Re: Automatic Generator cut in

["David Buckley" <db@xxxxxxx>]

--- In ukha_d@xxxxxxx, "Dan Lowe" <ukha@d...> wrote:
> Mick
>
> > How do I know if the genny produces clean enough power
> > for the UPS?

I've never done a "small" genny backed UPS, but have been
involved in
design and specification some "big" stuff, like 80-2000kva UPSs
with
genset backup.  When you are dealing with a genny supplying the
essential load of a large building with lifts and other such horrible
loads, the output of the genny is not stable either in voltage, but
especially in frequency.

What is important is that you have an on-line UPS, not a
ferromagnetic or other sort of (generally cheaper) UPS.  With an on-
line, all the mains does is charge the batteries, and provides a
synchronisation source.  If the input mains is horrible, its not
really important; it still keeps the batteries up.  The inverter,
which runs all the time, produces clean output.

The point about changeover is down to whether you want to DIY or
commercial.  Many commercial standby generators are avaiulable with
options that will do it all for you, you feed them mains, when they
detect mains stops arriving, they start, and switch.  When mains is
restored, they switch back and stop the genset.

If you want to DY, the absolute minimum is (as has been mentioned
elsewhere) mechanically interlocked contactors.  It is a legal
requirement that there is no posibility of backfeeding into the
mains, to prevent islanding, with the attendent death risks to
electicity workers.  If you are using a small generator, (anything
intended for building site or campsite use) it probably produces not
240V but 120-0-120, with L and N at the two ends of 110V, and a
centre tap which is earthed.  This is different to normal mains, and
you must switch the N as well as the L.

If you want to build your own controls, you can get mains failure
detect relays (RS), use a PLC, or something cute like a Splat!
controller with pretty LEDs.

When buying a genny note that some are rated for standby, and others
for continuous use.  However, I don t know how big a solution you are
looking for.  There are lots of options at 10kva plus, but at 3kva
the options are more limited.  There are small gennys with electric
start available, but you'll need to do the integration work yourself.

Good luck.

David.