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What is Power Factor Correction in HMIs


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#1 Guy Holt

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Posted 10 January 2010 - 07:39 PM

My question is why do PFC circuits cost so much? How do they work.


With a purely inductive and resistive load, you can solve the problem with capacitance - and vice versa. My guess is that the problem gets more expensive when the load deviates from a sine wave. It may be more in design than in components. This business is customer driven. The more we ask for it, the harder they'll think about delivering it.


John’s guess is correct. It is when the voltage waveform induced by the load deviates from a sine wave that power factor correction gets expensive. For instance, in the strictest sense magnetic HMI ballasts are “Power Factor Corrected.” In order to understand what I mean, it would help to understand some basic electrical engineering principles. If you haven't already, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it I cover some of the basic electrical engineering principles behind harmonic distortion and how it can adversely effect generators. The article is available on our website.

Here is a much simplified explanation of power factor and why it is necessary in HMI & Kino ballasts. With a purely resistive AC load (Incandescent Lamps, Heaters, etc.) voltage and current waveforms are in step (or in phase), changing polarity at the same instant in each cycle ( a high power factor or unity.) With “non-linear loads” (magnetic and electronic HMI & Fluorescent ballasts) energy storage in the loads, impedes the flow of current and results in a time difference between the current and voltage waveforms – they are out of phase (a low power factor.) In other words, during each cycle of the AC voltage, extra energy, in addition to any energy consumed in the load, is temporarily stored in the load in electric or magnetic fields, and then returned to the power distribution a fraction of a second later in the cycle. The "ebb and flow" of this nonproductive power increases the current in the line. Thus, a load with a low power factor will use higher currents to transfer a given quantity of real power than a load with a high power factor

As John correctly surmises, basic power factor correction brings the voltage and current waveforms back in phase (closer to unity power factor) by supplying reactive power of opposite sign, adding capacitors or inductors which act to cancel the inductive or capacitive effects of the load, respectively. For example, the inductive effect of motor loads may be offset by locally connected capacitors. If a load had a capacitive value, inductors are connected to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth on each AC cycle. The make up of a magnetic HMI ballast is very similar to an electric motor and hence, like an electric motor, has an inductive effect on the power supply. Between the power input and the HMI lamp is a transformer that acts as a choke coil. The transformer provides the start-up charge for the igniter circuit, rapidly increasing the potential between the electrodes of the head’s arc gap until an electrical arc jumps the gap and ignites an electrical arc between the lamp electrodes. The transformer then acts as a choke, regulating current to the lamp to maintain the pulsating arc once the light is burning.

Essentially a large coil of wire that is tapped at several places to provide for various input voltages and a high start-up voltage, the transformers of magnetic HMI ballasts exhibit high self-inductance. Self-inductance is a particular form of electromagnetic induction characteristic of coils (like those in magnetic HMI ballasts and electric motors) that inhibits the flow of current in the windings of the coil. This opposition to the flow of current is called inductive reactance. In the case of a magnetic HMI ballast, the multiple fine windings of the ballast transformer induces appreciable voltage and considerable current 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 supplied by the generator. Put simply, the ballast draws more power than it uses to create light. As John Sprung' suggests, the addition of capacitors will compensate for the high inductance of the transformer and bring the current partially back in phase with the voltage. For this reason a bank of capacitor is typically included in the design of magnetic HMI ballasts as a power factor correction circuit. In this sense magnetic ballasts are power factor corrected.

If, in the case of a magnetic ballast, you were to measure the current (using a true RMS 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. 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. The ratio of “true power” to “apparent power” is a measure of the “power factor” of the ballast and is expressed by a number somewhere between 0 and 1. 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). The favorite analogy electricians like to use to explain power factor is that if apparent power is a glass of beer, power factor is the foam that prevents you from filling it up all the way. 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.

By comparison to magnetic HMI ballasts, electronic HMI ballasts are quite a bit more complicated. In an electronic HMI ballast, AC power is first converted into DC. Then, a high-speed switching device (micro processor controlled IGBTs) turns the flat current into an alternating square wave. Hence, they are commonly referred to as square wave ballasts. Electronic square wave ballasts utilize solid state electronic components (rectifiers, capacitors, and IGBTS) which use only portions of the input power sine wave. Put simply, they place all their load on the peak values of the power waveform. These devices then return the unused portions to the power stream as harmonic currents. These harmonic currents stack on top of one another creating harmonic distortion that likewise creates an opposition to the flow of current, pulls the voltage and current out of phase, and when the power is supplied by a generator can lead to severe distortion of the voltage waveform in the power distribution system.

