Portable Generators in Motion Picture Production

Portable Generators in Motion Picture Production

All Generators are notcreated Equal

All Loads are not createdEqual

Harmonics & PowerDistortion

Clean & Ample LocationPower

2009 Guy Holt ------- All Rights Reserved ------- May not be reproduced without written permission.

Introduction

Given the wide variety of generators manufactured for different markets, it is important to understand the benefits and drawbacks to eachwhen it comes to their use in motion picture production. Especially, given that the increasing use of personal computers and microprocessor-controlled recording equipment in HD production has created an unprecedented demand for clean, reliable power on set at a time when thetrend in lighting is toward light sources that generate dirty power. The power waveform below left is typical of what results from theoperation of a couple of 1200W HMIs with non-power factor corrected ballasts on a conventional portable generator. The adverse effects ofthe harmonic distortion exhibited here, can take the form of overheating and failing equipment, efficiency losses, circuit breaker trips,excessive current on the neutral wire, and instability of the generator voltage and frequency. Harmonic noise of this magnitude can alsodamage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference.

Left: Distorted power waveform created by Non-PFC 1200W HMI ballasts on conventional generator. Right: Near perfect power waveform created by the same lights as part of a new production system.

Why is harmonic distortion suddenly an issue in motion picture electrical distribution systems? First, one must appreciate that the powergeneration and electrical distribution systems developed for motion picture production were never designed to deal with an abundance ofnon-linear loads like the electronic HMI and Fluorescent lighting ballasts prevalent today. In the past, attention was given to portablegenerator features such as automatic voltage regulation and speed regulation. But, given the increasing prevalence of harmonic currents andthe problems they cause, an increasingly more important feature today is the quality of the generated power waveform and how well itinteracts with today's light sources. As production gets more electronically sophisticated, a thorough understanding of the demands placed onportable generators by such production equipment is necessary in order to generate power that is safe and reliable.

It is the intent of this article to establish a foundation of knowledge that will enable us to build a new production system that generates theclean stable set power (seen in the waveform above right) capable of operating larger lights (HMIs up to 6kw or Quartz lights up to 5kw), ormore smaller lights, off of portable gas generators than has ever been possible before. But, before we can begin to build the edifice of thisnew production system (pictured below), we must first lay a foundation with the basics of power generation.

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Generator Basics

Principles of Operation

An electric generator is a device or machine that is used to convert mechanical energy into electrical energy. It is based on the principle ofelectromagnetic induction, a scientific law that was discovered by British scientist Michael Faraday and American scientist Joseph Henry in1831. The principle states that when an electric conductor, such as a copper wire, is moved through a magnetic field, electric current willflow through the conductor. The mechanical energy of the moving wire is converted into the electric energy. Faraday and Henry also foundthat when you move a magnet in a coil of wire, electric current is generated.

A rudimentary electrical generator with static magnets and rotating current carrying coils

A generator produces an Electromotive Force (emf) by changing the number of Magnetic Flux Lines (Lines of Force), passing through aWire Coil. In the rudimentary electrical generator illustrated above and below, when the Coil is rotated between the Poles of the Magnet bycranking the handle, an AC Voltage Waveform is produced.

A generator operates on the principle of Electromagnetic Induction, which is defined by Faradays Law, which states:

Faraday's Law

The Electromotive Force, (emf) induced in a Coil is proportional to the number of turns, N, in the Coil and the Rate of Change of thenumber of Magnetic Flux Line passing through the surface (A) enclosed by the Coil. In the rudimentary generator illustrated here, the Coil isunder a Stationary Magnetic Field. The Magnetic Flux Density, B, is constant and so Lines of Force is proportional to the Effective Area,Aeff, of the Loop (Lines of Force = B x Aeff.) As the Loop rotates at different angles, there is a change in Aeff which is shown in theillustration below.

Effective Area of the Wire Loop at Different Rotational Angle

The Rate of Change of the Lines of Force is the largest at the zero points of the Waveform and is the smallest at the peaks of the Waveform.Since, an Induced Effect is always opposed to the cause that produced it, the Induced emf is maximum at the zero points and minimum atthe peaks as illustrated below. To see why that is, lets look more closely at what happens as the loop rotates.

Different Rates of Change of the Magnetic Flux at Various Rotational Angles

In the loop diagrams below, the loop is rotating in a clockwise direction. At position A, the top leg (black) is moving toward the south pole,and the lower leg (white) toward the north pole. In position A, no flux lines are being cut since both legs are moving parallel to the lines offlux. Since no flux is cut, no voltage is induced. In position B, the loop has rotated 1/4 of a turn (90). The black leg is now movingdownward, and the white leg is moving upward. In this position, both legs are cutting across a maximum number of lines of flux, and theemf is maximum. At position C the loop has rotated 1/2 of a turn. The two legs are once more moving parallel to the lines of flux, and againno voltage is induced. At position D, the black leg is moving upward, and white leg downward. Both legs are again cutting a maximumnumber of lines of force, but in the direction opposite to that of position B. Since the legs are cutting the field in the opposite direction, theemf induced causes the current to flow in the opposite direction. The next 1/4 turn brings the loop back to position A, and the cycle startsover again.

Position of the Rotating Wire Coil Plane to the Magnetic Field Direction and the Induced Electromotive Force

If we were to plot on a graph this induced emf against coil rotation, we would get the sinusoidal waveform that appears below the loopdiagrams in the illustration above. Line X-X' is the zero line. All the area above this line is positive (+), and the area below is negative (-).A careful plotting of induced emf through one rotation of the coil reveals that a sinusoidal voltage waveform is the natural result of themechanical motion of a generators coils. For example, in position A on the illustration of the coil rotation, the loop is cutting no lines offorce so the induced emf is zero (point 1 on the graph.) One quarter turn later, the loop is in position B. It is cutting a maximum number oflines of force, so the emf is maximum (point 2 on the graph). At position C, the loop has completed 1/2 of a turn, and no lines of flux arebeing cut, so the emf is back to zero at point 3 on the graph. In position D, the loop is cutting the field in the direction opposite to that ofposition B. The emf induced in the coil i s maximum, but in the opposite direction (point 4 on the graph). Position E is the same as A, sothe loop is ready to start over again. If we were to summarize what happens during one full rotation of the coil: it starts at zero, rises tomaximum in one direction (+), falls back to zero, rises to maximum in the opposite direction (-), and then comes back to zero. Since, analternating emf causes the current to flow first in one direction and then the other it is called, Alternating Current, or just plain A.C. A complete rotation is called a Cycle. If the generator coil is made to turn 60 complete rotations in one second, the Frequency of rotation is 60Cycles per second. If we plot induced emf against coil rotation at 60 Cycles per second we get the familiar AC voltage sine wave - theAlternating Current (AC) used in commercial electrical power systems.

Generator Anatomy

In order to obtain a larger emf, modern generators use stronger rotating Electromagnets instead of the fixed permanent magnet of ourillustration. The electromagnets are mounted on a shaft (called the Rotor) and rotated within electrical coils (called the Stator.) DC power isused to Excite the electromagnets of the Rotor. The voltage of the AC output is a function of the level of the excitation of the Rotorselectromagnets, and controlled by the Exciter. Illustrated below is the anatomy of a Honda conventional generator. It consists of a stationaryStator and a two pole Rotor that spins inside the Stator.

The Rotor contains magnetic fields which are established and fed by the Exciter. When the Rotor is rotated, electrical current is induced inthe armature coils of the Stator. The voltage of the electrical current generated is proportional to the strength of the magnetic fields, thenumber of coils (and number of windings of each coil), and the speed at which the Rotor turns. And, since the Rotor rotation producesdifferent directions to the +/- poles of the magnetic field at different points in time, the voltage generated is sinusoidal (AC), and each fullengine rotation produces one complete AC sine wave. Consequently, the engine must spin the generator Rotor 3600 RPM to produce the60Hz AC frequency required in North America (60 cycles/second x 60 seconds/minute = 3600RPM). If, because of varying loads, the Rotorspins faster or slower, the voltage and frequency of the output vary in step. The quality of the electricity a conventional generator puts outthen is determined by the quality of the engine, how smoothly it runs, and how well the engine is capable of maintaining a constant speed.

The Stator assembly consists of insulated windings (armature coils) positioned near an air gap in the Stator core in which the Rotor rotates.The number and the way the armature coils are connected determine the phase of the power generated. The Stator of a single phasegenerator, like the Honda EX5500 illustrated above, has two sets of armature coils which are spaced 180 degrees apart (a three phasegenerator has three sets of coils spaced 120 degrees apart.) As illustrated in the wiring schematic below, one end of each coil is connected toa common neutral terminal. The other end of each coil is connected to separate terminals. Conductors attached to the three terminals (hot,hot, neutral) carry the current to the generators distribution panel (load bus) and on to the electrical load.

