Radar System Engineering

SEC. 143] DIRECT-DRIVEN ALTERNATORS 557
14.2. Wave Shape.-4ince all electric power used for radar is converted
to direct current or is used for heating, there is no inherent requirement
for sinusoidal wave shape. 1 However, an alternator furnishing
voltage of poor waveform may show very marked changes in waveform
when its load is changed. Since a stable d-c voltage is the final requirement
in most radar power supplies, it is necessary to maintain a fixed
relationship between the maximum voltage and the rms voltage, whose
ratio is called the “crest” or *‘amplitude factor. ” The output d-c
voltage f or rectifier circuits employing condenser input will be a function
of the crest voltage. For rectifiers employing choke input, the ratio
of rms voltage to average voltage is important; this ratio is called the
“form factor. ” These two ratios should be as constant as possible under
all load conditions. This condition is more easily met by the use of
alternators with low subtransient reactance. Prac~ically, it-is preferable
to employ conventional salient-pole rotating-field synchronous alternators
designed for 400 cps, rather than high-impedance inductor alternators
operating at higher frequencies. Although the reactance of an
inductor alternator can be neutralized by a series capacitance (as is done in
the Model 800-1-C Bendix 800-cps motor-alternator) such balance is completely
effective only at one value of a load exhibiting fixed power factor.
Changes in load cause changes in wave shape. Thus, even if the voltage
regulator (Sec. 14.5) behaves perfect 1y and maintains constant E.. or
E ,~, the output d-c voltage applied to the radar may vary excessively.
(See Fig. 14.1.)
Furthermore, individual machines of the inductor-alternator type
show wide variation in characteristics. Crest factors measured for
different machines of a given type have shown variations in the range from
1.15 to 1.75. When power is to be derived from an alternator of this
type, transformers for radar rectifiers should be provided with primary
taps, and filament supplies should be obtained from separate transformers.
14.3. Direct-driven Alternators.-Since all power is ultimately
derived from the rotation of the aircraft engine, fewer devices that can
cause trouble are required if mechanical rotation is used directly to
drive an alternator. A direct-driven alternator will normally furnish
alternating current whose frequency varies with engine speed. This is
usually satisfactory for radar operation. In some complex components,
such as equation-solving circuits, and for synchro applications, fixed
frequency is necessary. In these cases it is often desirable, particularly
where the a-c loads are heavy, to obtain that part of the load which can
be allowed to vary in frequency from an engin~ alternator, using a small
inverter set to obtain the fixed-frequent y power needed.
10ne exception to this statement should be mentioned. In an a-c resonance
charge modulator, the generator must produce a good sine wave.