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For example, the power waveform below left (from my article) is typical of what results from the operation of a 2500W non-Power Factor Corrected load (electronic HMI & Kino ballasts) on a conventional portable generator (a Honda EX5500 with a Barber Coleman Governor.) The severe harmonic noise exhibited here can cause overheating and failing equipment, efficiency losses, circuit breaker trips, excessive current on the neutral return, and instability of the generator's voltage and frequency

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Left: Conventional generator power w/ pkg. of non-PFC Elec. HMI Ballasts & Kino Flo Wall-o-Lite. Right: Inverter generator power w/ Pkg. of PFC Elec. Ballasts & Kino Flo Parabeam 400.


The opposition to the flow of current caused by harmonic distortion is called capacitive reactance. Capacitive reactance acts on the waveform in a way opposite to inductive reactance. It causes current to lead voltage. Since an electronic ballast also puts current and voltage out of phase with one another, it also has a power factor. 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. Where a typical 1200W non-power factor corrected electronic HMI ballast takes 18.5 Amps at 120 Volts to generate 1200 Watts of light the power factor is .54 (18.5A x 120V= 2220W, 1200W/2220W= .54).

When using a lighting package with low power factor (like the pkg. of non-PFC electronic HMI & Kino ballasts depicted above), the conventional wisdom in the past has been to not load the generator beyond 75% for more than a short period. Where the maximum recommend continuous load on a 6500W generator is 5500W, the de-rated continuous load rating would be roughly 4000 watts. By de-rating the load capacity in this fashion, the Gaffer minimizes the adverse effects of high THD so that both the generator and the loads placed upon it operate more reliably. However, this conventional wisdom no longer holds true if the HMI & Kino ballasts are power factor corrected and powered by an inverter 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. What this means is that an inverter generator can be loaded to capacity with PFC HMI and Kino Flo ballasts. The substantial reduction in line noise that results from using PFC ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the continuous load you can put on a portable gas generator (in the case of our modified Honda EU6500is generator a capacity of 7500 Watts.)

Since power factor correction can be of tremendous benefit when operating HMIs and Kinos on portable gas generators, it is worth understanding in more detail. Since, as exhibited in the waveform above, capacitive reactance distorts the shape of the voltage waveform from a sine wave to some other form, the addition of linear components such as inductors cannot counteract the harmonic currents as the addition of capacitors counteracted the inductive reactance of magnetic HMI ballasts. In the case of electronic ballasts, other more complicated (translate expensive) means of power factor correction is required to smooth out the power waveform they induce.

To understand how power factor correction circuits work in electronic HMI ballast it helps to understand the source of the harmonic currrents. The harmonic currents produced by electronic HMI ballasts are primarily generated by the diode-capacitor section of the ballast. As you may recall from our discussion above, the diode-capacitor section rectifies the AC input power into DC, which is then used by the power module to create the square wave. The diode-capacitor section accomplishes this by first feeding the AC input current through a full wave bridge rectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into a bank of capacitors which removes the 60 Hz rise and fall and flattens out the voltage-making it essentially DC. The required DC is then fed from these capacitors to the power module where the IGBTs switch it into an alternating square wave.

(ILLUSTRATION COURTESY OF HARRY BOX)
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Step 1: Rectifier Bridge converts AC power to rectified sine wave. Step 2: capacitors flatten the rectified sine wave to DC. Step 3: micro processor switching alternates polarity of DC creating an AC square wave.


The source of harmonics currents lies in the rectifying circuit of the diode-capacitor section of the ballast. The rectifying circuit only draws current from the AC line during the peaks of the supply voltage waveform, charging the capacitors to the peak of the line voltage. Since the capacitors only charge when input voltage is greater than its stored voltage, a non-PFC circuit only charges the capacitors for a brief period of the overall cycle time. After 90 degrees, the half cycle from the bridge drops below the capacitor voltage; which back biases the bridge, inhibiting further current flow into the capacitor. During this brief charging period, the capacitors must be fully charged, requiring large pulses of current to be drawn for a short duration. As can be seen in the illustration below, electronic ballasts draw current in high amplitude short pulses. The remaining unused current feeds back into the power stream as harmonic currents.

