ppt machines

7/30/2019 Ppt Machines

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Contents

1.Electric Machines

2.Transformer

3.Induction motor


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Transformer

Made up of inductors.

Not electrically connected.

An ac voltage applied to the primary induces an ac voltage in the secondary.


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3

Types of Transformer

Step-up transformer

- provides a secondary voltage that is

greater thanthe primary voltage.

Step-down transformer

- provides a secondary voltage that is

less thanthe primary voltage.

Isolation transformer- provides a secondary voltage that is equal

tothe primary voltage.

- to isolate the power supply electrically from

the power line, which serves as a

protection.


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Transformer secondary voltage

The turns ratio of a transformer is equal to the voltage ratio of the component:

)(

)(

1

2

1

2

tv

tv

N

N

or )()( 11

2

2 tvN

N

tv

For example:acac

VVtvN

Ntv 30)120(

4

1)()( 1

1

22


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Transformer secondary current

Assuming the transformer is 100% efficient, then

or

12

PP

)()()()(1122 titvtitv

)()(

)()(

1

2

1

2ti

tv

tvti )()( 1

2

1

2 tiN

Nti


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ExampleConsider the source, transformer, and load shown in the circuit below. Determine

the rms values of the currents and voltages (a) with the switch open and (b) with

the switch closed.

VrmsV 110)(1

Solution

Voltage applied to the primary,

VrmsVN

NrmsV

22)110(5

1

)()( 11

2

2

(a) With the switch open, the secondary current is zero. Hence, the primary

current is also zero.

(b) With the switch closed, AR

rmsVrmsI

L

2.210

22)()( 2

2

ArmsIN

NrmsI 44.0)2.2(

51)()( 2

1

21


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Transformer Rating

The rating of a transformer is statedas Volt Ampere (VA) that it cantransform without overheating.

The transformer rating can becalculated as either V1I1 or V2I2where I2 is the full load secondary

current.


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DC Machines


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Electric Machines

Machines that convert mechanicalenergy to and from electric energy.

- Motor convert electrical energyinto rotational mechanical energy

- Generator convert mechanicalenergy into electrical energy


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Faradays Law

MOTOR PRINCIPLE

When current carrying conductor isplaced in magnetic field itexperience a mechanical force.

10


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Faradays LawIf a flux passes through a turn of coil of wire, a voltage will be

induced in the turn of wire that is directly proportional to therate of change in the flux with respect to time.

If a coil has N turns and if the same flux passes through all of

them, then the induced voltage across the whole coil is given by

dt

de

dt

dNe


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For

ce

Left Hand Rule


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Faradays Law


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Current reverses


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Simple Diagram


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Consider further 90 degree rotation

17


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Commutator


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Electric Motor Basic construction

Basic components:

1. Stator

stationary part2. Rotor rotating part

3. Shaft coupled the machine to the mechanicalload.


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In most types of motor, a given windings canbe classified as field winding or as armaturewinding.

Field winding - to set up the magnetic fieldrequired to produce torque.

Armature windings - carry currents that varywith mechanical load. When the machine is

used as a generator, the output is taken fromthe armature windings.

Electric Motor


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Electric Motor - Basic classification


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DC Machines - Construction


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DC Machines - Construction


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LP11 25

A Two Pole DC Motor


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LP11 26

A Four Pole DC Motor


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DC Motor

Can be divided into 2 types:

(a) Wound type

shunt series

compound

(b) Permanent magnet type


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DC motor

Shunt wound motor- field is connected in parallel with the armature

- has good speed regulation

Series wound motor

- field winding in series with the armature- very high starting torque and poor speed regulation.

Compound wound motor

- field winding has both series and shunt components

- offers better starting torque than the shunt motor butworse speed regulation


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LP11 29

Field Coil

Armature

RA

Vf

Separately Excited DC Machine

+

-


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LP11 30

Shunt Field Coil Armature

RA

Shunt Excited DC Machine


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LP11 31

Series Field Coil

Armature

RA

Series Excited DC Machine

C d E it d DC M hi


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LP11 32

Shunt Field Coil Armature

RA

Compound Excited DC Machine

Series Field Coil

If the shunt and series field aid each other it is called aCumulatively excited machine

If the shunt and series field oppose each other it is called a

Differentially excited machine


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DC motor

Permanent magnet

- field windings are replaced by permanent

magnets

- adequate starting torque

- speed regulation somewhat worse than that of

the compound wound motor


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Characteristics of DC Electrical Motors


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DC GENERATOR


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