A typical 1200W power factor corrected electronic HMI ballast has a power factor of .86 which means it will draw 11.5 Amps at 120 Volts to generate 1200 Watts of light (11.5A x 120V= 1380W, 1200W/1380W= .86). While not a huge advantage when plugging into house power, the added efficiency of a PFC 1200 ballast can make a huge difference when powering a lighting package off of a portable generator. For example, when you consider that a Kino Flo Parabeam 400 draws only 2 amps, the 8 Amp difference between using a PFC 1200W electronic ballast and standard non-PFC 1200W electronic ballast, can mean the difference between running four additional Parabeam 400s on a portable generator or not – I think you would have to agree that is a major boost in production capability.

Clearly, the first step in designing a production system that mitigates the problems caused by harmonic currents is to use only Power Factor Corrected ballasts. The next step, as we will see next, is to start with as pure a power waveform as possible.

A Whole New World

Common questions I hear are: Why are harmonics suddenly an issue in motion picture electrical distribution systems? And, why haven’t we needed Power Factor Correction in HMIs until now? To answer these questions, one must appreciate the historical interplay between power generation and load in the past. The lagging power factor (current lags behind voltage) caused by the inductive reactance of magnetic ballasts had a considerably less adverse effect on conventional AVR generators than the leading power factor (current leads voltage) caused by the capacitive reactance of electronic ballasts. That was because Power Factor Correction in the form of capacitor banks brought voltage and current in phase enough that magnetic ballasts operated reliably for the most part; while the type of voltage waveform distortion they generated did not have an adverse effect on the relatively simple linear loads making up production packages of the day (principally the motor drives of film cameras and quartz lighting instruments.)

However, we are no longer in our parent’s linear world. The power generation and electrical distribution systems developed then were not designed to deal with the abundance of non-linear loads like electronic HMI and Kino Flo ballasts that make up lighting packages today. It’s a problem that has only recently begun because of the increasing use of non-linear lighting loads (for a comprehensive overview see the just released 4th Edition of Harry Box's "Set Lighting Technicians Handbook" - send me a self addressed stamped envelope & I will return it with a discount coupon good for 30% off the 4th Edition through the publisher's website.) The problem is being further compounded by the increasing prevalence on set of sophisticated electronic production equipment like HD cameras, computers, hard drives, and monitors which are not only sensitive to harmonic distortion, but are themselves sources of harmonic distortion.

For instance, the self-excited AVR systems of conventional generators were not designed to operate with leading power factor loads. If you will recall from our previous discussion, in AVR systems the AC voltage generated is controlled by DC excitation of the electro-magnets of the generator's Rotor. The amount of DC excitation required is a function of generator load; or, put another way, the excitation required to maintain constant voltage increases with load. The type of load also affects the amount of excitation required. Lagging power factor loads (magnetic ballasts) require more excitation than a unity power factor load (Quartz Lights.) Leading power factor loads (electronic ballasts) require less excitation than unity power factor loads.

Rudimentary AVR systems like those in portable generators are ill equipped to deal with leading power factor loads like electronic ballasts because the harmonic currents they generate create flux in the armature coils of the Stator that reacts additively with the Exciter flux in the field poles of the Rotor to increase saturation and produce a higher terminal voltage than called for a given load. Consequently, the AVR system responds erroneously to control voltage by reducing excitation. The end result is that the regulator goes to its minimum excitation capability while the additive excitation of the armature flux from the leading power factor causes the terminal voltage to continue to rise and not be controlled by the voltage regulator.

Erroneous regulation of voltage is just one example of the more severe effect that leading power factor loads have on conventional AVR generators than do lagging power factor loads. In the next section, where we compare the characteristic voltage waveform distortion created by different lighting loads on different generators, we will see that leading power factor loads also have a more severe effect on other production equipment operating on the same power. And, that after partial Power Factor Correction with capacitors, the lagging power factor of magnetic ballasts can actually have a positive effect on the already distorted power waveform of conventional AVR generators like the Honda EX5500. For these reasons, as long as you could shoot at one of the safe “flicker free” frame rates, magnetic ballasts worked reasonably well on conventional AVR generators with frequency governors until the introduction of electronic square wave HMI ballasts.

When electronic square wave HMI ballasts came on the market, they were at first thought to be the solution to all the problems inherent in running HMI lights on small portable generators. Since they are not frequency dependent, it was thought at first that electronic square wave ballasts would operate HMIs more reliably on small portable generators – even those without frequency governors. By eliminating the flicker problem associated with magnetic ballasts, they also eliminated the need for the expensive AC governors required for flicker free filming with magnetic HMI ballasts and portable gas generators.

For these reasons, as soon as electronic square wave ballasts appeared on the market, many lighting rental houses replaced the expensive crystal governed Honda EX5500 with the less expensive non-synchronous Honda ES6500. The theory was that an electronic square wave ballast would operate reliably on a non governed generator and allow filming at any frame rate, where as a magnetic HMI ballast operating on an AC governed generator allowed filming only at permitted frame rates. In practice, electronic square wave ballasts turned out to be a mixed blessing. As we have seen in this section, the leading power factor caused by the capacitive reactance of the new electronic ballasts proved to have a more severe effect on conventional AVR generators than did the old magnetic ballasts.

Since magnetic ballasts worked reasonably well on AVR generators with governors, in the past, attention was only given to portable generator features such as automatic voltage regulation, speed regulation and AC Frequency. But, given the increasing prevalence of leading power factor loads and the problems they cause, an increasingly more important feature today is the quality of the generated waveform and the impedance of the power system. For this reason, it is imperative that today’s power generation and electrical distribution systems be designed for non-linear lighting loads, not just linear lighting loads. This is especially true of the systems to be used in low budget independent production because these productions have traditionally relied upon portable gas generators that are more susceptible to the adverse effects of harmonic distortion. These productions are also increasingly embracing the use of HD digital cinema production tools, like inexpensive HD camcorders, laptop computers and hard drives, that require cleaner and more reliable power on set to operate effectively.

__________________________________________________________________