Radar System Engineering

584 PRIME POWER SUPPLIES FOR RADAR [SEC. 14.11
minimum of two hours per month at full load. For such emergency
service as that outlined, gasoline-powered prime movers are reliable and
satisfactory.
Voltage Stability of Supply.—If the service is obtained from a source
unstable in voltage, Iocalvoltage stabilization must reemployed. If the
voltage changes take place slowly, without transients, they can be eliminated
by means of induction regulators, with automatic or manual control;
tapped autotransformers, manual or servo-driven; or (for small
loads) variacs. For example, consider the use of commercial power by a
remote beacon station served by a line shared with other power users.
As night falls, increasing load on the line lowers the voltage at the beacon.
Servo-driven variacs have proved useful in such a situation.
Interference from Common Loads.—If the loads sharing power service
with the radar impose high intermittent demands, and especially if the
total capacity of the system is low with respect to these loads, serious
transient interference may be encountered. An isolating motor-generator
is the only satisfactory solution for such difficulties. If synchronous
frequency is not essential, induction-motor drive is preferable to the
greater complexity of a synchronous motor. It is good practice, where
possible, to make input and output services identical, and to provide
means for bypassing the isolating set in case of trouble. That is, if the
main service is 220 volts, 3-phase, 60 cycles, the motor-generator set
should be 220-volt 3-phase input (60 cycles) to 220-volt 3-phase output
(58 cycles).
14.11. Large Systems Where No Commercial Power Is Available.—
Where 24-hour operation of a prime mover is required, it is necessary to
provide duplicate power generation equipment, regular and emergency.
Such equipment should be interchangeable, and the two sets should be
used alternately to ensure reliable service. Where extreme reliability is
desired, three engines should be furnished; one can then be disassembled
for maintenance without risking complete system breakdown if the service
engine fails. Generators of a size at least t~vice the estimated load should
always be provided, to permit future additions of auxiliary devices not
initially specified. Diesel engines are definitely more reliable for continuous
operation than gasoline engines. Diesel fuel and diesel exhaust
fumes are less hazardous.
Electric power for loads larger than 7.5 kva should be 208/120-volt
3-phase 60-cycle alternating current. It is desirable to use 3-phase
60-cycle because electric motors of this rating are compact and simple to
operate. Weight savings in rectifier-filter supplies can usually be achieved
with 3-phase power, if this is important.
For large mobile systems it is well to investigate availability of standard
U.S. Army power-supply units. Unfortunately, the voltage