Radar System Engineering

586 PRIME POWER SUPPLIES FOR RADAR [SEC. 1414
to a permanent-magnet generator. The speed is controlled by an
air-vane
governor.
Judson produces several small models, one of which is rated 150 watts
400 cps and weighs about 15 lb. The general design is similar to that of
the Jacobsen machine, except that the engine is belt-connected to the
generator.
There is no way of regulating the output voltage of permanent-magnet
generators, but capacity in series with the output will hold the output
voltage constant to about i 5 per cent despite changing load. The
voltage can be altered to compensate for temperature changes by manual
adjustment of the speed governor on the engine.
Work has been initiated on the development of small prime movers
other than conventional gasoline engines. Small steam engines that
were capable of about 300 mechanical watts output were built by Radiation
Laboratory, but no satisfactory boiler had been developed at the
close of activity. Small steam engines and gas turbines are attractive
because of their ease of control and flexibility. Further development
would probably produce some worth-while small prime movers.
14.14. Ship Radar Systems.—The problem of providing power for
large shipborne radar systems is principally that of stability. Most
modern warships have 440-volt, 3-phase, 60-cps power supply which is
subject to transient fluctuations of 10 to 20 per cent in voltage and perhaps
5 per cent in frequency. Transients are caused by the sudden
application of large loads such as gun turret drives and airplane elevators.
Good results have been obtained by powering the radar system from a
60-cycle motor-generator set. An induction motor is used to drive the
alternator; when not heavily loaded, the mot or shows a relatively small
change in speed for a change in input voltage. Motor speed tends
to follow a change in frequency, but the mechanical inertia of the set is
usually adequate to hold up the speed during the usual short-period
frequency transients. The induction motor rating should be about
50 per cent larger than that required to drive the alternator. It is
desirable to have the alternator output the same as the ship’s power so
that the radar system can be operated directly from the ship’s supply in
case of breakdown or maintenance shutdown of the motor-alternator set.
The motor may be direct-coupled or belt-connected to the alternator;
usually direct coupling is preferred because of its simplicity. The output
frequency of a direct-connected alternator will be less than 60 cps by the
slip of the induction motor. This results in an output frequency of
57 to 58 cps, which is normally satisfactory for the operation of transformers
or other 60-cycle equipment.
There is no really satisfactory means now available for regulating the
output voltage of the alternator. Most of the available voltage regu-