Classifications
of DC machines : (DC Motors and DC Generators)
Each DC machine can act as a generator or a motor. Hence,
this classification is valid for both: DC generators and DC motors.
DC machines are usually classified on the basis of their field
excitation method. This makes two broad categories of dc machines; (i)
Separately excited and (ii) Self-excited.
§ Separately excited DC machines: In separately excited dc
machines, the field winding is supplied from a separate power source. That
means the field winding is electrically separated from the armature
circuit. Separately excited DC generators are not commonly
used because they are relatively expensive due to the requirement of an
additional power source or circuitry. They are used in laboratories for
research work, for accurate speed control of
DC motors with Ward-Leonard system and in few other
applications where self-excited DC generators are unsatisfactory. In this type,
the stator field flux may also be provided with the help of permanent magnets
(such as in permanent magnet
DC motors). PMDC (permanant magnet DC) motors are popularly used in
small toys, e.g. a toy car.
§ Self-excited DC machines: In this type, field winding
and armature winding are
interconnected in various ways to achieve a wide range of performance
characteristics (for example, field winding in series or parallel with the
armature winding).
In a self-excited type of DC generator, the field winding is energized by the current produced by themselves. A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.
Self-excited machines can be further classified as –
In a self-excited type of DC generator, the field winding is energized by the current produced by themselves. A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.
Self-excited machines can be further classified as –
§ Series wound dc machines – In this type, field winding is
connected in series with the armature winding. Therefore, the field winding
carries whole of the load current (armature current). That is why series
winding is designed with few turns of thick wire and the resistance is kept
very low (about 0.5 Ohm).
§ Shunt wound dc machines – Here, field winding is
connected in parallel with the armature winding. Hence, the full voltage is
applied across the field winding. Shunt winding is made with a large number of
turns and the resistance is kept very high (about 100 Ohm). It takes only small
current which is less than 5% of the rated armature current.
§ Compound wound dc machines – In this type, there are two
sets of field winding. One is connected in series and the other is connected in
parallel with the armature winding. Compound wound machines are further divided
as -
§ Short shunt – field winding is connected in parallel with only the
armature winding
§ Long shunt – field winding is connected in parallel with the
combination of series field winding and armature winding
