AC generators are widely
used to produce AC voltage. To understand how these generators operate, the
basic theory of operation must first be understood.
EO 1.2Given the
speed of rotation and number of poles, CALCULATE the frequency output of an AC
generator.
EO 1.3LIST the three
losses found in an AC generator.
EO 1.4Given the
prime mover input and generator output, DETERMINE the efficiency of an AC
generator.
Theory of Operation
A simple AC generator consists of: (a) a strong magnetic
field, (b) conductors that rotate through that magnetic field, and (c) a means
by which a continuous connection is provided to the conductors as they are
rotating (Figure 3). The strong magnetic field is produced by a current flow
through the field coil of the rotor. The field coil in the rotor receives
excitation through the use of slip rings and brushes. Two brushes are
springheld in contact with the slip rings to provide the continuous connection
between the field coil and the external excitation circuit. The armature is
contained within the windings of the stator and is connected to the output.
Each time the rotor makes one complete revolution, one complete cycle of AC is
developed. A generator has many turns of wire wound into the slots of the
rotor.
Figure
3 Simple AC Generator
The magnitude of AC voltage generated by an AC generator
is dependent on the field strength and speed of the rotor. Most generators are
operated at a constant speed; therefore, the generated voltage depends on field
excitation, or strength.
The
frequency of the generated voltage is dependent on the number of field poles
and the speed at which the generator is operated, as indicated in Equation
(101).
_{}
where
In
this manner, the units of frequency (hertz or cycles/sec.) are derived.
Losses
in an AC Generator
The
load current flows through the armature in all AC generators. Like any coil,
the armature has some amount of resistance and inductive reactance. The
combination of these make up what is known as the internal resistance, which causes a loss in an AC generator. When
the load current flows, a voltage drop is developed across the internal
resistance. This voltage drop subtracts from the output voltage and, therefore,
represents generated voltage and power that is lost and not available to the
load. The voltage drop in an AC generator can be found using Equation (102).
where