Generator Wiring Schematic

As such a single phase generator, like the EX5500, has two separate main power producing circuits. These two circuits supply equal powerto the receptacles shown below when the voltage selector switch is in the "120/240V" position. With single phase generators, when thedistribution panel has two or more receptacles, you must balance the total load on the generator by dividing the individual loads between thetwo main power circuits.

For example, the Honda EX5500 is rated for a continuous load of 5000W (41.7A total or 20.8A/main circuit). Now, if receptacle 2 (R2) inthe illustration above has a 2k light (a 16.8A load) connected to it and receptacle 3 (R3) has a 1k light (a 8.4A load) connected to it, the totalpower draw on Main Circuit 1 is 25.2A (greater than the 20.8A capacity of Main Circuit 1). This is a substantial overload to this circuit.Main Circuit 1 is substantially overloaded because both receptacles (R2 & R3) are powered by Main Circuit 1. To eliminate the excessivepower draw on Main Circuit 1, the load from receptacle 3 (R3) should be switched to receptacle 1 (R1). Now Main Circuit 1 is powering a16.8A load (less than 20.8A) and Main Circuit 2 is powering a 8.4A load (less than 20.8A).

In addition to the rotor and stator, a conventional generator has an excitation circuit (illustrated below) that consists of slip rings and brushesattached to the engine shaft (not illustrated.) DC flows from the Exciter, through the negative brush and slip ring, to the rotor field poles toestablish the magnetic fields. The return path to the exciter is through the positive brush and slip ring.

Rotor Electromagnet Excitation Circuit

Higher quality portable gas generators, like the Honda EX5500, use an automatic voltage regulator (AVR) as an Exciter. The AVR is anelectronic device that ensures constant voltage output regardless of the load applied to the generator (up to the rated load capacity). TheAVR accomplishes this by sensing the voltage in the stator coils and adjusts the DC excitation current, carried to the rotor electromagnetsvia the slip rings and brushes, to regulate the field pole flux to maintain constant voltage at the AC output receptacles.

ILLUSTRATION COURTESY OF HONDA POWER PRODUCTS

In small portable gas generators the generator end (called the alternator) is direct-coupled to the engine to provide smooth operation.Alternator housings are bolted directly to the engine providing precise rotor and stator alignment.

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Portable Generator Types

What differentiates generators is how they go about regulating the voltage and frequency (Hz) of the AC power they generate throughmagnetic induction. A generator that is intended to power only the universal motors found in power tools and the incandescent lights foundon construction sites requires very little power regulation because their intended loads are very forgiving. Where as, a generator that isintended to power sophisticated electronic equipment that is voltage and frequency sensitive, requires sophisticated and costly power

regulation. Where there is a direct trade-off between cost and power quality, the degree to which a generator regulates its power dependsupon the requirements of the loads it is intended to power.

For example, since it is less expensive to make a relatively simple generator that will satisfactorily operate most construction equipment andRV appliances (but not sophisticated electronics), there is not the cost/benefit return to warrant the incorporation of the more expensivepower regulation controls in generators manufactured for these markets. This explains why there are basically four types of generatorsavailable on the market to this day. Given this variety of generators manufactured for different markets, it is important to understand thebenefits and drawbacks to each when it comes to their use in motion picture production.

Where what differentiates one type of generator from another is the quality of its power it is important to understand the AC powerwaveform. AC Power is depicted using a sine wave.

The sine wave is a way for us to graphically represent how electricity works. The sine wave is measured using an oscilloscope. The verticalaxis represents amplitude (this may be represented in Volts.) The horizontal axis (degrees) represents time and is also known as wavelength.Notice how the voltage sine wave above starts at 0. It then reaches its peak at 90. This is where the voltage is at its positive maximum. Thewave then crosses 0 volts again at 180 (this is called the zero crossover) before peaking again at 270 in the negative and returning to 0volts at 360. This process is called a cycle. The frequency of cycles per minute is measured in Hz (Hertz). The standard in North Americais 60Hz.

Pure Sinusoidal Power Waveform

A pure sinusoidal voltage, like the one represented above, is a conceptual quantity produced by an ideal AC generator built with finelydistributed stator and field windings that operate in a uniform magnetic field. Since in reality neither the winding distribution nor themagnetic field can be uniform in a working AC generator, voltage waveform distortions are created, and the voltage-time relationshipdeviates from our conceptual pure sine function. The smoother the curve of the sine wave, the more stable the power. Any spikes or "blips"in the curve are caused by a fluctuation in the power. These can be bad for both your generator and the equipment being powered.

Here are the representative waveforms, and brief descriptions, of the four types of generators available on the market today. Given theimportance of understanding the benefits and drawbacks to each when it comes to their use in motion picture production we will examineeach type of generators, as well as the typical loads they will power on a set, in more detail latter.

Brushless Generators: Among the most common because of theirinexpensive construction, brushless generators have the least reliablevoltage control. Brushless generators can't react quickly to changingloads, either producing low power (a brownout) or high power.Fluctuations of this nature will cause voltage sensitive equipment likeHMI lights to shut off, or will damage sensitive electronics. With asubstantual voltage waveform distortion of 23%, brushless generatorsdo not interact well with HMI and Kino Flo ballasts. For this reasonbrushless generators are only suitable for powering incandescentlighting.

AVR Generators: AVR generators feature an Automatic VoltageRegulator designed to consistently control voltage. The AVR keeps theoutput voltage more or less constant, regardless of the load. With nolarge fluctuations in voltage resulting from changing loads, AVRgenerators will for the most part operate HMI lights reliably. Witholder magnetic HMI ballasts, AVR generators require frequencygovernors to eliminate flicker on film and scrolling in video. With anappreciable voltage waveform distortion of 19.5%, AVR generators donot interact well with non-power factor corrected HMI and Kino Floballasts.

MSW Inverter Generators: CycloConverter, Modified SineWave, Psuedo Sine Wave are different manufacturers trade namesfor modified square wave inverter generators. These generators useinverters to produce not a sine wave, but a modified square wave that,depending on their cost, more or less resembles a sine wave. Where themodified square wave is generated from switching DC power that isconverted from the AC power the alternator generates, the powerMSW Inverter generators generate is cleaner and more stable thanAVR generators. With a slight voltage waveform distortion, MSWInverter Generators will interact reasonably well with HMI and KinoFlo ballasts. However, a modified square wave will cause sensitiveelectronic equipment (computers, hard drives, video cameras) tooverheat. While, equipment that depends on peak voltage (batterychargers) will not operate as effectively on a modified square wave.For these reasons MSW Inverter Generators are less than ideal for HDdigital cinema productions.

PWM Inverter Generators: PWM Inverter Generators operate likeMSW Inverter Generators, but use a sophisticated pulse widthmodulation (PWM) logic to control a micro processor to switch IGBTsat high speeds to produce a near pure sine wave from the DC powerthat is converted from the AC power of the generator alternator. Witha negligible voltage waveform distortion of 2.5% (less than gridpower), PWM Inverter Generators interact well with HMI and KinoFlo ballasts. These units are ideal for sensitive electronics, such ascomputers, audio, and video recording equipment. PWM InverterGenerators offer a number of other benefits, including less noise,lower weight, and greater fuel efficiency as compared to conventionalAVR Generators.

WAVEFORMS COURTESY OF HONDA POWER EQUIPMENT

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Conventional Portable Generators

A conventional generator rotates two electro-magnets (energized wire coils) inside its stator core. Since the rotation produces differentdirections to the +/- poles of the magnetic field at different points in its circular motion, the voltage generated is sinusoidal (AC), and eachfull engine rotation produces one complete ac sine wave. By design, the engine must spin the generator rotor 3600 RPM to produce an ACfrequency of 60 Hz (60 cycles/second x 60 seconds/minute = 3600RPM). If, because of varying loads, the generator spins faster or slower,the voltage and frequency of the output vary in step. The quality of the electricity a conventional generator puts out then is determined bythe quality of the engine, how smoothly it runs, and how well the engine is capable of maintaining a constant speed.

Brushless Generators

Among the most common because of their inexpensive construction, brushless generators have the least reliable voltage control of allgenerators. The drawback to brushless generators in motion picture lighting applications is that they don't react quickly to changing loads.When a new load (light) is switched on, a brushless generator will alternately produce low voltage (a brownout) and then high voltage (asurge) as the engine slows down under the additional load, and then speeds ups again, before stabilizing under the greater load.

