D.C.Generators

Generator Principle:

An electrical generator is a machine which converts mechanical energy (or power) into electrical energy (or power).

The energy conversation is based on the principle of the production of dynamically (or motionally) induced e.m.f . As seen from fig-1.1,whenever a conductor cuts magnetic flux, dynamically induced e.m.f is produced in it according to Faraday’s Laws of Electromagnetic Induction. This e.m.f. causes a current to flow if the conductor circuit is closed.

Hence, two basic essential parts of an electrical generator are (i)a magnetic field and (ii)a conductor or conductors which can so move as to cut the flux.

Construction:

The fig.in above is shown a single-turn rectangular copper coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnet is or electromagnets. The two ends of the coil are joined to two slip-rings 'a' and 'b' which are insulated from each other and from the central shaft.Two collecting brushes (of carbon or copper ) press against the slip-rings.Their function is to collect the current induced in the coil and to convey it to the external load resistance R.

The rotating coil may be called 'armature ' and the magnets as 'field magnets'.

Practical Generator:

The simple loop generator has been considered in detail merely to bring out the basic principle underlying construction and working of an actual generator illustrated in fig.3.10  which consists of the following essential parts:

1.       Magnetic frame or Yoke

2.     Pole-Cores and Pole-Shoes

3.      Pole Coils or Field Coils

4.      Armature Core

5.      Armature Windings or Conductors

6.      Commutator   

7.      Brushes and Bearings.

Of these, the yoke, the pole cores, the armature core and air gaps between the poles and the armature core or the magnetic circuit whereas the rest from the electrical circuit.

Yoke:

 

The outer frame or yoke serves double purpose:

 

(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine 

    and

(ii) It carries the magnetic flux produced by the poles.

 

Pole cores and Pole shoes:

• The field magnets consists of pole cores and pole shoes.The pole shoes serve two purposes (i)they spread out the flux in the air gap and also,being of larger cross-section,reduce the reluctance of the magnetic path (ii) they support the exciting coils (or field coils) .

• The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened  to the pole face by means of counter sunk screws.

Armature core:

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets.In addition to this,its most important function is to provide a path of very low reluctance to the flux through the armature from a N-pole to a S-pole.
It is cylindrical or drum-shaped and is built up of usually circular sheet steel discs or laminations approximately 0.5 mm thick.It is key to the shaft.