AC GENERATOR THEORY
AC Generators
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 (10-1).
f =
(10-1)
NP
120
where
f
=
frequency (Hz)
P
=
total number of poles
N =
rotor speed (rpm)
120
=
conversion from minutes to seconds and from poles to pole pairs
The 120 in Equation (10-1) is derived by multiplying the following conversion factors.
x
60 seconds
1 minute
2 poles
pole pair
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 (10-2).
(10-2)
Voltage drop
IaRa
IaXLa
where
Ia
= armature current
Ra
= armature resistance
XLa = armature inductive reactance
ES-10
Page 6
Rev. 0