Fluctuations of this nature can result in the following scenario we have all probably experienced at one time or another when trying to runmultiple HMI lights with conventional portable generators. After turning on the first HMI light, you switch on a second light. The striking ofthe HMI arc creates a surge in the power load, this causes momentary engine instability, which results in a dip in output voltage. The dip involtage causes both HMI lamps (the one already running and the one striking) to cut out. When, within seconds, the engine stabilizes again,the power comes back up to full, which causes the HMI light that cut out to hot-restrike (because the ignition switch is still on.) But,because the lamp is hot, the strike doesnt take. The striking voltage returns to the ballast and fries delicate electrical components in theballast. As this nightmare scenario demonstrates, the voltage fluctuation of brushless generators are sufficient to cause voltage sensitiveequipment, like HMI lights to shut off, for this reason brushless generators are really only suitable for powering incandescent lighting andnot much else.

Another problem with brushless generators is that the power they generate exhibits significant voltage waveform distortion (see waveformabove). With an applied voltage waveform distortion of upwards of 23%, brushless generators do not interact well with HMI and Kino Floballasts, causing harmonic currents to be thrown back into the power stream, which results in a further degradation of the voltage waveform(more on that latter.)

Automatic Voltage Regulated (AVR) Generators

To be suitable for filming with all types of HMI ballasts, conventional generators must employ governor systems to maintain constant

voltage (V) and AC Frequency (Hz).

To avoid the nightmare scenario described above when striking multiple small HMIs (less than 1200W), a portable generator must have anAutomatic Voltage Regulator or AVR. An AVR keeps the output voltage more or less constant, regardless of the load. It accomplishes thisby first monitoring the output voltage. It then compares it with the desired set value and corrects any error by suitably changing the fieldexcitation current. By constantly adjusting the excitation to the brushes to increase or decrease the output voltage, the AVR ensures a moreor less consistent flow of power regardless of the load. Under normal circumstances an AVR system can ensure a voltage that is within 3%of the mean voltage. In this fashion, AVR systems eliminate surges and brown-outs that would otherwise occur when switching on and offsmall movie lights (both HMI & Quartz.)

Unfortunately, given the size of portable generators (usually less than 7000W) relative to common motion picture lighting loads (upwards of2000W), even the best AVR systems are still not responsive enough to always handle the changes in load created when switching on largermotion picture lights. Where the load placed upon the generator by a 1200W HMI (which draws anywhere from 13.5-19 Amps depending onthe type of ballast), or a 2000W Quartz light (which draws 16.8 Amps) can account for 30-60 percent of the capacity of the generator, thegenerators AVR system is more often than not simply overwhelmed. For this reason (and others), the general rule of thumb when usingconventional AVR generators is to oversize the generator by a factor of 2 to 1 relative to your total load. It also helps to use more smalllights than just a few large lights.

The second type of governor system a portable generator must have to be suitable for lighting with all HMI ballasts, as well as sophisticatedelectronic production equipment like laptops, hard drives, and HD monitors, is a AC Frequency governor.

Broadly speaking, HMI ballasts now come in two varieties. They are magnetic ballasts and electronic square wave ballasts, also calledflicker free ballasts. For the purpose of this discussion, I will not refer to electronic square wave ballasts as flicker free, because that impliesthat magnetic ballasts generate flicker, which they do not under controlled circumstances. To avoid flicker with magnetic HMI ballastsoperating on conventional generators, the generator speed must be tightly governed. The need for such tight control of the AC frequency hasto do with the fact that HMI lights are inherently arc lights whose output pulsates.

If you were to look at an HMI globe, instead of a coiled tungsten filament glowing, you would find an electrical arc spanning the gapbetween two opposing electrodes. On the most fundamental level, a magnetic HMI ballast is simply a variable transformer choke betweenthe power supply and the lamp electrodes. The transformer provides the start-up charge for the igniter circuit, rapidly increasing the potentialbetween the electrodes of the heads arc gap until an electrical arc jumps the gap and ignites an electrical arc between the lamp electrodes.The transformer then shifts gear and acts as a choke, regulating current to the lamp to maintain the pulsating arc once the light is burning.

As such, the light intensity of a HMI powered by a magnetic ballast follows the waveform of the supply power and increases and decreases120 times a second, twice every AC cycle. This fluctuation in the light output is not visible to the eye but will be captured on film or videoif the frequency (Hz) of the AC power is not precisely synchronized with the film frame rate or video scan rate. If the AC Frequency of thepower were to vary, a frame of film or video scan, would receive more or less exposure depending upon the exact correspondence of thefilm/video exposure interval to the cycling power waveform because the light intensity is pulsating at twice the AC frequency.

ILLUSTRATION COURTESY OF HARRY BOX

The normal sinusoidal 60Hz current of a magnetic ballast (left) creates a fluctuating light output (right)requiring that the camera frame rate be synchronized with the light fluctuations to obtain even exposure frame to frame.

In film production with magnetic HMI ballasts (as opposed to video), to avoid this flicker, you must also use a crystal controlled camera,run the camera at one of a number of safe frame rates (those that can be divided into 120 and result in a whole number), and use power thatis regulated at exactly 60 Hz +/- a quarter cycle (59.75 Hz - 60.25 Hz).

The problem one encounters when operating magnetic HMI ballasts on conventional generators is that by design the AC frequency theygenerate is a function of engine speed and their speed fluctuates. As the generator spins faster or slower, the frequency of the output variesin step. For this reason, when filming with magnetic HMI ballasts, a separate governor is required to ensure that the engine spins its core ata near constant 3600 RPM to produce the desired AC Frequency of 60 Hz (60 cycles/second x 60 seconds/minute = 3600RPM).

A Barber Coleman AC Frequency Governor in a Honda EX5500

An AC Frequency governor accomplishes this by first monitoring the engine speed, it then compares that reference signal with an internalquartz crystal reference, and corrects any error by adjusting the engine throttle through a mechanical linkage (see picture above.) Byconstantly adjusting the engine speed in this fashion the governor ensures a more or less stable 60 Hz AC Frequency. It is worth noting here,for the purpose of our latter discussion regarding the adverse effects of power waveform distortion, how the governor system obtains itsengine speed reference.

Larger generators that are designed to take AC frequency governors, have a magnetic pick up that senses the rotation of the core. However,since the AC frequency governors for portable gas generators are after market modifications, the engine speed reference signal is obtained bymeasuring the frequency of the output voltage inside the AVR unit. By sensing the zero-crossing information from the waveform, the ACfrequency governor can precisely regulate the engine speed and in theory eliminate erratic exposure of film frames or video scans.

In practice, AC governor systems work well in small portable generators only if the generator is well maintained, finely tuned, and carefullyprepped for each shoot. The carburetors of small generator engines are easily gummed up by old fuel making them run rough. For thisreason, it is important to bleed old fuel from the system and replace it if the generator as been sitting idle for an extended period of time. Asecond maintenance issue is that the generator battery must be at full capacity as well as fully charged. The reason for this requirement isthat the battery charging system of the generator was not designed for the additional electrical load of the AC Frequency governor. If thegenerator battery is not at full capacity and fully charged, the AC Frequency governor eventually runs the battery down to the point that itcan no longer regulate the engine because it is underpowered. Unfortunately, more often than not, the generators coming out of rental housesare poorly maintained and inadequately prepped making the AC governor system ultimately unreliable.


The refined square-wave signal of an electronic ballast (left) creates virtually even light output (right)

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

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

In practice, electronic square wave ballasts turned out to be a mixed blessing. Part of the problem with operating electronic HMI ballasts onportable gas generators in the past has to do with the purity of the power waveform they generate. With an applied voltage waveformdistortion of upwards of 19.5%, conventional AVR generators do not interact well with electronic HMI ballasts, causing harmonic currentsto be thrown back into the power stream, which results in a further degradation of the voltage waveform and ultimately to equipment failureor damage (more on that latter.)

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Inverter Generators

A conventional generator, one that runs at 3600 RPM, makes a pretty decent sine wave. This is because it generates power by rotating twolarge coils in a magnetic field, and as discussed above, sine waves are a natural product of rotating machinery. However the power thatconventional generators produce is considered dirty power.

ILLUSTRATION COURTESY OF KIRK KLEINSCHMIDT

Waveform of power output by conventional generator. Note the frequency error and noticeable distortion

Measured on an oscilloscope (pictured above), its sine wave appears jagged. Those small spikes in the sine wave indicate noise that can

cause HMI lights to act erratically and cause problems for sophisticated electronics, like video cameras, monitors, computers, and harddrives that need a clean sine wave to operate. With the increasing use of personal computers and microprocessor-controlled recordingequipment in motion picture production, the demand for clean, reliable power has reached new heights.


