DC GENERATOR working and construction

Generator Principle

An electric Generator is a machine that converts mechanical energy into electrical energy. 
An electric generator is based on the principle that whenever flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow if the conductor circuit is closed. The direction of induced e.m.f. (and hence current) is given by Fleming’s right hand rule.

Flying Electrical

 Therefore, the essential components of a generator are:
(a) a magnetic field
(b) conductor or a group of conductors
(c) motion of conductor w.r.t. magnetic field.

Construction of d.c. Generator

The d.c. generators and d.c. motors have the same general construction. In fact, when the machine is being assembled, the workmen usually do not know whether it is a d.c. generator or motor. Any d.c. generator can be run as a d.c. motor.
All d.c. machines have five principal components -
(i) field system
(ii) armature core
(iii) armature winding
(iv) commutator
(v) brushes

 Field system

The function of the field system is to produce uniform magnetic field within
which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the d.c. exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.

Armature core

The armature core is keyed to the machine shaft and rotates between the field
poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that
are stacked to form a cylindrical core as shown in Fig . The laminations (See Fig.) are individually coated with a thin insulating film so that they do not come in electrical contact with each other.
 The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to cross between the pole face and the armature “teeth”.

Armature winding

The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding. This is the winding in
which “working” e.m.f. is induced.

The armature conductors are connected in
series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current.

Commutator

A commutator is a mechanical rectifier which converts the alternating voltage
generated in the armature winding into direct voltage across the brushes. The
commutator is made of copper segments insulated from each other by mica
sheets and mounted on the shaft of the machine (See Fig ).

The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding.

Brushes

The purpose of brushes is to ensure electrical connections between the rotating
commutator and stationary external load circuit. The brushes are made of carbon
and rest on the commutator. The brush pressure is adjusted by means of
adjustable springs (See Fig.).

If the brush pressure is very large, the friction produces heating of the commutator and the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking.         
Fig.
Multipole machines have as many brushes as they have poles. For example, a 4-
pole machine has 4 brushes. As we go round the commutator, the successive
brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and
the −ve terminal.