EE027-Electrical Machines 1-Th-Inst.pdf

7/25/2019 EE027-Electrical Machines 1-Th-Inst.pdf

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

ELECTRICAL MACHINES 1General

Theory

No: EE 027

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Electrical Machines 1-General

Theoretical Part

No.: EE 027

Edition: 2008Al l Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerst rasse 64 P.O.Box 36, A 4031 Linz / Aus triaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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ELECTRICAL MACHINES 1-GENERAL

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................4

1 INTRODUCTION..........................................................................................................5

2 SUBDIVISION OF ELECTRICAL MACHINES .............................................................6

2.1

According to their use ..........................................................................................6

2.2

According to the type of voltage...........................................................................6

2.3 According to the mode of operation.....................................................................6

2.4 According to the construction...............................................................................6

2.4.1

Material ............................................................................................................6

2.4.2

Mechanical design ...........................................................................................7

2.5 Systems of classifying electrical machines ..........................................................7

3

TYPES OF CONSTRUCTION OF ELECTRICAL MACHINES.....................................8

3.1 Construction designation .....................................................................................8

4

TYPES OF PROTECTION .........................................................................................10

4.1 Protection indication...........................................................................................10

5

COOLING...................................................................................................................13

5.1 Types of coolant.................................................................................................13

5.2 Methods of circulating the coolant .....................................................................13

5.3

Cooling high-output machines ........................................................................... 13

6 HEATING....................................................................................................................14

6.1

Insulation classes and temperature limits ..........................................................14

6.2 The installation height and temperature of the coolant ......................................15

7

LOSSES IN THE MACHINE.......................................................................................16

7.1 No-Ioad condition...............................................................................................16

7.2 On-load condition...............................................................................................16

8 METHODS OF OPERATION .....................................................................................18

8.1

Continuous operation.........................................................................................18

8.2

Short-time, duty-type operation..........................................................................19

8.3 Intermittent operation.........................................................................................20

8.4

Continuous Operation with intermittent Operation .............................................21

9 BALANCING...............................................................................................................22

9.1 Measuring the magnitude of vibrations ..............................................................22

10 RATING PLATE......................................................................................................25


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11

MAGNETISM .......................................................................................................... 27

11.1 The effect of magnetic poles..............................................................................27

11.2

Elementary magnets ..........................................................................................28

11.3 Remanence........................................................................................................29

11.4 The magnetic field..............................................................................................29

12 ELECTROMAGNETISM.........................................................................................31

12.1 Corkscrew rule ...................................................................................................31

12.2 Coil rule..............................................................................................................32

13

MAGNETIC QUANTITIES ......................................................................................35

13.1

Magnetomotive force (F)....................................................................................35

13.3 Magnetic flux density of magnetic induction (B).................................................38

13.4 Magnetic field strength (H).................................................................................39

13.5

Permeability ().................................................................................................41

13.6

Magnetization characteristics.............................................................................42

13.7 Magnetic reversal characteristics or hysteresis loop .........................................43

14

THE MAGNETIC CIRCUIT .....................................................................................46

14.1 A comparison of a magnetic circuit with an electric circuit .................................47

14.2

A comparison of magnetic quantities with electrical quantities .......................... 48

14.3 The magnetomotive force and electric voltage .................................................. 49

15

ELECTROMAGNETIC INDUCTION .......................................................................50

16 THE CURRENT-CARRYING CONDUCTOR IN THE MAGNETIC FIELD..............53

17 INDUCTANCE ........................................................................................................ 54


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ELECTRICAL MACHINES 1 - GENERAL

LEARNING OBJECTIVES

The trainee should

. . . explain the following terms, applied to electrical machines: construction form; types

of protection; cooling and air circulation types; insulation class; power loss;

Installation type; mechanical vibrations.

. . . explain the magnetism and electromagnetism.

. . . name the symbols for magnetic quantities.

. . . make calculations for a simple magnetic circuit.

. . . describe the method of production of voltage and power in electrical machines.

. . . state what is meant by the inductance of a coil and how inductance depends andimensions and materials.


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ELECTRICAL MACHINES GENERAL

1 INTRODUCTION

An electrical machine, in the narrower sense of the word, is a machine which converts

either mechanical energy into electrical energy, or electrical energy into mechanical

energy, or electrical energy of one kind into electrical energy of another kind by means of

rotary motion. In the wider sense of the word transformers are also electrical machines.


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2 SUBDIVISION OF ELECTRICAL MACHINES

Electrical machines are classified using several different methods. This subdivision is

accomplished by one of the methods indicated in the following subparagraphs.

2.1 According to their use

- Generators.

- Motors.

- Converters.

- Transformers.

2.2 According to the type of voltage

- Direct-current machines.

- Alternating-current machines (single-phase, three-phase).

2.3 According to the mode of operation

- Direct-current machines.

- Commutator machines (DC machines and universal machine).

- Asynchronous machines.

- Synchronous machines.

- Transformers.

2.4 According to the construction

2.4.1 Material

- Copper and aluminium for electrical conductors.

- Iron for the construction of the armatures.

- Insulating material.