(ILLUSTRATION COURTESY OF FAIRCHILD SEMICONDUCTOR)
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Thin Black Trace: Rectifier Bridge converts AC power to rectified sine wave. Thick Black Trace: Stored Capacitor Voltage. Red Trace: Current drawn by capacitors once input voltage is greater than voltage stored in the capacitor (thick black trace.)


Notice how big the input current spike (red trace) of the rectifier circuit is. All the circuitry in the ballast as well as the supply chain (the generator plant, distribution wiring, circuit breakers, etc) must be capable of carrying this high peak current. In order not to have these high amplitude current pulses, the capacitors in the diode-capacitor section of the ballast must charge over the entire cycle rather than just a small portion of it. The power factor correction circuitry of electronic HMI ballasts use a multi-stage boost converter topology to accumulate energy in the capacitors over the entire cycle, which averages out the peak load, and greatly reduces the huge peak current. In the “Active Power Factor Correction” circuits used in electronic square wave ballasts, a boost converter is inserted between the bridge rectifier and the main input capacitors. The boost converter maintains a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switch mode converter produces the desired output voltage from the DC bus. Where the input to the converter is the full-rectified AC line voltage, the output voltage of the boost converter is set higher than the peak value (hence the word boost) of the line voltage (a commonly used value is 385VDC to allow for a high line of 270VACrms.)

Now that the capacitors charge throughout the AC cycle rather than just a brief portion of it, harmonic currents are not generated. And, with the boost pre-converter voltage higher than the input voltage, the load is forced to draw current in phase with the AC main line voltage. In this fashion, the 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.

If you still don’t entirely understand how power factor correction works in electronic HMI ballasts, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. In it, is a more detailed explanation of the basic electrical engineering principles behind harmonic distortion and how it can adversely effect generators. The article is available on our website.

Guy Holt, Gaffer, ScreenLight & Grip, Boston
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#2 Guy Holt

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Posted 10 January 2010 - 10:43 PM

I disagree that asking for PFC ballasts from your rental house will make them appear, they'll appear when it's economically feasible for the rental house to upgrade it's gear. Just as they did when electronic ballasts replaced their more reliable magnetic predecessors.


Since power factor correction circuitry can add 20-25% to the cost of a ballast, rental houses will not buy them unless they are forced to by customer demand or regulatory agencies. In permanent installations, many jurisdictions are beginning to legally require power factor correction for all power supplies above a certain power level. Regulatory agencies such as the EU have also set harmonic limits as a method of improving power factor. The current EU standard EN61000-3-2 requires that all power supplies over 80w have a power factor of 0.9 or more.

… electronic ballasts replaced their more reliable magnetic predecessors.


I detect in JD’s comment a whiff of nostalgia for the solidly reliable magnetic ballasts of yesteryear. I agree with him. There is a popular misconception that you should only use electronic ballasts with portable generators. Where that is true with conventional generators without crystal governors, it is not true of inverter generators like the Honda EU series. In the interest of full disclosure, I should say at this point that in addition to being a gaffer, I own and operate ScreenLight & Grip – a lighting and grip equipment rental and sales company. If what I am about to say sounds like I’m hyping magnetic ballasts it is not because we rent and sell them exclusively. We are dealers and rental agents for just about all the major brands and I don’t stand to profit from the sale or rental of magnetic HMI ballasts since we have made the transition to PFC electronic ballasts. As a professional Gaffer of a lot of tight budgeted historical documentaries for PBS’ American Experience and The History Channel (see my “credit-entials” on Imbd), I think it is worth noting that magnetic ballasts are still a viable production tool when used with the new inverter generators because they offer low budget independent filmmakers a cheaper alternative to high priced rental house equipment.

Magnetic ballasts will operate reliably on the Honda EU series generators because Honda's sine-wave inverter technology provides much higher quality power than conventional (non-inverter) generators. With a waveform distortion factor of less than 2.5%, the power generated by Honda’s EU series of generators is quite often better than what you get out of the wall outlet. The power these machines generate is rock solid with a frequency variance of only hundredths of a cycle - which eliminates the need for costly crystal governors. The Honda EU series generators provide true sine wave power with low enough distortion, and frequency stability, to power HMI's with magnetic ballasts without problems. As long as you shoot at one of the many safe frame rates, magnetic ballasts are also “flicker free” (where the topic of safe frame rates for magnetic ballasts is discussed extensively elsewhere in this forum I won’t get into it here.) Besides the extra bulk and weight of magnetic ballasts, the smaller magnetic ballasts (575-2500W) offer the distinct advantage of being less expensive and drawing less power (once they have come up to speed) than the commonly available non-PFC electronic equivalents.

