The teacher opens the door; you enter by yourself. – Chinese proverb
It’s not often that you meet female road warriors – I mean real road warriors who have spent their entire careers in buses and arenas. So when Libby Gray ended up sitting across the dinner table from me in Kansas City during USITT, one of her first comments really caught me off guard.
I asked her how she got into the lighting industry, and her answer was, in effect, “I ran away with the circus.” Her first job was running the portable generators for the circus. She explained that the generators were critically important, because in the event that they were to lose power in the middle of, say, a trapeze act, it could be fatal.
I knew right away that she and I would get along well, because anyone who starts a conversation that begins with a circus and results in a discussion about electricity is my kind of tech.
Is it a 1kW Generator, Really?
Oddly enough, just before dinner, I had been searching for answers to a portable generator question. I had been puzzling over the question of how to properly size portable generators and wondering about why they’re rated they way they are. Most everybody who has operated a portable generator will tell you how many kilowatts a generator produces. Even manufacturers of portable generators publish specs online and in the literature that will tell you how many kilowatts a particular generator will supply. But I’m not sure I believe them.
If a generator is indeed able to produce a certain number of kilowatts, then it should be able to run a given load today, tomorrow, in Phoenix, in Vancouver, wherever, no matter what the conditions. Is a 1kW generator always a 1kW generator? I can give you a generator and a load with a matching (or slightly lower) kilowattage, and the generator, under certain conditions, won’t be able to run it.
For example, if I had a 1.2kW generator and a 1kW HMI fixture, then the generator should be able to run it, right? But what if the HMI fixture had a magnetic ballast and no power factor correction? Would the generator still be able to run the load?
What’s PF Correction?
Power factor correction is a way of bringing the voltage and current in the system back in phase with each other. If they are out of phase, then the power factor drops from 1 to some value less than 1, and the current draw is higher than what is needed to run a power factor corrected load of equal value.
So in a nutshell, our 1kW load without power factor correction would draw more current than “normal” for a load of that size. Whether or not the generator could supply the load would depend on how low the power factor is. If it was much lower than 1, say, 0.8, then the generator would be slightly over capacity and the HMI lamp might flicker badly or not stay on at all.
In this case, our 1kW load is undersupplied by a “1.2kW” generator. Is it still a 1.2kW generator? If it is, why can’t it run a 1kW load?
What’s the Answer?
Perhaps a better way of describing the supply capacity of a generator is to use a value that is unaffected by the power factor of the connected load. There is a unit of measure that is similar to kW, except it takes into account the increased current caused by a lower power factor. That unit of measure is kVA.
kVA is simply the product of the kV and the A, or the kilovolts times the amps. In the case of a 120V system, the kV is 0.12, so a 1.2kVA generator could supply 10A, regardless of the power factor of the load.
That makes sense when you consider that the higher the current, the higher the temperature of a conductor. In fact, the temperature of a conductor increases exponentially with the current. And a limiting factor in any system is how much heat it can handle before it melts into a puddle on the floor.
So when you re-evaluate the supply capacity of a portable generator from the perspective of how much heat can it handle, and thus, how much current it can supply, then for a given voltage you’re left with a pretty hard and fast limit on the amount of current it can generate. And it doesn’t care how many kilowatts that is, only how many kVAs it is.
Can You Show Me?
In our example of the 1kW HMI lamp, if the power factor is 1, then the kVA is 1. But if the power factor is 0.8, then the kVA is 1.25 (1kW ÷ 0.8 = 1.25kVA). It’s easy to see that a 1.2kVA generator will not run a 1.25kVA load, at least not for very long.
The supply capacity of a portable generator is especially critical because they are happier when they are running closer to full capacity than they are when they’re running lightly loaded. As a generator works harder, the temperature of the internal parts rises, and as they heat up, they also expand. When a generator is optimally heated, the parts expand sufficiently so that they fit together better – they run more smoothly, and they produce the least amount of friction. I don’t think it’s ever a good idea to max out the capacity of any system, but in the case of a portable generator, manufacturers say it should be run at around 80 percent of full capacity.
If you don’t understand the difference between kW and kVA, then you might be confused about what full capacity is for a portable generator. But if you simply keep an eye on the kVA and compare it to the kVA of the portable generator, you’ll be better off.
To measure the actual kVA of the system load, just use a true RMS clamp meter and measure the amperage in each leg of a three-phase system or in the hot leg of a single-phase system. Then multiply amperage by the voltage and divide by 1000 for each leg. For a three-phase system, add the results. Divide that number by the name plate rating of the generator and it should be around 0.8.
What About Libby?
Enough about portable generators. What you really want to know is more about Libby. Today, she is with a different kind of circus – she’s the lighting director for Styx. Now the clowns are behind the scenes, the animals are in the house and the ringleader is behind the console.
