DC generators are classified based on how their fields are excited (i.e. produced). There are three methods of excitation, and thus three main types of DC generators:
- Permanent Magnet DC Generators – Field coils excited by permanent magnets
- Separately Excited DC Generators – Field coils excited by some external source
- Self Excited DC Generators – Field coils excited by the generator itself
Self-excited DC generators can further be classified depending on the position of their field coils. The three types of self-excited DC generators are:
- Series Wound Generators
- Shunt Wound Generators
- Compound Wound Generators
A portable generator is an example of a practical application that utilises such technologies.
Permanent Magnet DC Generator
When the flux in the magnetic circuit is created through the use of permanent magnets, then it is known as a Permanent magnet DC generator.
It consists of an armature and one or several permanent magnets situated around the armature. This type of DC generator generates does not generate much power.
As such they are rarely found in industrial applications. They are normally used in small applications – like dynamos in motorcycles.
Separately Excited DC Generator
These are the generators whose field magnets are energized by some external DC source, such as a battery.
A circuit diagram of separately excited DC generator is shown in the figure below. The symbols below are:
- Ia = Armature current
- IL = Load current
- V = Terminal voltage
- Eg = Generated EMF (Electromagnetic Force)
Voltage drop in the armature = Ia × Ra (R/sub>a is the armature resistance)
Let,
Then,
Power generated is equal to
And power delivered to the external load is equal to
Self Excited DC Generators
Self-excited DC generators are generators whose field magnets are energized by the current supplied by themselves. In these type of machines, field coils are internally connected with the armature.
Due to residual magnetism, some flux is always present in the poles. When the armature is rotated, some EMF is induced. Hence some induced current is produced. This small current flows through the field coil as well as the load and thereby strengthening the pole flux.
As the pole flux strengthened, it will produce more armature EMF, which cause the further increase of current through the field. This increased field current further raises armature EMF, and this cumulative phenomenon continues until the excitation reaches the rated value.
According to the position of the field coils,self-excited DC generators may be classified as:
- Series Wound Generators
- Shunt Wound Generators
- Compound Wound Generators
Series Wound Generator
In these type of generators, the field windings are connected in series with armature conductors, as shown in the figure below.
Whole current flows through the field coils as well as the load. As series field winding carries full load current it is designed with relatively few turns of thick wire. The electrical resistance of series field winding is therefore very low (nearly 0.5Ω ).
Here:
- 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
Then,
Voltage across the load is equal to,
Power generated is equal to,
Power delivered to the load is equal to,
Shunt Wound DC Generators
In these type of DC generators, the field windings are connected in parallel with armature conductors, as shown in the figure below. In shunt wound generators the voltage in the field winding is same as the voltage across the terminal.
Here:
- 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
Here armature current Ia is dividing in two parts – one is shunt field current Ish, and another is load current IL.
So,
The effective power across the load will be maximum when IL will be maximum. So, it is required to keep shunt field current as small as possible. For this purpose the resistance of the shunt field winding generally kept high (100 Ω) and large no of turns are used for the desired EMF.
Shunt field current is equal to,
Voltage across the load is equal to,
Power generated is equal to,
Power delivered to the load is equal to,
Compound Wound DC Generator
In series wound generators, the output voltage is directly proportional with load current. In shunt wound generators, the output voltage is inversely proportional with load current.
A combination of these two types of generators can overcome the disadvantages of both. This combination of windings is called compound wound DC generator.
Compound wound generators have both series field winding and shunt field winding. One winding is placed in series with the armature, and the other is placed in parallel with the armature. This type of DC generators may be of two types- short shunt compound-wound generator and long shunt compound-wound generator.
Short Shunt Compound Wound DC Generator
Short Shunt Compound Wound DC Generators are generators where only the shunt field winding is in parallel with the armature winding, as shown in the figure below.
Series field current is equal to,
Shunt field current is equal to,
Armature current is equal to,
Voltage across the load is equal to,
Power generated is equal to,
Power delivered to the load is equal to,
Long Shunt Compound Wound DC Generator
Long Shunt Compound Wound DC Generator are generators where the shunt field winding is in parallel with both series field and armature winding, as shown in the figure below.
Shunt field current is equal to,
Armature current, Ia = series field current,
Voltage across the load is equal to,
Power generated is equal to,
Power delivered to the load is equal to,
In a compound wound generator, the shunt field is stronger than the series field. When the series field assists the shunt field, generator is said to be commutatively compound wound.
On the other hand, if the series field opposes the shunt field, the generator is said to be differentially compound wound.
