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HMIs and paper amps


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#1 Greg Boris

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Posted 07 December 2012 - 02:25 AM

Hello all,
Been lurkin for a while and haven't had a real reason to post yet, but here it is.

The situation
I was gaffing a short tonight at a very old city apartment. We were outside and had a character climbing through a window. Coincidentally that window was the only place we could run power from so it was going to take a fair bit of time to properly hide the stinger coming out. I was using a 575 and 1200. I knew that the paper amps would clearly be over 15amps, but when doin the math it came out just under 15 (with very very little wiggle room). While not really keen on splitting the stinger and running them both off the same wire we were trying to save the time of hiding another stinger. I had also done this before out of necessity (lack of accessible power in a different practical location) with tungsten heads.

The lights were working for 15mins before the breaker tripped. There were alot of people going in and out of the kitchen (where it was plugged in) so I wasn't sure if something else had been turned on/plugged in. I also figured that if the HMIs were to pop the circuit it would have been closer to when we sparked them up and not after a period of time. Being ready to shoot, we flipped the breaker back on and tried to get the shot off but about 3 minutes later they tripped again. Accepting that it was a dumb idea (and potentially bad for the lights/safety) we took the time to run another stinger and hide it. Next time I won't even bother trying to push it.

The Questions
That being said I would like some clarification on some things for the future.

1. Assuming I had total control over a circuit and knew that there was nothing else on it, is it ever ok (while not prime) to push the power so close to the limit? I don't want to be unsafe or damage equipment. Should I always just use paper amps to give myself a threshold?

2. I mentioned I "figured that if the HMIs were to pop the circuit it would have been closer to when we sparked them up". Is this true?

3. I found it odd that it took 15 minutes for the circuit to trip the first time. Does the power usage of HMIs fluctuate even when fully warmed up that they could have "drawn more" at a particular instance? Adding to that... hypothetically if they were tungsten heads would I have been less prone to tripping?

4. Do any of these older apartments have circuits that wouldn't be a full 15amps and/or just not be able to support such a load for an extended amount of time (old wiring etc etc)?

When we ran the second stinger we didn't really search out the circuit to make sure that there wasn't anything else on/plugged in as I wasn't worried once we split the load so I am not positive that it wasn't something else that tripped it. As it was so close to 15amps, it could have been anything.

After tripping it, I knew we were overloading the circuit but not being able to specifically point to the cause made realize I had a few other questions that I should know for the future. I'm still learning so any facts/tips about power/HMIs/safety are appreciated.

Thank you for your time,
Greg
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#2 Adrian Sierkowski

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Posted 07 December 2012 - 02:40 AM

You need to take into account the power factor of the ballast. a 1.2K Par doesn't draw 12 Paper Amps. It's midnight where I am; but the problem is, anything with a ballast requires you to figure out the actual amps of the unit based upon it's power factor. This should be listed on the ballast somewhere, or is readily available from the manufacturer's website.
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#3 Greg Boris

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Posted 07 December 2012 - 02:44 AM

Rad. That's a definite start.
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#4 Guy Holt

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Posted 09 December 2012 - 10:26 PM

I'm still learning so any facts/tips about power/HMIs/safety are appreciated.


There are a number of possible factors contributing to the problems Greg had. Besides the Power Factor of the ballast, you also have to take into account line loss, resistance, and harmonics. Which of these accounts for the problems Greg had depends on what type of HMI ballast he was using – magnetic, Power Factor Corrected (PFC) electronic, or non-PFC electronic ballasts.

It is probably safe to assume that Greg was using HMIs with non-PFC electronic ballasts. Electronic HMI ballasts without power factor correction draw current in large pulses and return harmonic currents to the power stream. The capacitive reactance of electronic HMI ballasts also causes current to lead voltage and so they also have a leading power factor. I won’t go into detail here on the adverse effects that the Leading Power Factor and harmonics generated by non-PFC electronic HMI ballasts can have, but anyone operating HMI, Kino, and even LEDs should make them selves acquainted with harmonics. (use this link for more details.)

