Radar System Engineering

SEC. 145] VOLTAGE REGULATORS 563
the normal full-load current. Such high starting currents cause: (1)
excessive brush wear, (2) voltage dip on aircraft electrical system,
(3) excessive load on contactor points unless an oversize contactor is used.
Extremely high currents may cause laminated brushes to explode.
Voltage dips on the electrical system can cause the release of vital relays
or contractors. To reduce the initial current surge a series resistor
(often anauxiliary series motor field) isadded. Whenthemotoris upto
speed, this resistor is shunted out by a contactor (see Fig. 14.4. ) This
reduces the initial starting current, but the current again rises sharply if
the secondary contactor closes too soon. ClOsing of the secondary
contactor can be further delayed by an auxiliary starting relay connected
as shown in Fig. 14.5. The curve of Fig. 14.6 indicates the
reduction in starting current effected by the use of the auxiliary starting
relay.
14.5. Voltage Regulators.—Since constant output voltage must be
maintained despite changes in load, shaft speed, and d-c input voltage,
some form of voltage-regulating device is used with an alternator. For
radar applications, an electronic voltage regulator is the most satisfactory
type. Although it weighs more than a simple mechanical regulator, an
electronic regulator should be employed on systems delivering 75o va
or more.
Electronic Voltage Regulators.—An electronic voltage regulator
consists of a voltage-sensitive element, an amplifier, and an output stage
that supplies d-c excitation for the generator or alternator field.
Three types of voltage-sensitive elements have been used in experimental
regulators. One consisted of a VR-tube bridge excited from a
transformer and rectifier connected to the output of the alternator to be
controlled. A suitable filter could be added to the rectifier so that
regulation was performed with respect to the peak value of the output
waveform. Similarly, other types of filters could be used to regulate
with respect to the average value of output voltage, or to some chosen
value between peak and average. The filter introduces a time constant
that, in some cases, is too long to achieve the desired rate of response.
The second voltage-sensitive element, developed by Bell Telephone
Laboratories, was a bridge network made of thermistors and excited
from alternating current. This gives an a-c output error voltage that
can be readily amplified, but has the disadvantage of a relatively long
time constant. The thermistor bridge, though rather difficult to compensate
for wide variations in temperature, regulates to the rms value of
the output wave, which is a considerable advantage for some applications.
The third form of voltage-sensitive element was a tungsten-filament
diode operated in the region of saturated emission. The filament was
heated by the alternator output through a transformer, and the anode