[ale] OT need 600-1000W power protection for 3 minutes - cheap

[Alex Carver]

On 7/10/2013 23:09, Ron Frazier (ALE) wrote:
> Hi Alex,
>
> You obviously have an engineering background and probably recent
experience with this type of equipment. I also have a EET engineering
degree, from the 80's, and less recent experience. I have more questions
and some minor disagreement with some of what you said. If I'm
fundamentally wrong, feel free to correct me (nicely). I may remember
just enough about this to get in trouble, and it's been a while since I
studied it. I'm also trying to put it into somewhat laymen's terms. I
know you know about what I'm trying to describe. I'm doing that for
others. Why bother, because I learn too.
>
> In your other thread to me, you made a statement that VA and watts
> (W)are the same thing. I think I have to disagree on that.

I say they're the same thing for the purposes of selecting a UPS (and 
partially due to marketing).  All the calculations should be in VA and 
then the UPS is selected based on VA.  If all the calculations are done 
in Watts then use the Watt rating on the UPS.  I am not saying nor would 
I ever say that the underlying physics makes them the same.  Watts is 
real power, VA is apparent power, and VAr is reactive power.  Three 
elements that describe a phasor (vector).  I actually did mention this 
in an earlier part of the thread.

There is an element of CYA on the part of the UPS manufacturer and I'll 
mention that a few times.  Basically the manufacturer is going to 
underrate their device to keep them from getting sued.  My statements 
assume (perhaps unwisely so) that the manufacturer does no such thing 
and that the VA rating is the total VA that the device can deliver into 
any arbitrary load.  If that happens to be a resistive load (unity power 
factor) then the total VA is total Watts.  If there's a non-unity power 
factor then the total VA is just the total VA.  If a manufacturer is 
putting an arbitrary limit in the CPU on a power measurement that ends 
up lower than the design limit then that's the CYA.  They don't want the 
thing to break until after the warranty expires.

In short, if a circuit is designed to be able to source 1500 VA total, 
then it can source 1500 Watts into a unity power factor (resistive 
load).  UPS manufacturers assume something worse and then derate.

> So, watts measures real power. Some descriptions say this is energy
flow that can do actual work, run your motor, etc.

That's correct, real power does work.  Reactive power can not perform 
work, it is a consequence of reactive energy storage.

> When you introduce energy storage elements, such as capacitors which
store and release electrons, or coils (inductors) which store and
release magnetic fields (which induce electron flow), you also introduce
additional oscillations in energy flow, which cannot do actual work for
you but which may be useful or required for the operation of the system.
Try operating a motor without a coil.
>
> Actually, I've never heard a really good explanation of this, but,
> theeffect of this extra oscillating energy that doesn't do work but
shuffles between the source and the energy storage components is
reactive power, and is measured in volt-amperes-reactive, or VAR.

That's correct, it's not that you're introducing oscillations into the 
system, you are introducing a phase shift between the sinusoidal voltage 
and the sinusoidal current.  An ideal capacitor or inductor causes a 
phase shift of 90 degrees between the voltage across its terminals and 
the current flowing through the device.  This is pure reactive (VAr) 
power.  When current is at its maximum, voltage is zero.  When voltage 
is at a maximum, current is zero.  Real work can't be performed under 
these conditions.

> So, the effect of the combination of the real power doing the work
(W), and of the reactive power not doing work (VAR), is called apparent
power, and is measured in volt-amperes (VA). This apparent power is a
real pattern of energy flow in the system. It heats up the components,
and affects the system components. It just doesn't turn your motor or
paint graphics on your computer screen. Since W and VAR are separate and
distinct energy flows, then VA is also a separate and distinct quantity
from W. The W rating on a UPS and the VA rating are two separate and
distinct types of energy flow patterns that the device can support
without exceeding its capabilities. Now, whether they rate the ups by
using a simple mathematical ratio of VA vs W, or whether they test it
for each type of energy flow, I don't know.

