In almost all applications, ac generators use an
where
electromagnetic field rather than a permanent magnetic
type of field. The strength of the electromagnetic field
K is a constant determined by the physical
may be varied by a change in the amount of current
flowing through the coil. This is accomplished by a
characteristics of the generator (the number of
variation in the amount of voltage applied across the
windings, their location in respect to the rotating
coil. When the excitation voltage to the rotor windings
field, the materials used in construction, and so
is varied, the ac generator field strength is also varied.
forth.)
Thus, the magnitude of the generated ac voltage depends
represents the strength or intensity of the rotating
directly on the value of the exciter output voltage. This
relationship allows a relatively large ac voltage to be
magnetic field, and
controlled by a much smaller dc voltage.
N represents the speed or frequency of the rotating
The next principle of voltage regulation that must
field and thus the frequency of the output.
be understood is how the dc excitation to the rotor field
The number of windings and their physical
winding is controlled. Voltage control in a dc generator
characteristics are fixed when the generator is
is obtained when the strength of the dc generator shunt
manufactured so K cannot be altered to produce changes
field is varied. This is accomplished by the use of a
in voltage.
number of different types of voltage regulators.
The various loads throughout the ship require a
A device that will vary the excitation current to the
constant value of generated output frequency; therefore,
rotor field winding as changes occur in the ac generator
voltage is called an ac generator voltage regulator. This
the speed of the rotating field must be held constant.
regulator must also maintain the correct value of exciter
This prevents the use of the generator speed as a way
shunt field current when no ac voltage corrective action
to control voltage output. Therefore, the only
is required (steady state output).
practical remaining method for obtaining voltage
control is to control the strength of the rotating magnetic
In figure 8-1, note that a pair of connections labeled
ac sensing input feeds a voltage proportional to the ac
field
Figure 8-1.--Simplified voltage regulator circuit.
8-3
