On 28 Oct 2020 at 10:24:31 GMT, ""NY"" wrote:

"charles" wrote in message
...
In article ,
Martin Brown wrote:
On 27/10/2020 16:41, David wrote:
Just doing my winter disaster planning.

One item was to be able to run a small generator to keep the freezer
going in the event of a short power cut.

For a short powercut you really just need to keep the freezer door
closed and then put it on superfreeze for a bit when power is restored.

After the first day it is worth rigging up local power. Most power cuts
are usually shorter than that unless something goes very badly wrong.


about 6 years ago there was a cable fault in out road. An emergency
generator was brought in and coupled up to our house at about 1am. had it
for 4 days.

When the electricity company were doing some work on high voltage feeds to
various substations in the area, they brought in loads of huge generators
(they moved them from one village to another as the work progressed) and fed
groups of houses from the genny. The constant throbbing of the diesel
engines, day and night, was a real problem. I think our power was on local
feed for about a week.


I've wondered about getting a UPS for electronic equipment that reboots at
the slightest interruption. We went through a phase last year of getting
brief ( 1 second) power breaks every few days - or maybe a sequence of them
every 30 seconds or so. The electricity company had not been going round
pruning overhanging trees near HV lines, and when one touched a wire (eg in
stronger than normal wind) a circuit breaker would trip and reconnect.

How good are UPSs at providing mains that is sufficiently glitch-free, when
switching to and from battery, that computers don't notice? The only UPS
I've tried to use was one that my wife bought at the same time as her big
PC, but we never got round to trying for a couple of years - and when we
did, the battery would not provide mains, even with negligible load, for
more than about a second. The battery was either dead on arrival or else had
failed through lack of use (maybe through being stored with no charge). So
I've never seen how seamless the changeover is from mains to battery. I
presume they are more sophisticated than a relay, where contact bounce, even
if mains cycles match, may cause problems. Do most UPSs run the inverter all
the time (without taking power from it) so it is ready at a moment's notice,
locked to mains frequency and phase for instant switchover. I presume the
big problem with a UPS is when the supply is switched back to mains, because
the inverter will have been running off a free-running oscillator that will
be a different frequency so there will be a discontinuity at the point of
switchover. Or do UPSs stay on battery after the mains has returned,
tweaking the local frequency and phase gradually until they are locked to
mains and then switch without a discontinuity.

This was a cheap consumer 700 VA APC UPS, so probably one that switched
over, rather than one that was always running off battery and inverter, with
the battery kept charged by the mains but with no direct connection from
input to output.

As you say, most consumer UPSs only switch on the inverter when power is lost.
In practice most PC power supplies seem to cope without catastrophic glitches
on their output when switching on and off. I suspect there are enough
capacitors on the DC outputs to maintain the voltage for the tens of
milliseconds the mains supply is disrupted. There are actually standards for
this which good quality PSUs will meet.

Also as you say, an SLA UPS battery will die if left to self-discharge for a
year or two, so you never had a fair test.


--
Roger Hayter