Radar System Engineering

560 PRIME POWER SUPPLIES FOR RADAR [SEC. 14.3
In many cases, a limit on the number of engine mounting pads will
force the use of a dual-purpose generator such as the Bendix Aviation
Corporation NEA-7, which delivers 125 amp of 27.5-volt direct current
and also, independently controlled in voltage, 2500 va of single-phase,
115-volt alternating current at 800 to 1600 cps for shaft speeds of 4000
to 8000 rpm. (See Fig. 142.) A typical example of a large 3-phase
direct-drive alternator is the General Electric Company 10-kva 3-phase
208/ 120-volt 400- to 800-cycle machine shown in Fig. 14.3. This
photograph shows the installation as designed for use in TBM-3 aircraft,
using an auxiliary gear box to obtain dual output from the TBM-3’s
single engine pad. This expedient is usually much more successful than
attempting to convert (with low efficiency) direct current to large
“bites” of alternating current. When pressed for output mounting-pad
space it is well to consider such “dual outlet” gear boxes. Often such
a box can be employed with older engines (2000- to 4000-rpm jack-shaft
speed) to utilize high-output 4000- to 8000-rpm alternators by incorporating
a step-up ratio in the box. Such a gear box was designed to
permit use of the same General Electric Company generator with the
low-speed jack shafts of the B-17-G aircraft engines.
Large aircraft of the future will probably have electrical systems
furnishing 3-phase 400-cPs fixed-frequency power at 208/120 volts
(120-volt phase to neutral, 208-volt phase to phase). This will give
200/1 15 bus voltages. Preliminary tests indicate that such systems,
especially those having several large generators in parallel, will be stable,
reliable, and directly usable for radar. 1
Summary of Alternatives to Direct-driven A lternators.—If directdriven
alternators cannot be employed in any given case, or if the power
requirements are so small as to render a special generator impractical,
there are five alternatives:
1. Motor-alternator sets. D-c motor input, 400- or 800-cps output.
2. Inverters. Same input and output as (1) but with a single magnetic
circuit—a true inverter.
3. Dynamotors. Low-voltage d-c input, high-voltage (or voltages)
d-c output.
I H. E. Keneipp and C. G. Veinott, “ A 40-kva, 400-cycle Ah-craft Alternator,”
.4ZEE Transactions, 63,816-820 (November 1944).
L. G, Levoy, Jr., “Parallel Operation of Main Engine-driven 400-cycle Aircraft
Generator,” A IEE ‘l%msactions, 64,811-816 (December 1945).
W. K. 130iceand L. G. Levoy, Jr., “Aircraft Alternator Drives,” .41EE Tran-racticms,64,
534-540 (July 1945).
M. M, Hubbard, “ Investigation of >phase 208/120-volt 400-cycle Aircraft
Alternators)” RL Group Report 56-061545.