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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.
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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.
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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.
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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.
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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
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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.
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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
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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).
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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.
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ILLUSTRATION COURTESY OF HARRY BOX
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
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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.
ILLUSTRATION COURTESY OF HARRY BOX
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
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ILLUSTRATION COURTESY OF JOHN DE ARMOND
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".
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ILLUSTRATION COURTESY OF JOHN DE ARMOND
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.
ILLUSTRATION COURTESY OF JOHN DE ARMOND
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.)
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ILLUSTRATION COURTESY OF JOHN DE ARMOND
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.
ILLUSTRATION COURTESY OF JOHN DE ARMOND
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.
ILLUSTRATION COURTESY OF KIRK KLEINSCHMIDT
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,
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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.
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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.
ILLUSTRATION COURTESY OF KIRK KLEINSCHMIDT
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.
__________________________________________________________________
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)
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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.
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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.
ILLUSTRATION COURTESY OF HARRY BOX
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.
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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.
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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.
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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
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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).
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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.
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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.)
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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
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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).
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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
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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