MM0704, Lesson 3
Single-Phase Generators. A single-phase AC generator (figure 3-12) has a stator made up of a number of windings in
series which form a single circuit in which an output voltage is generated. The principle of the single-phase AC
generator is described first, and the polyphase AC generator is described later.
Figure 3-12 is a schematic diagram of a single-phase AC generator having four poles. The stator has four polar groups
evenly spaced around the stator frame. The rotor has four poles with adjacent poles of opposite polarity. As the rotor
revolves, AC voltage is induced in the stator windings. Since one rotor pole is in the same position relative to a stator
winding as any other pole, all stator polar groups are cut by equal amounts of magnetic lines of force at any given time.
As a result, the voltages induced in all the windings have the same amplitude (value), at any given instant. The four
stator windings are connected to each other so that the AC voltages are in phase ("series aiding"). Assume that rotor
pole one, a south pole, induces a voltage in the direction indicated by the arrow in stator winding one. Since rotor pole
two is a north pole, it will induce a voltage in the opposite direction in stator winding two with respect to that in
Figure 3-12. Single-Phase AC Generator.
In order that the two induced voltages be in series addition, the two coils are connected as shown. Applying the same
reasoning, the voltage induces in stator winding three (clockwise rotation of the field) is in the same direction
(counterclockwise) as the voltage induced in coil one. Similarly, the direction of the voltage induced in winding four is
opposite to the direction of the voltage induced in winding one. All four stator winding groups are connected in series
so that the voltages induced in each winding add to give a total voltage that is four times the voltage in any one
Multiphase Generators. Multiphase AC generators have two or more single-phase windings symmetrically spaced
around the stator. In a two-phase AC generator, there are two single-phase windings physically spaced so that the AC
voltage induced in one is 90 degrees out of phase with the voltage induced in the other. The windings are electrically
separate from each other. When one winding is being cut by maximum flux, the other is