Radar System Engineering

sm. 145] VOLTAGE REGULATORS 569
burned because, as the film of moisture is gradually reduced, it may
break down at one particular point on the face of the disk and the high
current-density at that point may be sufficient to burn the disk. In
this event, the regulator pile must be removed and the damaged disk
replaced.
One fundamental defect of the carbon-pile regulator is the lack of any
means for insuring uniform voltage distribution across the stack. Some
carbon junctions may be tightly +d.c
mated, with low voltage drop;
Carbon.pile
others may be loose, with high voltage
regulator
voltage drop. If the drop per
115V
junction exceeds 1 to 2 volts, Rect.
Alternator
4oocps
sparking may occur which will field P s
ruin the pile.
I:Ilk
Under changing load condi- FIG. 14,13.—Schematic diagram of
tions, the regulator acts to in- s~abilizing transformer in voltage regulator
cmcuitof 400-cpsaircraft inverter,
crease or reduce the field current:
as in any action of this kind, hunting may result. In order to reduce
hunting, a stabilizing transformer (Fig. 1413) is now used with most of
the larger alternators. Such a transformer provides a correction depending
on the rate of change of excitation.
The resistance of the carbon pile ranges from 2 to 60 ohms.’ Its
electrical rating is based on the rate of allowable heat dissipation, and is
given in watts. The unit often used on aircraft inverters is rated at
35 watts. Smaller inverters, notably the Eclipse 100- and 250-va units,
use a smaller regulator rated at 20 watts, while 2500-va inverters use a
75-watt regulator, of the same physical size as the 35-watt unit, but
with fins to increase the rate of heat dissipation. For the purpose of
using a smaller regulator than would otherwise be possible, the carbon
pile can be shunted with a fixed resistor. Such an arrangement increases
the range of resistance over which the carbon pile must operate, and care
must be taken that the maximum stable range of the pile is not exceeded. z
The coil of the carbon-pile regulator usually used in aircraft inverters
has a resistance of 185 ohms and a d-c rating of 115 to 125 ma, which
gives a voltage drop of 21 to 23 volts. As the voltage to be regulated is
115 volts alternating current, rectification and voltage-dropping are
necessary. Rectification is provided by a selenium dry-disk rectifier,
in series with a “ globar” resistor which provides some temperature
1Although the 35-watt carbon-pile regulator can be made to operate over a range
of resistancefrom 2 to 60 ohms, it has been found desirable to use only the range from
2 to 30 ohms since the high-resistance end of the range has a tendency to be unstable.
2Mr. C,. Neild, “ (?arhon.pile Regulators for .kircraft,” AIEE Transactions, 63,
839-842 (November. 1944).