Generally DC generators are classified according to the ways of excitation of their fields. There are three methods of excitation.
- Field coils excited by permanent magnets – Permanent magnet DC generators.
- Field coils excited by some external source – Separately excited DC generators.
- Field coils excited by the generator itself – Self excited DC generators.
Permanent Magnet DC GeneratorWhen the flux in the magnetic circuit is established by the help of permanent magnets then it is known as Permanent magnet DC generator.
It consists of an armature and one or several permanent magnets situated around the armature. This type of DC generators generates very low power. So, they are rarely found in industrial applications. They are normally used in small applications like dynamos in motor cycles.
Separately Excited DC GeneratorThese are the generators whose field magnets are energized by some external dc source such as battery .
A circuit diagram of separately excited DC generator is shown in figure. Ia = Armature current IL = Load current V = Terminal voltage Eg = Generated emfVoltage drop in the armature = Ia × Ra(R/sub>a is the armature resistance) Let, Ia = IL = I (say) Then, voltage across the load, V = IRa Power generated, Pg = Eg×I Power delivered to the external load, PL = V×I.
Self-excited DC GeneratorsThese are the generators whose field magnets are energized by the current supplied by themselves. In these type of machines field coils are internally connected with the armature. Due to residual magnetism some flux is always present in the poles. When the armature is rotated some emf is induced. Hence some induced current is produced. This small current flows through the field coil as well as the load and thereby strengthening the pole flux. As the pole flux strengthened, it will produce more armature emf, which cause further increase of current through the field. This increased field current further raises armature emf and this cumulative phenomenon continues until the excitation reaches to the rated value. According to the position of the field coils the self-excited DC generators may be classified as…
- Series wound generators
- Shunt wound generators
- Compound wound generators