Step 1: Rectifier Bridge converts multi-phase AC power to rectified sine wave. Step 2: rectified sine wave is flattened to DC. Step 3: micro processor switching alternates wave polarity creating a modified square wave.

Inverter generators meet this demand for cleaner power by adding an additional component that completely processes the dirty AC powerfrom the generators alternator. An inverter module takes the raw power produced by the alternator and passes it through a microprocessorcontrolled multi-step process to condition it. But, rather than using simple two pole cores, the alternators of inverter generators use multi-pole cores and small stators to produce a raw AC power that is multiphase (more than 300 overlapping sine waves), high frequency (up to20000 Hz), and upwards of 200 Volts. This high voltage AC power is then converted to DC. Finally the DC power is converted back tolow voltage single phase AC power by an inverter. In the process the inverter cleans and stabilizes the power.

Not all inverter generators are equal (Modified square wave verses true sine wave inverters.)

There are 3 major types of inverters used in generators - sine wave, modified square wave, and square wave. One might wonder why thereare so many types of inverters. As John De Armond, explains in his informative article "The Hows and Whys of Inverters and InverterGenerators" the primary reason is cost. To paraphrase John's article, to make a nice sine wave from DC power is expensive. There is atrade-off between cost and waveform purity. An approximation of a sine wave may be created by outputting one or more stepped squarewaves with the amplitudes chosen to approximate a sine (a modified square wave). The more steps, the more like a sine wave the output is.However, each of the voltage steps requires its own voltage supply, its own transistor switch, plus the necessary control circuitry. Thebottom line is that the more steps, the more expensive the inverter. The two go hand in hand.

ILLUSTRATION COURTESY OF JOHN DE ARMOND

Ideal Sine Wave (black), Single Step Square Wave (blue), Three Step Square Wave (red)

Take a look at the figure above. The black trace is, of course our ideal true sine wave. The blue wave is a single step approximation orsquare wave. The red wave is a three step wave or modified square wave. As is intuitive, the three step wave produces a closerapproximation of a sine wave and thus will satisfactorily operate more devices than the single step one. The tradeoff is cost and complexity


Switch sequence of three step output stage of a modified square wave inverter.

The figure above is a line drawing of a typical three step output stage of a modified square wave inverter. The voltages V1 through V3 areincreasingly higher DC voltages converted from the AC power generated by magnetic induction. A microprocessor generates the pseudo sinewave (modified square wave) by sequentially switching S1 through S3 on, S3 through S1 off, S4 through S6 on, S6 through S4 off. Itrepeats this 60 times a second. Where each of the voltage steps requires its own voltage supply, its own transistor switch, plus the necessarycontrol circuitry, one can intuit that the more steps in the modified square wave, the more complicated and thus more expensive the inverteris.

Where it is less expensive to make a modified square wave that will satisfactorily operate most construction equipment and RV appliances,than it is to make a true sine wave there is not the cost/benefit return to warrant the incorporation of the more expensive true sine waveinverters in generators manufactured for these markets. This is why there are still three types of inverter generators available on the marketto this day.

Advantages and Disadvantages:

Square Wave Generators

While a square wave inverter will run simple things like tools with universal motors with no problem, they will not operate much else. Forthis reason, generators with square wave inverters are now found only in the construction trades, where they offer the benefit of beingcheaper, smaller, lighter, and running longer on a gallon of gas than conventional generators. For reasons I will explain below, square waveinverter generators have no application in motion picture production.

Modified Square Wave Generators

Modified Sine Wave, Psuedo Sine Wave, and Cycloconverter are all sales terms used for a modified square wave type of AC power.Modified square wave inverters are low in cost, slightly more efficient than conventional generators, and will satisfactorily operate almost allcommon household appliances and power tools. For this reason, they are typically used in the economy RV/Residential Standby andIndustrial lines of generator manufacturers.

Where the modified square wave is generated from switching DC power that is converted from the AC power the alternator generates, thepower MSW Inverter generators generate is cleaner and more stable than AVR generators. With a slight voltage waveform distortion, MSWInverter Generators will interact reasonably well with HMI and Kino Flo ballasts. However, a modified square wave will cause sensitiveelectronic equipment (computers, hard drives, video cameras) to overheat. While, equipment that depends on peak voltage (battery chargers)will not operate as effectively on a modified square wave. For these reasons MSW Inverter Generators are less than ideal for HD digitalcinema productions. John De Armond, clearly explians why that is the case using one of the more rudimentary inverter generators, thesimple three step modified square wave discussed above, as an example in his article "The Hows and Whys of Inverters and InverterGenerators".


Output waveform of a Honda EX350 square wave inverter generator

The photo above is an oscilloscope shot of the actual output of an older Honda EX350 modified square wave inverter generator. Notice theRMS voltage indication on the right side - 120 volts even though the peak voltage is only 142 volts. For a true sine wave, the peak voltagewould be 120 * 1.414 = 169 volts. This difference in peak voltage is what makes or breaks the operation of modified square wave invertergenerators in motion picture production applications where they work fine on construction sites.


Voltage and the current output waveforms of a Honda EX350 square wave inverter generator powering 300W incandescent light

The photo above shows a scope shot of both the voltage and the current output of this generator driving a 300 watt incandescent light (aresistive load.) As you see, a modified square wave works well for a resistive load like an incandescent light. Things get a whole lot moreinteresting when one connects a fluorescent lamp to the generator. As you can see in photo below the solid-state ballast of the fluorescentlamp slightly distorts the voltage waveform (creates a spike) and creates all kinds of current oscillation. This kind of harmonic activity cancause a noticeable audio buzz, equipment to malfunction, or shut off (more on harmonic noise latter.)


Voltage and the current output waveforms of a Honda EX350 square wave inverter generator powering fluorescent light

Another common problem with modified square wave generators like the Honda EX350 is encountered when they are used to chargebatteries on remote sets without grid power. John De Armond illustrates the problem in his informative article "The Hows and Whys ofInverters and Inverter Generators" by first examining how the battery charger works on grid power when plugged into a conventional outlet.

To paraphrase him a battery charger typically consists of a transformer, a rectifier and support electronics like charge control circuitry. Oneach half-cycle of the 60 hz line voltage, the voltage first increases and then decreases in the shape of a sine. The transformer secondary ofthe battery charger follows this voltage. Connected to the secondary is the rectifier that converts the AC to DC for battery charging. Onlywhen the instantaneous AC voltage exceeds the battery voltage plus the 0.7 voltage drop of the rectifier does current flow to charge thebatteries. Photo 5 illustrates this effect. The two lines at 1 and 2 mark on the voltage sine wave where the rectifier starts conducting andcausing current to flow.


Problems arise when a charger of this type is connected to a modified square wave inverter. Recall from the first photo above that the peakvoltage of a modified square wave does not rise as high as a sine wave (142 volts verses the 169 volts of a true sine wave.) The horizontalline in the photo above shows about where the square wave would reach. In this particular case, the square wave would never reach a

voltage sufficient to make the rectifier conduct and so the battery would never charge even though power is connected, the LED indicatorslight up, and a true RMS voltmeter would indicate about 120 volts. This is another fundamental problem with modified square waveinverters in production applications.

Audio/video production equipment, computers, and battery chargers require a nearly pure (low distortion) sine wave input. If these devicesare to be run from an inverter generator, then the generators inverter module must supply a sine wave or something pretty close to it. Asdiscussed, inverters of this sophistication are appreciatively more expensive - from 2 to 3 times - because of the number of and prohibitivecost of high power electronic switch devices and components required. However, recent rapid developments in the field of IGBT (insulatedgate bipolar transistors) electronics and miniaturization/mass production of microprocessor based digital control systems have reached thestage that Pulse Width Modulation (PWM) inverter modules are economically viable and affordable. Still not as cheap as modified sinewave inverter modules, generator manufacturers only put Pulse Width Modulation (PWM) inverter modules in their deluxe or Super Quietproduct lines. For instance, the Honda super quiet EU series of generators employ Pulse Width Modulation (PWM) inverter modules with awaveform distortion factor of less than 2.5% - which is considerably better than conventional generators and quite often better than what youget out of the wall outlet.

True Sine Wave Generators

Pulse width modulation (PWM) inverters provide a more sinusoidal current and for that reason are commonly called true sine waveinverters. Pulse Width Modulation (PWM) inverters use micro-processor control modules to produce AC power with a "true" sine wave(with full width and amplitude) from high voltage DC power converted from the AC power generated by magnetic induction in the core ofthe generator. PWM inverters are more efficient and typically provide higher levels of performance.


Waveform of power output of PWM inverter generator. Note there no discernable distortion or frequency error.