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2.4.2 Mechanical design

Active parts

Active parts are those parts carrying the electric current (windings) or the magnetic flux

(armature laminations). The rotating part is called the rotor. The part of the machine which

generates electrical voltage by means of rotation in a magnetic field is called the

armature.

Inactive parts

Inactive parts include such parts as the casing (enclosure), the shaft, the fan and other

construction members. The stationary part of the machine is called the stator or magnetic

frame.

2.5 Systems of classifying electrical machines

There is not yet a single, international standard system of classifying the construction of

electrical machines. Standard systems developed by the IEC and by DIN apply to

machines from a number of European countries and the BSI provides standards for

machines made in the United Kingdom. Other countries and individual manufactures may

use their own standard specifications. In the various sections of this Training Unit an

indication is given of the IEC classifications for machine construction, protection,

insulation heating and the mode of operation. These standards apply to machines

manufactured in Germany and Austria. British standards are now almost the same as

these. For machines manufactured in other countries, one should consult the standard

specifications of the country concerned and the

manufacturer's handbook.


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3 TYPES OF CONSTRUCTION OF ELECTRICAL MACHINES

Depending an their use, there are various mechanical designs for electrical machines

(casings, mountings, arrangement of the bearings, and operating position etc.).

3.1 Construction designation

The types of construction which are important for the design and for ordering purposes

are designated with letters and numbers.

The examples given below are those recommended by the International Electro technical

Commission (IEC) and by the British Standards Institution (BSI).The letters indicate the following:

B ..... horizontal design

V ..... vertical design.

The number which succeeds the letter indicates the following:

- The type of bearing.

- The type of mounting or installation.

- The design of the shaft ends.

- The design of the casing.

The chart overleaf furnishes a few examples of construction design.


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electrical machines types of construction

abbreviation graphic representation type ofbearing

type ofmounting

installation remarks

B3

2

End-shield

journal

bearings

Casing with

feetOn the foundation

Free shaft end for pulley,

gear wheel or clutch half

B5

2

End-shield

journal

bearings

Mounting

flange

Supported

by flange

Casing without feet,

free shaft end

B8

2

End-shield

journal

bearings

Casing with

feetCeiling attachment Free shaft end

V3

2

End-shield

journal

bearings

Mounting

flange on

the upper

bearing

Supported by flange

atthe top

Free shaft end on the top.

Under some circumstances

there is a thrust bearing to

support the rotor weight.

V5

2

End-shield

journal

bearings

Casing with

feet for wall

mounting

Wall attachment

Free shaft end at the bottom.

Under some circumstances

there may be a thrust

bearing to support the rotor.

V10

2

End-shield

journal

bearings

Mounting

flange on

the drive

side

Supported by the

flange

face

Free shaft end at the bottom,

mounting surface on the drive

side

IM = Standard designation for rotary electrical machine.

Example: IMB 3 = Rotary electric machine with 2 end-shield bearings, one free shaft end,

casing with feet and mounted on a foundation.


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4 TYPES OF PROTECTION

Those parts of construction carrying current (electrical machines, switches and circuits)

must be protected against water seepage and foreign matter. Designation is effected by

means of key letters and two key numbers in accordance with DIN.

4.1 Protection indication

Example: IP 45

IP = the code letter indicating a type of protection, in general.

4 = the first code number indicating protection against contact and foreign matter.

5 = the second code number indicating protection against water.

Abbre-

viation

Extent of

protection

against contact

Protection

against the

penetration of solids

Protection

against water seepage

IP 00 No protection No protection No protection

IP 11 Protection against

accidental contact with

large surfaces of body

(hand)

Solids with a

diameter over 50

mm

Protection against

dripping water


contact with finger

Solids with a

diameter over 12

mm

Protection against drops

of water falling up to 15

from the vertical


contact with tools etc.more than

1 mm thick

With diameter over

1 mm

Protection against water

splashing from anydirection


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In this table, a few examples of the types of protection are shown. The first key number

begins at zero and ends at six. The higher the number, the better the protection against

contact and foreign matter. The second key number begins at zero and ends at eight.

The higher the number, the better the protection against water seepage.

Types of motor protection:

Drip-proof

Sprinkle-proof

Spray-proof


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Three types of protection against water seepage are portrayed on the previous page. In

places where explosions or firedamp (methane) may occur, only machinery which is

sufficiently safeguarded may be used

Ex I = machinery protected against firedamp (methane).

Ex II = machinery protected against explosion.


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5 COOLING

In order to extract the heat produced in the windings and the iron core, electrical machines

must be cooled.

5.1 Types of coolant

The following types of coolant may be used:

- Air: fan cooling for motors.

- Liquids (water and oil): oil cooling for transformers.

- Gases (hydrogen and nitrogen): super-conductive cables.

5.2 Methods of circulating the coolant

Depending on the output of a machine the following methods are used:

- Free convection: for a low power output cooling is achieved by natural convection

and radiation without special aids.

- Self-circulation: the cooling air is moved by a fan mounted on the machine shaft.

- Separate cooling: cooling is effected by a ventilator with a separate drive. This

methods is used mainly in machines with a large speed range.

5.3 Cooling high-output machines

Machines with high-output usually have a separate cooling circuit.