If you don’t have access to the newest PFC electronic ballasts, you are better served by using the older magnetic ballasts on an inverter generator like the Honda EU 6500is over non-PFC electronic ballasts. I know this is contrary to the conventional wisdom, so I will quickly summarize why.

When electronic square wave HMI ballasts came on the market, they were at first thought to be the solution to all the problems inherent in running HMI lights on small portable generators. By eliminating the flicker problem associated with magnetic ballasts, they also eliminated the need for the expensive and ultimately unreliable AC governors required for flicker free filming with magnetic HMI ballasts and portable gas generators. Electronic square wave ballasts eliminate the potential for flicker by squaring off the curves of the AC sine wave supplying the globe. Squared off, the changeover period between cycles is so brief that the light no longer pulsates but is virtually continuous. Even if the AC Frequency of the power were to vary, a frame of film or video scan, would receive the same exposure because the light intensity is now not pulsating but nearly constant. Electronic square wave HMI ballasts allow you to film at any frame rate and even at a changing frame rate.

Since they are not frequency dependent, it was thought at first that electronic square wave ballasts would operate HMI more reliably on small portable generators – even those without frequency governors. For this reason, as soon as electronic square wave ballasts appeared on the market, many lighting rental houses replaced the more expensive crystal governed portable generators with less expensive non-synchronous portable generators. The theory was that an electronic square wave ballast would operate reliably on a non governed generator and allow filming at any frame rate, where as a magnetic HMI ballast operating on an unreliably AC governed generator allowed filming only at permitted frame rates.

In practice, electronic square wave ballasts turned out to be a mixed blessing. Part of the problem with operating electronic HMI ballasts on portable gas generators in the past has to do with the purity of the power waveform they generate. With an applied voltage waveform distortion of upwards of 19.5%, conventional generators do not interact well with the leading power factor (current leads voltage) of the capacitive reactance created by electronic square wave HMI ballasts. The net result is harmonic currents are thrown back into the power stream, which results in a further degradation of the voltage waveform and ultimately to equipment failure or damage (for the reasons discussed in my previous post.)

The oscilloscope shots of the power waveforms below is from my article mentioned above and is typical of what results from the operation of a 1200W HMI with non-power factor corrected ballast on grid power (left), on a conventional generator (middle), and inverter generator (right.) The adverse effects of the harmonic noise generated by non-PFC electronic ballasts and exhibited here in the middle shot, can take the form of overheating and failing equipment, circuit breaker trips, excessive current on the neutral wire, and instability of the generator’s voltage and frequency. Harmonic noise of this magnitude can also damage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference.

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Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec. Ballast.


As is evident in the oscilloscope shots below of a 1200W magnetic HMI ballasts on grid power, on power generated by a conventional Generator (Honda EX5500), and power generated by an inverter generator (Honda EU6500is), the lagging power factor caused by the inductive reactance of magnetic ballasts has by comparison only a moderately adverse effect on the power waveform. Outside of causing a voltage spike in the inverter power, magnetic ballasts actually show a positive effect on the already distorted power waveform of the Honda EX5500 conventional generator. For this reason magnetic ballasts work better on conventional generators with frequency governors than do non-PFC electronic square wave HMI ballasts.

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Left: Grid Power w/ 1.2Kw Arri Magnetic Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri Magnetic Ballast. Right: Inverter Power w/ 1.2Kw Arri Magnetic Ballast.


These oscilloscope shots show that if you don’t have access to the newest PFC electronic ballasts, the older magnetic ballasts are in fact cleaner running on portable gas generators than non-PFC electronic ballasts. And, where inverter generators like the Honda EU6500is do not require crystal governors to run at precisely 60Hz, you can operate magnetic HMI ballasts reliably on them. In addition, the smaller magnetic ballasts (575-2500W) offer the distinct advantage of being less expensive and draw less power (once they have come up to speed) than the commonly available non-PFC electronic equivalents (13.5A versus 19A for a 1.2kw.)

