Subject Name: Basic Electrical Engineering Module Name: 5A.Three Phase Synchronous Generator

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Subject Name: Basic Electrical Engineering

Module Name: 5A.Three Phase Synchronous

Generator

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www.cambridge.edu.in Department of Electrical & Electronics Engineering

Module : 5 A – Three phase Synchronous

Generator

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Introduction

• A Generator is an electrical machine that converts mechanical energy into electrical energy.

There are different types of generators .

1) Dc generators

2) Ac generators (both single phase and three phase)

• Three phase synchronous generators are also called as alternators

or AC generators.

• They operate on the principle of Faraday’s laws of electromagnetic

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• The electromagnetic induction states that electromotive force (EMF) is induced in the armature

coil if it is rotating in the uniform magnetic field.

• The EMF will also be generated if the field rotates and the conductor becomes stationary. Thus,

the relative motion between the conductor and the field induces the EMF in the conductor.

• To generate emf either,

1) armature winding is rotating and field winding is stationary.

2) or armature winding is stationary and field winding is rotating.

• In the case of alternators, standard construction consists of armature winding mounted on a

stationary element called stator and field windings on a rotating element call rotor.

Contd…

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• As everywhere AC is used, the generation level of AC voltage is higher about 11kV or 33kV. This

gets induced in the armature.

• For stationary conductors or armature large space can be provided to accommodate large

number of conductors and the insulation.

• It is always better to protect high voltage winding from the centrifugal forces caused due to

rotation. So armature is kept stationary.

• It is easier to collect the output current directly from the fixed terminals on the stator to load

without passing it through brushes or slip rings.

Advantages of stationary armature and rotating

magnetic field:

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• The problem of sparking at the slip rings can be avoided by keeping field rotating and armature

stationary.

• It is easier to insulate stationary armature windings for high AC voltages.

• The ventilation arrangement for high voltages side can be improved if armature is kept

stationary.

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Constructional

features:

Basically an alternator consists of two parts ,

1) Stationary part called stator (armature winding) and

2) Rotating part called Rotor (field winding).

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• Stator consists of cast iron or steel frame which supports armature

core having slots on its periphery for housing armature conductors.

• The stator core used a laminated construction in order to reduce

eddy current loss.

• The material used special magnetic iron steel alloy so as to keep the

hysteresis losses minimum.

• The frame does not carry any flux and serves as the support to the

core.

• Ventilation is provided with the help of holes (ducts) casted in the

frame.

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Types of Alternators:

Depending on the type of rotor, we have two types of alternator. They are,

1. Salient pole alternator

2. Non salient pole alternator or smooth cylindrical type of rotor.

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Salient pole type Rotor :

• This is also called projected pole type as all the poles are

projected out the surface of the rotor.

• The Poles are made up of thick steel laminations and are bolted to

the rotor as shown in the fig.a

• The field winding is placed on the pole core and is supported by

the pole shoe.

• These rotors have large diameters small axial length.

• The mechanical strength of salient pole type less, hence this is

preferred for low speed alternators.

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Smooth cylindrical / Non salient type Rotor

• This is also called as non projected pole type of rotor.

• The rotor consists of smooth solid Steel cylinder having

number of slots to accommodate the field coil.

• The unslotted portions of the cylinder itself act as the poles.

• The poles are not projected out and the surface of

the

rotor is smooth which maintains uniform air gap

between

stator and rotor.

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Smooth cylindrical / Non salient type Rotor

• These rotors have small diameter and large axial length.

• These types of rotors are mechanically very strong and are

preferred for high speed alternators up 3000rpm.

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Comparison of different types of Alternators:

Salient/Projected Pole Type

Non-salient/Smooth cylindrical type

Poles are projected out from the

surface.

Unslotted portion of the cylinder acts

as poles hence poles are

non-projecting.

Air gap is non-uniform

Air gap is uniform.

It has large diameter and small axial

length

It has smaller diameter and large axial

length.

Mechanically weak.

Mechanically strong

Preferred for low and medium speed

alternators.

Preferred for high speed alternators.

Prime mover used are water turbines,

IC engines.

Prime movers used are steam turbines,

electric motors.

For same size, the rating is smaller

than cylindrical type.

For same size, the rating is higher than

salient pole type.

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Principle of operation:

• The alternators work on the principle of electromagnetic induction.

• For the purpose of better understanding, let us consider the armature conductors are rotating

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Principle of operation:

• Consider a relative motion of a single conductor under the magnetic field produced by two

stationary poles.

• Let the conductor start rotating position 1 at this instant the entire velocity component is

parallel to the flux lines the conductor is not cutting any flux lines and the emf induced in the

conductor is zero.

• As the conductor moves from position 1 to position 2, the flux lines cut by the conductor

increases, accordingly the magnitude of emf induced also increases.

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Principle of operation contd…

•

_{At position 2 velocity component is perpendicular to the flux lines and at this instant, the}

induced emf is maximum.

• As the conductor changes its position from 2 to 3, the velocity component perpendicular to

the flux starts decreasing and hence the magnitude of emf also decreases. At position 3 the

emf induced in the conductor is zero.

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• But the direction of velocity component is opposite in direction of velocity component

existing during the movement of conductor from position 1 to 2 . Hence the induced emf in

the conductor increases.

• At position 4, the velocity component becomes perpendicular to the flux lines, hence the

induced emf is maximum. But the direction of the velocity component is opposite hence the

induced emf is opposite to that of in position 2.

• As the conductor moves from position 4 to 1, the induced emf decreases and finally becomes

zero at position 1.

Principle of operation contd…

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Principle of operation

1 rotation = 360

0

_{= Mechanical angle}

1 cycle= { one positive cycle of induced emf} + {

one negative cycle of induced emf}

= 360

0

_{= electrical angle}

For 2 pole alternator, one mechanical angle corresponds to one electrical angle.

But in general,

I

0

_{mechanical angle =}

𝒑

𝟐

electrical angle.

Where p = no. of poles

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• One mechanical revolution = P/2 Electrical Cycle

• Electrical cycles/revolution = P/2

• As speed is N rpm ,in one second ,rotor will complete N/60

• Frequency = No of electrical cycles per second

= (No of electrical cycles per revolution)x(revolutions per second)

= (P/2)x(N/60) = PN/120

Therefore f = PN /120 Hz

Principle of operation contd…

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Winding Terminologies

Conductor: The part of the wire ,which is under the influence of the magnetic field and responsible for

the induced emf is called active length of the conductor.

Turn: A conductor in one slot , when connected to a conductor in another slot forms a turn. So two

conductors constitute a turn.

Coil: The number of turns are grouped together to form a coil. The coils may be a multiturn coil or

single turn coil.

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