- Direct conduction: the coolant is led directly to the winding (internal cooling).

- Indirect conduction: the coolant is led along the casing or the jacket

(surface cooling).


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6 HEATING

The unavoidable losses in the electrical machine cause all pats of the machine to heat.

The permissible temperature limit shown in the table for the insulation may not be

exceeded.

6.1 Insulation classes and temperature limits

The various insulating materials have a certain heat resistance. The permissible

temperature of electrical machines is limited accordingly. If the maximum permissible

temperatures are exceeded, the windings will be destroyed.Subdivision of insulating materials into classes and respective temperature limits.

Class Assigned temperature limit (C)

Y 90

A 105

E 120

B 130

F 155

H 180

C over 180

The assigned temperature limit is the highest temperature which is permissible at the

hottest spot. The table given is valid for an installation height H 1,000 m and for a

coolant temperature of 40C. The maximum temperature rise is given by the assigned

temperature limit minus the coolant temperature.

Example:

The maximum temperature rise for the insulation class F is 155 - 40 = 115C.


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6.2 The installation height and temperature of the coolant

If the coolant temperature is 30C and the installation height is

1,000 m, the maximum temperature rise will be equal to the maximum temperature rise

for a coolant temperature of 40C, plus 10C.In the insulation class F, the maximum temperature rise is then 115C + 10C = 125C.

If the insulation height is between 1,000 and 4,000 m, and the temperature of the coolant

does not exceed the values, given in the table below, no correction of the maximum

temperature rise is necessary, compared with that for a coolant temperature of 40C. The

table applies to the insulation class B. There are differences for the other insulation

classes.

Installation height (in m) Coolant temperature (in C)

0 - 1,000 40

1,000 - 2,000 32

2,000 - 3,000 24

3,000 - 4,000 16

If the installation height is between 1,000 and 4,000 and the coolant temperature is 40C,

a reduction in output is necessary. For every 100 m over the installation height of 1,000 m,

the maximum temperature rise must be reduced by 1 % relative to a coolant temperature

of 40C. This 1 % applies to the insulation class F. There are differences for the otherinsulation classes.


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7 LOSSES IN THE MACHINE

7.1 No-Load condition

Pin is equal to the idling losses, which consist of the iron losses

Pfe and the frictional losses Pfrand copper losses, the iron plus frictional losses is

called "stray losses ( PFe)".

7.2 On-load condition

Under load, there are in addition, the higher current heat losses PCu(copper losses) and

the additional losses Pa.

The total losses =

If the total losses and cooling are constant, the following formula for temperature applies

to the heating process:

T= thermal time constant Th, TC

eq= equilibrium temperature

heating cooling

Pin= PFe+ Pfr+PCu

TC

Pt= PFe+ Pfr+ PCu+ Pa


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Power flow diagram or power division diagram

Example:

- Friction losses occur in the bearings of electrical machines.

- Iron losses are known as eddy-current and hysteresis losses.

- Copper losses or winding losses are dependant upon the strength of the current.

- Additional losses are windage losses.


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8 METHODS OF OPERATION

The method of operation of an electric machine means the kind of load and time of its

operation.

Example:

Continuously running, intermittent duty, full load and no-load etc.

8.1 Continuous operation

The rated load PIast until the permanent operating temperature (equilibrium temperature)

is reached. The total losses Pt are constant. The temperature rises up to the final

temperature in accordance with a function


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The final temperature must lie below the maximum temperature for which the machine is

designed.

Pout = output power

Pt = total losses

= temperature

m = max. Temperature

t = time

8.2 Short-time, duty-type operation

The constant load P does not last long enough for the equilibrium temperature to be

reached. During the interruption which follows, the machine cools to the initial

temperature.

to= operation time


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The time resulting from the Operation time toand the de-energised time tstis termed the

period or cycle time tc.

8.3 Intermittent operation

Intermittent periodic duty-type operation

This type of operation is composed of a sequence of identical cycles. The cycle time tc

consists of the load time towith constant load and de-energised time tst . The starting

current has only a slight influence on the heating process.

to = operation time

tst = de-energised time or stopped time

tc = cycle time


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8.4 Continuous Operation with intermittent Operation

This type of operation is composed of as sequence of identical cycles.

The cycle time consists of the load time towith constant load and no-Ioad time ti . As

losses occur during the no-load time, the machine cannot cool off to the same extent as in

the intermittent periodic duty-type operation.

The ratio is called the cyclic duration factor.


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9 BALANCING

Due to the uneven mass distribution, vibrations occur in the rotor during running and the

rotor is unbalanced. For example an uneven distribution of the winding will cause

unbalance. Rotors must be balanced before they are installed.

A vibration frequency of thus results.

At 1,200 rpm, the vibration frequency is = 20 Hz.

9.1 Measuring the magnitude of vibrations

The magnitude of the vibration can be measured by many instruments. A common way is

by measuring the deflection. For example, the magnitude of the deflection is measured at

both bearings of a running machine in all three directions x, y, and z.

In the VDI standards (Verein Deutscher Ingenieure = Association of German Engineers)

there are standard classifications for mechanical vibrations in machines.

