know of any graph of feeder currents and, as we learned in this thread: http://www.controlbooth.com/forums/question-day/8986-why-two-neutral-camlok-inputs.html, measuring exact feeder current, particularly on the neutral, is rather an advanced subject.
     

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    Last edited: Oct 16, 2008
  3. David Ashton

    David Ashton Active Member

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    here is a short excerpt from the pre-heating debate [which got shut down just as I was winning] however it shows how you can make your own graph very easily, your meter will be a little inaccurate at low current but not enough to be significant, and a little demonstration is worth a thousand words

    Please consider, a 1000 watt 240 volt T19 on a dimmer{for 120v thinkers just halve all the figures} It has a cold resistance of 3.8 ohms and a hot resistance of 57.6 ohms. A typical dimmer will feed 1.65v at "0" and drive .35A putting 2.24 W into the lamp of preheat.
    @ 13V you can see the barest glow in the filament and are drawing .96A and 12.6W
    @30V you can see a real glow and are drawing 1.5A and 45W
    at 60V you begin to get some output but at 1/4 voltage you are drawing 2A or nearly half the "full" current
    @80V which is 1/3 voltage you are drawing 2.3A which is more than half the full load current and is also the point at which the dimmer starts to interact with the other dimmers on other phases.
    This is why running all your dimmers at 1/3 is the worst thing you can do to your neutrals.
    hope this helps
     
  4. garyvp

    garyvp Active Member

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    Great question. My issue with techies is that they think that during long rehearsals that they are using 50% of the power by setting the grand master at 5 when in fact they are using 80%. It takes a lot of current just to light that filament.

    I have installed digital ammeters (a good idea for $1000) on each leg of my dimmers in our 3-phase system (6 legs, 2 per phase to monitor) to help techies balance the grid and to make them aware of consumption. It is also interesting tool to observe the 'cascading' nature of SCR dimmers. A single 500w lamp consumes about 4.2 amps at full glow. At a dimmer setting of 1, the current is about 2 amps, at level 7 it is almost 4 amps. The other discussions that follow are perhaps a better explanation.
     
  5. STEVETERRY

    STEVETERRY Well-Known Member

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    The ammeters are a great idea for any derated feed to a dimmer-per-circuit system!

    And of course, they are True-RMS-responding ammeters, right?

    ST
     
  6. garyvp

    garyvp Active Member

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    They are clamp based Murata digital RMS meters - the readings match my RMS Fluke clamp. The observed change in amperage across the 3 phases, and especially the oversized neutral, highlight the effects of subtle changes in the local line voltage and the non-linear behavior of the SCR dimmers.
     
    Last edited: Oct 18, 2008
  7. David Ashton

    David Ashton Active Member

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    There may be a misunderstanding here, it's not the dimmers which are causing this very non linearity, it's the lamps, and the low resistance at low voltage which increases the current.Remember that the cold resistance is 10 times less than the hot resistance and this is what causes this distortion of the curve.You get the same effect with any type of dimmer.
     
  8. mnfreelancer

    mnfreelancer Active Member

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    I realized this when I looked at the document Derek cited that contained voltage curves. I stopped and thought about how lamp filament resistances change with temperature and how that directly affects current due to ohm's law...
     
  9. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    See here.

    I may now have to buy a Variac and conduct this experiment on my own. Where's my Kill-A-Watt and Amprobe?:lol:

    Danger. Electricity kills. When in doubt, consult a qualified professional.
     
    Last edited: Oct 17, 2008
  10. David Ashton

    David Ashton Active Member

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    Of course, one should mention that those using carbon filament lamps will have no problem as the resistance goes down as the temperature increases, all we need now are some theatre grade carbon filament lamps.
     
  11. church

    church Active Member

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    a good question with more than one answer and some very complex issues.

    Steve Terry has already noted that using an ammeter unless it is true RMS will produce meaningless current readings uless you are using an autotransformer or a resistance dimmer or a true sinewave dimmer. If your dimmers use SCRs or triacs to commutate the supply waveform then life becomes interesting. The light output from a bulb is a non-linear function between voltage, filament resistance which is a function of filament temperature. Which is why a lamp may produce no visible light but still be drawing current as already pointed out earlier.

    Generic curves can be produced for dimmer setting to light output but you do need the true RMS ammeter and a lightmeter. You may also want to look at the voltage waveform on an oscilloscope - you will see a sine wave with sections missing depending on the dimmer setting. Also note that if your dimmer rack can be set for different response curves this will also mess up your experiment you need to be set to a linear response.

    Only individuals who are trained and qualified to perform repairs on electrical equipment with the potentially lethal hazards involved and the appropriate Personal Protective Equipment should attempt to connect tsuitably fused and insulated ammeters and or oscilloscopes to this type of equipment. Mistakes can result in serious injury or death.
     
  12. David Ashton

    David Ashton Active Member

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    I compared a $1000 Fluke, a $100 Chinese true rms and a $20 s/h moving iron meter and they all read within a 3% range, not enough to throw out a general idea of what was happening.Now convincing people that what they are seeing is true, that is the hard part.
    Because of all the variables Church spoke of, doing your own graph of your own systems is the best way to go, and you will learn something.And it's fun.
     
  13. garyvp

    garyvp Active Member

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    Thanks for that piece of information. I had always thought it had to do with the electronics of the dimmers. With the metering on, the observed neutral value peaks midway through a cross-fade before 'settling-in'. I assume that this phenomenon is due to the filament effect.

    GVP
     
  14. garyvp

    garyvp Active Member

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    The CT driven MuRata meters closely match my RMS Fluke clamp to the first decimal. And you are right, I have learned something (like we had some interesting legacy wiring) and it is fun.
     
