[mythtv-users] OT: apc backup

[Simon Hobson]

Mike Thomas <mt3 at pfw.demon.co.uk> wrote:

> 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.

Getting even more off-topic now ...

I agree they are not exactly sophisticated units - in part that's what you get for an offline unit as most of their units are. In the case I mentioned, it was a rack mount 3kVA unit (with extra batteries) supplying about 2kW, and I ran it fine from a 3kVA generator. Initially it did just as you described - as soon as it loaded the genny, the supply went out of tolerance and it switched to battery, rinse and repeat. Once we changed the setting to maximum tolerance then it stayed on genny power and recharged it's batteries. Had it not done so, then my plan was to shut some servers down, get it going on genny, and then power them back up one at a time.

Something else I agree with is that they are "lazy" in their design. I've had discussions with them regarding limiting battery charge rate when mains supply is limited (as we have at work*). Their simplistic response is "wrong sort of supply" - like many other manufacturers, it's easier to just demand an over specified supply rather than design something properly. In our case, we have a 63A main fuse which isn't upgradable, but they insist all warranties are void if the supply is less that 125A for the 16kVA unit we were looking at. And a 3kVA unit shouldn't *NEED* a 32A supply either. Induction hob manufacturers deal with this issue just fine !
At least they didn't just lie about it like BPC did.

There is one upside to them. When they do pack in, there's a fair bit of metal to weigh in at the scrapyard once it's stripped down :-)