CLASSIFICATIONS OF DC MACHINES:
Each DC machine can act as a generator or a motor. Hence, this classification is valid for both: 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: In separately excited dc machines, field winding is supplied from a separate power source. That is, field winding is electrically separated from the armature circuit. This type of dc machines is not commonly used.
- Self excited: In this type, field winding and armature winding are interconnected in various ways to achieve wide range of performance characteristics. Depending on that, self-excited machines can be further classified as –
- Series wound – In this type, field winding is connected in series with the armature winding. Therefore, the field winding carries whole 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 – Here, field winding is connected in parallel with the armature winding. Shunt winding is made with 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 – 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.
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