Considering just the higher amperage draw of HMI electronic ballasts without PFC, an electronic square wave HMI ballast typically has a power factor less than .6 meaning the ballast has to draw 40 percent or more power than it uses. How much current a ballast will draw is indicated on the manufacturer’s nameplate. Let’s take a look at one of these nameplates, since deciphering them can be as difficult as deciphering Egyptian Hieroglyphs if you don’t know how to read them.

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Manufacturer’s nameplate from an Arri 575/1200 Electronic Ballast specifying its’ electrical characteristics (learn how to read it.)


Above is the nameplate from an Arri non-PFC 575/1200 Electronic Ballast. The first thing to look for is the ballast’s Volt-Amperes (VA), which is called “Pmax” here. Calculated as the RMS voltage times the current measured at the input to the device, Volt-Amperes is the measurement of "Apparent Power" delivered to a load, which is different than Wattage. Wattage is the True Power dissipation of the lamp calculated by integrating the product of current through the lamp and voltage over time. These may sound like they would be the same (they are in the case of incandescent lamps), however one characteristics of HMI, Flourescent, and LED power supplies is that some of the current flowing into them is discharged back into the power line, without actually doing work, which in the case of ballasts is the generation of light. The relationship between True and Apparent Power is called the Power Factor (PF.) Since, the Wattage will always be lower than or equal to the Volt-Amperes, PF varies from 0.0 to 1.0.

As indicated on its’ nameplate, this ballast has an Apparent Power of 2290VA - which means it draws nearly twice the load of its’ 1200W output. Next to the Pmax, it also indicates that the Power Factor is .6 (cos@=.6), or that the greater Apparent Power drawn by the ballast consists not only of high amplitude short pulses of current, but also harmonic currents that the ballast returns into the distribution system.

The next thing to look for is the relationship between Voltage (U) and Current (I). As this nameplate indicates this ballast will operate with line voltages ranging from 90 to 125 volts (US), and 180 to 250 volts (European.) You will also notice that the current (I) the ballast draws varies with the supply voltage. That is because this ballast is a “Constant Power” ballast. With constant power ballasts, if voltage drops the ballast will draw more current to maintain a constant Apparent Power - 2290VA in this case. For example, according to the nameplate it will draw 18A of current (I) at 125 Volts (U) (2290VA/125V = 18.32A.) If the voltage drops to 90V, it will draw over 25 Amps (2290VA/90V = 25.44A.)

This is an important characteristic of this ballast that should be taken into account when used outside the studio on location. With an Apparent Power of 2290VA, this non-PFC constant power HMI ballast will operate very close to the threshold of a 20A circuit – too close to operate reliably unless precautions are taken. For example, according to its’ nameplate, it takes 19 Amps at 120 Volts to generate 1200 Watts of light (2290W/120V = 19A). If there is any line loss from a long cable run, the ballast will possibly draw over 20 Amps in order to compensate for the voltage drop. For instance, at 110V it will draw 20.8 Amps. To the problem of line loss, usually there is also increased resistance from an overheated plug end, which makes the voltage drop even further. Since most stinger plug-ends are only rated for 15 Amps they tend to overheat with 1200W non-PFC electronic ballasts. The increased resistance that results from the heat causes the voltage to the ballast to drop even further and so it will draw more power to maintain the 2290VA load. If the light is operating on a small generator, there will also be voltage drop on the generator because of the greater load. The voltage output of generators can drop 5-10 volts under load. At 105V, the ballst that drew 19A at 120V, will now draw 21.8 Amps and cause circuit breakers to overheat and potentially trip.

If Greg was running standard electrical cord (14/3) with 15A Edison plug ends for a distance outside the windows, the load on the breaker could have been as high as 30Amps with the addition of the 575W HMI – which explains why it eventually overheated and tripped.

For more facts/tips about power/HMIs/safety, check out an article I wrote for our company newsletter on the use of portable generators in motion picture production. Harry Box, author of “The Set Lighting Technician’s Handbook” has cited my article in the just released Fourth Edition of the handbook. In addition, he has established a link to it from the companion website for the Fourth Edition of the Handbook, called “Box Book Extras.”


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"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."