The "true" sine wave these generators deliver is more suitable for computers, solid-state equipment with built-in computer functions ormicrocomputer-controlled functions. Unlike the simple two-pole alternators of AVR generators, an inverter generator uses a core thatconsists of multiple stator coils and multiple rotor magnets. Each full rotation of the engine produces more than 300 three phase ac sinewaves at frequencies up to 20 kHz, which is considerably more electrical energy per engine revolution than produced in conventional twopole AVR generators.

PHOTO COURTESY OF SUBARU/ROBIN POWER PRODUCTS

Core parts from PWM Inverter Generator. Note the multiple windings of the core stator.

The power generated by the multi-pole core next goes to the inverter module. A basic PWM inverter consists of a converter, DC link,

control logic, and an inverter.

Basic wiring schematic of PWM Inverter

The converter section consists of a fixed diode bridge rectifier which converts the more than 300 three phase ac sine waves at frequencies upto 20 kHz to a DC voltage (about 200 V in at least one unit).

Converter and DC Link

AC Output is then generated from the high voltage DC by the inverter section with voltage and frequency set by a PWM control logic. Ahighspeed microprocessor switches IGBTs (insulated gate bipolar transistors) on and off several thousand times a second according to thePWM control logic to create a variable voltage and frequency.

Control logic and Inverter Section

PWM inverter control logic goes something like this: to generate the positive half cycle of a true AC sine wave, an IGBT connected to thepositive value of the DC voltage from the converter is switched on and off by a micro-processor at variable rates and for variable intervals tocreate current to flow of a variable voltage.

ILLUSTRATION COURTESY OF SIEMENS CORP.

PWM Voltage and Current

In other words, the IGBT is switched on for a short period of time, allowing only a small amount of current to build up and then is switchedoff. The IGBT is switched on and left on for progressively longer periods of time, allowing current to build up to higher levels until thecurrent reaches a peak. The IGBT is then switched on for progressively shorter periods of time, decreasing current. The negative half of theAC sine wave is generated by switching an IGBT connected to the negative value of the converted DC voltage. The fixed DC voltage (200VDC) is modulated or clipped in this fashion to provide a variable voltage and frequency. Where IGBTs can turn on in less than 400nanoseconds and off in approximately 500 nanoseconds, they are ideal for the high switching speed necessary to create a true sine wave inthis fashion. The fixed DC voltage (200 VDC) is modulated or clipped in this fashion to provide a variable voltage and frequency.


The three phases of the inverter generator process: high frequency AC converted to DC; DC inverted to stable clean 120V, 60 Hz AC.

To summarize this complex process: the generator's multi-pole core produces high voltage multiphase AC power. The AC power is thenconverted to DC. Finally the DC power is converted back to AC by an inverter. Since the inverter completely processes the raw powergenerated by the alternator, the voltage and frequency of the power it generates is no longer linked to engine speed (RPM) as is the casewith conventional AVR generators. Rather, using microprocessor controlled IGBTs the inverter module switches the high voltage DCaccording to PWM control logic to provide AC power with a voltage stability within 1%, and frequency stability within 0.01 HZ. Theend result is a nearly pure sine wave with a wave distortion of only 2.5%; which, is as clean or cleaner than commercial power.

As discussed above, developments in this direction began a long time ago, but a techno-economical solution could not be found tomanufacture true sine wave inverters until recently because of the prohibitive cost of high power electronic devices and components.However, recent rapid developments in the field of IGBT electronics and miniaturization/mass production of microprocessor based digitalcontrol systems have reached the stage that Pulse Width Modulation (PWM) inverter modules are economically viable and affordable.

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Lighting Load Types

All loads are not created equal

All lighting loads are not the same. Incandescent, Fluorescent, LED, and HMI lights fall into two broad categories. Those that are linearloads and those that are non-linear loads. Non-linear loads further break down into two categories: those that exhibit high inductive reactance(magnetic HMI ballasts) and those that exhibit high capacitive reactance (electronic HM, Fluorescent, & LED ballasts). Because each type ofload has an effect (mostly adverse) on the power supply, their individual characteristics are worth exploring in more detail. Even more so,because they adversely affect generated power more than they do grid power.

Linear Loads

Incandescent Lights (Purely Resistive Loads)

An incandescent light is a simple resistive load. The high resistance of its tungsten filament creates heat until the filament glows - creatinglight. The current in such a simple resistive AC circuit increases proportionately as the voltage increases and decreases proportionately as thevoltage decreases. Changes in alternating current (AC) and the relationship between voltage and current in a purely resistive circuit(Incandescent Lights) can be represented graphically by the sine waves below.

Unity Power Factor: Voltage & Current are in Phase.

For a sinusoidal voltage, the current is also sinusoidal. For a purely resistive load like incandescent lights, the current is always proportionalto the voltage. The voltage and current are in phase and so have a Power Factor of 1 or unity power factor (power factor will be explained indetail below.)

Non-Linear Loads

HMI Lights with Magnetic Ballasts

The make up of a magnetic HMI ballast is relatively simple by comparison to the newer electronic HMI ballasts. Between the power inputand the lamp is a transformer that acts as a choke coil. The transformer provides the start-up charge for the igniter circuit, rapidly increasingthe potential between the electrodes of the heads arc gap until an electrical arc jumps the gap and ignites an electrical arc between the lampelectrodes. The transformer then acts as a choke, regulating current to the lamp to maintain the pulsating arc once the light is burning. Assuch, the light intensity of an HMI follows the power waveform and increases and decreases 120 times a second, twice every AC cycle. Thisfluctuation is not visible to the eye but will be captured on film or video as a steady pulsation if the camera is not in precise synchronizationwith the AC power frequency. With magnetic HMI ballasts, to avoid this flicker, you must use a crystal controlled camera, run the cameraat one of a number of safe frame rates (those that can be divided into 120 and result in a whole number), and use power that is regulated atexactly 60 Hertz (cycles per second.)

Transformers of a 12k Magnetic HMI Balllast

Essentially a large coil of wire that is tapped at several places to provide for various input voltages and a high start-up voltage, thetransformers of magnetic HMI ballasts exhibit high self-inductance. Self-inductance is a particular form of electromagnetic induction thatinhibits the flow of current in the windings of the ballast transformer, pulls the voltage out of phase with the current, and reduces the powerefficiency (power factor) of the ballast. Because the high self-inductance inherent in magnetic HMI ballasts adversely effects the powergenerated by small portable generators, it is a topic worth exploring in more detail.

Self-Inductance

Self-inductance is defined as the induction of a voltage in a current-carrying wire within a coil when the current in the wire itself is changing

as it alternates. Taking a close look at a simple circuit with a coil will help us to understand how voltage is induced by changing current.The alternating current running through a coil creates a magnetic field in and around the coil that is increasing and decreasing as the currentalternates. The magnetic field forms concentric loops that surround the wire and join to form larger loops that surround the coil as shown inthe image below. When the current increases in one loop the expanding magnetic field will cut across some or all of the neighboring loopsof wire, inducing a voltage in these loops. This voltage causes a current to flow in the windings of the coil.

ILLUSTRATION COURTESY OF THE NDT RESOURCE CENTER

Magnetic fields created in and around a coil with alternating current running through it.

By studying this image of a coil, it can be seen that the number of turns in the coil will have an effect on the amount of voltage that isinduced into our simple circuit. Increasing the number of turns or the rate of change of magnetic flux thereby increases the amount of currentinduced. The current induced by this voltage has a direction such that its magnetic field opposes the change in magnetic field that inducedthe current. Or, in other words, the current induced in a conductor will oppose the change in current that is causing the flux to change.

Inductive Reactance

By taking an even closer look at a coil of wire it can be seen how induction reduces the flow of current in our simple circuit. In the imagebelow, the direction of the primary current is shown in red, and the magnetic field generated by the current is shown in blue. It can be seenthat the magnetic field from one loop of the wire will cut across the other loops in the coil and this will induce current flow (shown ingreen) in the circuit.

ILLUSTRATION COURTESY OF THE NDT RESOURCE CENTER

Induced current works against the primary current in a coil.

Note that the induced current flows in the opposite direction of the primary current and accomplishes no actual work other than to createenergy circulating back and forth between the coil and the power source. The induced current working against the primary current results ina reduction of current flow in our simple circuit. This opposition to the flow of current is called inductive reactance.

Since inductive reactance reduces the flow of current in a circuit, it appears as an energy loss just like resistance. However, it is possible todistinguish between resistance and inductive reactance in a circuit by looking at the timing between the sine waves of the voltage and currentof the alternating current. As we saw above, in AC circuits with resistive loads, the voltage and the current are in-phase, meaning that thepeaks and valleys of their sine waves occur at the same time. When there is inductive reactance present in the circuit, the phase of thecurrent will be shifted so that its peaks and valleys do not occur at the same time as those of the voltage. As illustrated below, inductivereactance causes current to lag behind the voltage. The degree to which the two waveforms are put out of phase depends on the relativeamount of resistance and inductance offered by the coil.

