For some applications, the magnetic field of the generator may be provided by permanent magnets. The rotor structure can consist of a ring of magnetic iron with magnets mounted on its surface. A magnet material such as neodymium-boron-iron or samarium-cobalt can provide a magnetic flux density in the air gap comparable to that produced with field windings, using a radial depth of magnet of less than 10 millimetres. Other magnet materials such as ferrite can be used, but with a considerable reduction in air-gap flux density and a corresponding increase in generator dimensions.
Permanent-magnet generators are simple in that they require no system for the provision of field current. They are highly reliable. They do not, however, contain any means for controlling the output voltage. A typical example of use is with a wind turbine where the generator output of variable voltage and frequency is supplied to a power system through an electronic frequency converter.
Induction generators
An induction machine can operate as a generator if it is connected to an electric supply network operating at a substantially constant voltage and frequency. If torque is applied to the induction machine by a prime mover, it will tend to rotate somewhat faster than its synchronous speed, which is equal to 120 f/p revolutions per minute, where f is the supply frequency and p is the number of poles in the machine. The rotor conductors, moving faster than the air-gap field, will have induced currents that interact with the magnetic field to produce a torque with which to balance that applied by the prime mover. A stator current will then flow into the supply network delivering electrical power. The amount of power delivered is approximately proportional to the difference between the rotor speed and the field speed. This difference is typically of the order of 0.5 to 2 percent of rated speed at rated load.
An induction generator cannot normally provide an independent electrical power source because it does not contain a source of its own magnetic field. Stand-alone induction generators can, however, operate with the aid of appropriate loading capacitors.
Induction generators are frequently preferred over synchronous generators for small hydroelectric sites because they are not subject to loss of synchronism following transient changes in the power system.
Inductor alternators
An inductor alternator is a special kind of synchronous generator in which both the field and the output winding are on the stator. In the homopolar type of machine, the magnetic flux is produced by direct current in a stationary field coil concentric with the shaft. In the heteropolar type, the field coils are in slots in the stator.
Voltage is generated in the output windings by pulsations in the flux in individual stator teeth. These pulsations are produced by use of a toothed rotor, which causes the reluctance of the air path from the rotor to each stator tooth to vary periodically with rotation.
Inductor alternators are useful as high-frequency generators. They also are useful in situations requiring high reliability, a feature achieved by their having no electrical connections to the rotor.