The “Box Book Extras,” site is also worth checking out because it includes other source material used for the handbook, articles by Harry Box published in other periodicals, related websites, as well as more in depth discussion of topics touched upon in the handbook. You can log onto the Box Book Extras site FOR FREE at http://booksite.foca...x/setlighting/ with our pass-code "setlighting." Use this link for my FREE news letter article on the use of portable gas generators in motion picture production.

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston
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#5 Greg Boris

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Posted 12 December 2012 - 11:21 PM

Guy Holt, thank you very much for your time and explanation. It's comforting finding people who so willingly give up this information... and in such a comprehensive manner.

I reread your post a few times to make sure I understood the basics/math behind it for future reference and have a few more questions regarding the application of all the information you gave me. By the way, thank you... again.

1. I'm assuming the 2290VA figure isn't applicable for the 575. In trying to understand the power factor more universally beyond the 1200 lamp I tried to figure out how the figure 2290VA came from the power factor of (cos .6). What is the formula for applying the power factor to the lamp? I didn't know the equation to start with so I basically played around until I got a close figure.
I did...
(cos(.6)) x 1200 + 1200 = 2190.4VA

assuming this is right (probably isn't) the 575 would be...
(cos(.6)) x 575 + 575 = 1049.57VA

2. Once the power factor of the ballast has been calculated I can move toward figuring out the actual voltage reaching the ballast. I had assumed it was at 120v, but you brought up line loss which I had forgotten about. Would a simple voltage reading at the end of my cable run reveal the dip? Would the "increased resistance that results from the heat" of using standard stingers show up in the voltage reading? Or is there a rough number I can use for the voltage drop from overheated plugs?


Thank you,
Greg Boris
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#6 Guy Holt

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Posted 15 December 2012 - 09:07 PM


 I reread your post a few times to make sure I understood the basics/math behind it for future reference and have a few more questions regarding the application of all the information you gave me.


The KVA above is for a 1200W HMI operating on an non-PFC electronic ballast. The KVA for a 575 operating on the same type of ballast would be roughly half that. The KVA would be different for an electronic ballast with Power Factor Correction or a magnetic ballast. Take a 1200W magnetic HMI ballast for example.In magnetic HMI ballasts, through a phenomenon called Inductive Reactance, the multiple fine windings of the ballast transformer induces considerable current, called Reactive Power, that is in opposition to the primary current, causing the primary current to lag behind voltage, a reduction of current flow, and an inefficiency in the use of power. Put simply, the ballast draws more power than it uses to create light.

If, in this situation, you were to measure the current (using a Amp Meter) and voltage (using a Volt Meter) traveling through the cable supplying the magnetic HMI ballast and multiply them according to Ohm’s Law (W=VA) you would get the “Apparent Power” of the ballast (expressed as KVA.) But, if you were to instead, use a wattmeter to measure the actual amount of energy being converted into real work (light) by the ballast, after the applied voltage overcomes the induced voltage, you would get the “True Power” of the ballast (expressed as KW.) The difference between Apparent Power and True Power, or the additional power required for the applied voltage to overcome the induced voltage, is the Reactive Power (expressed as KVAR.) The ratio of True Power to Apparent Power is called the “Power Factor” of the ballast.

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The favorite analogy electricians like to use to explain these terms is that if Apparent Power is a glass of beer, Reactive Power is the foam that prevents you from filling it up all the way, so that you are left with less beer or Ture Power. In other words, the thirst-quenching portion of your beer is represented by KW in the figure above. The foam is represented by KVAR. The total contents of your mug, KVA, is this summation of KW (the beer) and KVAR (the foam). In our beer mug analogy, Power Factor (P.F.) is then the ratio of Beer (True Power) to the entire volume of the mug (beer plus foam or Apparent Power.)

