by the prime mover speed, which in turn is
stationary part of the generator enclosed
controlled by the installed governor. By adjusting
in the generator housing and is the set of
the governor setting on two generators in parallel,
windings in which voltage is produced. It
an operator can equalize the active kW load and
supplies power to the ship's loads (via a
keep the frequency at 60 Hz.
SWBD). The rotor (field) is the rotating part
and is driven by the prime mover, either a
Chapter 4 of this TRAMAN provided
diesel engine or a GTE. Direct current is
information to help you understand the electrical
induced into the rotor by the voltage regulator
principles concerning electric power. Refer to
circuit to create the rotating magnetic field.
NEETS, modules 1, 2, and 5, for additional
The design speed of the generator determines
information.
the number of magnetic poles required on the
rotor and the frequency output.
Generators on gas turbine ships rotate at
MOTORS
1800 rpm. The rotors of these generators are
the salient pole type and have four magnetic
You will be called upon to perform preventive
poles, two positive and two negative. Field
maintenance on 60-Hz ac motors. The size of
excitation is provided by either slip rings and
these motors varies. They can be as large as a lube
brushes on a brush-type generator, or a
oil pump motor or as small as a cooling fan
silicon rectifier assembly on the newer brush-
motor in an electronics enclosure. Regardless of
less generators. The slip rings or the silicon
their size, motor operation is theoretically the
rectifier assembly allows a path of flow for
same.
the current from the voltage regulator to the
rotor. The magnetic field allows for the
The two major types of induction motors you
transmission of energy from the rotor to the
may encounter are the polyphase (3-phase) and
stator. Varying the current to the rotor allows you
the single-phase. The induction motor gets
to control the generator output voltage and the
its name from the fact that electromagnetic
reactive load (amperage).
induction takes place between the stator and the
rotor under operating conditions. Most of the
Brushless ac generators are used in the
large motors found on gas turbine ships are
fleet for ship's service and emergency power.
polyphase. Smaller motors, like those used for
Figure 9-2 compares the brush-type and brush-
electronics enclosure cooling fans, are normally
less ac generators. In the brush-type ac
single-phase.
generator (fig. 9-2, view A), the current is
transferred from the rotating member of the
machine to the stationary member and vice
Polyphase Motors
versa by the commutator, slip rings, and
brushes. Brushless ac generators, on the
The driving torque of ac motors comes
other hand (fig. 9-2, view B), have a silicon
from the reaction of current-carrying con-
rectifier assembly that replaces this coupling.
ductors in a magnetic field. In induction
This development is simple, compact, free of
motors, the rotor currents are supplied by
sparking, and greatly reduces the maintenance
electromagnetic induction. The 3-phase stator
of generators. Silicon rectifiers are mounted
windings (coils) receive power from the external
on the rotating shaft to furnish dc to the
source via a controller. They produce a con-
ac generator rotor field. In the brush-type
tinuously rotating magnetic field at constant
ac generator, the commutator serves as a
speed (synchronous), regardless of the load
rectifier to perform this function. Feedback
on the motor. The rotor is not connected
from the 3-phase power output through a
electrically to the power supply. The revolving
voltage regulator control (not shown) furnishes
magnetic field produced by the stator cuts
the correct amount of excitation to the exciter
the conductors of the rotor. This induces a
field for ac generator voltage control.
voltage in the conductors and causes current
to flow. Hence, motor torque is developed
Generator frequency and resistive (kW) load
balance (for paralleled generators) are controlled
by the interaction of the rotor's magnetic
9-2