Poor Power Factor: Voltage & Current are in out of phase.

As we saw in our simple circuit above, the number of turns in the coil will have an effect on the amount of voltage that is induced into thecircuit. Increasing the number of turns increases the amount of induced voltage. In the case of a magnetic HMI ballast, the multiple finewindings of the ballast transformer induces appreciable voltage and considerable current that is in opposition to the primary current, causingthe primary current to lag behind voltage, a reduction of current flow, and an inefficiency in the use of power supplied by the generator. Putsimply, the ballast draws more power than it uses to create light. Capacitors are typically included in the design of magnetic HMI ballasts tocompensate for the high inductance of the transformer and to bring the current back in phase with the voltage.

Apparent Power Verses True Power = Power Factor

If, in this situation, you were to measure the current (using a Amp Meter) and voltage (using a Volt Meter) traveling through the cablesupplying the magnetic HMI ballast and multiply them according to Ohms Law (W=VA) you would get the apparent power of theballast. But, if you were to instead, use a wattmeter to measure the actual amount of energy being converted into real work (light) by theballast, after the applied voltage overcomes the induced voltage, you would get the true power of the ballast. The ratio of true power toapparent power is called the power factor of the ballast.

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 thatprevents 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 apparentpower (beer plus foam), even though only the beer (true power) counts as far as how much actual drinking is possible. Where a typical1200W 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).

Capacitive Reactance

Electronic HMI, Fluorescent, & LED ballasts belong to a category of power supplies, called Switch Mode Power Supplies (SMPSs), thatexhibit another type of opposition to the flow of current that is called Capacitive Reactance. SMPSs utilize electronic components that useonly portions of the AC power waveform. These devices then return the unused portions as harmonic currents that stack on top of oneanother, pull the voltage and current out of phase, and under the wrong conditions create distortion of the voltage waveform.

As illustrated in the wiring schematic above, all SMPSs consist of first a diode-capacitor section (consisting of a Bridge Rectifier andSmoothing Capacitor) that converts the AC input power to DC power; and then, in the case of HMI & Fluorescent lights, a Switch-modeConverter section that converts the DC power back to an alternating power waveform that ignites the lamp. In the case of High Output ACLED ballasts, the Switch Mode Converter further conditions the DC power the diode-capacitor section outputs. How HMI and Fluorescentballasts differ as SMPSs is by the shape and frequency of the alternating power waveform the Switch-mode converter generates. In the caseof electronic HMI ballasts the Switch-mode converter generates a low frequency (60Hz) square wave. In the case of electronic Fluorescentballasts, the Switch-mode converter generates a high frequency (>20kHz) sine wave. Regardless of what circuits are in the green box in theillustration above, all SMPSs utilize a diode-capacitor section to first convert the AC line input power to DC power. The diode-capacitor

section of a SMPS is the source of the capacitive reactance that opposes the flow of current and contributes to its poor power factor.


The capacitive reactance of SMPSs act on power in a way opposite to inductive reactance. It causes current to lead voltage. SMPSs typicallyhave a power factor less than .6, meaning that the ballast (whether HMI, Fluorescent, or LED) has to draw 40% - 50% more power than ituses. Where capacitive reactance leads to an inefficient use of power (lots of foam, not much beer), and the harmonic currents generated canhave adverse effects on other equipment operating on the same power, it is worth exploring the cause of capacitive reactance and the sourceof the harmonic currents in more detail. To understand the cause of the capacitive reactance of SMPSs, and its effect on the power supply,lets look first at the operation of fluorescent ballasts in more detail.

Fluorescent Ballasts (Electronic vs. Electromagnetic)

The ballast of a fluorescent light functions very much like an HMI ballast. It provides the lamp with high voltage during start-up to ignite anarc between the lamp electrodes, and then stabilizes the arc by limiting the electrical current to the lamp. As in the case of HMI lights, thereare two basic types of fluorescent ballasts: magnetic and electronic.

A magnetic fluorescent ballast works very much like a magnetic HMI ballast. It uses a magnetic transformer of copper windings around asteel core to convert the input line voltage and current to the voltage and current required to start and operate the fluorescent lamp. Likemagnetic HMI ballasts, they exhibit high inductive reactance and have a poor power factor. The power factor of magnetic ballasts is usuallyless than .5 and they typically account for 18% to 35% of total harmonic distortion in the power supply of offices where they are commonlyused. Like magnetic HMI ballasts, the output frequency of a magnetic fluorescent ballast is the same as the input AC line frequency (60 Hz),which means that (as was the case with an HMI magnetic ballast) the camera frame rate must be synchronized with the AC frequency of thepower supply in order to avoid the appearance of light intensity fluctuation in the image. For this reason fluorescent lights were seldom usedin motion picture production until the advent of high frequency electronic ballasts for fluorescent lamps.

Fluorescent Lights with Electronic Ballasts

Electronic fluorescent ballasts are a Switch-mode Power Supply (SMPS) designed to perform all the same functions as a magnetic ballastbut at a higher frequency. They first rectify the 60 Hz AC input to DC and then produce a very high frequency alternating current (20,000 -50,000 Hz depending on the fixture) using an inverter and power conditioning components.

Kino Flo 4 Bank Select Ballast

The high frequencies at which electronic fluorescent ballasts operate make them a suitable light source for film and television production. Byconverting the 60 Hz input frequency to between 20,000 - 50,000 Hz, electronic ballasts eliminate the problem of light intensity fluctuationassociated with standard magnetic ballasts. At those frequencies the period of time between the off and on pulse of each cycle is so short thatthe illuminating phosphors do not decay in light output.

Assorted High Frequency Fluorescent Lights Designed for Motion Picture Lighting.

Like the glowing tungsten coil of an incandescent lamp, the fluorescent phosphors become essentially flicker free. Electronic fluorescentballasts also weigh less and dont have the characteristic hum of magnetic ballasts. These characteristics of high frequency electronic ballastsmake them well suited for motion picture lighting. Developed first by Kino Flo (above), and now available from a number of manufacturers,motion picture fluorescent lights now come in a wide assortment of shapes and sizes.

Assorted CFL Fluorescent Lights Designed for Motion Picture Lighting.

Regardless of its shape or size, the ballasts of all high frequency fluorescent lights utilize a Diode-Capacitor circuit to first convert the ACline input to DC. Since it is the Diode-Capacitor circuit of an electronic ballast that generates a high level of capacitive reactance, whichleads to an inefficient use of power and the generation of harmonic currents, let us examine how they work in one type of fluorescent lightin more detail the self ballasted Compact Fluorescent Lamp (CFL) pictured below.

CFL Fluorescent Light being tested.

Since the Diode-Capacitor circuit of a self ballasted CFL is similar in design to those in most all fluorescent movie lights (Kino Flo, Lowel,etc.), a close examination of the power factor of CFLs will help us to understand the cause of the capacitive reactance in SMPSs in general,as well as its effect on the power supply.

circuit schematic of an Incandescent bulb.

To understand the power factor of a self ballasted CFL bulb it is helpful to compare it to an incandescent bulb. If you will recall from thebeginning of this section, an incandescent light is a simple resistive load (see circuit schematic above.) The high resistance of its tungstenfilament creates heat until the filament glows - creating light. As we see in the oscilloscope shot below, of a 25W incandescent bulboperating on grid power, the current is always proportional to the voltage (current is represented on the scope as the voltage drop on a 1Ohm resistor.)

Current and Voltage Waveform of a ACEC 25W Incandescent bulb.

If the applied voltage is sinusoidal, the current generated is also sinusoidal. That is, the current increases proportionately as the voltageincreases and decreases proportionately as the voltage decreases. Since the peak of the voltage corresponds to the peak in current, the voltageand current are also in phase and so have a unity power factor (Power Factor of 1.)

The voltage and current waveforms, below, of a CFL bulb operating on grid power is very different from that of the incandescent lightabove. The most noticeable difference is that the current, generated by the CFL bulb, no longer proportionately follows the nice smoothsinusoidal voltage waveform supplied to it by the power grid. Rather, it has been distorted by electrical components in the ballast so that itinstead consists of sharp spikes in power that quickly drop off over a short duration. A second distinguishing characteristic is that the peakof the voltage no longer corresponds to the peak in current. The current now leads the voltage by 1.7 milli seconds. The voltage andcurrent are no longer in phase, but instead exhibit what we call a Leading Power Factor.

