- Self Excited DC Generators: These generators initiate power generation using the residual magnetism in their rotor, which helps start the electromotive force needed for operation.
- Operation Mechanism: As the magnetic field increases due to the initial current, the voltage output of the generator rises, continuing this cycle until saturation occurs at the field poles.
- Types of DC Generators: DC generators are mainly classified as series wound, shunt wound, and compound wound, each differing in coil arrangement and voltage regulation characteristics.
- Compound Wound Generators: These generators use a combination of series and shunt windings to overcome the limitations of each type, ensuring a more consistent output voltage.
- Voltage Regulation: In compound DC generators, voltage stability is maintained by adjusting the magnetic fields in response to changes in load, demonstrating effective voltage regulation.
Self-excited DC generators supply their own field windings from the armature output. Residual flux in the stationary field poles lets the rotating armature generate a small initial EMF. This EMF drives current through the field winding. If that current strengthens the residual magnetic field, the added magnetic flux raises the armature EMF. The feedback continues until the field-circuit line meets the machine’s open-circuit characteristic, which sets the no-load voltage at that speed.
What Are Self Excited DC Generators?
A self-excited DC generator does not need a separate steady field supply. Residual magnetism in the field poles produces the initial armature voltage when a prime mover rotates the armature.
The initial field current must reinforce the residual flux. Voltage then builds towards the intersection of the open-circuit characteristic and the field-resistance line. Magnetic saturation limits the slope of the characteristic, while the field-circuit voltage drop rises with current. Reliable build-up requires residual magnetism, correct field polarity, adequate speed and field-circuit resistance below the critical value for that speed. If one condition is absent, the generator may fail to build voltage.
Types of DC Generators
Self-excited DC generators are classified by how their field windings connect to the armature and load: series, shunt or compound. This wound-field DC-machine classification should not be assumed from the product label used for portable generators; identify the output type and excitation system separately.
Series Wound Generators
In a series-wound generator, the field winding, armature winding and load carry the same current. The series field uses a few turns of thick conductor and has low resistance so it can carry the full load current.
As load current first rises, the stronger magnetic field raises the generated EMF and terminal voltage. After the iron approaches saturation, armature reaction and internal voltage drops can make terminal voltage fall as current continues to rise. This strongly load-dependent characteristic limits the applications of these types of generators.
Shunt Wound DC Generators
In a shunt-wound generator, the field winding connects in parallel with the armature and has many turns of fine wire. Its relatively high resistance limits field current.
For the current relation, 
The terminal voltage usually falls as load current rises. The armature-circuit voltage drop, armature reaction and reduced shunt-field current all contribute to the droop; the armature resistance itself does not need to increase.
Compound-Wound DC Generators
A compound-wound generator has both shunt and series field windings. The shunt field provides the main excitation, while load current in the series field changes the net flux as load changes. The connection may be long-shunt or short-shunt, and the series field may aid or oppose the shunt field.
Long Shunt Compound Generator
In a long-shunt compound generator, the shunt field is connected across the combination of the armature and series field. The series field is in the armature-current path before the current divides between the load and shunt field.
Short Shunt Compound Generator
In a short-shunt compound generator, the shunt field is connected directly across the armature. The series field is outside that shunt connection and normally carries load current.
Voltage Regulation of a Compound DC Generator
In a cumulatively compounded generator, increasing load current strengthens the series-field magnetic field. This added flux can offset some or all of the terminal-voltage reduction caused by armature resistance, brush drop and armature reaction. The selected series-field turns determine the degree of compounding. An under-compounded generator falls in voltage, a flat-compounded generator has similar no-load and full-load voltages, and an over-compounded generator rises in voltage.
Cumulative and Differential Compound DC Generators
A cumulative connection makes the series field aid the shunt field. A differential connection makes the two fields oppose. Differential compounding causes terminal voltage to fall more sharply with increasing load and is unsuitable where stable voltage is required. Field polarity and current direction therefore determine the compound characteristic.
