Chap 2 Transformer 1

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EBB2133 Electrical Machine 1Chapter 2: Transformer

Lecturer:

Dr Zuhairi BaharudinRoom: 22 03 09 Ext: 7810

email: zuhairb etronas.com.m


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Students should be able to :

a) describe

the

operation

and

the

different

types

of

transformers

escr e t e c aracter st c o an ea trans ormer

c) analyze circuits containing ideal transformers

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What is a TRANSFORMER ?

It is a device that converts ac electric energy at

one eve nto ac e ectr c energy at anot er eve(can be either voltage or current).

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In electric power generation and distribution ,

make electricit eneration ossible at the most economic generator voltage level

make electric power transmission take place at the most

supply electric power to users at a safe and suitable voltage.

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Load-Residential

Power Plant

Transmission l ine

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In electronic and communication systems , trans ormers are use to :

match the impedance for maximum power transfer w y

transfer signals.

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PrimaryConnected to voltage

SecondaryConnected to load Windings, N P and N S

i P iS

source generator

v v N P N S

++

__

Core

7

Laminated electrical steel core Contains the flux flow,


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How does a Transformer Work ?

Primary coil N P excited byan AC source, v P

Secondary coil N S surroundstotal flux,

Resulting current producestotal flux, , flowing throughthe core

An AC voltage, vS , is inducedin the second coil

i P iS

v P vS N P N S

++

__

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Ideal Transformer For ea trans ormer, we assume t at :

The permeability of the core, r approaches infinity

No resistance in the windings No leaka e lux

Therefore,

S P N t i )(i P iS

++

P S N t i )( v P vS P S

__

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Transformer schematic symbols:How do we determine the

iS (t )i

P (t ) P S

++polarity of the secondary

voltage ?

P S __

If v P is positive at the dotted end of the winding with respect to theundotted end, then vS will bepositive at the dotted end.

the primary winding, iS will flow outof the dotted end of the secondary

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Types of Transformersower rans ormers are u n one o ways :

a) Shell type transformer

i P iS ++

v P vS N P N S

__

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a) Shell type transformer

Lower voltage regulation

b) Core type transformer Large gap between primary

and secondary windings Regular use in high voltage

Simple to construct Smaller in size

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Since the power losses are ignored, the input power must be equal to output power :

v P i P = v S iS or a N t vt v

S

P

S

P ==)(

where a is called the turns ratio .

transformer.

= V cos = V cos since P = S = P in = P out

Therefore and of an ideal transformer have the same power factor .

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Reactive power going into the primary winding is also equal to reactive power going into the secondary winding.

Q in = V P I P sin = V S I S sin = Q out

The same relationship also applies to apparent power S.

in = P P = S S = out

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Impedance Transformation through A Transformer A transformer changes voltage or current levels. This also

changes the ratio between voltage and current.

L

L I

VZ =

impedance of an element.

How ?

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Impedance TransformationThe im edance of the load is

S

S L Z I

V=

P

P L Z I

V='

However, the source sees animpedance between theprimary terminals as

P

P L Z

I

V='

We now t at P = a S an P = S a , t ere ore

S S aa 22

' ===VV

LS S

L a / II

the im edance seen b the source is a 2 times the real im edance.

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Impedance Transformation

modify the value of anycomponent. P P

L Z IV

='

Example : A 1000

resistor placed across the secondary of an ideal

having a resistance of 1000 x (1/ 5)2 = 40 . It is also possible to use the ideal transformer to match the magnitude

ratio.

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Introduction

Important relationships

transformer : Volta e level

Apparent impedance seenby the primary

a N t v P P

==)(

Current We will use these

S S L L Z a Z ' =

relationships to analyzecircuit with ideal

a N

t it i S P 1)( ==

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Analysis of Ideal Transformer The easiest way to analyze a circuit containing an ideal

transformer is to replace one side of the transformer by an .

This process is known as referring or shifting the first side of the transformer to the second side.

Using this method, values of voltages on the side being

replaced are scaled using V / V S = a and values of impedances are also scaled using Z L = a 2 Z L.

Lets look at an example !

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a) What is the load voltage for the system shown below?b) What are the transmission losses of the system ?

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c) If the generator is directly connected to the load, what is the load voltage?

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Chap 2 Transformer 1

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