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Mysterious line loss after swapping a light


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#1 Rob McGreevy

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Posted 08 June 2014 - 05:18 PM

So, here's a scenario I ran into recently...I was best boy electric on a feature, and one of our 4ks went down (bad ballast). We grabbed a new ballast from the rental house, same fixture. This new ballast had only cam-lok input, no 60A bates option. So we grabbed a few extra sticks of banded and hooked it all up. Shortly into shooting, the light went down. In addition, one of the lunch boxes started buzzing really loudly - the same buzz you get on a hand squeezer when it's turned to a pretty low setting. I traced the problem back to the genny and found that voltage had dropped to 108v - still enough to power the M18s with their shiny new Arri ballasts, but enough of a drop to kill the older 4k and turn the lunch box into a resonating chamber. I cranked up the knob on the genny and the problem was solved, but I'm still left wondering - what happened? The genny was never touched, why the voltage drop? I know that with the older ballast power factor-related line loss could have been involved, but we also sized the cable way up to banded #2 instead of 60A, so I'm still a bit flabbergasted on what happened...any thoughts?

 

Also hi, I'm new to the forum :)


Edited by Rob McGreevy, 08 June 2014 - 05:22 PM.

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#2 Guy Holt

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Posted 08 June 2014 - 06:55 PM

So, here's a scenario I ran into recently...I was best boy electric on a feature, and one of our 4ks went down (bad ballast). We grabbed a new ballast from the rental house, same fixture. This new ballast had only cam-lok input, no 60A bates option. So we grabbed a few extra sticks of banded and hooked it all up. Shortly into shooting, the light went down. In addition, one of the lunch boxes started buzzing really loudly - the same buzz you get on a hand squeezer when it's turned to a pretty low setting. I traced the problem back to the genny and found that voltage had dropped to 108v - still enough to power the M18s with their shiny new Arri ballasts, but enough of a drop to kill the older 4k and turn the lunch box into a resonating chamber. I cranked up the knob on the genny and the problem was solved, but I'm still left wondering - what happened? The genny was never touched, why the voltage drop? I know that with the older ballast power factor-related line loss could have been involved, but we also sized the cable way up to banded #2 instead of 60A, so I'm still a bit flabbergasted on what happened...any thoughts?

 

Also hi, I'm new to the forum :)

 

Welcome Rob, we'll see if we can't help you solve this enigma. But, first we need some more information. You say that "one of our 4ks went down", implying you were using more than just one. How many were there? What make and model number were the ballasts of these 4ks and the ones that went down? Besides the M18's what other lighting loads were you running on the same distro? Please list all of them no matter how small and please list the make and model number of any other HMI ballasts. What size generator were you using (ie. 300A, 500A, 1000A, 1200A?) Were you operating it single phase or three phase? What size feeder were you running from the genny and how far of a run was it to your main distro box? Finally, how many amps did you have on each leg when the ballast went down?

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston


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#3 Rob McGreevy

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Posted 08 June 2014 - 09:07 PM

Well it was a while ago so I may not remember all those details...and the project being long wrapped it's of course no longer relevant, just something I've been curious about ever since it happened...but I'll do my best to recall what I can.

We were shooting at a football field. It was a 500 amp Lite Power genny set to single phase. We ran 100ft of 2/0 to a main distro box at the 50 yard line. From there it was 150' of banded into a snake bite, then 100ft of 100A to a lunch box, and from the lunch box I orginally used a 60A cable to the 4k and a stinger to M18. These lights were on a lift at the goal post, we mirrored this set up on either side of the field. We also had two M18s burning in the announcers booth, that was a 50' 100A to a distro box underneath the booth, ballasts on the ground, head cable up to the booths. Load balance was beautiful, almost dead even between the legs.

 

This was the initial set up, the ballast that went down was I think a power gems unit, don't recall the exact model or year but it was an older one. The one we replaced it with was an even older Sunray ballast. Because I used to actually work at the rental house that supplied the ballast I'm well aware of how shitty those models are. And since they are only cam-lok in, we wound up having to rent a couple extra sticks of banded to supply it with power. Once we had it burning I put a meter on the line. Wish I could tell you the exact numbers but I don't recall, but amperage-wise it seemed like a pretty normal draw for a crappy old Sunray ballast, and factoring in the step up to #2 cable my line loss calc told me we should have been in the clear...but shortly into shooting we lost the light. It was only that light, and once I heard the buzzing from the lunch box and traced the VD back to the genny and cranked it up we didn't have any issues the rest of the shoot (we shot at that field with that same set up for almost 2 weeks). So it really wasn't even much of an issue, it took 5 minutes of trouble shooting and we were fine...it's just been bugging me ever since because I have yet to figure it out.


