Single-phase generator (also known as single-phase alternator) is an alternating current electrical generator that produces a single, continuously alternating voltage. Single-phase generators can be used to generate power in single-phase electric power systems. However, polyphase generators are generally used to deliver power in three-phase distribution system and the current is converted to single-phase near the single-phase loads instead. Therefore, single-phase generators are found in applications that are most often used when the loads being driven are relatively light, and not connected to a three-phase distribution, for instance, portable engine-generators. Larger single-phase generators are also used in special applications such as single-phase traction power for railway electrification systems.
Designs
Revolving armature The design of revolving armature generators is to have the armature part on a rotor and the magnetic field part on stator. A basic design, called elementary generator, is to have a rectangular loop armature to cut the lines of force between the north and south poles. By cutting lines of force through rotation, it produces electric current. The current is sent out of the generator unit through two sets of slip rings and brushes, one of which is used for each end of the armature. In this two-pole design, as the armature rotates one revolution, it generates one cycle of single phase alternating current (AC). To generate an AC output, the armature is rotated at a constant speed having the number of rotations per second to match the desired frequency (in hertz) of the AC output.
The relationship of armature rotation and the AC output can be seen in this series of pictures. Due to the circular motion of the armature against the straight lines of force, a variable number of lines of force will be cut even at a constant speed of the motion. At zero degrees, the rectangular arm of the armature does not cut any lines of force, giving zero voltage output. As the armature arm rotates at a constant speed toward the 90° position, more lines are cut. The lines of force are cut at most when the armature is at the 90° position, giving out the most current on one direction. As it turns toward the 180° position, lesser number of lines of force are cut, giving out lesser voltage until it becomes zero again at the 180° position. The voltage starts to increase again as the armature heads to the opposite pole at the 270° position. Toward this position, the current is generated on the opposite direction, giving out the maximum voltage on the opposite side. The voltage decrease again as it completes the full rotation. In one rotation, the AC output is produced with one complete cycle as represented in the sine wave.
More poles can also be added to single-phase generator to allow one rotation to produce more than one cycle of AC output. In an example on the left, the stator part is reconfigured to have 4 poles which are equally spaced. A north pole is adjacent to the two south poles. The shape of the armature at the rotor part is also changed. It is no longer a flat rectangle. The arm is bent 90 degrees. This allows one side of the armature to interact with a north pole while the other side interacts with a south pole similarly to the two-pole configuration. The current is still delivered out through the two sets of slip rings and brushes in the same fashion as in the two-pole configuration. The difference is that a cycle of AC output can be completed after a 180 degree rotation of the armature. In one rotation, the AC output will be two cycles. This increases the frequency of the output of the generator. More poles can be added to achieves higher frequency at the same rotation speed of the generator, or same frequency of output at the lower rotation speed of the generator depending on the applications. This design also allows us to increase the output voltage by modifying the shape of the armature. We can add more rectangular loops to the armature as seen on the picture on the right. The additional loops at the armature arm are connected in series, which are actually additional windings of the same conductor wire to form a coil in rectangular shape. In this example, there are 4 windings in the coil. Since the shapes of all windings are the same, the amount of the lines of force will be cut at the same amount in the same direction at the same time in all windings. This creates in phase AC output for these 4 windings. As a result, the output voltage is increased 4 time as shown in the sine wave in the diagram.
Revolving field
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