Page 96 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
94 Chapter Three

Motor B is rated at 70 kW with a full load current of 123 A at 0.89
power factor. At starting it takes 861 A at a power factor of 0.15, and
the peak accelerating power is 175 kWe at a power factor of 0.6.
In this example the following assumptions are made:

The generator efficiency is 94 percent.
The generator has a permanent magnet pilot exciter and the volt drop
on application of full-rated low-power factor kVA is 11 percent.
The maximum permissible voltage drop is 12 percent.
The load as stated includes all the electrically driven engine auxil-
iaries and the charging of any UPS batteries.

The loading sequence may be set out in tabular form, the com-
pleted table appearing as Table 3.1. It includes a great deal of infor-
mation and an explanation of how it is compiled follows. The
left-hand section entitled “Applied load” is compiled first, and it will
be found that, for each line of this part of the table, two items of infor-
mation are available. For the mixed load, the kWe and cos are
known; for the starting and running conditions of the motors, the
current and cos are known; and for the peak accelerating power,
the kWe and cos are known. From the two known figures in each
line it is possible to calculate the other three values.
The last line of the table indicates the load at the end of the loading
sequence, and two values can again be extracted from the preceding fig-
ures. The figure for kWe is the sum of the mixed load and the two motor
running loads, that is, 240 76 65 381, and for kVAr is similarly
180 39 33 252. From these two figures the other three values can
be calculated; neither current nor kVA should be added arithmetically
because they include phasor quantities.
The right-hand section entitled “Cumulative Load” can now be com-
piled. The mixed load is the first to be applied and the cumulative load
figures are the same as for the applied load. The second line down is the
motor B starting transient condition and the kWe figure is the sum of
the mixed load and the motor starting load, that is, 240 89.5 329.5,
and the kVAr figure is similarly 180 590 770. The kVA figures can-
not be obtained by arithmetic addition and have to be calculated if they
are required.
The third line down is the motor B accelerating transient condition
and the kWe figure is the sum of the mixed load and the motor acceler-
ating load, that is, 240 175 415, and the kVAr figure is similarly
180 233 413.
The fourth line down is the motor B steady state running condition and
the kWe figure is the sum of the mixed load and the motor running load,
that is, 240 76 316, and the kVAr figure is similarly 180 39 219.

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