Of course there are downsides to using magnetic ballasts. One down side is that you are restricted to using only the safe frame rates and shutter angles. But when you consider that every film made before the early 1990s was made this way, you realize it is not such a limitation. Another downside to magnetic ballasts is that you can’t load the generator to full capacity because you must leave “head room” for their higher front end striking load. When choosing HMIs to run off portable generators, bear in mind that a magnetic ballasts draws more current during the striking phase and then they “settle down” and require less power to maintain the HMI Arc. By contrast, an electronic ballasts “ramps up”. That is, its’ current draw gradually builds until it “tops off.”

For example, even though a 2.5kw magnetic ballast draws approximately 26 amps you will not be able to run it reliably on the 30A/120V twist-lock receptacle on a 6500W generator’s power panel. As mentioned above, magnetic ballasts have a high front end striking load. For this reason, you must always leave “head room” on the generator for the strike. But, even though the twist-lock receptacle is rated for 30 Amps conventional 6500W generators are only capable of sustaining a peak load of 27.5 Amps per leg for a short period of time. Their continuous load capacity (more than 30 minutes) is 23 Amps per leg. And if there is any line loss from a long cable run the draw of a 2.5kw magnetic ballast will climb to upward of 30 Amps. To make matters worse, the lagging power factor caused by the inductive reactance of the magnetic ballast kicking harmonic currents back into the power stream causes spikes in the supply voltage that can cause erratic tripping of the breakers on the generator or ballast. For a more detailed explanation of why that is I, again, suggest you read my newsletter article. The article is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html. In my experience the load of a 2.5kw magnetic ballast is too near the operating threshold of a 6500W generator for it to operate reliably.

The only sure way to power a 120V 2.5kw (or even a 4kw) HMI magnetic ballast on a portable gas generator is from its 240V circuit through a 240v-to-120v step down transformer like the one we manufacture for our modified Honda EU6500is. Our 60A Full Power Transformer/Distro steps down the 240V output of the generator to a single 60A 120V circuit that is capable of accommodating the high front end striking load, and even the voltage spikes, of either a 2.5kw or 4kw magnetic ballast at 120V. And, by splitting the large front end striking load of 2.5/4kw HMIs evenly over the two legs of the 240V circuit of the generator, the transformer reduces the impact on the generator when you first switch on the light. The same holds true when you switch on large tungsten lights like 6000W Molepar Six Lights or 5ks. And since, magnetic HMI ballasts will operate flicker free at all standard frame rates on an inverter generator (without the need for a crystal governor), our 60A Full Power Transformer/Distro gives new production life to older 2.5kw & 4kw HMIs with 120V magnetic ballasts. It provides an affordable way of powering more affordable HMIs.

Guy Holt, Gaffer, ScreenLight & Grip, Boston
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#3 John Sprung

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Posted 11 January 2010 - 04:31 PM

OK, a wild crazy question -- What if we could generate and distribute DC instead of AC, like we did back in the carbon arc days? Incandescents and halogens work fine on DC. We wouldn't need the bridge rectifier and capacitor input, though we could leave them in the circuit. The bridge would merely idiot-proof the polarity. The sound guys would be happy not to have so much 60 Hz running around. And the big one -- DC doesn't have a power factor or phases or any of that, maybe just starting surges once in a while.

The things that would need AC would be the ones that have a transformer first in their power supply, mostly video village stuff, battery chargers, etc. We'd need nice clean little tech power inverters for that stuff, but the total of such loads is really small compared with what we could run on DC.





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#4 Michael Collier

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Posted 11 January 2010 - 07:23 PM

OK, a wild crazy question -- What if we could generate and distribute DC instead of AC, like we did back in the carbon arc days?


Damn it John, I thought of that about halfway through the read and thought I would be original in that idea.

There are a few things that would need working. First is the long power runs a film set often has. In long runs, AC transfers power more effeciently than DC, and that line loss in a long DC run could negate any efficiencies seen in a DC set.

If we were to hybrid the set, both DC and AC, perhaps we could have a 3 phase feeder going to a distro box that has a PFC AC-DC converter. If there has to be one to handle all of the loads, then it could effectively reduce the cost of the PFC, rather than having it in every ballast. Then ballasts could be easily designed to run on both (the DC line providing power after the non PFC- AC section)

Since its common on long runs to go into a distro box somewhere near the hot set, it wouldn't require any extra steps to set up. The distro box would just be a bit more expensive (but allow for cheaper ballasts.) Because it seems (from reading the above) the best technique would be to individually rectify all 3 phases, and then sum the voltage into one single bus, there would be no need to ballance a DC power, since all load would be equally shared by the phases as their individual peaks are reached.

