Uninterruptible Power System 800 Guía de instalación descargar pdf (Pagina 4)

PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998

Notice that the wattage is not known nor is the power factor. Since in the U.S., 120 VAC is the

nominal value of input voltage, we can calculate the CPU’s volt-amperes (VA) directly:

IBM CPU – 120 V x 1.5 A = 180 VA

If we use our rule of thumb and assume a power factor of 0.8, then

180 VA x .8 = 144 Watts (W)

The value difference between 180 VA and 144 W, 36 VA circulates as

unused power between the input power source and the computer.

So at this point we have two numbers 180 VA and 144 W, which one do we use? It depends on

how the UPS capacity is stated.

The SCI Powerbase UPS have their capacities stated in Volt amperes, but the DataShield UPS

have their capacities stated in watts. Use whichever number is appropriate to the UPS system

selected.

So far, we’ve only covered the CPU but the calculations for the monitor are similar:

IBM Monitor - 120 V x .9 A = 108 VA108 VA x .8 = 86 W

IBM model 30

CPU 180 VA 144 W

IBM B/W

Monitor 108 VA 86 W

System Total 288 VA 230 W

Thus we should be looking at a UPS with a V rating of 300 VA or a wattage rating of approximately

240 W.

Notice we’re being conservative in that the derating factor of 20% was not used. It is good design

practice to oversize the UPS to take care of those equipment additions such as tape drives and

modems that invariably get added to the computer system later.

In larger computer systems such as those supplied by IBM, DEC and Data General, detailed

computer printouts listing the volt ampere, wattage and peak current of each device can be

obtained from the computer manufacturer’s local sales office.

These print outs are essential

for accurate, comprehensive sizing of the UPS system!

We have not mentioned three phase power which is found in larger computer systems, but the

principles are much the same; i.e., the individual device volt ampere (VA) ratings can be directly

added to produce a composite value. In these larger 3 phase systems (typically above 10,000 VA

or commonly written 10 KVA) wattage is almost never used as sizing measure.

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Uninterruptible Power System 800 Guía de instalación Pagina 4