Dc generator (Direct current generator)is an electrical machine that converts mechanical energy into direct current (DC). The mmf necessary to establish the flux in the magnetic circuit of a dc generator can be obtained by:
- permanent magnets
- field coils excited by some external sources and
- field coils excited by the generator itself.
Types of DC Generators
- Separately -Excited DC Generators
- Self-Excited DC Generators
- Series Wound Generators
- Shunt-wound Generators
3. Compound wound Generators
- Short-shunt compound Wound Generators
- Long-Shunt compound Wound Generators
Separately-Excited DC Generators
A dc generator whose field winding is excited from an independent external dc source such as the battery, the generator is called a separately Excited generator. The current flowing through the armature Ia and loads IL is the same and the terminal voltage(voltage across the load) v is equal to the generated emf Eg, less voltage drop in the armature, IaRa i.e
Self-Excited Dc Generators
A DC generator whose field winding is excited by the current supplied by the generator itself is called a self-excited generator. The field coils are interconnected with the armature winding either in series with the armature or in parallel with the armature, or partly in series and partly in parallel with the armature. Due to the residual magnetism, some flux is always present in the pole of this machine.
When the armature is rotated, a small voltage is induced in the armature winding. This induced voltage causes a small current to flow in the field coils and increases flux per ole. An increase in flux per ole causes an increase in induced voltage, which further increases the field current and so flux per pole and generator builds up to rated voltage.
Classification of Self-excited Generators
Self-Excited generators may be classified as:
Series wound generators
One field winding is connected in series with the armature winding so that the whole current flows through the field winding as well as the load. Since series field winding carries the full load current,series field winding is designed with a few turns of thick wire or strips. The resistance of the series field winding Rse is very low, in the order of 0.5.
One field winding is connected across the armature circuit in shunt or parallel. The voltage across the field winding is the same as the terminal voltage of the generator. The total armature current Ia flowing from the +ve brush divides between two parallel paths, i.e., the external circuit and the field circuit. The load current IL and the shunt field current Ish close the circuit at the -ve brush, and their sum is equal to the armature current, i.e.
fig: shunt wound generator
The shunt field current must be as small as possible because the effective power of a generator is proportional to the current delivered to the external load circuit. In the case of a shunt wound generator, the shunt field current is 2.5% of the rated armature current. The resistance of the shunt field winding Rsh will naturally be high, in the order of 100.
Compound wound Generators
There are two field windings in compound-wound generators. One of them is connected across the armature, and the other is connected in series with the armature winding. Compound generators may be connected either in a short shunt with the shunt field winding in parallel with the armature or in a long shunt with the shunt field winding in parallel with both the armature and series field winding. The major portion of excitation is supplied by the shunt field.
compound generators are also of two types:
- cumulative compound wound
- Differential compound wound
- In cumulative compound wound generators series field assists the shunt field.
- In differential compound wound generators series field opposes the shunt field.
Short shunt compound wound generators
fig:short shunt compound wound generators
Long shunt compound wound Generators
fig: Long shunt compound wound Generators
Characteristics of DC Generators
curve giving the relationship between various quantities such as field current, generated emf, terminal voltage, and the load current are known as generator characteristics. mainly there are three most important characteristics of a dc generator:
Open circuit characteristics or magnetic: This characteristic gives the relation between the generated emf in armature on no load Eo and field current If at a given speed. This characteristic is also known as the no-load saturation characteristic.
fig:Magnetic or OCC(open-circuit characteristics) Fig:saturation or magnetisation curve
Emf generated in the generator is given by the expression:
i.e., the generated emf in the generator is directly proportional to the product of flux per pole and speed. If the generator runs at a constant speed, then the generated emf is directly proportional to the flux per pole. since the generated emf is proportional to flux per pole if speed is constant and field ampere turns are proportional to field current.
If a curve is drawn between generated emf on no load and field current at a constant speed, the curve obtained is known as magnetic characteristics or open circuit characteristics (OCC). The curve starts from point A instead of point O when the field current is zero, which is due to residual magnetism.
The curve from point b to point c is practically a straight line and is called the straight line portion of the magnetization curve. Point c is called the knee of the curve, where the saturation of the magnetic circuit begins. The point from point c to point d is above the knee point.
Internal or Total characteristics: This curve gives the relation between the emf actually generated in the armature E and the armature current. This curve lies below the open circuit characteristics (OCC).
External characteristics: This curve gives the relation between terminal voltage V and the load current I under the given conditions of speed and excitation. The terminal voltage is always less than E, as there is a voltage drop in the armature. This curve lies below the internal characteristics. This characteristic is important as it helps determine the suitability of a generator.