eletric drives

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INDUSTRIAL APPLICATIONS

UNIT VII


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STEEL MILLS(Rolling mills)

Rolling of steel, the process during

which the cross section of the metal getsreduces , while increasing its length

proportionately , is the major function of steel

mills

1.Continuous cold rolling mills

2.Reversing cold rolling mills3.Continuous hot rolling mills

4.Reversing hot rolling mills


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Reversing mill:

There is only one stand carrying the rolls that

press the metal and the metal is passedthrough this stand alternately forward and

backward several times in order to reduce it

to desired size

Pass:Each motion or travel is known as pass


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Continuous mill:A continuous mill or tandem mill consists ofseveral stands, each one of them carrying

pressing rolls. The metal passes through all the

stands in only one direction and gets rolledsimultaneously by some or all of hem


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REVERSING HOT ROLLING MILLS


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CONTINUOUS HOT ROLLING MILLS

Process:

Have two group of stands-roughing

stand and finishing stand.Each stand may have 2 or 4 rolls

In 4 roll high stand , the inner rollsare used to vary and maintain the

gap between the working rolls.


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Requirement of drive for continuous hot rolling mill

1. In order to produce billets of different dimensions ,

different gap lengths between the working rolls are to

be set

2.In order to avoid sag of the metal between stand , the

speed of the stand motors should be controlled to a

high degree accuracy. As the metal comes into contact

with the working rolls at its speed of operation , this

gives rise to a sudden application o f load on the stand

motor, its speed nay drop and cause sag of the metalbetween rolls


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Reversing cold rolling mill

process:the strip to be rolled is received by the mill in

the form of a reel, wound on a mandrel,

mounted on one of the reel standsanother stand carries an empty mandrel. The

end of the strip is threaded manually through

the stand rolls and round the empty mandrel.


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Reversing cold rolling mills

Requirements of driveThe mill consists of a reversing stand two reversible coilers and in most cases

an additional uncoiler for the pass.

The mill stand is often of the four high type, i. e it has to work rolls with

diameter between 200mm and 500mm.

One of the characteristic of the rolling mill is that the armature inertia of the

mill motor would be larger than that of the rolls. Hence in large capacitymills to keep the ratio of motor inertia to the load inertia low motors with

multiple armatures are chosen.

The coilers and uncoilers have made t maintain constant tension during

various modes of operation of the mill.

They should also be able to provide the necessary acceleration fromthreading to normal operating speed.


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Continuous cold rolling millsThe metal in such mills passes in only one direction

through several successive stands ,

The stands are often of the four high type,

The strips are subjected to a large interstand tensionwhich is kept at constant value depending upon the

desired thickness of the sheet to be produced

The threading of the metal through the stands is

done at a low speed

Depending on the products to be obtained a wide

variation in speed of the mill drive is called for.


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Paper mills

Pulp making and paper making are the two main

processes that take place in a paper mill and thedrives required are quite different for each one of

them.


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Pulp makingPulp is made in two ways purely by mechanical means and by both

mechanical and chemical processing.

The former involves grinding logs of wood of about a meter length on largegrind stones

Pulp can also be made by cutting logs of wood into chips of several

centimeters length and treating them with alkalies along with other raw

materials like grass rags etc.

During the chemical treatment the material is continuously beatenThe end products of the grinders as well as beaters are refined and stored in

large tanks as pulp ready for making papers

Depending on the size of the mill the ratings of motors used for chipping ,

beating, refining and storing range from several hundreds to thousand kilo

watts.


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Paper makingThe machine that makes paper in a paper mill has to

perform the job of forming sheets , drying of sheetspressing of sheets and reeling up of sheets. Therefore

the paper is made in the following five sections ,

Couch section (wire section )

Press section

Dryer section

Calander section

Reel section


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Requirements of paper machine driveFor the actual formation and production of sheets and

from the point of view of the economy it is necessaryto maintain the speed of paper machine constant.

For a paper machine to be multipurpose its speed

should be adjustable over a range as large as 10:1

While cleaning the wire it has to be moved forward afew centimeters at a time so that it can be cleaned and

inspected properly. Hence the motor should be capable

of running at inching speeds of 10-25m/min as long as

particular button is pressed.Smooth and quick starting of the sections without

causing excessive starting current are required.


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Types of drive used for paper machine:

There are two types of drives employed for making

paper from pulp

Line shaft driveSectional drive


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Line shaft drive

In this form of drive the various sections of the paper machineare driven from a line shaft running the full length of the paper

machine

Cone pulleys and belt combinations drive the various sections of

the machine from line shaft through right angled gear

reductions.

Sectional drives

In sectional drive each section of the paper machine has its own

electrical motor.

All the motors are operated from a common supply busBy varying bus bar voltage the speed of the paper machine can

be controlled

By adjusting the field excitation of any motor it is possible to

vary speed of that particular motor with respect to other motors


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Cement makingThe raw materials for producing cement contain lime and silica as main

components and alumina and ferric oxide as fluxing components,

The lime stone mined from the quarries is crushed and transported to the

plant by dumper wagons , trucks or rope ways depending on the area and

distance involved

In fact if the quarry is within 1 2 km from the plant the crushers might be

located right next to the plant and in the line of the supply of the lime stone.


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Textile millsFrom the stage cotton is

picked up in the field to the

stage it leaves the mills as

finished cloth it undergoes

different processes which are

explained below.Cotton to silvers


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