Current and Voltage Waveform of a Brelight 25W CFL Bulb.

Like all electronic fluorescent ballasts, the ballasts of CFLs are a Switch-mode Power Supply that converts line-frequency power (60Hz) to ahigh frequency alternating current. In the case of self-ballasted CFL bulbs, what is in the green Switch Mode Converter box of the SMPSillustration above, are a pair of MOSFETS (metaloxidesemiconductor field-effect transistors) that act as a high frequency DC to ACinverter. For the purpose of this discussion, what's in the green Switch Mode box, or what the power supply ultimately does with the DCpower put out by the diode-capacitor circuit is not important. What's important is that like all SMPSs, CFL ballasts consist of first a diode-capacitor section that converts the AC input power to DC power. Since, the capacitive reactance of all SMPSs is caused by this diode-capacitor circuit, how it operates in self-ballasted CFL bulbs and the affect it has on power quality is representative of SMPSs in general(fluorescent, HMI, & AC LED.)

Typical schematic of CFL electronic ballast: L-to-R consists of half-bridge rectifier, conditioning capacitor, DC/AC Inverter.

The distorted current waveform and Leading Power Factor exhibited by CFLs is caused by the Diode-Capacitor circuit of its electronicballast. To quickly summarize the cause of this current distortion, the Diode-Capacitor circuit uses only the ascending portion of the supplyvoltage waveform - which pulls the current out of phase with the voltage. As seen in this scope shot, it also draws current in quick bursts,and returns the unused portions as harmonic currents that stack on top of one another creating harmonic distortion of the power waveform.These harmonic currents, combined with the Leading Power Factor, creates the capacitive reactance that opposes the flow of current in thecircuit that leads to an inefficient use of power by the ballast. Since, the harmonic currents generated can have an adverse effect on otherequipment operating on the same power, it is worth exploring the cause of this capacitive reactance and the source of the harmonic currentsin more detail.

Step 1: Rectifier Bridge converts line frequency AC power to rectified sine wave. Step 2: rectified sine wave is flattened to DC by conditioning Capacitor.Step 3 (not shown): Inverter alternates wave polarity creating a high frequency alternating power to excite lamp gases.

As illustrated above, the diode-capacitor section converts the AC power to DC power by first feeding the AC input current through a bridgerectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into a conditioningcapacitor that removes the 60 Hz rise and fall and flattens out the voltage - making it essentially DC. The DC is then fed from theconditioning capacitor to the Switch-mode converter which in the case of a fluorescent ballast is a high frequency inverter that utilizes a pair

of MOSFETs to generate the high frequency (20-50kHZ) AC power.

Yellow Trace: Rectifier Bridge converts AC power to rectified sine wave. Blue Trace: Stored Capacitor Voltage. Red Trace: Current drawn by capacitorsonce input voltage is greater than voltage stored in the capacitor (Blue trace.)

As shown in the illustration above, the diode-capacitor circuit only draws current during the peaks of the supply voltage waveform as itcharges the conditioning capacitor to the peak of the line voltage. Since the conditioning capacitor can only charge when input voltage isgreater than its stored voltage, the capacitor charges for only a very brief period of the overall cycle time. That is because, after peaking, thehalf cycle from the bridge drops below the capacitor voltage; which back biases the bridge, inhibiting further current flow into the capacitor.Since, during this very brief charging period, the capacitor must charge fully, large pulses of current are drawn for short durations.Consequently, electronic fluorescent ballasts (and SMPSs in general), draw current in high amplitude short pulses. The remaining unusedcurrent feeds back into the power stream as harmonic currents.

Given this method of operation, the diode-capacitor circuits of CFLs (and SMPS in general) create two artifacts that can effect power qualityadversely. First, since the conditioning capacitor starts to charge when input voltage is greater than its stored voltage, and stops after theinput voltage peaks, it pulls current out of phase with voltage. As we can see in the oscilloscope shot above, it causes current to leadvoltage or creates a "Leading Power Factor." Second, the unused portions of the voltage waveform that return into the power stream asharmonic currents can have a severe effect on power quality under certain conditions. Where, it is the combination of a Leading PowerFactor and harmonic currents generated by diode-capacitor circuits that constitute the capacitive reactance of SMPS that opposes the flow ofcurrent it is worth exploring the effect of both in CFLs in more detail.

Components of a CFL ballast

These simple diode-capacitor circuits are used in CFL bulbs and in many fluorescent movie lights because they are compact andinexpensive. However, they have a number of drawbacks. For instance, notice how large the input current spike (red trace above) of thediode-capacitor circuit is. Without power factor correction, the in-put bridge rectifier requires a large conditioning capacitor at its output.This capacitor results in line current pulses (as seen in our oscilloscope shot above) that are very high in amplitude. All the circuitry in theballast as well as the supply chain (the generator, distribution wiring, circuit breakers, etc) must be sized to carrying this high peak current(the foam in our analogy).

For a rather amusing demonstration of the greater current drawn by SMPSs for the same wattage of light check out this You-Tube videoCompact Fluorescent verses The Generator." In this video, lighting designer Kevan Shaw, first operates a 575W ETC Source Four Lekowith Quartz Halogen bulb on an 850W two stroke gas generator without problem. However, when he tries to operate an equivalent wattageof CFLs (30 x 18W bulbs = 540W) the generator goes berserk. Kevan then turns off the 18W CFL bulbs one at a time until the generatorstabilizes. Only after turning off half the CFL Bulbs does the generator operate normally with a remaining load of 15 - 18W CFLs (270 W.)What accounts for the erratic behavior of the generator in this video under the smaller load of CFLs? It is a combination of the poor PowerFactor of the CFL bulbs and the harmonic currents they generate. Even though the 15 CFL bulbs have a True Power of 270W, the Wattindicator on Kevan's generator indicates that they draw twice that in Apparent Power (535W), or have a Power Factor of .5 (270W/535W=.504.)

Another drawback to the diode-capacitor circuits used in SMPSs is that when they draw current it is for only a fraction of the half cycle ofthe voltage waveform. If we return to the illustration above, we see that the pulses of current are narrow, with fast rise and fall times. Since adiode-capacitor circuit uses only the very peak of the voltage waveform, they generate high harmonic content as the unused portions of thevoltage waveform are returned as harmonic currents (see graph below.)

Distribution of Harmonic Currents generated by CFL bulb

The fast rise time of these current pulses can cause Radio Frequency Interference (RFI) problems. For this reason, Lowel Light warns ontheir website that their compact fluorescent (CFL) fixture, the Lowel Ego, that: The lamps may cause interference with radios, cordlessphones, televisions, and remote controls. If interference occurs, move this product away from the device or move to a different outlet(http://www.lowel.com/ego/lamp_info.html.)

Harmonic currents can also stack on top of one another creating excessive current on the distribution system neutral (see below.) And, sincethe neutral conductor of a distribution system is not fused, it can cause the neutral to overheat and possibly catch fire.

In one study, substituting incandescent lamps with the equivalent wattage of CFLs in a small single phase distribution system substantially increased the current on the system neutral as a result of the 3rd harmonics generated by the CFL Bulbs.

For this reason, on their website Kino Flo cautions users of their older style fixtures, that the ballasts will draw double the current on theneutral from what is being drawn on the two hot legs. On large installations it may be necessary to double your neutral run so as not toexceed your cable capacity. (http://www.kinoflo.com/FYI/FAQs.htm#2"]FAQ Why is the neutral drawing more than the hot leg.)

Finally, when the power is supplied by a conventional AVR generator, these harmonic currents can also lead to severe distortion of thevoltage waveform in the power distribution system. When you plug an electronic ballast (fluorescent, HMI, or LED) into a wall outlet youneed not be concerned about current harmonic distortion producing voltage distortion. The impedance of the electrical path from the powerplant to the outlet is so low, the distortion of the original applied power waveform so small (less than 3%), and the power plant generatingcapacity so large by comparison to the load, that harmonic currents fed back to it will not effect the voltage at the load bus (electricaloutlet.) However, it is an all together different situation when plugging an electronic ballast (fluorescent, HMI, or LED) into a portablegenerator. In this case, the impedance of the power generating system (generator and distribution cable) is sufficient enough that a harmoniccurrent will induce a voltage at the same frequency. For example, a 5th harmonic current will produce a 5th harmonic voltage, a 7thharmonic current will produce a 7th harmonic voltage, etc. Since, as we saw above, a distorted current waveform is made up of thefundamental plus one or more harmonic currents, each of these currents flowing through an impedance will, result in voltage harmonicsappearing at the load bus, a voltage drop, and distortion of the voltage waveform.

