Dc generators

Dc Generator

Dc generator (Direct current generator)is the electrical machine which converts the 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:

1. permanent magnets
2. field coils excited by some external sources and
3. field coils excited by the generator itself.

Types of Dc Generator

1. Separately -Excited DC Generators
2. Self-Excited DC Generators

• Series Wound Generators
• Shunt wound Generators

3.Compound wound Generators

• Short-shunt compound Wound Generators
• Long-Shunt compound Wound Generators

1.Separately-Excited DC Generators

A dc generator whose field winding is excited from an independent external dc source such as battery ,the generator is called 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 armature,IaRa i.e

fig:Seperately Excited DC generator

2. self-Excited Dc Generators

A dc generator whose field winding is excited by the current supplied by the generator itself ,is called 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 small voltage is induced in the armature winding.This induced voltage causes a small current to flow in the field coils and increases in flux per ole,thus increase in flux per pole causes increase in induced voltage which further increases the field current and so flux per pole and generator builds up to rated voltage.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 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 series field winding Rse is very low and in order of 0.5Ω.

fig:Series Wound generators

• shunt wound Generator:one field winding is connected across the armature circuit in shunt or parallel.The voltage across the field winding is same as the terminal voltage of the generator.Total armature current Ia flowing from +ve brush divides between two parallel paths i.e 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 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 case of shunt wound generator shunt field current is 2.5% of the rated armature current.The resistance of the shunt field winding Rsh will naturally be high in order of 100Ω.

3.Compound wound Generators

There are two field windings in compound wound generators.one of them is connected across the armature and other is connected in series with the armature winding.compound generators may be connected either in  short shunt with the shunt field winding in parallel  with the armature OR 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.

Fig:cumulative compounding Fig:Differential compounding

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:

1.open circuit characteristics or magnetic :This characteristics gives the relation between the generated emf in armature on no load Eo and field current If at a given speed.This characteristics is also known as no-load saturation characteristics.

fig:Magnetic or OCC(open-circuit characteristics)    Fig:saturation or magnetisation curve

Emf generated in generator is given by expression:

i.e generated emf in generator is directly proportional to the product of flux per pole and speed.If the generator runs at a constant speed then 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.If a curve is drawn between generated emf on no load and field current at constant speed ,so curve obtained are known as magnetic characteristics or open circuit characteristics(OCC).The curve starts from point a instead of O when the field current is zero which is due to residual magnetism. The curve from point b to point c is practically 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.Th point from c to point d is above the knee point.

2.Internal or Total characteristics:This curves gives the relation between the emf actually generated in the armature E and the armature current.This curves lies below the open circuit characteristics(OCC).

3.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 voltage drop in the armature.This curve lies below the internal characteristics.This characteristics is important as it helps to determine the suitability of a generator.