Lt NS Wickramasinghe BSc(E & E Eng)
Electrostatic generators are used for scientific experiments requiring high
voltages. Because of the difficulty of insulating machines producing very high
voltages, electrostatic generators are made only with low power ratings and are
never used for generation of commercially-significant quantities of electric power.
Before the connection between magnetism and electricity was discovered,
generators used electrostatic principles.
An electrical generator is a device that
converts mechanical energy to electrical
energy, generally using electromagnetic
induction. The source of mechanical
energy may be a reciprocating or turbine
steam engine, water falling through a
turbine or waterwheel, an internal
combustion engine, a wind turbine, a
hand crank, or any other source of
Two ends of the coil are joined to slip-rings
are insulated from each other and from the
central shaft. Two collecting brushes 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 magnet as FIELD MAGNETS.
When the plane of the coil is at the right angles to lines of flux. Flux linking with
the coil is maximum but rate of change of flux linkages is minimum. It is because
the coil sides RED and VILOT do not cut the flux. There is no induced e.m.f in the
When the coil is rotating further rate of change of flux linkage increase till coil
plane is parallel to the lines of flux. The flux linkage with coil is minimum but rate
of change of flux is maximum. Hence maximum e.m.f induced.
From 90 to 180, the flux linkage with the coil is increases but the rate of change of
flux linkage decreases. Gradually the induced e.m.f decreases till the angle 180.
This is same as position
This is same as position a.
BASIC PARTS OF A GENERATOR
Magnetic frame or York
Pole cores and pole shoes
Pole coils or field coils
Armature winding or conductors
Brushes and bearing
MAGNETIC FRAME OR YORK
York serves double purpose
1. It provides mechanical support for
the pole and acting as protecting
cover for the whole machine.
2. It carries the magnetic flux produced
by the poles.
York is made of cast iron ,cast steel or
rolled steel. It must have sufficient
strength and have high permeability.
POLE CORES AND POLE SHOES
Laminated pole core
Solid pole core
The pole core itself may be a
solid piece made out of cast
iron or steel but pole shoe is
laminated and fastened to
Complete pole cores and shoes
are build of lamination of
annealed steel which are
riveted together under hydraulic
pressure. The thickness is
POLE COILS OR FIELD COILS
Pole coils which consist of copper wire or strip, are former –wound for correct
dimensions. When current is passed through these coils ,they electromagnetise
the poles which produce the necessary flux tat is cut by revolving armature
A complete circular lamination is made
up of four or six or even eight segmental
laminations. The purpose of using
laminations is to reduce the loss due to
ARMATURE WINDING OR
Armature winding basically divided in to Two.
1. Lap winding
2. Wave winding
ARMATURE WINDING OR
Pole – pitch -: The distance in between two adjacent poles equal to the number of
armature conductors or armature slots per – pole.
EX : if there are 36 conductors and 4 poles, the pole pitch is
36/4 = 9.
Pitch of a winding (Y) -: It is the distance between the beginning of two consecutive
Y= Yb -Yf ----------------- Lap winding
Y = Yb +Yf -------------------Wave winding
Back pitch (Yb) -: The distance, measured in terms of the armature conductors
which a coil advances on the back of the armature is called
Front pitch (Yf) -: The distance between the second conductor of one coil
first conductor of the next coil which are connected
together at the front.
ARMATURE WINDING OR
Resultant pitch (Yr)-: It is the distance between the beginning of the next coil to
which it is connected.
Commutator pitch (Yc)-: It is the distance between the segments to which the
two ends of the coil are connected.
The function of the commutator is to
facilitate collection of current from the
G = generator
VG=generator open-circuit voltage
RG=generator internal resistance
VL=generator on-load voltage
All 3-phase generators use a rotating magnetic field. In the
picture to the left we have installed three electromagnets
around a circle. Each of the three magnets is connected to its
own phase in the three phase electrical grid.
As you can see, each of the three
electromagnets alternate between
producing a South pole and a North pole
towards the centre. The letters are shown
in black when the magnetism is strong,
and in light grey when the magnetism is