They contain six groups, (K, M, G, T, D, s) which identify the type of machine or

equipment.


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Within each of these groups are the classifications: good, satisfactory, borderline,

unsatisfactory.

In practice, the deflection and the frequency of the mechanical vibration are measured.

These measured values are then applied to the assessment curves. If the assessment is

unsatisfactory, the cause of the vibration (e.g. unbalance, loose foundation screws) must

be removed. If there is unbalance, balancing must be performed again. If a pulley is

unbalanced, if no longer runs smoothly, due to the centrifugal force which occurs.

If the pulley is left free of external forces, it will remain stationary in one position only,

where the unbalance mass mis under the wheel. In static balancing, the same mass

ma (balancing mass) must be attached an the opposite side. In so doing, the pulley

remains stationary in any position. When running, the centrifugal forces cancel each other

out, the machine runs smoothly.


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The roller-shaped rotor depicted above is statically balanced.

The resultant centrifugal forces F1and F2cause vibration while it is running. In dynamic

balancing, compensating masses are attached opposite the masses m1and m2

There are also balancing machines for balancing.


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10 RATING PLATE

The most important data are:

1 The name of the manufacturer and the company sign

2 The designation of the model and the size

3 The type of current

4 The type of machine

5 The production number and the year of production

6 The type of winding connection

7 The rated voltage

8 The rated current

9, 10 The rated output and the unit of output

11 The method of operation

12 The power factor13 The direction of rotation

14, 15 The rated speed and the rated frequency

16 The word "excitation" for DC machines, synchronous machines

and "rotor" for asynchronous machines

17 The connection system


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18 The nominal exciting voltage for separately-excited DC machines

or nominal exciting voltage for synchronous machines

19 The rotor current under nominal operation for induction motors

or exciting current for synchronous machines under nominal operation

20, 21 The class of insulation material and the type of protection

22 Weight

23 The engineering standards used in the manufacture of the

machine (these may not always be present an the rating plate).


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11 MAGNETISM

Ferromagnetic materials (magnetic materials) are attached by magnets. Such materials

include iron, cobalt, nickel and some alloys. As iron is the most important material the

term "ferromagnetic" is derived from the Latin equivalent iron (ferrous). Permanent

magnets are made from ferromagnetic materials by means of a special process. The two

sides of a magnet are named the "North" and the "South Pole". The magnetic effect is

greatest at these sides. If a magnet is pivoted, it settles in a position where its North Pole

points to the geographical North Pole and its South Pole points to the geographical South

Pole.

11.1 The effect of magnetic poles

If two bar magnets are brought together with the North Pole facing the South Pole, they

attract each other. If two bar magnets are brought together with the two North Poles facing

each other, they repel each other. Poles of like name repel each other - poles of unlike

name attract each other. This explains why the North Pole of a pivoted compass needle

points to the geographical North Pole.


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11.2 Elementary magnets

The division of a bar magnet into several small magnets.

If a bar magnet is cut in half, two new bar magnets with a North and a South Pole each

are produced. If these magnets are divided further, several smaller bar magnets with a

North and a South Pole each are produced. The original bar magnet is composed of

several smaller magnets. This division can be continued, in theory, until one reaches the

smallest magnets, called elementary magnets.


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If for example, a piece of iron is not magnetized, the elementary magnets are not

orientated. The iron is not magnetic. The elementary magnets can be aligned by bringing

a permanent magnet close to them. The North Poles of the elementary magnets then

point in one direction, the South Poles in the other. The iron is then magnetized.

The more elementary magnets which are aligned, the greater the magnetic strength of the

magnet.

11.3 Remanence

If materials are magnetized and the magnetizing cause is removed, some of the

elementary magnets return to the unorientated state. If most of the elementary magnets

remain aligned, these materials are called "hard magnetic".If only a small part of the elementary magnets remain aligned, these materials are called

"soft magnetic".

The remaining magnetism in the work piece is called residual magnetism or remanence.

By heating the material to a higher temperature or by severe mechanical shock, the

remaining magnetism can be removed.

11.4 The magnetic field


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Imaginary lines, indicating the magnetic effect, emitted from the North Pole of the bar

magnet are called "magnetic flux lines". They form continuous lines starting from the

magnet's North Pole and ending at its South Pole. They also run through the inside of the

magnet. Magnetic flux lines can be made visible with iron filings. The filings align

themselves along the flux lines. The space, through which the magnetic lines pass, is

called "magnetic field". The number of the flux lines per unit of area is called the "magnetic

flux density".


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12 ELECTROMAGNETISM

If a current flows through a conductor, a magnetic field is formed around the conductor. Its

direction depends on the direction of the current.

12.1 Corkscrew rule

The electric current was defined as flowing from plus to minus. The electrons move in the

opposite direction. The direction of the current can be depicted by an arrow. If one looks

at the arrow from behind, one sees the end of the arrow which is symbolized by a cross.

From the front, one sees the tip of the arrow which is symbolized by a dot. If a corkscrew

is turned in the direction of the current, the direction of rotation indicates the direction of

the flux lines. This rule is called the "corkscrew rule" or "screw rule".