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Thus, for a given KVA: the more foam you have (the higher the percentage of KVAR), the lower your ratio of KW (beer) to KVA (beer plus foam). Thus, the lower your power factor. Or, the less foam you have (the lower the percentage of KVAR), the higher your ratio of KW (beer) to KVA (beer plus foam). In fact, as your foam (or KVAR) approaches zero, your power factor approaches 1.0. When lights with a low power factor are used, a generator must be sized to supply the apparent power (beer plus foam), even though only the beer (true power) counts as far as how much actual drinking is possible. Where a typical 1200W magnetic HMI ballast takes 13.5 Amps at 120 Volts to generate 1200 Watts of light the power factor is .74 (13.5A x 120V= 1620W, 1200W/1620W= .74).

Our beer mug analogy is a bit simplistic. In reality, when we calculate KVA, we must determine the “vectorial summation” of KVAR and KW. Therefore, we must go one step further and look at the angle between these vectors.

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To understand this concept let’s use the analogy of a man dragging a heavy load as illustrated above. The man’s Working Power (or True Power) in the forward direction, where he most wants his load to travel, is KW. Unfortunately, the man can’t drag his load on a perfect horizontal (he would get a tremendous backache), so his shoulder height adds a little Reactive Power, or KVAR. The Apparent Power the man is dragging, KVA, is this “vectorial summation” of KVAR and KW. The “Power Triangle” below illustrates this relationship between KW, KVA, KVAR, and Power Factor:

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In an ideal world (one without gravity), the man wouldn’t have to waste any power along his body height and so the KVAR would be very small (approaching zero.) KW and KVA would be almost equal and so the angle (formed between KW and KVA) would approach
zero and the Cosine would then approach one. Power Factor would then approach one. For a light to be considered “efficient”, the Power Factor should be as close to 1.0 as possible. Where a typical 1200W magnetic HMI ballast takes 13.5 Amps at 120 Volts ( KVA =13.5A x 120V= 1620W), to generate 1200 Watts of light (KW), the Power Factor is .74 (PF = KW/KVA=1200W/1620W= .74). In other words, our 1200W magnetic ballast wastes roughly 25% of the power that it uses in Inductive Reactance.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Sales and Rentals in Boston
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#7 Edward Lawrence Conley III

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Posted 22 January 2013 - 03:44 PM

Hi Greg,

Circuit Breakers have a time frame/amperage limit above their listed amperage before they trip so that they can handle start up loads or brief surges in power but if the load exceeds the amperage for a certain time period then they will open.

Also you have to remember that the Power Company can have a 10 % + or - on the actual voltage in the wall so paper calculations could be off if you assume 120v at the wall receptacle and the actual voltage is lower.
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#8 Greg Boris

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Posted 23 January 2013 - 04:26 AM

Circuit Breakers have a time frame/amperage limit above their listed amperage before they trip so that they can handle start up loads or brief surges in power but if the load exceeds the amperage for a certain time period then they will open.


This is a nice tidbit of information to know. Thank you.

I would also like to give another "super thank you" to Guy for taking the time to compile and explain all that information.


Everyone has been very helpful and I truly appreciate it!
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#9 Guy Holt

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Posted 23 January 2013 - 05:11 PM

The lights were working for 15mins before the breaker tripped. There were alot of people going in and out of the kitchen (where it was plugged in) so I wasn't sure if something else had been turned on/plugged in. I also figured that if the HMIs were to pop the circuit it would have been closer to when we sparked them up and not after a period of time. Being ready to shoot, we flipped the breaker back on and tried to get the shot off but about 3 minutes later they tripped again.


The poor Power Factor of HMIs have been vexing set electricians for years. For more information about the adverse effects these loads can have on generators and power distribution systems, and how to remedy them so that you can operate bigger, or more small, lights on portable generators or house power than has ever been possible before, join me in a workshop I am teaching on Feb. 16th titled “Lighten Up: Doing More with Less without Compromise.”

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As part of the same workshop series, New England Studios, Talamas Broadcast, and Production Hub will be sponsoring a workshop on “Video Lighting Design” by L.D. Richard Cadena on Feb. 9th. Noted Focal Press Author, ETCP Trainer, and Founder of the Academy of Production Technology, Richard’s workshops are both lively and informative. Log onto bit.ly/nptwkshps for more workshop information and registration details.

Guy Holt, Gaffer, New England Studios, Lighting & Grip Rental in Massachusetts

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