Since electronic ballasts consume current only at the peak of the voltage waveform (to charge the smoothing capacitor), voltage drop due tosystem impedance occurs only at the peak of the voltage waveform. In this fashion, the pulsed current consumed by electronic ballastsproduces voltage distortion in the form of flat-topping of the voltage waveform.

The pulsed current consumed by electronic ballasts produce voltage distortion in the form of flat-topping

The measurement of this distortion is designated as the Total Harmonic Distortion (THD) of the distribution system. While self ballastedCFLs generate the most severe harmonic noise, all fluorescent ballasts (both magnetic & electronic) generate harmonic noise (see tablebelow.)

The severe voltage waveform distortion exhibited above can cause overheating and failing equipment, efficiency losses, circuit breaker trips,and instability of the generator's voltage and frequency. In addition to creating the radio frequency interference (RFI) mentioned on theLowel Light website, harmonic distortion of this magnitude can also cause component level damage to HD digital cinema productionequipment and create ground loops. We will explore how harmonic distortion of the power waveform adversely effects equipment operatingon it in more detail in subsequent sections, but first lets continue our survey of lighting loads with electronic HMI ballasts.

HMI Lights with Electronic Ballasts

Like the development of electronic fluorescent ballasts, the development of electronic HMI ballasts was a major advance in lightingtechnology because they eliminate the flicker problem associated with magnetic ballasts, as well as the need for expensive frequencygovernors in small generators. They allow you to film at any frame rate and even at changing frame rates. An electronic HMI ballasteliminates flicker by creating a virtually constant output of light over the AC cycle by squaring off the curves of the AC sine wave. Thechangeover period is so brief that the light is virtually continuous.

By comparison to magnetic HMI ballasts, electronic HMI ballasts are quite a bit more complicated. As another example of a Switch-modePower Supply (SMPS), they, in fact, operate in a very similar fashion to electronic fluorescent ballasts. Like a fluorescent ballast, AC poweris first converted into DC. Then, a high-speed switching device (micro processor controlled IGBTs) turns the DC current into alternatingcurrent. The difference between an electronic HMI ballast and an electronic fluorescent ballast is that the HMI ballast generates a squarewave where the electronic fluorescent ballast generates a high frequency sine wave.

Since an electronic ballast completely processes and regulates the input power they can tolerate fairly wide voltage and Hertz ratediscrepancies. A 120V electronic ballast can take an input from 95V to 132V with out effecting the output signal and the fixture's colortemperature, and it will not be affected by the fluctuations in frequency (Hz) of conventional AVR generators without governors.

Where they are not frequency dependent and will tolerate voltage fluctuations, at first it was thought that electronic square wave ballastswould operate more reliably on small portable generators even those without frequency governors. For this reason, as soon as electronicsquare wave ballasts appeared on the market, many lighting rental houses replaced the more expensive crystal governed portable generatorswith less expensive non-synchronous portable generators. The theory was that an electronic square wave ballast would operate reliably on anon-synchronous generator and allow filming at any frame rate, where as a magnetic HMI ballast operating on a crystal controlledsynchronous generator allowed filming only at permitted frame rates. In practice, electronic square wave ballasts turned out to be a mixedblessing.

Like all SMPSs, electronic HMI ballasts without power factor correction draw current in large pulses and return harmonic currents to thepower stream. The capacitive reactance of electronic HMI ballasts also causes current to lead voltage and so they also have a leading powerfactor. An electronic square wave HMI ballast typically has a power factor less than .6, meaning the ballast has to draw 40 percent or morepower than it uses. For example a 1200W non-power factor corrected electronic HMI ballast takes 18.5 Amps at 120 Volts to generate 1200

Watts of light and has a power factor of .54 (18.5A x 120V= 2220W, 1200W/2220W= .54).

Above is the nameplate from an Arri 575/1200 Electronic Ballast with DMX Control. You can see that it is marked that it will draw 18A ofcurrent ("I") at 125 Volts ("U"). You will also notice that it states that the ballast has a cos@=.6 which mean that the Power Factor is .6. Itis important to understand that this greater Apparent Power consists not only of the high amplitude short pulses of current drawn by theballast. Like a CFL, a non-PFC electronic HMI ballast also returns the unused portion of the voltage waveform into the distribution systemas harmonic currents. That is, when a wattmeter measures the actual amount of energy being converted into real work (light) by the ballast(the True Power of the ballast), it is not measuring the power that goes into the generation of harmonic currents. Before exploring in moredetail how the Leading Power Factor and harmonics generated by electronic HMI ballasts can adversely effect equipment operating on it, Idlike to conclude our survey of lighting loads by saying a few words about "High Output" AC LEDs.

High Output AC LEDs

An LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flowseasily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. As illustrated below, when the opposingelectrodes of the p-n junction have different potentials, electrons fall into the lower energy level, releasing energy in the form of a photons orlight. LEDs, by nature, require direct current (DC) with low voltage, as opposed to the mains electricity from the electrical grid that suppliesa high voltage with an alternating current (AC).

LED lights used in motion picture lighting applications fall into a category of LED technology called AC LED lighting. The term AC LEDlighting refers to illumination generated by High Power LED (HPLED) light engines supplied with a sinusoidal AC voltage sourcetypically the utility line voltage (e.g., 120 V in the U.S., 100 V in Japan, 220 V in Europe). AC LEDs present many advantages overincandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, andgreater durability and reliability. For these reasons, but principally because of its high luminous efficacy, AC LED lighting has tremendouspotential to become the dominant type of lighting in motion picture production. However, they are relatively expensive and require moreprecise current and heat management than traditional motion picture light sources.

One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit power input. White LEDsquickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs availablewith a luminous efficacy of 1822 lumens per watt [lm/W]. For comparison, a conventional 60100 W incandescent light bulb producesaround 15 lm/W, and standard fluorescent lights produce up to 100 lm/W. In September 2003, Cree, Inc. introduced a white LED lightgiving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient asstandard incandescent lights. In 2006 Cree, Inc. demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20mA, which is even better than standard fluorescent lights. However, these efficiencies are for the LED chip only, held at low temperature ina lab. In a lighting application, operating at higher temperature and with drive circuit losses, efficiencies are much lower. United StatesDepartment of Energy (DOE) testing of commercial LED lamps showed that average efficacy was still about 46 lm/W in 2009 (testedperformance ranged from 17 lm/W to 79 lm/W).

Cree's high-power LED XLamp 7090 XR-E Q4

As of September 2009 some High Output LEDs manufactured by Cree Inc. now exceed 105 lm/W (e.g. the XLamp XP-G LED chip picturedabove) at room temperature; and Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lumensper watt at room temperature (the correlated color temperature was reported to be 4579 K.) Without a doubt AC LEDs have become themost efficient light source available. But, before the full potential of AC LED lighting can be realized for motion picture lightingapplications, the AC LED manufacturers must overcome some key barriers: color rendering, cost, power quality and versatility.

The Color Rendering/Cost Trade-Off

At this point in time, manufacturers of LED Lights for motion picture applications seem to trade off color rendering for cost. Theinexpensive motion picture LED lighting instruments are affordable because they use High Power AC LED chips that are mass produced forhome and industrial lighting applications. The problem is that the color rendering of these LED chips is less than optimum for motionpicture lighting applications (to see how poor the color rendering of LEDs is compared to traditional tungsten lights use this link to theSolid State Lighting Project Technical Assessment generated by the Academy of Motion Picture Arts and Sciences). Expensive LED lighting

instruments, like the MoleLED 12 Pack (pictured below), are expensive because they use LED chips, like the OSRAM KREIOS stagelight module, that are specifically designed for motion picture lighting applications and hence are not produced on a mass scale.

The MoleLED 12Pack

Until the recent development by OSRAM of their KREIOS LED technology, the color rendering of LED fixtures was generally pretty poor- they exhibit significant green output. Where most manufacturers of LED fixtures for motion picture production have chosen to either buildinto the fixture minus green gel filters (Litepanels) or provide them to apply separately (CoolLights, Nila), Mole-Richardson instead chose touse the new OSRAM KREIOS stage light module (pictured below) in their MoleLED 12 Pack fixture.

The OSRAM Kreios stage light LED module

The KREIOS stage light module is a metal core circuit board with 20 high-output blue LEDs each topped with a remote phosphor dome.The phosphor domes are an OSRAM proprietary design, which are blue light activated to produce light in two exact color temperatures,Tungsten and Daylight. While, remote phosphor technology has been used for years to extend the short wavelength of Blue LEDS to create afuller color spectrum, OSRAM was the first LED manufacturer to use remote phosphor technology to exactly match the spectral sensiti

Portable Generators in Motion Picture Production

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