In his posting of Mon, 1 May 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?= writes
The starting point is to try to reduce the power consumption, if
possible.
Let's look first at a battery solution. Which avoids all the problems
and costs of a generator and can easily be configured to automatically
take over supply.
Many thanks for the very comprehensive posting, much appreciated.
I should just say that this is for a one man office work shop and so
funds are rather limited. The computer part of the load amounts to
3-4KW 240v UK and the additional being 4.5KW for HID lighting and
running the central heating etc.....but this is not critical but would
be nice to be able to run these in the event of a long power failure.
At present the UPS units hold up the computer related stuff for about 40
minutes. Most of the power outages amount to less than one minute, but
when the failure is longer then it can last for hours or all day
occasionally. This is always a worry as what I produce has tight
deadlines involved and sod's law the long failures happen at the most
inconvenient time! Any hold up is obviously costly of course in simply
being unable to carry on working as normal.
5kW for, say 8 hours, is about 2000 AHr at 24 volts. This could be met
by 40 6v Trojan T-125s plus charger plus inverter. If you can get the
power consumption down, you can manage with fewer batteries. Also, if
set up with a genny, as below, fewer batteries are needed.
Now the genny situation.
Inverter gennies are good for continuous running on varying loads and
are much more fuel efficient on light load. Basically, the engine runs
at a speed determined by the load and can go to tick-over at very light
load. The inverter produces a constant output voltage and frequency,
irrespective of engine speed. So they are great for, say, powering a
house, where the loads can vary. You don't need this as you will only
be running it now and again and never on light load.
The waveform from low power gennies (eg a few kW) on or near full load
is broader and flatter than mains power - this can cause a UPS to
reject it and refuse to switch over from battery backup. For 5kW
loading via UPS, I would suggest a 10 - 12.5kVA generator as a minimum.
It probably won't be an inverter-type at this sort of power. AVRs don't
do anything to the waveform shape, so won't fix the UPS problem.
Not too sure what you mean here..... Are you saying that I am going to
have a problem with all AVR regardless of their capability? BTW I have
contacted several sellers of these cheapish small generators and they
say their kit will run computers without any problems but I am very
wary!
One setup to consider is a smaller set of Trojans linked to a very
high-output charger and inverter, plus a genny. The inverter
automatically takes over, without a flicker, if the mains fails and you
can size the batteries to run what you need for 4 hours or so. Also
have a genny which you can connect to the high-output charger. Start
the genny if the power has been off for a couple of hours and you need
power for more that the next two hours. The charger produces enough
output to power the loads *and* recharge the batteries. So the genny is
always on full load, when running. When the genny load starts to reduce
as the batteries reach full charge, stop it and run off the batteries again..
The biggest thing to watch out for is buying too small a genny. Next
thing to watch out for, in a system only using UPS and a genny, is not
being able to get the genny started fast enough and the UPS not
"liking" the output from the genny and either refusing to switch over,
or, worse, cycling back and forth between battery and genny. A set of
batteries and inverter are far, far easier...and usually the power
comes back on before you have to start the genny.
Yes I like the simplicity of the battery and inverter route!
Many thanks