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This rule determines the flux lines in our diagram. They form concentric circles around the

conductor carrying direct current. The magnetic field becomes weaker further from the

wire. If the direction of the current is reversed, the direction of the flux lines also reverses.

Magnetic fields also form around current flowing in gases or liquids.

12.2 Coil rule

If a conductor is bent to form a loop, the magnetic field is greater in the middle.

A stronger magnetic field can be produced by a stronger current or by the same current

flowing in a coil made of several loops or turns. The resultant field of the coil is composed

of the sum of the magnetic fields of the individual turns.


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The right hand is placed an the coil in such a manner, that the fingers point in the direction

of the current. The thumb, when extended, points in the direction of the flux lines inside

the coil (that is, it points to the end of the coil which is the North Pole).

The North Pole is where the flux lines leave the coil, the South Pole is where they enter it

(as it is with a permanent magnet).

Flux lines of a coil

The diagram above shows a cross section of a coil.


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If a ferromagnetic core is inserted into the coil, the magnetic field can be greatly

increased. The cause is the alignment of the elementary magnets in the core. The

magnetic flux can be increased many times over.


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13 MAGNETIC QUANTITIES

In order to be able to understand the connections of magnetism with electricity, it is

necessary to know some magnetic quantities.

13.1 Magnetomotive force (F)

This quantity is the product of current and the number of turns. The number of turns Nhas

no unit, so the magneto motive force is measured in amperes (A or Ampere-turns At).

A coil with 2,000 turns, carrying a current of 1.5 A has a magneto motive force of1.5 x 2,000 = 3,000 At.

The magneto motive force is an important quantity for the creation of a magnetic field.

As one can see from the formula, the same magneto motive force can be produced by a

low current and a large number of turns or by a high current and a small number of turns.

F= I . N


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13.2 Magnetic flux

The sum of the flux lines is called the magnetic flux.

= Greek letter phi

Unit of magnetic flux:

Wb = weber 1 Wb = 1 volt-second (Vs).

The magneto motive force (m.m.f.) produces a magnetic field, which consists of magnetic

flux lines. The number of flux lines is proportional to the electric current. In the figure,

several flux lines are drawn corresponding to the current in the three coil turns. Let the

cross-sectional area of the coil be A. The sum of all of the flux lines which pass through

this area is called "magnetic flux ", measured in Webers.

In order to be able to determine the unit of the magnetic flux, one must use one of the

effects of the magnetic field.


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The stationary loop of the conductor encloses all the lines of flux.

The loop of the conductor is

in motion; the lines of flux are

cut. Voltage is generated

(motional induction).


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The loop of the conductor no

longer cuts the lines of force.

If the motion of the loop of the conductor lasts for 1 second, and if the voltmeter indicates

1 V, this corresponds to a magnetic flux of 1 Vs = 1 Wb.

13.3 Magnetic flux density of magnetic induction (B)

Unit of measurement: T = Tesla.

If, in our example, we divide the magnetic flux inside the coil by the cross section of the

coil, we obtain a certain density of the magnetic flux. This is called flux density. The

magnetic flux passing perpendicularly through an area Aof 1 m is called the magnetic

flux density B. The unit of measurement of the magnetic flux density is Tesla (T).


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With a magnetic flux = 0.009 Wb and a cross-sectional area of the coilA= 4 cm,

a magnetic flux density B= = = 2.25 T

With a surface of the same size:

13.4 Magnetic field strength (H)

Unit of measurement:

Ampere per meter = A/m.

In our example of a coil, the Length of a line of flux is designated with lin meters.


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The magneto motive force produces the magnetic field along the entire flux line Length l.

The magnetic field strength His calculated from the magneto motive force divided by the

Length of the flux lines.

The shorter the Length of the magnetic flux lines e.g. the shorter the coil the more the

magnetic field strength increases. The longer the magnetic flux lines, the smaller the

magnetic field strength becomes. The unit of measurement of the magnetic field strength

is . lmis the mean Length of the flux lines of the magnetic circuit.

In the iron core 1, a magneto motive force of 100 At, is used to produce magnetic flux in

each centimetre of magnetic circuit, whereas a magneto motive force of only 50 At is

available for each centimetre of the iron core 2.


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13.5 Permeability ()

The term permeability means the magnetic conductivity of materials.

For coils with an iron core, the connection between the magnetic field strength and the

magnetic flux density is B =xH

or B = oxr x H.

The permeability is equal to the product of the absolute permeability o and the

relative permeabilityr. The permeability has the same unit of measurement

aso ( which simplifies to Henry per meter H/m) and the relative permeability has no

units. If an iron core is placed in a coil which carries current, the magnetic flux is greatly

increased since iron has a high relative permeability.

Absolute permeability ()

This is the connection between the magnetic field strength and the magnetic flux density

in air or a vacuum and is given by the expression B =ox H.

The size ofo = = 1.257-6

x10 H/m.

The magnetic flux density increases proportionally to the magnetic field strength. Inside an

air-cored coil, a magnetic field strength of 1.000 produces a magnetic flux density of

B =oxH = 1,257x10-6

x1,000 T = 0.001257 T.

