VA
and Watts made simple
People often become perplexed by the two expressions ‘VA’ and ‘Watts,’ however
it is important to understand both and their constraints and limitations, in
order to match a UPS to an individual load.
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Figure
3:2 VA and Watts in a Linear Circuit
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As demonstrated in the top drawing of the above diagrams, the VA rating
can be found by multiplying the supply voltage (V) and the current (A) drawn
from the supply using the rms (root mean square) values of voltage and current
in each case.
Watts (W) are calculated by multiplying the load current
and the supply in the direct current (dc) circuit, this is done in an identical
way to that
described for VA. For example: (V × A= W.) The circuit values of VA
and Watts are in some circumstances identical for instance when the supplied
voltage and load current are in phase in an ac circuit which is feeding a
purely resistive load. The bottom drawing of the diagram above demonstrates
this.
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Figure
3:3 VA and Watts in a Non-linear Circuit
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More often than not the equipments attached to an alternating current (ac)
circuit are not linear. Conventional ac loads are inductive in character;
this often causes the equipment to be behind the practical voltage amount
by a total relative to that of the equipments reactance. The diagram above
(Figure 3.3) clearly illustrates and gives example of this.
Observe in the illustration above that the Volt Ampere rating is the same
as that in the previous diagram. This is due to the fact that root mean square
(rms) values of voltage and waveforms are not affected by the relative phase
shift. Additionally in both instances given in this explanation the current
drawn from the supply is identical. Nevertheless the bottom illustration
in figure 3.3 presents a declining 960W in Watts’s rating, this was
previously at a figure of 1200W thus specifically the equipment is dissolving
far less Watts in comparison to VA, which stays the same.
The decreased wattage is a result of the phase connection involving the current
waveforms and the voltage; this is because a negative value is created by
the product of V × A, as the limits are of dissimilar polarities. This
is again demonstrated in Fig 3.3.
Negative power trips in the illustration are symbolising
what is known as wasted power, this is power that is not dispersed in a
functional way from
the operating load. This wasted power is a result of and comparative to the
combined load’s reactance, this is why it is occasionally mentioned
as reactive waste also.
(pf)
Power Factor
This
can be described in simple terns as the connection between real and relative
power in a alternating current (ac) circuit.
It is also the ratio of Watts to Volt Ampere.
| Power Factor (pf) |
= Watts |
| |
VA |
By working out the cosine of the Phase angle between the current waveforms
and voltage the power factor can be easily established.
E.g. As shown in figure 3.3 in which the phase angle is 36° the load
power dispersion can be worked out using the following method:
Watts = VA × cos36 = 1200 × 0.8 |
Watts = 960 |
Obviously, if there are two loads each sharing an identical
wattage ratting but with different (pf) power factor’s and they are
both connected to the same supply of power, the load which has the higher
pf is going to
take out a lesser amount of current from the shared supply as its partner
load.
It is routine that a Uniterutped Power Supply is often referred to in relation
to its output VA (kVA) rating. It is standard to presume a number of 0.8
in the case that there is no given output power factor on a UPS. An example
illustrating this is: If a UPS is 1kVA and has a maximum power rating of
800W, 0.8 is the outcome. Also under these conditions neither of these given
values should be exceeded by the total load.
Measures are taken from time to time in order to advance the complete power
factor and bring it closer to harmony, this is usually in situations where
large or highly reactive loads are concerned. This process is commonly known
as, ‘power factor correction’ and it is in the main part accomplished
by putting in a capacitive load in addition to the load equipment to reduce
the complete circuit reactance.
What
Uninterruptible Power Supplies are available to you
There
is an extensive assortment of different UPS modules currently on the market,
options range from the extremely compact desktop models that are now available
up to vast units that hundreds of kVA . The systems power rating can now
easily be increased to way over a thousand kVA, this is achievable when some
manufacturers create their USP with the capability to be paralleled with
other units. For example 2 or 3 MVA units are doable.
Desktop
Units
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Figure
3:4 Desktop UPS Models
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Micro
Models- maximum of 250VA
Micro
systems in this low power range are purpose-built in order to supply and
protect only a single PC. These are usually accommodated in a mini-tower
box, which is around only half the size of an average PC system unit. Because
of the micro models small dimensions and lightness in weight it can be considered
to be transportable. It should be connected to a mains power supply outlet,
for example in the UK a three-pin 13A, wall socket. Units at a power level
below 250V offer a solitary point solution to a specific need and include
on-line, off-line and interactive designs.
