Radar System Engineering

SEC. 144] MOTOR-ALTERNATOR SETS 561
4. Mechanical vibrators. Low-voltage d-c input, output direct
current or 120-cps alternating current or any combination.
5. Electronic inverters. Low-voltage d-c input, output as desired.
The options (2) and (5) can bediscarded at once. Electronic inverters,
although theoretically the most desirable, are practical only for
power levels of a few watts because of low efficiency arising from tube
plate-drop. Inverters are usually undesirable because they cannot be
regulated. This leaves motor-alternators, dyrmmotors, and vibrators.
Practical limitations on commutator diameter, voltage per bar, and
operation at altitude restrict the use of dynamotors. For example,
it is never wise to attempt to use dynamotors with outputs above 1200
volts. Motor-alternators with voltage regulators are the most reliable
conversion means.
14.4. Motor-alternator Sets.—Table 143 gives common sizes of
motor-alternator sets now in use.
Capacity, va
100
250
500
750
840” (1OOO)
1500
2500
T.mL~ 14,3.—~w@Io~ SIZM OF ~lOTOW.ILTEhN.tTOI? S~Ts
—
Frequency of
output, Cps
400
400
400
400
800
400
400
Phases
3
lor3
1
10r3
1
1
1
l}-eight,
lb
13
23
38
32 t
.0 5-
’75
19.2”””
21.8
19.8
26 2 (31 .5)
27.2
33,4
* 840 is namqlat. rating, FoToperation below 10,OOQ ft with blast cooling, 1000 w. are ohtsimable.
i Includes weight of external r-f filter and 12-pf series condenser. See Figs. 148, 1416, 14 17, 14.21
for pictures of some commonly used 400-cycle motor-alternators.
For heavy-current duty, particularly at high altitude, brush wear
becomes a problem. Care should be taken to see that all rotating
devices using brushes requiring operation at altitudes above 20,000 ft
have suitable “high-altitude” brushes.1
Starting currents are often a serious problem in the case of motoralternator
sets rated at 750 va or more since they may be 5 to 10 times
lL. M. Robertson, “Effectof Altitude on Electric Ap~>aratus,''Electrical Engineerin~
(June, 1945).
Paul Lebenbaum, Jr., “Altitude Rating of Electric Apparatus,” Transactions
AIE.E,63,955-960 (December, 1944).
D. Ramandanoff and S. IV. Glass, “High-altitude Brush Problem,” l’ransactions
.41EE,63,W$829 (November, 1944).
Howard M. Elsey, “ Treatment of High-altitude Brushes,” Transactions All?.!?
(.kugust, 1945).