A few years ago I bought a book called Standard Handbook For Electrical Engineers, but I never read it. It just sat on the shelf taunting me, making fun of me behind my back, sticking its $150 tongue out at me. It's a big, mean book. It's literally two inches thick and it weighs five pounds. It has a lot of ugly formulas and information about electricity, power systems, motors, power electronics and a lot more. It's the kind of book that will jump you from behind and force you into wearing pocket protectors and tape on your eye glasses.
Recently, a reader e-mailed me about an article I wrote entitled "That's a Load of Watts" (PLSN, May 2010, page 40).The gist of the article is that generators are capable of delivering a limited number of amps at a given voltage, which means that the kVA rating is much more useful than the rated load in kilowatts, which can change depending on the power factor of the load.
Other Factors at Play
Marc Shapiro of Sound Illumination in Eugene, Oregon, wrote, "I enjoy reading your tech notes, ‘That's a Load of Watts' being the latest. However, I think you are letting the lighting/stage tech off the hook too easily by only addressing the watt/VA rating of an alternator. Aside from the ability of the electrical side of the device to dissipate its heat, I think you need to consider the electro-magnetic capacity of the alternator and the instantaneous power available from the engine. My concern would be starting current for motors, and inrush current for incandescent lamps.
"I have experienced generators stalling out due to the load of trying to start a compressor motor, where the generator's rating could carry the full load of the compressor, but it couldn't deliver the starting current. It is even possible that the rating is sufficient to supply the starting current but the engine cannot respond quick enough to pick up the starting load without stalling. This is significantly affected by the size of the flywheel. If there is enough inertia in the flywheel to get the load running then the engine has time to throttle up to carry the continuous load."
His email gave me impetus to face the big book in hand-to-hand combat. So I worked up my courage, snuck up on it, wrestled it off the shelf and attacked its voluminous pages. What I found will give you chills.
Two Types of Generators
There are basically two types of generators: synchronous generators and induction generators. Synchronous generators produce an AC waveform that is exactly in synch with the speed of rotation of the generator. If the generator slows down or speeds up, so does the frequency of the AC it generates. The idea is to keep the rotational speed constant regardless of the connected load on the generator.
Induction generators spin at speeds slightly higher than the AC frequency and the speed of rotation varies according to the connected load. Wind generators usually use an induction generator while most of the portable generators we use in the live event production industry are synchronous generators. That means that for the gennys we use, the speed of rotation should stay the same regardless of how many lights, motors, or coffee pots are connected to the power distro. And when the load changes quickly the generator should have some muscle on reserve in order to handle any sudden changes to the load. But that's easier said than done.
Some of the loads we use every day exhibit high inrush currents when they are first turned on. An incandescent lamp with a cold filament can draw almost ten times its normal operating current on startup. I took a snapshot of the inrush current in a 500-watt PAR 64 using a Fluke 43B power quality meter. It measured a peak inrush current of 41 amps at 120V before it settled down to 5.7 amps in half a second. Discharge lamps also have a large inrush current. I measured it in a 575-watt MSR lamp fixture at 20 amps at 120V, settling down to 7.1 amps in 0.6 seconds. Chain hoists and other motors also produce high inrush currents. The chain hoists that we most often use in this industry are induction motors. When they are started from a stand still, it requires a lot of torque to start lifting a load. Even if they are lowering a load with the aid of gravity the windings still have to be energized before the motor will start moving. A 1-ton CM hoist can draw as much as 30 amps at 230V on startup before settling down to 11.4 amps (3.8A 3-phase) in about half a second.
A Complex Calculation
The typical show has all of these elements and more, multiplied by dozens and dozens of fixtures and motors. Automated lights are typically struck all at once and left on. The manufacturer usually builds in a staggered start using a short random time delay in each fixture so that the combined load doesn't bring down the entire power distro. But when we move truss pods and trigger conventional lighting cues the instantaneous current can reach extremely high peak levels. And that translates to a huge amount of torque on a portable generator. It does its best to maintain synchronous speed but unless it has enough muscle to power through the surge in demand, it could stall.
If you didn't already have enough of a good reason to build in some overhead capacity in your generators now you have even more reason – to handle sudden shifts in the load. Manufacturers tell me that generators like to be "fully" loaded, which to me means 80 percent because a load of 100 percent is just asking for trouble.
After an hour of doing battle with the big book, we both lay on the floor, mentally drained. My once curious brain was shredded into little pieces and strewn across the room. But I'm a better person for it and now I have even more respect for its two solid inches of densely packed information.
