[mythtv-users] apc backup

[Mike Thomas]

On Mon, 21 Mar 2016 08:17:16 +0000
Simon Hobson <linux at thehobsons.co.uk> wrote:
> Mike Thomas <mt3 at pfw.demon.co.uk> wrote:
> > - They are incompatible with generators. They switch off, on, off,
> > on, off, on until they blow a fuse. They create huge voltage spikes
> >  whilst doing this.
> 
> There's a setting to widen the mains voltage tolerance. We've
> successfully run them off generators - once this has been changed.
> Well actually it just means the load runs off the genny and the UPS
> recharges it's battery. A couple of years ago one of our customers
> had  several pre-announced mains outages - whole day jobs. Several
> times I've been out with my small genny to keep their servers going
> (for remote users). Now they've fitted a mains input and changeover
> switch by the meter and share next door's genny (a bit 80kVA job they
> use for running electrically powered diggers if there's no mains
> available) and just carry on as normal.

Dear Simon,

I am aware of the setting. My remark still applies. It all comes down
to the behaviour of the generator's governor to sudden load changes.
There are some APC models which are now advertised as
"generator-compatible". Mine are not.

The APC UPSes are not very clever. When they switch from running on
battery to running on the mains they immediately start charging the
batteries. This creates a sudden load increase of several amps. A
generator which can source 100 Amps might hardly notice this, but a
smaller generator would. The voltage dips, the frequency drops and the
APC unit switches back to battery. Rinse and repeat.

In the process they create huge switching transients and demand large
inrush currents. One of my units (supplying just a 4 amp load)
sometimes causes a dedicated 32A type C breaker to trip when it
flip-flops between mains and battery, irrespective of whether it is
running from a generator or the public utility supply. In fact, it has
been known to do this when the UPS performs a self-test. Other UPSes
start charging their batteries a split second later to avoid this
inrush.

Mike