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Types of DC Generators
A DC generator is an electrical machine that converts mechanical energy into DC electrical energy. This conversion is based on the principle of Faraday's Law of Electromagnetic Induction. A DC generator essentially consists of an armature, a field magnet, a commutator, and brushes. The classification of DC generators is primarily based on their method of field excitation, which is crucial for understanding their operational characteristics and applications. This guide provides a detailed and structured overview of the different types of DC generators, a vital topic for JEE aspirants.
1.0What Is a DC Generator?
A DC generator is a device that produces unidirectional current by rotating a conductor within a magnetic field. The mechanical rotation induces an electromotive force (EMF), which is then converted into DC output using a commutator and brushes.
2.0How DC Generators Work
A rotating armature (coil) inside a magnetic field experiences changing magnetic flux, inducing an EMF (Faraday’s law).
The commutator reverses the connection of the coil to the external circuit at appropriate intervals, ensuring the current flows in a single direction.
Field windings, powered either by an external source or the generator itself, establish the necessary magnetic field.
3.0Classification of DC Generators
DC generators are classified into two main categories based on how their field windings are supplied with current:
- Separately Excited DC Generators: The field winding is powered by an external DC source.
- Self-Excited DC Generators: The field winding is supplied by the generator's own output.
4.0Separately Excited DC Generators
In a separately excited DC generator, the field winding is electrically isolated from the armature winding. It is connected to an independent external DC source, such as a battery. This external source provides the necessary field current (If) to create the magnetic flux.
Circuit Diagram:
Working Principle:
- An external DC source provides a constant current to the field winding, creating a constant magnetic field.
- The armature is rotated by a prime mover (e.g., an engine), cutting through the magnetic flux.
- An EMF is induced in the armature, which is then converted to a DC voltage by the commutator.
Characteristics:
- The field current (I_f) is independent of the armature current (I_a).
- The terminal voltage (V_t) can be controlled by varying the external field current.
- Terminal Voltage Equation: V_t=E_g−I_aR_a, where Ra is the armature resistance.
Applications:
- Used where a wide range of output voltage control is required.
- Commonly used in laboratories for testing and in special applications like electroplating and battery charging.
5.0Self-Excited DC Generators
In self-excited DC generators, the field winding receives its power from the generator's own output. A small amount of residual magnetism in the field poles is required to start the generation process. The initial rotation of the armature generates a small EMF, which then flows through the field winding, increasing the magnetic flux and leading to a rapid build-up of the generated voltage.
Self-excited generators are further sub-divided based on how the field winding is connected to the armature:
DC Shunt Generator
A DC shunt generator has its field winding connected in parallel (shunt) with the armature winding. This means the entire terminal voltage is available across the field winding. The shunt field winding has a large number of turns and a small cross-sectional area, giving it a high resistance.
Circuit Diagram:
Here,
Working Principle:
- Residual magnetism in the poles induces a small initial EMF in the armature.
- This small EMF drives a small current through the high-resistance shunt field winding.
- The field current strengthens the magnetic field, which in turn induces a larger EMF. This process continues until the voltage stabilizes.
- The terminal voltage equation is
- The armature current is the sum of the load current and the shunt field current:
Characteristics:
- Provides a relatively constant terminal voltage.
- The terminal voltage drops slightly as the load current increases due to the armature resistance drop (I_aR_a).
- They are susceptible to self-excitation failure if the residual magnetism is lost or the field resistance is too high.
Applications:
- Commonly used where a constant voltage supply is needed, such as in battery charging and general lighting applications.
DC Series Generator
A DC series generator has its field winding connected in series with the armature winding. The series field winding carries the entire load current. It is made of a few turns of thick wire with a low resistance.
Circuit Diagram:
Here,
Working Principle:
- The field current is the same as the armature current and the load current I_{se}=I_a=I_L
- As the load current increases, the field current increases, which strengthens the magnetic field and boosts the induced EMF.
- This results in a terminal voltage that rises with the load.
Characteristics:
- The terminal voltage increases with increasing load current.
- The voltage at no load is very small, as there is no current flowing through the series field.
- The terminal voltage equation is where is the series field winding resistance.
Applications:
- Not suitable for applications requiring a constant voltage.
- Used as boosters in distribution systems to compensate for voltage drops in the lines.
DC Compound Generator
A DC compound generator combines the features of both shunt and series generators. It has two field windings: a series field winding and a shunt field winding. The overall magnetic field is a combination of the fields produced by both windings.
Compound generators are further classified based on the connection of the shunt winding:
- Long Shunt Compound Generator: The shunt field winding is connected in parallel with both the armature and the series field winding.
- Short Shunt Compound Generator: The shunt field winding is connected in parallel with the armature only.
Compound generators are also classified based on the direction of the magnetic flux produced by the two field windings:
- Cumulative Compound Generator: The series and shunt field fluxes aid each other, i.e., they are in the same direction.
- Terminal voltage characteristics fall between those of a shunt and a series generator.
- Used for applications requiring a stable voltage under varying load conditions, such as industrial power supplies.
- Differential Compound Generator: The series and shunt field fluxes oppose each other, i.e., they are in opposite directions.
- The terminal voltage drops rapidly as the load increases. This makes them unsuitable for most applications but useful for special purposes like electric arc welding where a large current with a sharp voltage drop is needed.