  15. David Ashton

    David Ashton Active Member

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    garyvp Now the real value for you is that when someone does ask you these questions you won't have to give them some theoretical hypothesis, you now know, because you did the experiment and saw the result, if more folk did this instead of relying on what someone said,[or they thought someone said] we would have a lot fewer eccentric ideas on these fora, and it's fun.
     
  16. church

    church Active Member

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    "Thanks for that piece of information. I had always thought it had to do with the electronics of the dimmers. With the metering on, the observed neutral value peaks midway through a cross-fade before 'settling-in'. I assume that this phenomenon is due to the filament effect."

    Garyvp the effect you refer to above is actually to do with the design of the contol circuit that controls the triggering of the TRIAC or SCRs. In the 70s Strand used to heavily promote their dimmers and consoles as "dipless" because you did not get this effect. Many other manufacturers also have their own design to achieve this and this is what we expect from a high quality console and dimmer however on the lower end equipment you do often see a dipping in the light level and a corresponding reduction in current as you crossfade and the level settings between the two subs or scenes. This is one reason why the lower end equipment is cheaper it is not necessarily to do with build quality it may just be the elimination of features.

    In the simplest implementation of a dimmer a comparator circuit is used to supply a trigger to the TRIAC selecting the highest value from scene A or scene B. The output of the comparator will be whichever is the higher of scene A or scene B. The "dip" occurs when Scene A is the same as Scene B because the comparator output is is zero. Because you are moving through this point it only occurs for a fraction of a second and because the lamp filaments do not cool instantly you will only see a reduction in light intensity for a fraction of a second.

    This was really noticeable with analogue consoles that used potentiometers in presets where the presets were connected through "ORing" diodes. As control consoles and dimmers have become truly digital this is not so obvious because this is done in software (actually implemented in firmware on a PIC) and digital dimmers have eliminated the comparator circuit.

    Of course having said all of this if you are changing from one scene with a high lighting intensity to a scene with low intensity it is easy to mistake this for a "dip" in levels or in the reverse scenario it can appear as a momentary "flasH" of brightness.

    However measuring current at the transistion from one scene to another will show changing current levels.
     
  17. David Ashton

    David Ashton Active Member

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    Sorry to say, Church but you're confusing different issues, the "dipless" crossfades on Strand consoles was effected by using variable mark/space ratio circuits and is absolutely nothing to do with the dimmers.
    Modern consoles do the same thing digitally but again it has nothing to do with the dimmers.
    While you can have different curves on a dimmer to match the use to which it is put, there is no dipless function, this is purely a control function.
    The comparator circuit is a part of the dimmer which switches the triac relative to the control voltage, it is a dumb circuit, it only sees the control voltage and does not "see" the presets.
    The "dipless" circuits were electrically dipless but this does not relate to optically dipless, in these circuits all lamps going down fade over the given time and the lights going up do so in a given time and lights which are not changing do not move.This was the first "tracking" system.
    Having said all that it has absolutely zero relevance to this thread.
     
  18. garyvp

    garyvp Active Member

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    This subject gets more and more interesting--------I have been TD/ME in a vacuum for years. This site is terrific. Thank you all.

    The metering system has been in place for one show and already has had impact. Watching the neutral is its own show. Today I ran the current show (took 5 minutes to analyze 60 cues) and found: a maxed-out phase on one of the dimmers (rebalanced it away) and a double-up circuit (two lamps on single extension cord where there was only one plug in the channel - lazy designer). The metering brought this home to the designer. I have alerted all designers (in this theatre, a designer will hang, cable, plug, and patch their fixtures and program all subs and cues) that I will run all there shows under metering. It is the best investment I have made in this place in a while.

    Also, found a dead channel from a newly wired cable (neutral and line flipped) - always great fun to pull a pack apart and solder in new SCRs.

    Thanks again while I think of my next question.

    Gary
     
  19. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    You meant "...one plug in the dimmer [not channel]," didn't you? If tufering two 1000W luminaires puts you that close to (or over) the edge, I'd say something is wrong. For curiosity's sake, what is your supply and quantity and capacity of dimmers?
     
    Last edited: Oct 22, 2008
  20. garyvp

    garyvp Active Member

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    Ah...yes.
    In short, we have much more face plate capacity than we have in available power and this confuses designers. But 24 dimmers underpowered still allows for a well lit show with 60 lamps in our small basement space.

    And it is not that doubling/tufering is bad, but requires more care in balancing in our environment; we have about four designers, soon to be 8, and some are more careful than others.

    We have NSI 9800's 8 x 2.4 dimmer (channel in their vernacular), 80 amp packs supported by a 100 amp three phase feed, although the packs are really two single phases of 50 amps each, well under faceplate. Each dimmer is a 2.4k. Therefore, there is 50 amps for each cluster or four dimmers, which limits us to about 12 500w lamps(most of our lamps are that or less), which means that you can plug 4 lamps, but this limits you elsewhere in that cluster. I try to insist on three per dimmer, and 10 for the group of four. This really adds up to about 40 amps which is about 80% of our 50 amp breakers in the panel. Our aging lighting grid is all conduit and 84 circuits with duplex receptacles and some with shared neutrals. The lighting guys are supposed to plug one lamp per circuit, but sometimes they plug 2 (Well, there are two sets of holes there!). While this seems logical to them to plug two lamps they can forget to compensate and can overload the dimmer with 5 lamps which trips the NSI magnetic breaker, or 4 which I can detect on the metering system - 4 x 4 x 500w trips the panel breaker. If they double up, they need to make sure that the total load to the dimmer is no more than 2k and the cluster is 5k.

    Garyvp
     

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