Radar System Engineering

SEC. 14.7] DYNAMOTORS 579
14.7. Dynamotors.—To supply smaller radar sets with moderate
power at not over 1000 or 1200 volts, a dynamotor is often more economical
in weight and efficiency than an inverter. A dynamotor has conventional
d-c shunt- and series-field windings on the stator and one or
more armature windings on the rotor. The rotor winding is excited
through brushes and a commutator from the input voltage.
A single rotor winding can be tapped to give the required secondary
voltage. Alternatively, separate rotor windings can be used, with
turns-ratios to the primary winding chosen to provide the correct output
voltages. The required d-c voltages are brought out through commutators
from the secondary windings. Dynamotors have been built
i
FIG. 14.23 .—Eicor Inc. aircraft dynamotor,
to give voltages as high as 2000 volts, but such machines meet severe
difficulties with commutation and insulation. Dual- and triple-output
dynamotors are very common, and several machines having four outputs
(Fig. 14.23) have been built, but it is impractical to go beyond this
because of the large number of windings and commutators involved.
Dynamotors for military aircraft are built for an input voltage of
27.5 volts. For automobiles, commercial aircraft, and miscellaneous
use, dynamotcrs are built for input voltages of 6, 12, 32, and 110 volts.
Output ratings vary from 14 or less to over 1200 volts, and from 30 ma
to 5 amp, with any combination of two, three, or four at a time. Weights
vary from 5 lb, for a total output of 20 watts, to 26 lb, for an output of 400
watts. Efficiencies range from 40 per cent to 60 per cent.
The dynamotor suffers from poor regulation. Furthermore, variations
in input voltage may be as great as 10 per cent. The dynamotor