I'll interject a rather odd problem I came across with whole-house generators.
A friend of mine has business offices that are in a semi-rural area, he needs reliable power for the office. He got quotes from the industrial/commercial suppliers, and they were something like 5x to 10x the cost of something like Generac installed. He went with Generac.
In some cases, it is "you get what you pay for", and if Generac level fits your needs, you are set. But why the cost delta? For one thing, the commercial units are rated for many more hours use than the home-based units. Again, probably little/no concern for occasional use, so it's all good.
But one day my friend calls me, he says he can't figure this out. All their office computers are on their own UPSs so they survive a glitch, or short outage before the generator kicks in, and they've always worked just fine for short outages. But they had an overnight outage, and all the computers were down, and the UPSs were dead and beeping their warning tones. But the generator was running, the lights were on, all seemed well. So why would the computers go down when they also had UPS backup?
Ah-hah! As I'm talking, he says "The weird thing is, and it just happened - when the well pump comes on, the UPSs stop beeping!".
So I'm in Sherlock Holmes mode to figure this out. Turns out, one way the home-level units manage to squeak out more power and run time for a limited amount of cost is, they play loose with frequency regulation. So there is a weird positive feedback speed regulator board. If you are not calling for much power, the engine slows down to conserve fuel and reduce wear and tear. And when there is a high demand, it speeds up (you'd expect an engine to slow down with a heavy load, but the positive feedback over-rides this). By speeding up, it can produce more power than if it ran a speed to provide a constant 60 Hz. So that's how they can say it provides XX.X kW.
If you're still reading, you may be wondering, what does this have to do with their computers on UPSs?
Well, that's the second piece of the puzzle. The UPS is programmed such that if the line looks unstable (low or high frequency), something must be wrong, so it switches to battery. So when their Generac kicked in, but the frequency drifted, the UPSs all switched to battery, and stayed on battery, and were all dead by the next morning. But when their well pump kicked in, that drew a "Goldilocks" amount of load to cause the Generac to run very near 60 Hz, and the UPSs were happy, and switched back to the line and started charging their batteries. But the well pump didn't run at all overnight, and not long enough to recharge the batteries, so the UPSs were all dead.
If you search the Generac forums, you'll find ways to modify the control board to 'tame' that positive feedback and maintain a more stable 60Hz, but it's rarely needed. Until it is.
Carry On. - ERD50
