WHAT IS GENERATOR?WHAT IS THE PRINCIPLE ? • An generator is a machine which converts mechanical energy into electrical energy. • Principle of generator is the production of dynamically induced e.m.f according to FARADAY’S LAWS OF ELECTROMAGNETIC INDUCTION.
MAIN PARTS OF D.C GENERATOR • YOKE. • POLE CORE AND POLE SHOE. • FIELD COIL. • ARMATURE CORE. • ARMATURE WINDING. • COMMUTATOR. • BRUSHES AND BRUSH HOLDERS. • BEARINGS. • SHAFT.
YOKE: • It provides mechanical support for the poles and as a protecting cover for the whole machine. • It carries the magnetic flux produced by the poles.
POLE CORE AND POLE SHOE: • The main function of pole core is to act as support for the field coils carries current which surrounds it and establish magnetic flux. • The main function of pole shoe are it supports the field coil it spreads out the magnetic flux over a larger area.
FIELD COILS: • Field coils are also called pole coils. • When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.
ARMATURE CORE: • Armature is the rotating part. • It provides a path of very low reluctance to the flux through the armature from NORTH pole to SOUTH pole. • It is cylindrical or drum shaped.
ARMATURE WINDING OR CONDUCTORS: • Armature windings consists of conductors or copper wires which are placed in slots. • It is of two types • Open coil winding and • Closed coil winding. • Open coil winding is used in A.C machines. • Closed coil winding is again divided into 2 types. • Ring winding and Drum winding.
Drum winding is again divided into two types. • Lap winding and Wave winding. • Lap winding is used for low voltages and high currents. • Wave winding is used for high voltages and low currents.
In lap and wave winding back pitch and front pitch are equal to the pole pitch. • Number of commutator segments is equal to the number of slots or coils. • Pole pitch is equal to the number of conductors per pole.
COMMUTATOR: • The function of commutator is to facilitate collection of current from the armature conductor. • It also converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit.
BRUSHES AND BRUSH HOLDERS: • The main function of brushes is to collect the current from commutator. • Brushes are housed in the brush holders. • These brush holders are used for the protection purpose to the brushes. • Total number of brushes is equal to the number of poles.
BEARINGS AND SHAFT: • Bearing and shaft are mainly used for smoothing operation because of their reliability. • Ball bearings are frequently employed, through for heavy duties, roller bearing are preferable.
ARMATURE REACTION: • Armature reaction is meant the effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. • This armature reaction has two effects. • 1.DEMAGNETISING EFFECT. • 2.CROSS MAGNETISING EFFECT.
DEMAGNETISING EFFECT: • It demagnetises or weakens the main flux. • It is produced in opposite to the main field flux. • It is denoted by OFd
This demagnetising effect is decreased by placing extra ampere-turns to the main field winding. • It is essential to calculate their number, it is denoted by ATd. • Demagnetising ampere turns are neutralized by adding extra ampere turns to the main field winding.
First is the figure of demagnetising effect and the latter is cross magnetising effect
CROSS MAGNETISING EFFECT: • It distorts the main field flux. • It is denoted by OFc. • Cross magnetising effect is neutralized by compensating windings.