While that is true in John’s present circumstances, it would not be true if he were using non-power factor corrected HMIs, Kinos, and LEDs instead of Tungsten lights. These lights will draw harmonic currents that could double the voltage drop. Voltage drop tables and calculators are for the 60Hz primary only and so do not take into account the effect of harmonic frequencies. On top of additional voltage drop, these lights operating on a conventional AVR generator, like the one John originally proposed, would cause voltage “flat topping.”
Let’s look at the voltage drop component first. As you can see in the power quality meter reading above of a non-power factor corrected 1200W HMI it draws a distorted current waveform that is rich in harmonics. The higher harmonic frequencies create what is known as "skin effect" in the cable. Skin effect is a phenomenon where the higher frequencies cause the electrons to flow toward the outer sides of a conductor. Since the flow of the electrons is no longer evenly distributed across the cross sectional diameter of the conductor, more electrons are flowing through less copper and the resistance of the conductor increases. The increase in resistance reduces the ability of the conductor to carry current, resulting in greater voltage drop over shorter distances and overheating of the conductor.
The area of the cross sectional diameter of a conductor used by DC current (left), Low Frequency AC Current (center), High Frequency AC Currents (right).
The graphs below illustrates the difference of varying harmonic numbers on both Proximity Effect and Skin Effect for 12 AWG and 4/0 cable (the cable spacing used to obtain the Proximity Effect values is based on National Electric Code (NEC) insulation type THHN.) Comparing the graphs, it is immediately apparent that Skin Effect is more significant in the smaller cable, than the large cable. Within the range of the triplen harmonics alone (i.e. 3rd, 9th, 15th), Skin Effect increases by 60%.
Which means that the increase in voltage drop due to harmonics is appreciably more significant in the jacketed multi-conductor cables (10/2 or 12/3) commonly used with small portable generators, than with the larger gauge feeder cables (single conductor #2, 2/O, & 4/O) used to distribute power from tow plants. The increase in resistance due to Skin Effect reduces the ability of stingers (12/3 cable) to carry current, resulting in overheating of the conductors and greater voltage drop over shorter distances than with larger feeder cables.
Increased voltage drop from harmonics is not the only problem you face when running non-power factor corrected HMIs, Kinos, & LEDs on conventional AVR generators. The other problem is voltage “Flat Topping” caused by the high impedance of the generator.
Left: Non-pfc SMPSs powered by grid power. Right: Same Non-pfc SMPSs powered by conventional AVR Generator (Honda EX5500) Note different effect that the same non-linear load harmonics have on grid power and power from conventional AVR generator.
The oscilloscope shot above right shows the voltage flat topping caused by a non-pfc 1200W HMI ballast on a conventional AVR generator (a Honda EX5500.) What causes this flat-topped voltage? According to Ohm’s Law current reacts with impedance to cause voltage drop. Since electronic ballasts consume current only at the peak of the voltage waveform, voltage drop due to system impedance occurs only at the peak of the voltage waveform. The zig-zag saw tooth pattern in the right oscilloscope above suggests the flat topping of the voltage waveform we see here is caused not only by the 60hz fundamental but also by the harmonic currents drawn by the non-pfc SMPSs at higher frequencies that also create voltage drops as they encounter the system impedance of both the cable and generator. For example, when encountering the high impedance of a conventional AVR generator, a 3rd harmonic current will produce a voltage drop at a 3rd harmonic voltage. Likewise a 5th harmonic current will produce a voltage drop at a 5th harmonic voltage, etc. Each harmonic current drawn by a non-pfc load flows through the system impedance resulting in a voltage drop at that harmonic frequency. In other words, where a distorted current waveform is made up of the fundamental plus one or more harmonic currents, each of these currents flowing into an impedance will according to Ohm’s Law, result in a voltage drop resulting in voltage harmonics appearing at the load bus and distortion of the voltage waveform. This pattern does not appear in the voltage waveform of the grid power on the left in the oscilloscope shots above because of its’ much lower impedance. The impedance of a generator can be 5 to 100 times (depending on its’ size and design) that of a utility transformer and it will change as the load changes.
The first step in mitigating these problems caused by harmonic currents is to eliminate the currents. Using only power factor corrected lights will go a long way in reducing the number of harmonic currents in the power stream.
Left: Conventional AVR Generator w/ 1200W non-pfc HMI. Right: Inverter Generator w/1200W non-pfc HMI
A second step in mitigating the problems caused by harmonic currents on small putt-putt generators is to operate them on only inverter generators. For example, the power waveform above on the right, is the same non-pfc 1200W HMI ballast operating on our modified Honda EU6500is Inverter Generator. As you can see, the difference between the resulting waveforms is startling. Even though the load is the same, the fact that the harmonic currents it draws now encounter the relatively low impedance of an inverter generator, results in considerably less power waveform distortion - making it possible for the generator to operate a larger total load.
The extremely low line noise exhibited in the inverter generator power waveform above (right) creates a new math when it comes to calculating the load you can put on a portable generator. Where before you could not operate more than a couple of non-PFC 1200W HMIs on a conventional 6500W putt-putt generator because of the consequent harmonic distortion, now you can load an inverter generator to capacity. And if the generator is one of our modified Honda EU6500is inverter generators, you will be able to run a continuous load of up to 7500W as long as your gear is Power Factor Corrected.
These power quality issues have vexed film electricians for years. If you haven’t already, you might want to read an article I wrote for our company newsletter that explains the electrical engineering principles behind these issues and how to resolve them. http://www.screenlig...generators.html
Guy Holt, Gaffer,
ScreenLight & Grip,
Edited by Guy Holt, 26 October 2014 - 11:02 AM.