Relative permeability (r)

The relative permeability indicates by how much the magnetic flux density is increased

compared with the air-cored coil (coil without iron core). The relative permeability is not a

constant value. lt depends on the magnetic field strength and on the material. For air and

non-magnetic materials r = 1. The relationship between the magnetic

flux density Band the magnetic field strength Hfor magnetic materials is indicated in the

magnetization curves.


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13.6 Magnetization characteristics

Magnetic flux density in T

The characteristic curves for dynamo sheet and cast steel, alloyed sheet, and grey cast

iron are shown above. The magnetization curves are ascertained by tests. The

magnetization characteristic of air is a straight line through zero.


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H = magnetic field strength in and is the cause of the formation of the flux lines.

13.7 Magnetic reversal characteristics or hysteresis loop


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If a coil with an iron core is connected to an alternating voltage, the elementary magnets

often have to change their direction. The hysteresis loop shows the relation between B

and H during magnetic reversal. With a hard magnetic material, start from zero, the

demagnetized condition. As the magnetic field strength increases the magnetic flux

increases, along the rise path, from point 0 to point 1. This curve shown the B - H

relationship when a magnetic field is applied for the first time to a completely

demagnetized material. The field strength is now decreased (upper curve). At H= 0, Bstill

has a certain positive value, the residual magnetism. lt shows that some elementary

magnets are still aligned. His now made negative.

The value of Hat which Bbecomes zero is called the coercive force.

lt is the amount of magnetic field strength required to wipe out all the residual magnetism.

When the value of H is decreased still further, one reaches point 2. The negative field

strength is now reduced to zero, then made positive again (lower curve) and thus one

reaches point 1 from point 2.

During this transition starting from zero, the field strength must be increased again. The

magnetic flux density remains lower than the magnetic flux density in the first reversal,

due to the hysteresis or magnetic friction in the iron core.

The losses occurring during the magnetic reversal are called magnetic reversal or

hysteresis losses. The area enclosed by the hysteresis curve is a measure of these

losses.

If, in the example, point 1 was the saturation value of the material (all elementary magnets

were aligned), then the residual magnetism is called the remanence of the material.

Similarly the coercive force is then called the "coercivity" of the material.

Hard magnetic materials are used for permanent magnets. After a single magnetization

process, they have a large magnetic flux density, remanence and coercivity.

If magnetic reversal is performed continuously, hard magnetic materials would have very

high losses and thus heat up considerably.


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Soft magnetic materials are used for the laminations or sheets of magnetic materials used

to make the rotors and stators of electric machines. Due to the continuous magnetic

reversals in these magnetic materials the hysteresis curves have low coercive force. The

area enclosed by the hysteresis loop should be small, so that the losses are low and the

heating up of the material is reduced.


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14 THE MAGNETIC CIRCUIT

The magnetic circuit is the path of the magnetic lines of flux. The magnetic flux is

generated by the magneto motive force I xN.

The flux takes a path through the iron, since flux passes more easily through iron than

through air (the reluctance of the iron is lower than that of air). An air gap is built into the

iron core. A certain amount of magneto motive force is necessary to drive the magnetic

flux through the iron and the air gap. The magneto motive force Fis equal to the sum of

the magnetic field strength in the iron HFe multiplied by the mean iron length plus the

magnetic field strength in the air Hamultiplied by the length of the air gap.

F = HFe x lFe +Ha x la

This Law is similar to Kirchhoff's Second Law for an electric circuit. lt is generally written:

F = H1 x l1+H2x l2+H3 x l3 + . . .


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The products H1x l1 , H2x l2and H3x l3are called "partial magneto motive forces". The

total magneto motive force is equal to the sum of the partial magneto motive forces.

14.1 A comparison of a magnetic circuit with an electric circuit

From the above comparison we can see that in magnetism, the magneto motive force is

analogous to the e.m.f. in electricity. This is the reason why it is termed the "magneto

motive force" or m.m.f.


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14.2 A comparison of magnetic quantities with electrical quantities

Magnetism Electricity

Quantity Unit Quantity Unit

Magnetomotive force =

magnetic voltage

Electric voltage

F0= F1+ F2+ F3+ . . .

F = H xl

F = x

A U = U1+U2+U3+. . .

U = E xl

U = I xR

V

Magnetic field strength Electric field strength

A

Magnetic flux Electric currentWb

or

Vs

A

Magnetic induction Current density

Permeability Electric conductivity

=0xr

Magnetic resistance Electric resistance

* H = Henry=1

The equivalent of resistance in a magnetic circuit is called "reluctance" R.


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14.3 The magneto motive force and electric voltage

Magnetomotive force = current x turns

F= magneto motive force in At

I = electric current in A

N= Number of turns

Electric voltage = rate of change of magnetic flux

N = number of turns

= magnetic flux (Wb)

f = frequency (Hz)

4.44 = constant

* Magnetic flux continuously changes its magnitude and direction (alternating).

F = IxN

V = 4.44 x x Nx f


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15 ELECTROMAGNETIC INDUCTION

This refers to the generation of an electric current with the aid of a magnetic field.

