5 a] With a neat diagram explain the constructional details of DC generator.
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
1. 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.
2. 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
1. Pole core basically carries a field winding which is necessary to produce the flux.
2. It directs the flux produced through air gap to armature core and to the next pole.
3. 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,
1. Armature core and
2. 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:
1. 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 commutator.
2. 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:
1.Brushes collect current from commutator and make it available to the stationary external circuit.
2. Ball bearings are usually used as they are more reliable.
3. For heavy duty machines, roller bearings are preferred.