Edited by Rob McGreevy, 08 June 2014 - 09:11 PM.

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#4 Guy Holt

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Posted 10 June 2014 - 06:42 AM

Well it was a while ago so I may not remember all those details...and the project being long wrapped it's of course no longer relevant, just something I've been curious about ever since it happened...but I'll do my best to recall what I can.

 

Thanks Rob, what you do remember confirms my suspicion that you were using older non-power factor corrected electronic 4k ballasts on a small generator operating single-phase.  Your problem was not only voltage drop but also voltage “flat topping” caused by the harmonic currents generated by the 4k ballasts encountering the unusually high impedance of your distro system and the small generator.

 

Harmonics_Non-PFC_4kw_Sm.jpg

 

Let’s look at the voltage drop component of your problem first. Non-power factor corrected HMI ballasts like those you used draw a distorted current waveform that is rich in harmonics. The higher harmonic frequencies create what is known as "skin effect" in the cable. Skin effect is a phenomenon where the higher frequencies cause the electrons to flow toward the outer sides of a conductor. Since the flow of the electrons is no longer evenly distributed across the cross sectional diameter of the conductor, more electrons are flowing through less copper and the resistance of the conductor increases. The increase in resistance reduces the ability of the conductor to carry current, resulting in greater voltage drop over shorter distances and overheating of the conductor.

 

skin_effect.jpg

The area of the cross sectional diameter of a conductor used by DC current (left), Low Frequency AC Current (center), High Frequency AC Currents (right).

 

But, you are correct that voltage drop alone was not the cause of your problem. The other contributing factor was voltage “Flat Topping” caused by the high impedance of your distro system. Always remember, there are two components to the impedance of a distro: cable and generator. The skin effect caused by the harmonic currents generated by the non-pfc ballast increased the impedance of the cable. The second contributing factor was that you were operating single phase on a small generator. A 500A generator operating three-phase provides about 166A/leg. The same generator operating single phase does not provide 250A/leg, but rather the same 166A/leg but just single phase. As you have described your load, you had about 100A on each leg. So, the generator was pretty well loaded and half that load (52A) was generating harmonic currents.

 

waveform_com_grid-AVR.jpg

Left: Pkg. of 2-1200 HMI Par w/ non-pfc ballasts & Kino Wall-o-Lite powered by grid power. Right: Same Lighting Pkg. powered by conventional AVR Generator (Honda EX5500) Note different effect that the same non-linear load harmonics have on grid power and power from conventional AVR generator.

 

Under these circumstances one could expect similar voltage “Flat Topping” to that in the example above. What causes flat-topped voltage? According to Ohm’s Law current reacts with impedance to cause voltage drop.  Since electronic ballasts consume current only at the peak of the voltage waveform, voltage drop due to system impedance occurs only at the peak of the voltage waveform. The zig-zag saw tooth pattern in the right oscilloscope above suggests the flat topping of the voltage waveform we see here is caused not only by the 60hz fundamental but also by the harmonic currents generated by the ballasts at higher frequencies that also create voltage drops as they pass through the system impedance. For example, when encountering the high impedance of a loaded generator, a 3rd harmonic current will produce a voltage drop at a 3rd harmonic voltage. Likewise a 5th harmonic current will produce a voltage drop at a 5th harmonic voltage, etc. Each harmonic current drawn by the non-power factor corrected 4k ballasts flows through the system impedance, resulting in a voltage drop at that harmonic frequency. In other words, where a distorted current waveform is made up of the fundamental plus one or more harmonic currents, each of these currents flowing into an impedance will according to Ohm’s Law, result in a voltage drop resulting in voltage harmonics appearing at the load bus and distortion of the voltage waveform.

 

This pattern does not appear in the voltage waveform of the grid power on the left in the oscilloscope shots above because of its’ much lower impedance.  The impedance of a generator is not an easily known quantity. Depending on its’ size and design, the impedance of a generator will be 5 to 100 times that of a utility transformer and it will change as the load changes. But where you were using a fairly small generator for the harmonic load generated by the 4k ballasts, the internal reactance of the generator would have been sufficient to cause appreciable voltage flat-topping as well as a voltage drop.

 

Now for the reason the voltage output of the generator dropped even though no one touched its’ voltage regulator. The voltage output of the generator did not drop, rather the flat topped voltage caused erroneous reading of the voltage by the meters. The flat topped voltage manifests itself as low voltage on the generator’s meter because conventional electrical meters like those on most generators are designed to read only sinusoidal waveforms accurately – not distorted waveforms. Flat-topped voltages introduce errors into the measurement circuits of these meters which result in low readings. Since the consequences of under measurement can be significant - overloaded cables may go undetected, bus-bars and cables may overheat, fuses and circuit breakers will trip unexpectedly - it is important to understand why meters based on "true rms" techniques should be used on power distribution systems supplying harmonic generating loads. 

 

Most analogue meters and a large number of digital multi-meters are designed to read voltage and current quantities based on a technique known as “average reading, rms calibrated”. This technique entails taking a measurement of the average (or mean) value (0.636 × peak) and multiplying the result by the form factor (1.11 for a sine wave). The result is 0.7071 times the peak value, which is displayed as “rms”. This assumption is valid only for pure sinusoidal waveforms like the one pictured below. 

 

Meters_Haromonics_2.jpg

 

To accurately measure waveforms distorted by harmonics, a meter that will measure the true rms value is required. For example, if you were to use a conventional “average reading, calibrated rms” meter to measure the waveform below distorted by a non-linear power supply (with a peak value of 2.6 A and an average of 0.55 A), its' display would give a “rms” current of 0.61 A. 

 

Meters_Haromonics_3.jpg

Distorted current drawn by non-pfc 4k ballasts

 

A meter that measures true rms will give a more accurate measurement of 1.0 A for the distorted waveform above. By comparison, the reading of a conventional “average reading, calibrated rms” meter is almost 40% lower than the real current value.

 

Now for the reason the older non-power factor corrected 4k ballasts were failing when the newer power factor corrected M18 ballasts were not. One adverse effect of flat-topped current is that it causes non-power factor corrected equipment to draw more current to maintain the power rating (watts) of the unit. This, in turn, can cause protective fuses on electrical boards of the equipment to blow. I experienced this first hand, when I first tried some years ago to operate a 4k HMI Par on a Honda ES6500 (a conventional AVR generator) with the first generation of electronic square wave ballasts - a Lightmaker.  The ballast inexplicably failed when it had never given us problems on mains power. Upon closer inspection back in our shop, we found that a protective fuse on the main board had blown. We replaced the fuse and continued to operate the ballast off of grid power without incident. But as soon as we tried to run it again on the Honda the fuse blew. Since the Lightmaker ballasts are not Power Factor Corrected (PFC), the cause of the ballast's erratic behavior was the amount of harmonic distortion it was feeding back into the power stream generated by the ES6500. The harmonic currents were not a problem on grid power because, given the extremely low impedance of grid distribution, they did not induce voltage distortion.  But, fed back into the power stream generated by our Honda ES6500, the same harmonic currents created voltage distortion and sufficient voltage drop from skin effect to cause sufficient increase in current to blow the protective fuses on the ballast boards.

 

Ballast performance has improved remarkably since that first generation of electronic Power Gems & Lightmaker ballasts. The latest generation of power factor corrected electronic ballasts have filters built in that reduce the number of harmonic currents the ballast will feed back into the power stream. However, since power factor correction is still not commonly found in HMI ballasts smaller than 2500W, voltage waveform distortion still causes problems when operating HMI lights on conventional portable generators. Especially, since a 1200W non-power factor corrected HMI ballast can draw upwards of 19 Amps on a portable generator, it doesn’t take much increase of circuit impedance to push its load over the 20A threshold of distribution circuit breakers. And, given that 1200W ballasts are commonly wired with u-ground Edison plugs rated for 15 Amps, resulting in overheating of the plug end, and an increase of resistance even under normal conditions, even a slight voltage drop from "skin effect" caused by high THD values is what probably accounts for the inexplicable tripping of breakers  that anybody who has tried to power 1200W HMIs on portable generators is familiar with.

