Theory of Operation:

A simple AC generator will consists of: (a) a strong magnetic field, (b) conductors which rotate by that magnetic field, and (c) a means through that a continuous connection is given to the conductors as they are rotating (Figure).  The strong magnetic field is generate through a current flow by the field coil of the rotor.  The field coil in the rotor receives excitation by the use of slip brushes and rings. Two brushes are spring-held in contact along with the slip rings to gives the continuous connection among  the  field  coil  and  the

Figure: Simple AC Generator

external excitation circuit.  The armature is contained within the windings of the stator and is connected to the output.  Every time the rotor makes one complete revolution, one complete cycle of AC is established.  A generator has several turns of wire wound into the slots of the rotor.

The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor.  Many generators are operated at a constant speed; therefore, a generated voltage depends on field excitation or strength.

An 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 = NP/120                                                                (10-1)

where

f  = frequency (Hz)

P = total number of poles

N = rotor speed (rpm)

120 = denotes conversion from minutes to seconds and from poles to pole pairs

The 120 in given Equation (10-1) is derived through multiplying the subsequent conversion factors.

 60 seconds/1 minute x 2 poles/ pole pair

Within this manner, the units of frequency (hertz or cycles/sec.) are derived.