GENERATORS

BY Lt NS Wickramasinghe BSc(E & E Eng)

INTRODUCTION

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 mechanical energy.

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.

BASIC CONSTRUCTION

Two ends of the coil are joined to slip-ringsare 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 ARMATUREand magnet as FIELD MAGNETS.

R

Brushes

Rings

WORKING

0 90 180 270 360

a b c d e

emf

WORKING cont…

a 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 coil.

b 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.

c 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.

d This is same as position b.e This is same as position a.

WORKING cont…

N S

segments

brushes

WORKING cont…

0 90 180 270 360

a b c d e

emf

BASIC PARTS OF A GENERATOR

1. Magnetic frame or York2. Pole cores and pole shoes3. Pole coils or field coils4. Armature core 5. Armature winding or conductors6. Commutator7. Brushes and bearing

MAGNETIC FRAME OR YORK

York serves double purpose

1. It provides mechanical support for the pole and acting as protectingcover for the whole machine.

2. It carries the magnetic flux producedby the poles.

York

feet

York is made of cast iron ,cast steel orrolled steel. It must have sufficient strength and have high permeability.

POLE CORES AND POLE SHOESSolid pole core Laminated pole core

Pole shoeScrews

Rivets

York

Pole

The pole core itself may be asolid piece made out of cast iron or steel but pole shoe is laminated and fastened to the pole.

Complete pole cores and shoesare build of lamination of annealed steel which are riveted together under hydraulicpressure. The thickness is

(0.25-1)mm .

POLE COILS OR FIELD COILSFrame

Wire

Exciting coil

Laminated pole core

Laminated Pole shoe

Pole coils which consist of copper wire or strip, are former –wound for correctdimensions. When current is passed through these coils ,they electromagnetise the poles which produce the necessary flux tat is cut by revolving armature conductors.

ELECTROMAGNETIC POLES

ARMATURE CORE

SlotKey way

Air holes

Lamination

Teeth

A complete circular lamination is madeup of four or six or even eight segmentallaminations. The purpose of using laminations is to reduce the loss due to eddy current.

ARMATURE WINDING OR CONDUCTORS

Armature winding basically divided in to Two.1. Lap winding2. Wave winding

Lap winding Wave winding

Yb

YrYf

Yc

YbYf

Yr

Yc Commutator

Conductors

ARMATURE WINDING OR CONDUCTORS cont…

Pole Pole –– pitch pitch --: The distance in between two adjacent poles equal to the number ofarmature conductors or armature slots per – pole.

EX : if there are 36 conductors and 4 poles, the pole pitch is36/4 = 9.

Pitch of a winding (Y) -: It is the distance between the beginning of two consecutive turns.

Y= Yb -Yf ----------------- Lap windingY = 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 back pitch.

Front pitch (Yf) -: The distance between the second conductor of one coilfirst conductor of the next coil which are connected together at the front.

ARMATURE WINDING OR CONDUCTORS cont…

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.

COMMUTATOR

End clamp

Commutator lugs

Insulated coppersegments

The function of the commutator is tofacilitate collection of current from the armature conductors.

EQUIVALENT CIRCUIT

G = generatorVG=generator open-circuit voltageRG=generator internal resistanceVL=generator on-load voltageRL=load resistance

G

RG

RL VLVG

GENERATORS

Synchronous Asynchronous

SYNCHRONOUS GENERATORS

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 weak.