Explain the principle of operation and construction of a dc generator.

5.A] Explain the principle of operation and construction of a dc generator.

• Working principle of D.C.Machine as a generator:
  • It is based on the principle of dynamically induced e.m.f .
  • Whenever a conductor cuts magnetic flux, dynamically induced e.m.f. is produced in the conductor according to the Faradays laws of Electromagnetic Induction. This e.m.f. causes a current to flow in the circuit, if the conductor circuit is closed.
  • the emf is given by o e=Blv*Sinθ volts/coil side where, B=the flux density in Tesla,
    l=the active length of the coil side in meters
    v=the velocity with which the coil is moved in meters/sec and is the angle between the direction of the flux and relative velocity.
  • The direction of the induced voltage can be obtained by applying Fleming’s right
    hand rule

Construction of DC Machine

• Salient parts of a D.C.Machine are:
• (i)Yoke
• ii) Field system (poles)
• (iii) Armature
• (iv) Commutator
• (v)Brushes

• Yoke:
  • It is made of cast iron or silicon steel
  • It serves the purpose of outermost cover of the D.C. machine. So that the insulating materials get protected from harmful atmospheric elements like moisture, dust and various gases like SO2, acidic fumes etc.
  • It provides mechanical support to the poles, It forms a part of the magnetic circuit and it provides a path of low reluctance for magnetic flux.
• Poles:
  • It is made cast iron or cast steel laminations which are stamped together.
  • Each pole is divided into two parts a) pole core and b) pole shoe
  • Pole core basically carries a field winding which is necessary to produce the flux.
  • It directs the flux produced through air gap to armature core and to the next pole.
  • Pole shoe enlarges the area of armature core to come across the flux, which is necessary to produce larger induced emf. To achieve this, pole shoe has given a particular shape

• Field winding [F1-F2]:
  • It is made of conducting material like copper or Aluminum. The field winding is wound on the pole core with a definite direction.
  • It carries current due to which the pole core behaves as an electromagnet and produces necessary flux. As it‟s exciting the pole as electromagnet hence it is also called Exciting winding.

• Armature:
  • It is further divided into two parts namely,
    •  Armature core and
    •  Armature winding
  • Armature core is cylindrical in shape made up of iron and mounted on the shaft. It is provided with of slots on its outer periphery to place the conductor and the air ducts to permit the air flow through armature which serves cooling purpose.
  • In order to collect the Emf generated in each conductor they are connected in certain pattern called armature winding.

• Commutator:
  • The basic nature of Emf induced in the armature conductors is alternating. This needs rectifications in case of D.C. generator which is possible by device called
    a commutator.
  • It is cylindrical in shape made of hard drawn copper segments. These segments are insulated from each other by a layer of mica.
• Brushes and brush gear:
  • Brushes collect current from commutator and make it available to the stationary external circuit.
    Ball bearings are usually used as they are more reliable.
  • For heavy duty machines, roller bearings are preferred.