I believe Chris is referring to the dynamic nature of power amp loads where in a given situation the average load would be less than the generator's capacity, however the instantaneous load could be significantly more. There is some rotational inertia in the generator that can provide some burst capacity for the first kick drum hit or two, but that can be depleted with repeated hits , causing loss of voltage or frequency, or possibly stalling the generator.
Thanks for the clarification. I was wondering if it wasn’t for the same reason film electrics de-rate generators by 50% when supplying non-linear lighting loads like some HMIs, Fluorescent, and LED Fixtures. Where it appears that you guys use a lot of gear operating on Switch Mode Power Supplies (SMPSs), you might be interested in the graph below from a Caterpillar Technical Service bulletin on sizing gen-sets. It is a “Limit Characteristic” graph that illustrates the effects that non-linear loads have on generators. The fluctuations in the Engine Stability line in the illustration below represent the generator’s operating limits depending on whether its load has a leading or lagging Power Factor. As you can see by the right quadrant the effect of harmonics on a generator is a significant limiting factor.
A Limit Characteristic graph for a generator illustrates the effect of leading
or lagging Power Factor on the generator's output.What the Limit Characteristic graph above tells us is that, operating a capacitive load (the leading power factor quadrant right of the Unity Power Factor center line), an AVR generator like the Mulitquips first reach a thermal limit as a consequence of heat generation in the generator's rotor from harmonic currents. And, since increasing harmonic content leads to a tendency toward erroneous self-excitation (SE), the generator output is inevitably affected by instability of the Excitation circuit from higher leading power factor loads. Finally, since there comes a point as the Power Factor of the load decreases, when harmonics inhibit the successful operation of the generator’s Automatic Voltage Regulator all together, and hence the generator’s capacity to generate any power at all, the kW output eventually drops to zero. This Limit Characteristic graph also tells us that operating an inductive load with lagging Power Factor (the left quadrant of the Limit Characteristic graph) the generator’s output is more stable; but will eventually be limited by both the rotor (field) and stator (armature) heating (many times the field temperature limit is lower than the armature limit, especially at low power factor values.)
These same power quality issues have vexed film electricians for years, which is why most movie gaffers still oversize their generators by 100%. For more details see an article I wrote for our company newsletter that explains the electrical engineering principles behind these issues and how to resolve them. The newsletter article is available at
http://www.screenlightandgrip.com/html/emailnewsletter_generators.html. Guy Holt, Gaffer,
ScreenLight & Grip,
www.screenlightandgrip.com
