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1.9 MAIN ALTERNATOR
(A-C GENERATOR)
Figure 1-6. Main Alternator (A-C Generator)
1.9.1 Principle of Operation
The MAC (Lima) brushless A-C generator is a
self-regulated, rotating field synchronous unit with the
rotor having a salient pole construction with amortisseur
windings. The generator stator and exciter stator are
combined in a common housing. The generator field,
exciter rotor and rotating rectifier assembly are mounted
on a common shaft. The output of the exciter rotor is
applied to the generator field winding through a rotating,
full-wave bridge, silicone rectifier unit.
The exciter pole pieces contain residual magnetism,
setting up lines of force across the air gap to the exciter
armature. When the exciter armature begins to rotate a
voltage is induced and current flow is initiated in the
exciter armature AC windings. This voltage is fed to the
rotating rectifier assembly, rectified and fed to the
alternator field coils. This DC voltage is sufficient to
magnetize the laminated alternator field which will set up
lines of force across the air gap to the alternator stator. As
the generator rotor rotates a voltage will be induced and
current will flow in the alternator stator windings and to
the output circuit.
All connections between the exciter stator windings
and the generator stator windings are internally
connected within the stator housing. Only the output
power leads of the generator unit are brought out to the
generator terminal box.
1.9.2 A-C Generator (Main Alternator)
Circuit Diagram
Figure 1-7 shows the internal schematic diagram of
the generator, exciter and rectifier unit. The generator is
a three phase unit and the exciter stator and exciter rotor
also have three phase windings. A portion of the exciter
stator windings is connected across a tap on the generator
stator winding. This exciter shunt winding provides the
generator field excitation power required for the
generator no load voltage. Another portion of the exciter
stator windings is connected in series with the output of
the generator and provides a compounding excitation
characteristic.
The rotor is, in effect, the secondary of a rotating
current transformer induction frequency converter. The
exciter rotor output voltage is applied to the generator
field windings by a three phase full wave rotating silicone
rectifier unit. The response time of the excitation system
is very fast since the exciter stator carries an alternating
current corresponding to the load current which appears
immediately on the exciter primary. An increase in load
current will cause an immediate increase in the exciter
secondary output voltage which is rectified and applied to
the generator field windings. The inherent compounding
characteristics of the excitation system provide excellent
voltage regulation even under heavy overload conditions.
FIELD
STATOR
GENERATOR
RECTIFIER
ASSEMBLY
Figure 1-7. A-C Generator Circuit Diagram
1-8
SHUNT
SERIES
STATOR
L
O
A
D
ROTOR
EXCITER
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