PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
Selecting from our standard sizes, we find that we must supply a three phase, 40 kVA UPS which
has a 111 amp AC output per phase. One can see that “A” phase is 84% loaded, while “B” and “C”
phases are only 27% loaded, which means that there is substantial room for additional single
phase loads for future expansion – but only limited expansion room for three phase loads.
In the previous example, we have been looking at continuous full load currents. Any load which
has a significant inrush current must also be identified. In order to avoid an overload of the UPS, it
may be necessary to power-up the various loads sequentially. It may also mean that the way the
loads are balanced on a three phase UPS may need to change so that all the high inrush loads are
not on the same phase, if possible.
In the earlier example, if the 12 amp, three phase continuous load had an inrush of five times its
continuous rating (60 amp inrush), and if all other loads on “A” phase were “on” and we attempted to
start the load, a total load of 141.5 amps would be seen on “A” phase. This would cause an overload
and all loads would be transferred to the bypass source until the overload was cleared. If this is likely to
happen often, the consideration should be given to installing a large UPS. Note that SCI inverters are
designed to transfer to the bypass source upon an overload of 120-125% of full load.
As shown in the previous example, if the 5 and 7 amp loads on “B” phase both had inrushes of ten
times their rating and both could start at the same time, then an overload would occur. In this case,
consider moving on of the loads to “C” phase and perhaps an equal amount of load could be
moved from “C” phase to “B” phase.
There are usually many load considerations which must be looked at in sizing the UPS. Be sure that all
possibilities which might jeopardize the integrity of the continuous power have been considered.
It may be necessary, for quotation purposes, to estimate the size of a UPS required for a particular
installation where time does not permit a detailed Load Study.
1. With all equipment operating, one method would be to use a clamp-on ammeter.
For a single phase, two-wire system, measure the hot leg. The kVA rating is calculated by
multiplying the measured current times the line voltage, divided by one thousand:
kVA = amps x voltage
1000
For a single phase, three-wire system (240/120), measure each hot leg. The kVA rating is the
highest current reading multiplied times the line-to-neutral voltage, multiplied times two, divided
by one thousand:
kVA = amps (highest) x voltage (line-to-neutral) x 2
1000
For three phase systems, measure all three hot legs. The kVA rating is calculated by
multiplying the highest current reading times the line-to-neutral voltage, times three, divided by
one thousand:
kVA = amps (highest) x voltage (line-to-neutral) x 3
1000
(12 paginas)
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