The magnitude of this voltage depends an the following:

- the length of the conductor in the magnetic field.

- the strength of the magnetic field.

- the speed with which the conductor moves in the field or with

which the field moves past the conductor.

Movement of a conductor in the magnetic field.

movement of a conductor in the magnetic field

As demonstrated in the drawings 1 to 3 above, the magnetic flux lines pass from the North

Pole to the South Pole through the air gap. If the conductor is moved from point 1 to point

3, a voltage is induced in the conductor. An electron surplus (-) forms at the front end of

the rod, and an electron deficit (+) at the rear end. In position 1, the conductor loop

encloses or links all of the magnetic flux.


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In position 2, the conductor loop links half of the magnetic flux. In position 3, it does not

link any of the magnetic flux. The magnetic flux linkage has thus decreased. lt has

changed. A change of the magnetic flux linking the conductor loop induces voltage in the

loop.

The voltage induced depends on the rate of change of the flux and on the number of turns

N. The current Icaused by the induced voltage flows in such a manner that its magnetic

field opposes the change in flux. This rule is called Lenz's Law. Note that U has a

negative sign. If the velocity of the conductor is constant in a uniform flux density the

formula can be written as follows:

B = flux density in T

l = length of conductor in m

v = conductor velocity in m/s

N = number of turns

U = induced voltage in V

With a generator, the right-hand rule is used to determine the direction of the current.

U= Nx B x lx v


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If the right hand is held so that the lines of flux strike the palm and the thumb points in the

direction of movement of the conductor, the extended fingers indicate the direction of the

current in the conductor. The flux change in the formula

can be achieved in two ways.

In motional induction (principle of the generator), the conductor moves inside the magnetic

field. lt does not matter whether the conductor moves and the magnetic field is stationary

or vice versa.

In stationary induction (principle of the transformer), the magnitude of the magnetic field

changes. This is achieved by altering the current in the coil or by altering the reluctance of

the magnetic circuit.


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16 THE CURRENT-CARRYING CONDUCTOR IN THE MAGNETIC FIELD

By means of the left-hand rule, the direction of motion of the current-carrying conductor or

the direction of the force acting on it can be determined. If the left hand is held so that the

lines of flux strike the palm and the extended fingers point in the direction of the current in

the conductor, the thumb shows the direction of force on the conductor.

The deflecting force can be calculated from the following formula:

B = flux density in T

l = length of conductor in m

I = current in A

z = number of conductors B. . . , I . . .A, l . . .m,

F = force in N F. . .N

The force is proportional to the magnetic flux density, the current flowing through the

conductor, the length of the conductor in the magnetic field and to the number of

conductors.

A current-carrying conductor in a magnetic field produces motion. This principle is used in

every motor.

F= B x lx Ixz


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17 INDUCTANCE

This is defined as follows:

= flux in Wb

I = current in A

L = inductance in H

In a coil without a core the magnetic flux increases in proportion to the current. The

inductance L is constant, independent of the current flowing. In a coil with an iron core,

the magnetic flux increases Iinearly at first, then more slowly as current increases.


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lnductance is dependent on the current. At the beginning, in the straight part of the curve,

the inductance is and higher up in the curve, the inductance becomes

smaller.

In saturation, the inductance decreases. For a coil with N turns, the inductance is

calculated as L= N x .

Cylindrical coil with no core

For a cylindrical coil with no core the inductance Lis calculated as:

L= 1.257 x10-6x N x ; A =

lin m, Lin H.

With a cross-sectional area of 500 mm2, a length, l= 0.2 m and a number of turns,

N = 300, the inductance is L= 0.00028 H.

Coil with a closed iron core

For a coil with an iron core, the inductance is L=rx0xNx .

The relative permeability may have very high values up to 100,000 depending on the

material.

. . . m


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EE027Electrical Machines 1 - General

Theoretical Test


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E L E C T R I C A L M A C H I N E S 1 - G E N E R A L

T E S T 1

1. State the definition of an electrical machine.

2. State the definition of active parts in an electrical machine.

3. A machine is of the construction classification IM B 8. State the meaning of the letter B

in this classification.

4. On the rating plate of a machine are the characters IP 42.

- State the meaning of the letters IP.

- State the meaning of each of the figures 4 and 2.

5. - Name three methods of cooling rotating electric machines.

- For each method of cooling, name a typical type of electric machine for which the

method would be suitable.

6. On the rating plate for some machines, the indication "insulation class E" is found.

State the information that this indication gives about the way in which this machine

may be run.

7. - Name two of the constant power losses (losses independent of the load) which

occur in an electric machine.

- Name the two power losses which vary with the load applied to an

electric machine.

8. State what is meant by the equilibrium temperature of an electrical machine.

9. Draw the power flow diagram of an electrical machine.

10. State the difference between "short-time duty-type operation" and "continuous-running

duty-type operation" of a machine.


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T E S T 2

1. State the reason why a machine may be unbalanced dynamically although lt is in

static balance.

2. State two reasons for assuming rating plates on electric machines.

3. State the reason for assuming that a magnetic South Pole is situated near the

geographic North Pole.

4. State the change which takes place inside a piece of ferromagnetic material when it is

magnetized.

