DC Generator

Principle of DC Generator:

• A DC generator produces direct power based on fundamental principle of Faraday laws of electromagnetic induction.
• According to these laws, when a conductor moves in a magnetic field it cuts magnetic lines force, due to which an e.m.f is induced in the conductor.
• The magnitude of this induced emf depends upon the rate of change of flux (magnetic line force) linkage with the conductor.
• This e.m.f will cause current to flow if the conductor circuit is closed.
• The two essential parts of a generator are
  a) a magnetic field and
  b) conductors which move inside that magnetic field.

• Consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis ab.
• When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field.
• During the movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an e.m.f induced in both sides (AB & DC) of the loop.
• As the loop is closed there will be a current circulating through the loop.
• The direction of the current can be determined by Fleming right hand Rule.
• The waveform of the current through the load circuit is as shown in the figure.
• This current is unidirectional in nature.

Construction of Direct Current Machines

The main parts of DC machine are:

1. Frame or Yoke,
2. Field system,
3. Armature core and Armature winding
4. Commutator
5. Brushes

1. Frame or Yoke:

• The cross section of the yoke can be solid or it can be fabricated.
• Small machines use solid yokes.
• For large machines it is fabricated.
• Yoke protects the inner part of the machine from the atmosphere.
• Yoke carry a unidirectional flux.
• The flux always moves from South Pole to North Pole.
• Yoke flux is always half of the pole flux.
• Thus yoke provides a return path for the pole flux.
• Therefore the cross section of the yoke can be half of pole cross section.
• Since yoke flux is unidirectional no e.m.f is induced in it.

2. Field system:

• Field system consists of the following parts: Pole core, Pole shoes and Field coils.
• Pole core is the part of the field system where the magnetic flux is set up.
• Pole core can be solid in cross section or it can be laminated.
• Machines of smaller power rating uses pole core of solid cross sections.
• Larger machines uses laminated pole core.
• In laminated pole, laminations of steel sheets are stacked together to give the pole core.
• Lamination is done to reduce eddy current loss.

• Field windings are made of copper or aluminum conductor.
• They carry current and produce magnetic field.
• All field windings on the poles are connected in series and carry same current.
• Depending on the current directions in the coil we get the polarity of the pole as north or south.
• Field windings are of two types: Series field and shunt field windings.
• Series field winding will have smaller number of turns of thick wire and needs a large current to produce a required value of flux.
• The shunt field winding will have large number of turns of fine wire and requires small current to set up required flux.
• Series field winding will be connected in series with the armature.
• And shunt field winding is connected in parallel with the armature.

3. Armature core and Armature windings:

• The armature core is cylindrical in shape.
• High permeability silicon steel stampings are used for the armature core.
• The laminations are circular in shape with outer periphery slotted to receive the armature conductors.
• The laminations of the core are separated from each other by a layer of varnish coating.
• The varnish coating acts as an insulator for the flux and limits the induced eddy currents to the lamination.
• This reduces the eddy current loss.
• The armature conductors are interconnected to form the coils.
• The coils are interconnected to form the Armature winding.
  
• The armature windings will have coils connected in parallel paths.
• All the coils in a path will be in series.
• There are two types of winding: Lap and Wave winding.
• In Lap winding, the number of parallel paths (A) equal to number of pole (P) and is used in high current, low voltage machines.
• In Wave winding, the number of parallel paths is always two.
• Thus each path will have more coils compared to lap winding.
• Therefore wave winding is used for high voltage, low current machines.

4. Commutator:

• As the coil rotates in the magnetic field, the e.m.f induced in the conductors varies sinusoidally and results in sinusoidally alternating current in the conductors.
• This sinusoidally alternating current must be made unidirectional so that the output current is a direct current.
• Commutator converts ac current into unidirectional current.
• Commutator is cylindrical in shape, made of V shaped copper or brass segments insulated from each other by a mica layer.

5. Carbon brushes:

• Carbon brushes are used in DC machines to collect the current from the rotating armature.
• These brushes, under spring force always make contact with the commutator surface.
• These brushes are placed in brush holders supported on brush arm.

EMF equation of DC Generator

• Flux cut by a conductor in one revolution = Flux per pole x No. of poles
• Time for one revolution = 1/ N minutes = 60 / N seconds.
• In one revolution a conductor moves under all P poles
• Time taken by the conductor to move under one pole = Time for one revolution / number of poles.
  dt = (60/N)/ P = 60 / NP

• If Z is the number of conductors on the armature, they are connected A parallel paths, with each parallel path having Z / A conductor in a path all connected in series.
• Emf generated between the terminals of a parallel path, Eg = e.m.f induced in one conductor x No. of parallel paths

• For Lap wound armature, A = P, no. of poles

• For wave wound armature, A=2.

Voltage equation of generator:

• The induced e.m.f Eg of the generator while allowing a current of Ia through the armature conductors causes an armature resistance drop IaRa and due to the bush contact with commutator a small amount of voltage is also dropped (usually around 2 volts).
• The remaining voltage is available at the terminals of the generator as terminal voltage, V.
• Thus terminal voltage is given by Eg= V + IaRa + brush drop
• A = Armature

Types of DC Generators

1. Separately Excited DC Generators:

• In this generator, the field winding is connected to a separate source.
• The current in the field winding is independent of the generated voltage.

2. Self Excited DC Generators:

• The field winding is connected to the armature of the generator.
• The current required for producing the magnetic flux is given by the induced voltage of the machine.
• Therefore the generator is called self excited.
• Following are different types of self excited generators
• Shunt wound DC generator:

• A field winding with large number of turns of fine wire is connected in parallel with the armature.
• It takes a small current from the armature to produce required value of the magnetic flux.
• The flux produced will almost remains constant.
• Series wound DC generator:

• A field winding with small number of turns of thick wire is connected in series with the armature.
• It always carries the armature current.
• The flux produced will be proportional to the armature current.
• The flux will vary with the armature current.
• Compound wound DC generator:

• This machine will have both series field and shunt field windings.
• Based upon the connection of the field windings, the compound generator can be short shunt or long shunt.

Shunt Excited DC Generator:

Series Excited DC Generator:

Compound wound DC generator:

• will have both series field and shunt field windings.
• Based upon the connection of the field windings can be:
  (i)short shunt – Shunt to armature only
  (ii)long shunt-Shunt to Armature & Series field

Short Shunt:

Long Shunt: