I believe the power companies make everything come out exactly even (to NTIS precision) once every thirty days. Even if they have to temporarily switch to dc or 400 Hertz every now and then.
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LOL! thanks, Tom
Correct. The instantaneous frequency is within a few tenths of a percent. In my neighborhood, under frequency load shedding on some rural substations is set for 59.5 Hz.
Over a day, the regional system operators keep the cumulative cycle count as close to 60 Hz as possible. Its been a while since I looked at the performance numbers, but +/- 1 second per day is a typical target for operation.
They do this by counting the total number of cycles per day and scheduling some additional generation during the night to catch up if the system is slow, or cut some if its fast.
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You're only interested in comparing intervals.
The delay from the satellite to you is the same at the beginning of your interval as it is at the end of your interval, so they cancel out. You don't care about absolute time here.
:)
It depends.
If your local power utility is connected to a national grid through an AC link, the utility is going to have the stability of the national grid, which in many countries is very precise.
Most utilities are AC conencted and individual utilities CANT vary in frequency at all-- they have to be phase-locked to the grid or otherwise huge currents would flow and trip the breakers on the link line. All the generators are effectively electrically and mechanically in parallel and the frequency is very stable-- if one generator slips back even a few degrees in phase, current flows in from other generators to compensate. Same thing if a generator gets an extra burst of steam-- it will just slip ahead a bit of the average and try to push more current into the utility and grid. Of course there are myriad bits of instrumentation that try to keep the swings within reasonable limits or else the generator breakers trip it off line.
Back in the 1940's there was some utility in Texas that wasnt on a large grid and some problem caused its generators to slow down to 40 cps for a few seconds. Allegedly several hundred thousand AC motors in refrigerators and air conditioners burnt out.
Some are linked by DC lines and the frequency of those utilities might be less stable than the national grid.
It would take more than a few seconds to burn up the heavy duty motors used in the '40s. How many air conditioners were there in the '40s, anyway?
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Don't confuse size with "heavy-duty". The motors back then were wound with wire insulated with plain old organic varnish which would burn up at really low temperatures. Up til the late 1960's you could burn up a Skilsaw in two seconds if the blade stalled. The good old days.... For motor rewinders.
Did you ever see a skill saw motor on a refrigerator? Most were made with a lot more iron to conduct the heat away than modern motors. A locked rotor on a crappy universal motor on a cheap portable circular saw is very different than any induction motor running below the rated frequency for a few seconds. Tell me, how you can change the frequency from 60 Hz to 40 Hz and back to 60 Hz in a few seconds without the alternator ripping itself out of its mount? As far as the Skill saw burning up, you should have used B&D. I've seen smoke pour out of an overworked tool. The owner let it cool down and went back to work. Also, locking the rotor in a circular saw is a good way to be hurt, or lose a limb if it comes flying out of the bad cut and starts running again.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
This isnt a debatable issue-- the Texas event really happened, it's covered in many power EE textbooks, alongside the 1965 NYC blackout.
The way you described it is suspect, unless it was a 5 KW generator in the bed of a pickup truck.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Here's some data I took a few years ago:
only a couple of days, but you get the idea.
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