What are the Types of DC Generators?
In this section, we will be discussing the types of dc generators. The types of dc generators are classified according to the methods used in the excitation of the field system. Following are the types of dc generators:
- Permanent Magnet DC Generator
- Separately Excited DC Generator
- Self-excited DC Generators:
Series Generator, Shunt Generator, and Compound Generator.
Different Types of DC Generators and Their Application
The different types of dc generators along with their application are as follows:
1. Permanent Magnet DC Generator
A permanent magnet dc generator is one of the types of dc generators in which permanent magnets are used to excite the field coils. These types of generators don’t generate sufficient power.
Applications of PMDC (Permanent Magnet DC Generator)
These types of dc generators find their application as dynamos in automobiles and applications requiring portable dc power generations.
2. Separately Excited DC Generator
A separately excited dc generator is one of the types of dc generators in which the field winding of the generator is excited with help on an independent external dc source.
From the EMF equation of dc generator we have,
From this relation, we can observe that output EMF from the generator directly depends on the speed of rotation of armature N and field current (as field current is directly proportional to Φ).
Separately excited dc generators are rarely used and self-excited dc generators are preferred over separately excited ones.
Applications of Separately Excited DC Generator
Separately excited dc generators find their application in the laboratory where we require to generate a wide range of output voltage.
3. Self-Excited DC Generator
A self-excited dc generator is one of the types of dc generators in which the excitation current to the field winding is supplied from the output of the generator itself. As the field winding of the generator is supplied by the generator itself, it is termed self-excited. Depending upon the connection of the field winding of the generator with the armature there are three types of self-excited dc generators.
- Series Generator
- Shunt Generator
- Compound Generator
a. Series Generator
A series generator is one of the types of self-excited dc generators where the field winding and armature winding are connected in series. So, the same current flows through the armature, field winding, and the load. In series generators, the field winding has a small resistance.
Applications of DC Series Generator
DC series generators find their application in electric vehicles for regenerative braking.
b. Shunt Generator
A shunt generator is one of the types of self-excited dc generators where the field winding and the armature winding are connected in parallel. Here, only a small portion of the current flows from the armature of the generator to the field winding while the rest flows through the load. The field winding has high resistance.
Applications of DC Shunt Generator
DC shunt generators are employed in applications requiring portable dc power generations and in exciting the excitation system of the alternation.
c. Compound Generator
A compound generator is one of the types of self-excited dc generators that employ two field windings. One of the field winding is in parallel to the armature and another field winding is in series with the armature. The compound dc generator is further divided into two types.
i) Short Shunt DC Generator
In this compound dc generator, only shunt field winding is ni parallel with the armature. The figure below shows the short shunt dc generator.
ii) Long Shunt DC Generator
In this compound dc generator, the series field winding is in parallel to the armature winding and the shunt field winding is in parallel with both the series field winding and armature winding. The figure below shows a long shunt dc generator.
Applications of Compound Generator
DC compound generator finds its application in arc welding.
Characteristics of Different Types of DC Generators
The characters of different types of dc generators are as follows:
1. Separately Excited DC Generator
The characteristics of separately excited dc generators can be discussed in terms of open circuit and load characteristics.
(i) Open Circuit Characteristics
It is referred to as the no-load characteristics of the generator. The emf equation of a dc generator is
For Z, P, and A to be constant in a generator, Eg α Φ N.
(a) Eg – N Characteristics
If the field current If is constant then the speed (N) and generated emf (Eg) relation is linear. The figure below shows this linear relation.
(b) Eg – If Characteristics
If the speed is held constant and the field current is varied then the flux (Φ) will vary in accordance with the B-H curve of the magnetic circuit. The figure below shows the relation of Eg and If.
(ii) Load Characteristics
We will discuss the two types of load characteristics for a separately excited dc generator.
(a) Internal Characteristics
This characteristic shows the relation between the generated emf (Eg) and the load current (IL). The Eg-IL curve has a droop nature. As Eg α Φ N, we may conclude that if N and If are maintained constant, Eg the will remain constant. Although the If is held constant, the flux per pole will reduce due to the effect of the armature reaction.
(b) External Characteristics
This characteristic of these types of dc generators shows the relation between the terminal voltage (V) and the load current (IL) in a dc generator. We can obtain this characteristic by subtracting the voltage drop occurring in the armature resistance from the internal characteristics.
2. Series Generator
DC series generator is one of the types of generators in which the same current flows through the armature and field winding. The figure below shows the characteristics of the dc series generator.
The figure clearly shows the open circuit characteristics (O.C.C), internal and external characteristics of the generator. All of these resemble the similar curves of the same family. The O.C.C shows the characteristics under the no-load condition while the internal characteristic is obtained from O.C.C by subtracting the armature drop. The external characteristic is obtained from O.C.C by subtracting the armature reaction drop and armature voltage drop.
3. Shunt Generator
The dc shunt generator is one of the types of dc generators in which the armature current gets divided into the load and shunt field winding. The figure below shows the characteristics of the dc shunt generator.
From this figure, we can observe that the open-circuit characteristic is the same as that of the series generator. From this point, we can also conclude that the O.C.C of all the types of dc generators we discussed (series, shunt, and separately excited) are similar.
The internal and external characteristics observation tells us that with an increase in the load current the generated emf and the voltage across the load decreases. The internal characteristics take into account the armature reaction whereas the external characteristics take into account the armature voltage drop also.
4. Compound Generators
The compound generators are the types of dc generators that employ long-shunt and short-shunt configuration and can be further categorized into two types as cumulatively compound and differentially compound. The figure below shows the characteristics curve of a cumulatively compound dc generator.
Here, in these types of generators if the winding is arranged in such a way that increases in current thereby increases the voltage across the terminal is termed as an over-compounded dc generator. The winding arrangement results in a flat voltage profile on changing the load current, such types of compound generators are termed as flat-compounded dc generators. If the winding arrangement is such that it results in the decrease of terminal voltage with an increase in load current such generators is known as an under-compounded generator.