On the back on most UPS is a IEC, which is a standard mains connector that
is protected by a fuse or circuit breaker, this should usually be connected
to the load equipment.
Batteries for UPS units of such a low power level are typically integrated
into the body of the unit; this means that it is very improbable that additional
battery cabinets will be available as an extra.
Micro models are indented to be located
nearby the load user, due to this factor it is not normally
required that a remote alarm facility be provided to alert
the user of the machine status of operation. Though, today’s
needs may require that an automatic control interface be
installed between the UPS and the personal computer.
Mini-models:
VA of between 500-2000VA
Mini UPS models are in numerous areas very comparable to the micro units
just mentioned. Both are intended to be used in offices and both are small
and light enough to be deemed portable. The lager power rating of the mini
UPS makes it appropriate for supplying not only a personal computer but its
connecting equipment such as printer, web-cam, scanner and so on.
Like the micro systems these are designed to be plugged into a 13A three-pin
socket and again can include on and off line and interactive designs.
On the back on most UPS is a IEC, which is a standard mains connector that
is protected by a fuse or circuit breaker, this should usually be connected
to the load equipment. However unlike the previous machine there is likely
to be some additional supply outlets for small items of load equipment E.g.
printers.
Again at this current power rating the batteries are likely to be integrated
into the design f the UPS box, some units however may be capable of connecting
supplementary batteries this would be enclosed in an additional purpose-built
battery box, this would then increase the overall back-up time of the batteries.
The charger for the external batteries will still be sufficient in providing
the power needed to re-charge any additional batteries. On the other hand
in extreme situation an external charger system may be required and thus
will also need to be connected to the main power supply.
Mini models are indented to be located nearby the load user, due to this
factor it is not normally required that a remote alarm facility be provided
to alert the user of the machine status of operation. Depending on whether
the load is critical or not, this system may often require SNMP or automatic
shutdown.
About
Systems between 3-20kVA (medium sized)
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Figure
3:5 medium sized systems
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These medium sized UPS are intended to supply much more than the desktop
units and are capable of supplying more than a single point power provision.
These UPS are often used to power loads such as office networks and communication
centres.
The medium sized model is not to be viewed as a portable machine like the
two precious UPS, as they are designed to be constantly wired via a switchgear
of medium power to the mains power supply, this may also need external input
overload protection. Some of the biggest models in this medium range could
require three-phase output as some may also produce a three-phase output.
As this is quite an extensive power range the issue of batteries changes
from one end to another. At the low end of the scope the explanation about
batteries concerning desktop UPS is still what to go by. However in the upper
end of this range for example from 15-20 kVA batteries may be housed in an
additional cabinet which is to be located adjacent to the UPS unit. These
are usually attractive to the eye as many manufacturers produce a matching
US and battery cabinet that fits well into most offices.
At the bottom end of the power ranger the UPS can be connected to a three-pin
13A socket similar to the desktop UPS units. In contrast to this the most
powerful pf UPS in this range are designed to be hard wired to distribution
busbars which will be fitting inside the cabinet provided, alternatively
a purpose built distribution system can be connected through the UPS output.
About
30-400kVA High power systems
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Figure 3.6 large systems |
Systems with a high power rating are capable for proving power to large
data centres, however they are not usually appropriate for use in an average
office environment as they do create high noise levels while in operation,
this is due to the cooling fan which is essential to lager and high-powered
loads. As a result such UPS unit are generally stored in a out-of-the-way
location. Their output will typically be connected to a very large number
of loads, which will use a devoted power supply, this must hold an external
switchgear and protective device.
These highly power system are almost always use for online design and
always three-phase input and output.
Unlike the prior equipment the batteries will not be housed inside the
UPS box. Depending on the required back-time they are to be stored in separate
box(s) neighbouring the UPS unit or in the circumstances of an extremely
large system put in a devoted battery room and rack-mounted.
A 12 pulse rectifier is in use by some of the largest UPS models in order
to decrease the quantity of main inference such harmonics, these are
created inside the Uninterrupted Power Supply entering the utility
mains supply.
With the employment of a 12-pulse rectifier a separate box is often needed
to accommodate it, it is essential that it is located next to the UPS.
A generator may also be added to the systems designs with the intention
that it back up the load when there is a utility mains failure. There
stand-by generators must be self-starting and also must be able to maintain
a clean
and reliable power supply with the UPS on full load. For more information
see the: About generators section.
As this UPS is likely to be located
in an isolated purpose-built room a remote alarm a necessary,
the majority models in this power rating range will come
with this remote alarm as a standard feature.
