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SECTION 1
INTRODUCTION AND
THEORY OF OPERATION
INTRODUCTION
The magnetic amplifier voltage regulator is designed
to operate with brushless synchronous 400 Hz
generators. The voltage regulator controls generator
voltage by regulating the amount of current it supplies
the exciter field.
SPECIFICATIONS
Input Power: single phase, 400 Hz, 120 Vac
Sensing: three phase, 400 Hz, 120-139, or 208-240
Vac
Output: continuous maximum: 55 Vdc, 4 amperes
Test Circuit: 50 Vdc (optional)
THEORY OF OPERATION
Note: Refer to drawings
A. Sensing
A sample of the generator output voltage is applied to
regulator sensing terminals E1, E2 and E3. Tap
setting H2 for 120 or 208 Vac and H3 for 139 or 240
Vac. This voltage is applied to sensing transformer T1
and T2, which proportionally reduces the voltage.
Regulators equipped for parallel operation also
include transformer T4. This circuit does not affect
voltage regulator operation in single unit operation if
the CT leads are shorted. (See B below.)
The reduced AC voltage is then fed to a rectifier
consisting of diodes D1 through D6. The rectifier
output, which is proportional to the sensing voltage, is
filtered by capacitor C1. It is then impressed across a
voltage divider circuit consisting of resistors R2 and
R4, voltage range set adjust potentiometer R3, and
the externally mounted voltage adjust rheostat. A
portion of this voltage is applied to the base of
transistor Q1 in the error detector circuit.
B. Sensing Circuit During Parallel Generator
Operation in Parallel Cross-Current Compensa-
tion Mode
Parallel cross-current compensation allows two or
more parallel generators to share reactive loads and
maintain constant system output voltage. The reac-
tive error signal developed in the external paralleling
circuit is added vectorially to the sensing voltage by
transformer T4. The circuit is phased such that a
generator with excessive excitation will decrease
excitation and an under-excited generator will in-
crease excitation. At balance, each generator will
carry its share of reactive load.
C. Error Detector
The error detector is a differential amplifier. Resistors
R11 and R14 in the emitter circuit of transistors Q1
and Q2 are of equal value. Therefore, when the base
voltage of Q1 is equal to the base voltage at transistor
Q2, Q1 and Q2 conduction will be equal. Transistor
Q2 is the reference voltage side of the error detector.
Its base voltage is kept constant by zener diode Z1.
The base voltage of Q1 is proportional to the sensing
voltage and the setting of the voltage adjust rheostat.
Q1 base voltage will therefore increase and decrease
in direct proportion to any increase or decrease in the
sensing voltage or change in the setting of the voltage
adjust rheostat. Operation of the error detector (Q1,
Q2), second stage differential amplifier (Q3, Q4), and
amplifier Q5, Q6 is as follows.
An increase of Q1 base voltage will decrease Q1’s
collector voltage and increase Q2’s collector voltage.
The differential collector voltage is directly connected
to the bases of Q3 and Q4. This voltage increases
Q3’s collector current and decreases Q2’s collector
current. The increase in Q3’s collector current is also
an increase in base drive to the control current
amplifier transistors Q5 and Q6. The collector current
of Q5 and Q6 and the magnetic amplifier control
current increases, resulting in decreased magnetic
amplifier output. Conversely, a decrease in sensing
voltage results in lower conduction of transistors Q1,
Q3, Q5, and Q6, a decrease in control current and an
increase in magnetic amplifier output.
D. Power Stage
The output power stage of the voltage regulator
rectifies 400 Hz power and feeds a regulated amount
of dc power to the exciter field. The power output
stage consists of an electromagnetic filter (RFI), two
full-wave rectifiers (BR1 AND BR2) and two toroid
magnetic amplifiers (MA-1 and MA-2). The magnetic
amplifiers contain two windings wound about a
ferromagnetic core. The windings are: a power or
“gate” winding and a control winding C1 and C2. Field
excitation is controlled by operation of the magnetic
amplifiers as described in the paragraphs that follow
The single phase, 400 Hz, 120 Vac power applied to
regulator terminals L1 and L2 magnetizes the mag-
netic amplifier core. During the time the core is being