Perhaps because all 3 reach their peaks at different times, the work that must be done to PFC a DC distro box compared to a ballast that works on a single leg of 3 phase AC power would be simpler, since the duration between non-peak times would be minimized and spread more evenly over time (6 peaks rectified at 60 degree intervals, rather than 2 peaks at 180 degree intervals).

That DC could then be re-inverted (or just a isolated tap off one of the phases, since the harmonics would be lessened with such a system) could be sent to devices that absolutely need AC current.

Might not be the cheapest upfront solution, but cheaper than stocking several PFC ballasts on set, and might even be cheaper than non-PFC ballasts eating up extra gas (diesel is expensive, and if we are talking 20-30% efficiency loss, it could add up.) hell if it became the norm, then the cost of DC only ballasts would go down compared to AC ballasts (since the whole AC to DC section can be omitted.)

Just a thought that was totally original until John beat me to it.
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#5 Paul Bruening

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Posted 12 January 2010 - 12:29 AM

Wouldn't it be nice for those big arcs to come back. Wow, what lovely light they made. Didn't someone here recommend a computer regulated arc system? Linear steppers can push the rods. Sensors could manage the arc quality and uniformity. Gaffers would only have to change out the rods and the computer could tell them how soon that would have to be. Low maintenance, lovely, daylight exterior light.

Build the distribution rectifier into a Yaris, hatch back car and drive it up to the best distro point. Plug the AC lines in from a distant (therefore quieter) 3 phase (Y-wired?) genny. Sure sounds easier than all that complex AC lighting management at every usage point.
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#6 John Sprung

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Posted 12 January 2010 - 02:17 AM

First is the long power runs a film set often has. In long runs, AC transfers power more effeciently than DC, and that line loss in a long DC run could negate any efficiencies seen in a DC set.


I know that was the reasoning back in the Edison vs. Tesla/Westinghouse days, but consider the Dalles-Sylmar DC line. They must have solved the problem to go with such a big DC transmission line. Here's a quote and URL:

"The Pacific DC intertie is a 1000kV (+/- 500kV ) DC line from Celilo, near The Dalles, Oregon, to Sylmar, in southern California. It is rated at 3100 MW bidirectional."

http://www.wecc.biz/...ts/path6665.pdf

Sylmar isn't all that far from here, which is why I'm aware of it.




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#7 John Sprung

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Posted 12 January 2010 - 02:19 AM

Wouldn't it be nice for those big arcs to come back.


Alas, the problem now is getting good quality carbons. They require rare earth element that we can't get at the moment for political reasons.




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#8 David Rakoczy

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Posted 12 January 2010 - 07:50 AM

Wouldn't it be nice for those big arcs to come back. Wow, what lovely light they made. Didn't someone here recommend a computer regulated arc system? Linear steppers can push the rods. Sensors could manage the arc quality and uniformity. Gaffers would only have to change out the rods and the computer could tell them how soon that would have to be. Low maintenance, lovely, daylight exterior light.

Build the distribution rectifier into a Yaris, hatch back car and drive it up to the best distro point. Plug the AC lines in from a distant (therefore quieter) 3 phase (Y-wired?) genny. Sure sounds easier than all that complex AC lighting management at every usage point.



If you ever ran a Set Arc you would know 'computers' would not and will not be used to run ARCs... and it is more than Carbon Rod availability that hinders their use. They belch smoke! So, for exteriors they are fantastic as they are so hardy you can just leave them scattered around a backlot.... but interior Set work... no. Not anymore.

Related Arc Thread

Phil, there is a huge difference between a Carbon Arc sitting nicely in a projection room with proper ventilation and someone to feather dust it off once a week... and a Set Arc that takes a TREMENDOUS amount of abuse.
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#9 John Sprung

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Posted 12 January 2010 - 01:26 PM

They belch smoke!


Yeah, and that smoke is .... Carbon!

