Excitation Methods of DC machine
The performance characteristics of a dc machine are greatly influenced by the way in which the field winding is excited with direct current.
The excitation for establishing the required field can be of two types
- Permanent magnet excitation(PM): Permanent magnet excitation is employed only in extremely small machines where providing a field coil becomes infeasible. Also, permanent magnet excited fields cannot be varied for control purposes.
- Permanent magnets for large machines are either not available or expensive. However, an advantage of permanent magnet is that there are no losses associated with the establishment of the field.
2. Electro magnetic excitation: Electromagnetic excitation is universally used. Even though certain amount of energy is lost in establishing the field it has the advantages like lesser cost, ease of control.
Methods of Excitation
- Separately excited DC generators
- Self excitation
1. Separately excited DC generators
- The total mmf required can be produced by a coil having large number of turns but taking a small current.
- Such winding has a high value of resistance and hence a large ohmic drop. It can be connected across a voltage source and hence called a shunt winding.
- Such method of excitation is termed as shunt excitation. one could have a few turns of large cross section wire carrying heavy current to produce the required ampere turns. These windings have extremely small resistance and can be connected in series with a large current path such as an armature.
- Such a winding is called a series winding and the method of excitation, series excitation.
- A d.c. machine can have either of these or both these types of excitation
- When both shunt winding and series winding are present, it is called compound excitation.
- The mmf of the two windings could be arranged to aid each other or oppose each other. Accordingly they are called cumulative compounding and differential compounding.
- If the shunt winding is excited by a separate voltage source then it is called separate excitation.Self excitation of series generators
2. Self excitation
- If the excitation power comes from the same machine, then it is called self excitation.
- These are the generators whose field winding is excited by the current supplied by generator itself. In such machine, field coils are interconnected with armature winding.
- Due to residual magnetism, some flux is always present in poles of such machines.
- When the armature is rotated, a small voltage is induced in armature winding owing the residual flux.
- The induced voltage cause small current to flow in field coils and further increase the field current and so flux per pole.
- The conditions for self excitation can be listed as below.
- Residual field must be present.
- The speed of operation of the machine must be above the critical speed.
- The polarity of excitation must aid the residual magnetism.
- The load resistance must be very large.
- The field circuit resistance must be below the critical value.
Self-excited machines can be differentiate 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).
2. Shunt wound – 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.
3. 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.
- it is further classified as –
- Long shunt – field winding is connected in parallel with series field winding and armature winding.
- Short shunt – field winding is connected in parallel with only the armature winding.