- DC Generator Types: DC generators are mainly divided into three types based on their field excitation methods: permanent magnet, separately excited, and self-excited.
- Self Excited Generator: A self-excited DC generator uses its own output to power its field coils, which can be arranged as series, shunt, or compound wound.
- Application Insight: Portable generators often use DC generator technology due to its reliability and efficiency in generating electricity.
- Generator Components: Essential components of DC generators include the armature, field coils, and the external circuit, each playing a critical role in power generation and delivery.
- Understanding Diagrams: Diagrams in the article help clarify the setup and function of different generator types, enhancing comprehension of technical descriptions.
DC generators are classified by the source and connection of their magnetic field. The three main types of DC generators are:
- Permanent Magnet DC Generators: permanent magnets provide the field flux.
- Separately Excited DC Generators: an independent DC source supplies the field winding.
- Self Excited DC Generators: the generator’s own output supplies its field circuit.
Self-excited machines are classified again by how their field windings connect to the armature and load:
- Series Wound Generators
- Shunt Wound Generators
- Compound Wound Generators
The linked portable generator page gives one application context, but portable generating sets can use several electrical machine and conversion arrangements.
Permanent Magnet DC Generator
A permanent magnet DC generator uses magnets to establish the flux in its magnetic circuit. It needs no wound field circuit or field power supply.
The permanent magnets surround or form part of the magnetic structure around the armature. Output depends on speed, flux and machine dimensions. Permanent excitation does not by itself impose a low power rating.
This arrangement suits machines that need fixed excitation. Its field cannot be adjusted electrically in the same way as a wound-field generator.
Separately Excited DC Generator
A separately excited generator receives field current from an independent DC source, such as a battery or controlled supply.
The diagram shows the armature circuit and its separate field supply. The symbols are:
- Ia = Armature current
- IL = Load current
- V = Terminal voltage
- Eg = Generated EMF (Electromotive Force)
The armature Voltage drop is Ia × Ra, where the final term is armature resistance.
For the circuit shown:
The terminal relation is:
Generated electrical power is:
Power delivered to the external load is:
Self Excited DC Generators
Self Excited DC Generators use part of their generated current to supply the field circuit. The field winding is electrically connected to the armature in a series, shunt or compound arrangement.
Residual magnetism leaves a small flux in the poles. Rotation then induces a small emf, which sends current through the connected field circuit and increases the pole flux.
The rising flux produces more armature emf and field current. Voltage builds only when the residual polarity supports the field and the field-circuit resistance and speed permit self-excitation. The build-up stops at the operating point set by magnetic saturation and the circuit.
According to the field-winding connection, self-excited DC generators are:
- Series Wound Generators
- Shunt Wound Generators
- Compound Wound Generators
Series Wound Generator
Series Wound Generators connect the series field winding in the same current path as the armature and external load.
The series field carries the full load current. It therefore uses relatively few turns of thick conductor. The electrical resistance of the winding is kept low to limit voltage drop and loss.
The symbols are:
- Rsc = Series winding resistance
- Isc = Current flowing through the series field
- Ra = Armature resistance
- Ia = Armature current
- IL = Load current
- V = Terminal voltage
- Eg = Generated EMF
The current relation is:
The load Voltage is:
Generated electrical power is:
Power delivered to the load is:
Shunt Wound DC Generators
Shunt Wound Generators connect the field winding in parallel with the armature terminals. The shunt field therefore has the same terminal voltage across it.
The symbols are:
- Rsh = Shunt winding resistance
- Ish = Current flowing through the shunt field
- Ra = Armature resistance
- Ia = Armature current
- IL = Load current
- V = Terminal voltage
- Eg = Generated EMF
Armature current Ia divides between shunt-field current Ish and load current IL.
The current relation is:
Compared with load current IL, shunt-field current is designed to be small. The shunt winding therefore uses many turns and relatively high resistance to establish the required field with limited current.
Shunt field current is:
Voltage across the load is:
Generated electrical power is:
Power delivered to the load is:
Compound Wound DC Generator
In a series-wound generator, output voltage tends to rise with load current before saturation and internal drops dominate. In a shunt-wound generator, terminal voltage tends to fall as load current rises.
A compound generator combines series and shunt fields so their effects can be chosen to support the required voltage-current characteristic.
Compound-wound generators have both field windings. The series field carries armature or load current, while the shunt field is connected in parallel. Connection determines the two forms: short-shunt and long-shunt compound generators.
Short Shunt Compound Wound DC Generator
In a short-shunt compound generator, the shunt field is connected across only the armature winding. The series field remains in the load path shown in the diagram.
Series field current is:
Shunt field current is:
Armature current is:
Load Voltage is:
Generated electrical power is:
Power delivered to the load is:
Long Shunt Compound Wound DC Generator
In a long-shunt compound generator, the shunt field is connected across the series field and armature winding together, as shown below.
Shunt field current is:
Armature current Ia is also the series-field current:
Load Voltage is:
Generated electrical power is:
Power delivered to the load is:
Compound Wound Dynamics: In a cumulative compound generator, the series field aids the shunt field. Its extra flux can offset internal voltage drop as load rises.
If the series field opposes the shunt field, the generator is differentially compound wound. Its terminal voltage falls more sharply with load.