Watts and Volt-Amps are both energy related only by a cosine function 
(the power factor).  When the power factor is unity, Watts and Volt-Amps 
represent the same quantity of real energy.  Under those conditions, 
Watts and Volt-Amps do happen to be exactly the same.  Realistically 
there's no such thing as perfect unity power factor since all circuits 
have some reactive component (stray capacitance and inductance in the 
wires alone) so the power factor is less than unity but not by much.  VA 
represents the final complex resultant vector of the two orthogonal 
vectors Watts and VAr (Watts is real, VAr is imaginary on the Re-Im 
coordinate plane).

In the case of the UPS, no one measured for Watts versus VA.  They 
simply rated the Watts based on an assumed power factor from the VA 
starting point.


> The power triangle, and the trigonometric equations that go with it,
allow us to determine the relationships of these quantities.
>
> Power Factor is the ratio of W to VA, or real power to apparent
> power.  So, PF = W / VA.



>
> If you have a APC BR1500G ups, which is rated at 1500 VA and 865 W
> (I
think), then it an simultaneously support those maximums of those types
of energy flows but cannot exceed either. Those numbers imply a certain
amount of reactive power (VAR), which could be calculated. Suffice it to
say that adding more reactive power, either capacitive or inductive
(VAR) also increases the apparent power (VA), but the increase is not
linear, and must be calculated with trigonometry. Actually, I did the
math, and if I did it right, the reactive power could be 1225 VAR.
>
> Let's say you connect a light bulb. Assume it draws only real power
and there is no inductance and no reactive power. Your light bulb can
draw a maximum of 865 W, since that's the ups limit. The apparent power
(VA) in this case, is also 865 VA, is well below it's limit, and is not
an issue. Now, let's replace the light bulb with a motor. The motor
requires real power (W) and reactive power (VAR) which combine to create
apparent power (VA) to run it. It is conceivable that this motor is
drawing large amounts of reactive power (VAR) and lesser amounts of real
power (W). The combination of the two could create an apparent power
(VA) which will exceed the 1500 VA rating of the ups.

Yes and no.  See, this is why I mentioned the marketing aspect.  The UPS 
can supply 1500 VA.  It can supply 865 Watts of real power assuming your 
load has a power factor of about 0.6.  That's what I'm getting at.  If 
your load was 100% resistive (power factor of 1) then the UPS can (or 
should by design assuming a lawyer isn't nearby) supply 1500 Watts 
(designed 1500 VA * unity power factor = 1500 Watts).  The limit of the 
UPS isn't the mystical 865 Watts, it's 865 Watts into a reactive load of 
0.6 PF.

Essentially they derated it somewhat to account for people.  Most 
computer power supplies are sold with Wattage ratings not VA ratings. 
So people add up the Watts.  The problem is that the switching supply 
does not have a unity power factor.  So the UPS can handle 1500 VA total 
no matter the form of the power (real or reactive).  But people in 
general don't know any better so they add up all the boiler plates in 
Watts.  Now the UPS manufacturer has to derate so that someone doesn't 
buy their product, slap six 250 Watt power supplies on it and expect it 
to work because the numbers added up to 1500.  The manufacturer assumes 
that most things connected will have a non-unity power factor, took a 
very conservative guess of 0.6 and stamped the boiler plate with that 
Wattage based on the designed VA rating.  It's just a game of CYA, 
nothing more.  (Once the lawyer shows up, the limit becomes mystical again.)


> In the first case, the real power rating of 865 W is clearly
> limitingwhat you can do with the device. In the other case, the apparent power
rating of 1500 W is limiting what you can do with it, which is the
combination of whatever real load is there and whatever reactive load is
there. Note that it is entirely possible that, in the second example,
the real power being drawn is nowhere close to the 865 W limit.
>
> In a residential setting, you are typically billed for the real
> power
only, the up to 865 W. In an industrial setting, you're typically billed
or penalized for using reactive power.
>
> (For extra credit, those interested can look up power factor
correction capacitors. Hint, running a motor uses inductive reactive
power. Adding a capacitor uses capacitive reactive power. If you do it
right, they cancel out. This lowers your bill in an industrial setting.)
>
> So, we see that we can exceed the real power rating of the ups, or
> we
can exceed the apparent power rating, and either one is a no no. Trust
me, the ups will scream about it, or shut down. Don't ask me how I know.

