Machines Lec#01

7/31/2019 Machines Lec#01

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Transformer Basics


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Electro-magnetism: Faradays laws

First Law: Whenever the magnetic flux linked

with a circuit changes, an electromotive force

(EMF) is always induced in it

Second Law: The magnitude of the induced

EMF is equal to the rate of change of flux

linkages.


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Two different ways of changing the flux:

Sweeping a magnet past a loop of wire (rotating

machines)

Changing the current that originates the field

(transformer)


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What is transformer?

A transformer is a device that

transfers electrical energy from one

circuit to another throughinductively coupled conductors

the transformer's coils.


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Transformer Applications

AC power transmission

STEP-UP Transformer

STEP-DOWN Transformer

Impedance matching

Electrical Isolation

These will be

discussed in later

lectures.


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Before invention of transformers, in initial daysof electrical industry, power was distributed asdirect current at low voltage. The voltage drop

in lines limited the use of electricity to onlyurban areas where consumers were served withdistribution circuits of small length. All theelectrical equipment had to be designed for the

same voltage. Development of the firsttransformer around 1885 dramatically changedtransmission and distribution systems.


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The alternating current (AC) power generated at

a low voltage could be stepped up for the

transmission purpose to higher voltage and

lower current, reducing voltage drops andtransmission losses. Use of transformers made it

possible to transmit the power economically

hundreds of kilometers away from thegenerating station.


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Step-down transformers then reduced the

voltage at the receiving stations for distribution

of power at various standardized voltage levels

for its use by the consumers.


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LARGE INDUSTRY

RESIDENTIAL

POWER STATION

11 KV/240 V

132/33 KV

400/132 KV132/11 KV


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Basic Principle of Transformer

As mentioned earlier the transformer is a static

device working on the principle of Faraday's law

of induction. Faraday's law states that a voltage

appears across the terminals of an electric coilwhen the flux linkages associated with that coil

changes. This emf is proportional to the rate of

change of flux linkages.


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Basic Operation

Basic Operation:

The primary winding is connected to an ac voltagesource. The magnetic field (flux) builds up (expands)

and collapses (contracts) about the primary winding.The expanding and contracting magnetic field aroundthe primary winding cuts the secondary winding andinduces an alternating voltage into the winding. This

voltage causes alternating current to flow through theload. The voltage may be stepped up or downdepending on the design of the primary and secondary

windings.


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Basic Operation


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Basic Operation


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Principle parts of a transformer and their functions are:

The CORE, which provides a path for the magnetic

lines of flux.

The PRIMARY WINDING, which receives energyfrom the ac source.

The SECONDARY WINDING, which receives energy

from the primary winding and delivers it to the load. The ENCLOSURE, which protects the above

components from dirt, moisture, and mechanical

damage.


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Constructional Features

Constructional aspects can be divided into three

categories.

1. Core construction

2. Winding arrangements

3. Cooling aspects


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Core Construction:

Transformer core for power frequency

application is made of highly permeable

material. The high value of permeability helps to

give a low reluctance for the path of the flux and

the flux lines mostly confine themselves to the

iron. Silicon steel in the form of thin laminationsis used for the core material.


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The steel has a permeability many times that of

free space, and the core thus confine the flux to

a path which closely couples the windings.

Powdered iron cores are used in circuits (such as

switch-mode power supplies) that operate above

main frequencies and up to a few tens of

kilohertz.


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The composition of a transformer core depends

on such factors as voltage, current, and

frequency. Size limitations and construction

costs are also factors to be considered.


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Depending upon core configuration transformer

is divided into two categories.

(a) core type.

(b) shell type


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Core Type Shell type


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Why Laminated Core?

To minimize the loss resulting from eddy currents,transformer cores are LAMINATED. Since the thin,insulated laminations do not provide an easy path forcurrent, eddy-current losses are greatly reduced.

The main strategy in mitigating these wasteful eddycurrents in transformer cores is to form the iron core in

sheets, each sheet covered with an insulating varnish sothat the core is divided up into thin slices. The result isvery little width in the core for eddy currents tocirculate in:


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Eddy current losses increase with frequency, so

transformers designed to run on higher-

frequency power (such as 400 Hz, used in many

military and aircraft applications) must usethinner laminations to keep the losses down to a

respectable minimum. This has the undesirable

effect of increasing the manufacturing cost ofthe transformer.


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Winding arrangement:

Windings form another important part of transformers.

In a two winding transformer one is connected to a

voltage source and creates the flux is called as a primarywinding. The second winding where the voltage is

induced by induction is called a secondary. If the

secondary voltage is less than that of the primary the

transformer is called a step down transformer. If thesecondary voltage is more then it is a step up

transformer.


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A step down transformer can be made a step up

transformer by making the low voltage winding its

primary. Hence it may be more appropriate to designate

the windings as High Voltage (HV) and Low Voltage(LV) windings. The winding with more number of turns

will be a HV winding. The current on the HV side will

be lower as V-I product is a constant and given as the

VA rating of the machines. Also the HV windingneeds to be insulated more to withstand the higher

voltage across it.


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Schematic Symbol

Air Core Ferrite Core Iron Core


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Classification of Transformer

By power level (from fraction of a watt to manymegawatts),

By application (power supply, impedance matching,circuit isolation),

By frequency range (power, audio, RF)

By voltage class (a few volts to about 750 kilovolts)

By cooling type (air cooled, oil filled, fan cooled,

water cooled, etc.) By purpose (rectifier, arc furnace, amplifier output,

etc.).

By ratio of the number of turns in the coils


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Transformer Losses

Transformer losses are divided into losses in the windings,termed copper loss, and those in the magnetic circuit, termediron loss. Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistive heating of theconductors. At higher frequencies, skin effect and proximity effect createadditional winding resistance and losses.

Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost

due to hysteresis within the core. For a given core material, the loss isproportional to the frequency, and is a function of the peak flux densityto which it is subjected.


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Transformer Losses

Eddy currents

Ferromagnetic materials are also good conductors,

and a core made from such a material also

constitutes a single short-circuited turn throughoutits entire length. Eddy currents therefore circulate

within the core in a plane normal to the flux, and are

responsible for resistive heating of the core material.

The eddy current loss is a complex function of thesquare of supply frequency and inverse square of the

material thickness.


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Transformer Losses

Magnetostriction

Magnetic flux in a ferromagnetic material, such as

the core, causes it to physically expand and contract

slightly with each cycle of the magnetic field, aneffect known as magnetostriction. This produces the

buzzing sound commonly associated with

transformers, and can cause losses due to frictional

heating.


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Transformer Losses

Mechanical losses

In addition to magnetostriction, the alternating

magnetic field causes fluctuating forces between the

primary and secondary windings. These incitevibrations within nearby metalwork, adding to the

buzzing noise, and consuming a small amount of

power.


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Stray losses

Leakage flux that intercepts nearby conductive

materials such as the transformer's support structure

will give rise to eddy currents and be converted toheat.

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Machines Lec#01