TM11-6125-256-34
Section II OVERALL CIRCUIT ANALYSIS
shunt field the motor would overspeed The regulator
applies the shunt field as the speed approaches the
(fig FO-1)
equivalent of a 400-Hz generator output The shunt field
The electromechanical section of the motor-generator
average current is varied from this point on to maintain a
consist of the motor and the generator Both units share
400-Hz generator output
the same housing, with separate stationary windings The
armature of the motor and the exciter of the generator
share a common shaft
b Generator The ac generator is a rotating-field type
generator Direct current from the regulator is fed to the
rotor through the slip rings and the four ac contact
brushes
a Motor The motor is a four-pole series-shunt
motor with windings wound in magnetic addition to each
other It operates on a 28-volt dc input drawn from the
aircraft electrical system
(1) When the motor starts it turns the exciter of the
generator As the exciter rotates, residual magnetism and
then the magnetic fields set up by the current through the
exciter windings pass conductors in the ac stator and
induce an alternating current
(1) Power for operation of the motor is initially
applied through the series coil Four dc brushes are
located at the neutral point between the four coils (poles)
(2) Alternating current is then passed through radio
Therefore adjacent commutator bars on the armature
interference filter RF1 and made available at the ac
reach a dc brush when no voltage (neutral point) is
power connector A portion of this alternating current is
between brushes The neutral point is set by the position
fed to the electronic section for use in regulating the
of the interpole windings
alternating current amplitude and frequency
(2) When 28 volts dc is impressed upon the series
coil current will flow from the positive (+) terminal
through the series and interpole coil and through the
armature and brushes The magnetic field resulting from
the current flow through the series coil reacts with the
magnetic field set up by the current flow through the
a The regulator senses one phase of the motor-
armature of the motor causing the armature to rotate
generator ac output separate parts of the regulator
compare magnitudes of the sensed ac voltage and
frequency to their respective present values If an error
(3) By rotating through a magnetic field, the
armature causes a counter electromotive force (cemf) to
exists between either of the motor-generator controlled
entities and its preset comparator the regulator will
be induced in the armature winding The speed of
armature rotation continues to increase until the cemf is
function to minimize that error maintaining ac output
almost equal to the dc input The small difference
voltage and frequency with the desired tolerance
between the cemf and the dc input permits the flow of
enough current to keep the armature rotating
alternately switching the field transistor into saturated on
(4) The motor starts and comes up to speed as a
and off states Due to the method of sensing used, this
series motor As the motor speed increases the
switching rate is proportional to the ac frequency The
armature voltage rises The rises in armature voltage is
proportion of on-time and off-time, which determines the
sensed by the series field cutout circuit which latches and
average field voltage (and so field current), is a function
shorts out the series field Without the application of the
of the error signal
