page_title: Single-phase generator
section_title: Revolving field
hierarchical_section_title: Single-phase generator / Designs / Revolving field
context_page_description: Single-phase generator 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.
context_section_description: The design of revolving field generators is to have the armature part on stator and the magnetic field part on rotor. A basic design of revolving field single-phase generator is shown on the right. There are two magnetic poles, north and south, attached to a rotor and two coils which are connected in series and equally spaced on stator. The windings of the two coils are in reverse direction to have the current to flow in the same direction because the two coils always interact with opposing polarities. Since poles and coils are equally spaced and the locations of the poles match to the locations of the coils, the magnetic lines of force are cut at the same amount at any degree of the rotor. As a result, the voltages induced to all windings have the same value at any given time. The voltages from both coils are "in phase" to each other. Therefore, the total output voltage is two times the voltage induced in each winding. In the figure, at the position where pole number 1 and coil number 1 meet, the generator produces the highest output voltage on one direction. As the rotor turns 180 degrees, the output voltage is alternated to produce the highest voltage on the other direction. The frequency of the AC output in this case equals to the number of rotations of the rotor per second.
  This design can also allow us to increase the output frequency by adding more poles. In this example on the right, we have 4 coils connected in series on the stator and the field rotor has 4 poles. Both coils and poles are equally spaced. Each pole has opposite polarity to its neighbors which are angled at 90 degrees. Each coils also have opposite winding to its neighbors. This configuration allows the lines of force at 4 poles to be cut by 4 coils at the same amount at a given time. At each 90-degree rotation, the voltage output polarity is switched from one direction to the other. Therefore, there are 4 cycles of the AC output in one rotation. As the 4 coils are wired in series and their outputs are "in phase", the AC output of this single-phase generator will have 4 times the voltage of that generated by each individual coil.
  A benefit of the revolving field design is that if the poles are permanent magnets, then there is no need to use any slip ring and brush to deliver electricity out of the generator as the coils are stationary and can be wired directly from the generator to the external loads.
caption: Diagram of revolving field single-phase generator with two poles