If the UPS is screaming because the measured Wattage is exceeding the 
stamped Wattage then one of two things has happened:

1.  Your power supplies aren't the 0.98 PF that you think they are

or

2.  The manufacturer has installed an alarm for a much lower power.

I bet both, but that's just me. :) The reality is another CYA issue.  If 
the UPS is truly designed to supply 1500 VA then it can supply 1500 
Watts resistive.  Anything less is simply someone applying an arbitrary 
limit.


> You also said that you didn't think I was running with a high power
factor. I've done some testing and I still think I am.
>
> I'm taking readings from the Kill-A-Watt EZ and from the APC Smart
> UPS
display panel itself. There is more stuff on the ups than just the pc,
and the Kill-A-Watt measures just the pc, but the data is still useful.
>
> Test 1: Idle PC Kill-A-Watt: 193 W, 195 VA, .98 PF UPS Display: 265
> W, 277 VA, .96 PF (calculated)
>
> Test 2: Loaded PC Kill-A-Watt: 445 W, 448 VA, .99 PF UPS Display: 485
> W, 502 VA, .96 PF (calculated)
>
> Presumably, the ups measures VA more accurately than the
> Kill-A-Watt,
so it will know when to shut down if overloaded. However, all my tests
show that I'm running in excess of .95 power factor in both idle and
loaded conditions.
>
> I don't think the advice to do calculations all in VA is a good idea
for a couple of reasons.
>
> First, is accessibility to the data. I looked at the name plates on
> a
power brick for my laptop, my monitor, and my pc power supply. Granted,
this is a limited statistical sample, and granted, you might not want to
attach your laptop to a ups, but you get the idea.
>
> Laptop: no VA rating, no W rating, V = 100 - 240, A = 1.5 This makes
> no sense to me in that the A rating is not different for
240 V. It should be substantially less. In any case, 120 V * 1.5 A = 180
W. That sounds way off to me, but it's what the label says.

120V * 1.5A = 180 VA not Watts.  AC circuit.  It's only Watts if it's 
resistive.

> Monitor: V = 100 - 240, A = 1, calculate 100 V * 1 A = 100 W
>
> PC Power supply, in the PC, no ratings visible, box says 750 W.
>
> Basically, there is not a VA anywhere in site. Nada.
>
> Also, there are many wall warts running phones and hubs around that
> it
would be almost impossible to reach and read the labels on.
>
> The second reason I don't think you should gauge by VA especially
using the .6 conversion factor, is that you get the wrong numbers.
>
> Let's take the figures above. They add up to 1030 W. Using your
conversion factor of .6, this equates to 1716 VA. So, you might buy an
1800 VA / 1100 W ups, if one existed. This would be pretty pricy, but
would probably work. It would probably be overkill.
>
> Let's take another example. Let's say I know I'm pulling 800 VA
> based
on the Kill-A-Watt, including pc and all the stuff I want to keep
running. Let's assume the reading is fairly accurate. So, we take 800
VA, and multiply by .6, and conclude that I can buy an 800 VA / 480 W
ups. This assumes I'm running at .6 power factor. This is totally wrong.
I have demonstrable evidence that I'm running at .95 power factor. So, I
don't need 480 W, I need 760 W. So, I've just dramatically undersized my
ups.

No, you haven't undersized it other than to trip up the arbitrary alarm 
programmed into the unit (as mentioned above).  If the inverter can 
handle 800 VA total, it can handle 760 Watts assuming the firmware will 
let it (and it probably won't).

> This is exactly the situation I'm in, although the load grew to
> exceed
the ups. I need 540 W minimum, and the ups only has 500W.
>
> So, if you start with W, convert to VA, and buy that VA of ups,
assuming they use the same conversion factor, you'll be OK.
>
> But, if you start with VA, and don't account for the W, your ups
equipment will be way too small, especially if running at a power factor
of greater than .6. The greater the power factor, and hence the more
real power, the more undersized your ups will be.
>


The whole issue comes down to lawyers anyway.  If lawyers didn't exist, 
that 1500 VA UPS could supply 1500 W to an incandescent light bulb no 
problem because physics (and the supposed actual design) says so.  But 
lawyers exist so the UPS can only supply 865 W because the lawyer said 
so.  The lawyer doesn't believe in physics, just checkbooks. :)