 

The first step in mitigating the problems caused by harmonic currents is to eliminate the currents. Where customarily the largest source of harmonic currents in a typical lighting package are HMI and Fluorescent lights, using only ballasts with power factor correction (PFC) circuitry will go a long way in reducing the number of harmonic currents in the power stream. A PFC circuit realigns voltage and current and induces a smoother power waveform at the distribution bus. PFC circuits successfully increase the power factor to as much as .9, making ballasts with it near linear loads. As a result, the ballast uses power more efficiently with minimized return current and line noise and also reduces heat, thereby increasing their reliability.

 

HD_PP_DemoWS.jpg

Wide Shot of Night exterior scene lit with a pkg. consisting of PFC 2.5 & 1.2 HMI Pars, PFC 800w Joker HMI, Kino Flo Flat Head 80, 2 ParaBeam 400s, and a ParaBeam 200 powered by a modified Honda EU6500is.

 

A second step in mitigating the problems caused by harmonic currents on small putt-putt generators is to operate them on only inverter generators. The combination of improved power factor and the nearly pure power waveform of inverter generators makes it possible to reliably power larger lights, or more smaller lights, than has been possible before on a small portable gas generator. For example, the power waveform above on the right, is the same 2500W load but with power factor correction operating on our modified Honda EU6500is Inverter Generator. As you can see, the difference between the resulting waveforms is startling. Even though the load is the same, the fact that it is power factor corrected and the power is being generated by an inverter generator, results in virtually no power waveform distortion. For this reason, sensitive electronic production equipment will operate reliably and without damage on the same power. And, the generator is capable of operating larger, or more smaller, lights than has ever been possible before on a portable gas generator.

 

HD_PP_Demo_SetUp_Night.jpg

 The PFC 2.5 & 1.2 HMI Pars, PFC 800w Joker HMI, Kino Flo Flat Head 80, 2 ParaBeam 400s, and a ParaBeam 200 of our HD P&P Pkg. powered by our modified Honda EU6500is through our 60A Full Power Transformer/Distro

 

The extremely low line noise exhibited in the inverter generator power waveform above (right) creates a new math when it comes to calculating the lighting load you can put on a portable generator. Where before you could not operate more than a couple 1200W HMIs with non-PFC ballasts on a conventional putt-putt generator because of the consequent harmonic distortion, now you can load an inverter generator to capacity. And if the generator is one of our modified Honda EU6500is inverter generators, you will be able to run a continuous load of up to 7500W as long as your HMI and Kino ballasts are Power Factor Corrected.

 

HD_PP_Demo_Transformer-Distro.jpg

A Distro System consisting of a 60A Full Power Transformer/Distro, 2-60A GPC (Bates) Splitters, 2-60A Woodhead Box distributes power from a modified Honda EU6500is. Even though the generator is 100' away to reduce noise, plug-in points remain conveniently close to set.

 

According to this new math, when you add up the incremental savings in power to be gained by using only PFC HMI ballasts, add to it energy efficient sources like the Kino Flo ParaBeam fixtures (which are also PFC), and combine it with the pure waveform of inverter generators, you can run more HMI lights on a portable gas generator than has been possible before. For example, the 7500W capacity of our modified Honda EU6500is Inverter Generator can power a lighting package that consists of a PFC 2.5kw HMI Par, PFC 1200, & 800 HMI Pars, a couple of Kino Flo ParaBeam 400s, a couple of ParaBeam 200s, and a Flat Head 80. Given the light sensitivity of HD cameras, this is pretty much all the light you will need to light both the foreground and deep background of night exteriors. Use this link for more information about the benefits of low line noise.

 

As Rob experienced first hand, the distortion of the voltage waveform that harmonic currents can cause (Harmonic Noise) can severely limit the total number of lights that can be reliably operated on conventional generators. These power distribution issues have been vexing set electricians for years.  If you haven’t already, you might want to read an article I wrote for our company newsletter that explains the electrical engineering principles behind these issues and how to resolve them. 

 

BoxBookLinkGenSetFP.jpg

 

This article is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website Box Book Extras.”  Of the article Harry Box exclaims:

 

"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 article is available online for free at http://www.screenlig...generators.html. I hope this helps

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston.


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