MIL-HDBK-1004/5
except when the system can reuse existing motor-generator sets that meet, or
can be adapted to meet, criteria. In such cases use new motor-generator sets
which match the output voltage of the existing sets.
b) Unit capacity. Normally, provide 312-kVA generators (the
largest capacity now being produced as a standard by more than one
manufacturer), since this size is usually the most economical and has the
maximum full load efficiency (see Table 9). Use other unit capacities when
adequately justified.
c) Vertical shaft construction. Vertical-shaft construction
minimizes floor space requirements. A 312-kVA vertical motor-generator set,
weighing as much as 6 tons (5500 kilograms), is approximately 4 feet (1.2
meters) square by 6.5 feet (1.98 meters) high. The same size horizontal unit
can require a 6-foot (1.8 meters) by 7-foot (2.1 meters) floor space and can
be almost as high. These areas and loads do not include the rest of the
assembly requirements. Provide a clear space of at least 3 feet (0.9 meter)
above the motor generator to allow for maintenance of the vertical unit.
Table 9
Typical Full-Load Efficiencies
______________________________________________________________________________
Input
Output
Efficiency
Horsepower (1)
Percent
kVA
kW
______________________________________________________________________________
400
312
250
88
300
250
200
87
250
219
175
86
250
187
150
85
200
156
125
83
150
125
100
80
100
93.8
75
78
100
75
60
76
75
62.5
50
75
______________________________________________________________________________
(1) Nearest standard size. Actual input horsepower may vary, depending
upon the individual manufacturer.
Other Components. Figure 5 shows the other components that are
3.3.4.2
provided as a part of a packaged frequency conversion unit. This ensures that
units are factory designed to meet performance requirements.
a) Voltage step-up. Match the kVA of the low-to-medium-voltage
step-up transformer specifically to the generator capacity. Provide voltage
sensing devices on the transformer output to regulate the voltage of the
motor-generator set. This regulation ensures that the voltage level at the
medium-voltage bus of the 400-Hz feeder distribution center remains constant
under any steady-state load condition.
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