Putting UPS's in series?

[YKhan]

I'm wondering if there are any dangers or precautions to putting a
couple of UPS's in series to increase their power-on time?

Yousuf Khan

 

[Robert Redelmeier]

I'm wondering if there are any dangers or precautions to putting
a couple of UPS's in series to increase their power-on time?

Even presuming sufficient VA (=W/PF), few UPSes produce
nice sine waves on their outputs while most expect them on
their inputs. There may be problems [overheating] with the
second UPS. UPS should be able to handle poor powerfactor
draws since most computing devices are miserable.

It would be vastly preferable to put them in parallel
where you can split loads, like one for the CPU,
one for monitor & wallwarts.



-- Robert

 

[YKhan]

I'm wondering if there are any dangers or precautions to putting
a couple of UPS's in series to increase their power-on time?

Even presuming sufficient VA (=W/PF), few UPSes produce
nice sine waves on their outputs while most expect them on
their inputs.  There may be problems [overheating] with the
second UPS.  UPS should be able to handle poor powerfactor
draws since most computing devices are miserable.

It would be vastly preferable to put them in parallel
where you can split loads, like one for the CPU,
one for monitor & wallwarts.

-- Robert

That's what I was afraid of. I remember hearing something about that
sometime back, but I couldn't remember if I heard it right or what the
reason was, so I asked here.

Yousuf Khan

 

[Strobe]

I'm wondering if there are any dangers or precautions to putting
a couple of UPS's in series to increase their power-on time?

Even presuming sufficient VA (=W/PF), few UPSes produce
nice sine waves on their outputs while most expect them on
their inputs.  There may be problems [overheating] with the
second UPS.  UPS should be able to handle poor powerfactor
draws since most computing devices are miserable.

It would be vastly preferable to put them in parallel
where you can split loads, like one for the CPU,
one for monitor & wallwarts.

If you do this, don't forget to put a small lamp on one of them.
Bad enough coping with a power outage without having to do it in the dark.

That's what I was afraid of. I remember hearing something about that
sometime back, but I couldn't remember if I heard it right or what the
reason was, so I asked here.

Have you looked into replacing the UPS battery with a larger capacity one?
This is the preferred (and more elegant) way to get longer power-up time.

 

[Rick Jones]

If you do this, don't forget to put a small lamp on one of them.
Bad enough coping with a power outage without having to do it in the dark.

That's what all the blinking lights are for - all real computers have
blinking lights right?-)

rick jones

 

[Bill Davidsen]

I'm wondering if there are any dangers or precautions to putting a
couple of UPS's in series to increase their power-on time?

In general it's more effective to split load or use a bigger UPS. With few
exceptions the output of a UPS is ugly with high frequency harmonics, resulting
on a bunch of losses.

 

[Steve Thompson]

That's what all the blinking lights are for - all real computers have
blinking lights right?-)

Absolutely. Anyway, we have a nice illuminated EXIT sign which is on
emergency power and so stays on when the power is out. The computers are
not, because the emergency power is turned off once every month for
"testing". Really.

Steve

 

[LSMFT]

I'm wondering if there are any dangers or precautions to putting a
couple of UPS's in series to increase their power-on time?

Yousuf Khan

Series would double the voltage and blow your stuff up.

 

[Bryce]

That's funny. I hadn't thought of connecting the OUTPUTS
in series.

In 'normal' mode, most consumer-grade UPS devices just pass
through the input mains power. Any attempt at connecting the
outputs in series would either result in total output
voltage equaling mains voltage, or zero and an immedite
shutdown by either unit's short-circuit protector.

When operating off-line (i.e., during a line power failure)
it would be possible to obtain much more than rated voltage
with the series connection, but it would be weird. The two
UPS units would not be phase synchronized and probably would
not even operate at exactly the same frequency. Output
voltage would vary unpredictably.