Mobs of environmentalists would come after you with pitchforks and torches. (Though they probably wouldn't light the torches....) ;-)





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#10 Paul Bruening

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Posted 12 January 2010 - 03:08 PM

If you ever ran a Set Arc you would know 'computers' would not and will not be used to run ARCs... and it is more than Carbaon Rod availability that hinders their use. They belch smoke! So, for exteriors they are fantastic as they are so hardy you can just leave them scattered around a backlot.... but interior Set work... no. Not anymore.

Related Arc Thread

Phil, there is a huge difference between a Carbon Arc sitting nicely in a projection room with proper ventilation and someone to feather dust it off once a week... and a Set Arc that takes a TREMENDOUS amount of abuse.




Using them outdoors was my principle interest. Competing with the sun is a real, "Sun of a bitch." (Sorry, couldn't help myself.)

I still don't get why they can't be modernized. Lots of outdoor equipment has boards and sensors regulating them.

The Asians are doing dirty industry like mad. Can they make the rods and sell them to us or is the rare earths restriction on manufactured product as well? Somebody over there has to be making welding, cut-off rods. They're the same idea as light producing rods.
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#11 Michael Collier

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Posted 12 January 2010 - 03:18 PM

I know that was the reasoning back in the Edison vs. Tesla/Westinghouse days, but consider the Dalles-Sylmar DC line. They must have solved the problem to go with such a big DC transmission line. Here's a quote and URL:


Gotta give you points for obscure link to prove your point. I can't help but think the 1000kv has something to do with their efficiencies. My question for their operation is how do you convert 1 million volts DC into anything usable? Without the ability to use transformers (without modulating the DC into an AC form) how does the average home owner use that power? Or what facility does use that power?

(in the course of writing I think I found my answer)

From wiki: "High-voltage direct current (HVDC) technology is used only for very long distances (typically greater than 400 miles)"

I assume the low loss has to do with extreme high voltage, which would reduce current greatly, and more importantly, would be constant with my understanding of the physics of electricity....so I don't have to rethink everything I have been taught. Crisis averted


But back to film--

Another advantage of a DC set: put a squeezer on a tungsten light and you (I assume) shouldn't hear the filament buzz like you do when dimming an AC supplied light.

I also imagine a DC set would be safer in general, but I am having difficulty formulating a reason why. Just seems to fit that it might be. I suppose in the hands of a qualified gaffer and genny op, any distribution of power should be safe, just as in the hands of a duntz any style of distribution could be dangerous.


as far as carbon arcs go, I don't think we ever need to revisit those things. but if we absolutely must, maybe we can claim efficiencies gained in electric usage would offset the smoke produced (local carbon offset. Taken to the extreme maybe one could finance their picture on offsets gained by saving electricity. It could work: "see all these lights I have up? I'll only strike half of them if you pay me", instant carbon credit. well, it makes as much sense as offsets gained by not clear cutting forests.)
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#12 JD Hartman

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Posted 12 January 2010 - 04:02 PM

But back to film--

Another advantage of a DC set: put a squeezer on a tungsten light and you (I assume) shouldn't hear the filament buzz like you do when dimming an AC supplied light.


Try using a variac next time, instead of a dimmer from HomeCheapo (or worse,.. Ikea). Better control and the ability to overdrive the lamp and boost color temp. as well.
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#13 Paul Bruening

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Posted 12 January 2010 - 06:16 PM

as far as carbon arcs go, I don't think we ever need to revisit those things.


It's the light they give. Especially, when compared to the sun. They are the brightest, closest color balanced, broadest spectrum, flicker-hassle free light still around. If only their antique characteristics could be re-engineered with some automation... well, and the rods thing.
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#14 John Sprung

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Posted 12 January 2010 - 08:01 PM

Another advantage of a DC set: put a squeezer on a tungsten light and you (I assume) shouldn't hear the filament buzz like you do when dimming an AC supplied light.


The buzz comes mainly from the fast rise time you get with a triac dimmer -- also very bad for power factor. The variac would give you dimming with a pure sine wave -- or just as good as the sine wave that went into it. And therefore no dimmer buzz. But neither the triac nor the variac work on DC. Old time resistance dimmers do work, and they keep you nice and warm in the winter.





-- J.S.
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#15 Guy Holt

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Posted 12 January 2010 - 08:48 PM

Another advantage of a DC set: put a squeezer on a tungsten light and you (I assume) shouldn't hear the filament buzz like you do when dimming an AC supplied light.

I also imagine a DC set would be safer in general, but I am having difficulty formulating a reason why. Just seems to fit that it might be. I suppose in the hands of a qualified gaffer and genny op, any distribution of power should be safe, just as in the hands of a duntz any style of distribution could be dangerous.


The intent of my original post was not to ferment a rebellion against the established order of Alternating Current. While older HMIs with magnetic ballasts are less expensive to purchase or rent, Power Factor Correction (PFC) makes the newest electronic ballasts worth the extra money when it comes to lighting with portable generators. For example, the substantial reduction in line noise that results from using power factor corrected ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the load you can put on a generator. In the past we had to de-rate portable gas generators because of the inherent short comings of conventional generators with AVR and Frequency governing systems when dealing with non-PFC electronic ballasts. The harmonic distortion created by non-PFC ballasts reacting poorly with the distorted power waveform of conventional AVR generators limited the number of HMIs you could power on a portable generator to 75% of their rated capacity (4000Watts on a 6500W Generator). But now, where inverter generators have virtually no inherent harmonic distortion or sub-transient impedance and power factor correction (PFC) is available in small HMI ballasts, this conventional wisdom regarding portable gas generators no longer holds true. Where before you could not operate more than a couple 1200W HMIs with non-PFC ballasts on a conventional generator because of the consequent harmonic distortion, now according to the new math of low line noise, 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.

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, and combine it with the pure waveform of inverter generators, you can run more lights on a portable gas generator than has been possible before. For example, as I mentioned in a previous post, on a Red shoot I operated a lighting package that consisted 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 on our modified Honda EU6500is Generator. Given the light sensitivity of the Red Camera, this was all the light we needed to light a large night exterior. For more details on how this is accomplished I suggest you read my newsletter article on the use of portable generators in motion picture production which I mention above. The article is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Boston
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#16 John Sprung

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Posted 12 January 2010 - 08:56 PM

My question for their operation is how do you convert 1 million volts DC into anything usable?


Very carefully.

They started with mercury arc tubes back in 1970, and have converted entirely to silicon thyristors, per this from Bonneville Power:

http://www.bpa.gov/c...5fs/fs1005b.pdf

Here's a start on how the thyristors work. It turns out that they use optical triggering to control stuff that's like 500kV above or below ground:

http://www.abb.com/i...8200323587.aspx





-- J.S.
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#17 Hal Smith

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Posted 12 January 2010 - 09:24 PM

Okay, after all the buzz about power factor I propose a huge Kudo Award for the first person to answer this question correctly:

What is the reason that a given load has a power factor other than 1.0?

Hint: It doesn't have anything to do with waveform although different waveforms will have a different power factors if the sine wave PF not 1.0

James Clerk Maxwell's acolytes will know the answer, Albert Einstein's will know even more about the answer.
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#18 JD Hartman

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Posted 13 January 2010 - 12:06 PM

I remember (and hated) this from physics. Power factor other than 1.0 is caused by the voltage and current in a circuit being out of phase. The relationship can be either lagging or leading.
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#19 Hal Smith

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Posted 13 January 2010 - 04:18 PM

I remember (and hated) this from physics. Power factor other than 1.0 is caused by the voltage and current in a circuit being out of phase. The relationship can be either lagging or leading.


75% credit. What's the reason for the voltage and current phase angle not being the same?

Hint: ELI the ICE man

(PS: Loathing and hating Physics loses points. Once upon a time I taught Physics at The University of the South and the University's Prep School, The Sewanee Academy).
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#20 JD Hartman

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Posted 13 January 2010 - 09:56 PM

75% credit. What's the reason for the voltage and current phase angle not being the same?

Hint: ELI the ICE man

(PS: Loathing and hating Physics loses points. Once upon a time I taught Physics at The University of the South and the University's Prep School, The Sewanee Academy).


Didn't hate my physics class any more than my Differential Equations classes. Instructors lose their students interest or attention, when they can not demonstrate how this knowledge will apply in the real world.

Why? An inductor or capacitor in an AC circuit will alter the phase angle between voltage and current. The inductor will resist the change in current and cause it to lag the rise in voltage (E Leads I). A capacitor will take time to charge and cause the voltage to lag the rise in current (I C E).

If it weren't for the capacitors affect on phase angle in AC circuits, we wouldn't have rotary phase converters either.
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