5. State the essential difference between "hard" and "soft" magnetic materials.

6.

The diagram represents the flux lines of a magnetic field.

- State two methods of producing a magnetic field of this shape.

- Name the unit of magnetic flux.


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7. Current flows in the loop of wire, shown in the diagram, in the direction indicated. A

magnet is suspended, freely pivoted, at the centre of the loop. State which pole of the

magnet will be nearest to the viewer when it rotates.

8. State two magnetic effects of placing a piece of ferromagnetic material inside a coil

which carries an electric current.

9. A coil of 250 turns carries a current of 5 Amperes.

Calculate the magneto motive force produced by the coil, giving the units of

measurement.

10. A loop of wire completely enclosed a magnetic field. When the loop is moved out of

the field, in a time equal to 1/10 second, the mean voltage induced in the loop is 2

volts.

Calculate the magnitude of the magnetic flux which was enclosed by the loop, stating

the units of measurement.


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T E S T 3

1. A magnetic field has a flux density of 0.5 T. A loop of wire, enclosing an area of

0.01 m perpendicular to the flux, is moved out of the field in a time of 0.2 seconds.

Calculate: The magnetic flux enclosed by the loop at the start.

The voltage induced on the loop, due to its motion.

2. A coil of 250 turns is used to magnetize an iron core which has a mean length of

0.5 m. Calculate the current which will be required in the coil in order to produce a

magnetic field of strength 1000 in the core.

3. The magnetic field strength in a magnetic core is 1000 .

If the relative permeability of the core is 1200, calculate the magnetic flux density

in the core (o= 1.257 x 10-6).

4. - State the effect that a high coercivity has on the shape of the hysteresis curve for a

magnetic material.

- State why it is important that a material should have a small area enclosed by its

hysteresis curve if it is to be used under conditions of frequent magnetic reversal.

5. State what is meant by "reluctance" of a magnetic circuit.

6. A magnetic circuit is shown in the diagram. State which of the following quantities

have the same value for each part of the circuit: flux density; magnetic flux;

field strength; reluctance; permeability.


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7. A conductor of length 20 cm is moved through a magnetic field of flux density 1.5 T.

Calculate the velocity which the conductor must have if a voltage of 2 V is to be

induced in the conductor.

8. Is the inductance of a coil with iron core dependent on the current flowing?

9.

The diagram shows a conductor, carrying a current in the direction shown, in a

magnetic field. By means of a sketch, show the direction of the force on the conductor.


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10. The rotor of an electric motor has 100 conductors, each 30 cm in Iength and carrying a

current of 2 A. They are situated in a magnetic field of flux density 1.2 T, perpendicular

to the conductors.

- Calculate the total force acting on the conductors.

- If the conductors are all at a distance of 20 cm from the centre of the motor shaft,

calculate the torque acting on the rotor.


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T E S T 1

( S o l u t i o n )

1. An electrical machine, is a machine which converts either mechanical energy into

electrical energy, or electrical energy into mechanical energy, or electrical energy of

one kind into electrical energy of another kind by means of rotary motion.

2. Active parts are those parts carrying the electric current (windings) or the magnetic

flux.

3. B indicates horizontal mounting and shaft.

4. - IP indicates that the following figures relate to the protection of the machine.

- First figure (4) indicates the degree of protection against solids

(solids over 1 mm diameter).

Second figure (2) indicates the degree of protection against damage by water

(protection against drops of water falling up to 15 from the vertical).

5. - Free convection; self-circulation; separate circulation.

- Very small machines; medium power machines; machines with variable speed

(high power).

6. Maximum temperature at which the machine may be run without damage is 120C.

7. Iron (hysteresis) losses and frictional (and windage) losses.

Copper (I x R) losses and additional losses.

8. The steady temperature which the machine will attain after running at constant load for

a long period.


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9.

10. Short-time duty-type operation permits the machine temperature to fall to coolant

temperature in the rest period.


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T E S T 2

( S o l u t i o n )

1. The balancing masses have equal moments about the shaft, but are not opposite

each other an the shaft.

2. - So that the machine may be installed with an appropriate supply.

- So that, in the event of a machine failure, an exact replacement may be obtained.

3. Poles of like name repel each other - poles of unlike name attract each other.

The North Pole of a pivoted compass needle points to the geographical North Pole so

there must be a magnetic South Pole there.

4. The elementary magnets become aligned.

5. Hard magnetic materials retain magnetism after the magnetizing force is removed.

Soft magnetic materials readily demagnetize.

6. - Permanent magnet (short bar) or short coil or loop of wire and current.

- Weber (Wb).

7. The South Pole.

8. The material becomes magnetized and the magnetic flux density increases greatly.

9. Magnetomotive force = 250 x 5 = 1250 Ampere-turns.

10. Magnetic flux = 2 x 1/10 = 0.2 Weber (or volt-seconds).


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T E S T 3

( S o l u t i o n )

1. - Flux = B xA= 0.5 x 0.01 = 0.005 Wb.

- Voltage = U= 0.005/0.2 = 0.025 V.

2. H = (I xN)/l

I = (H xl)/N= (0.5 x 1000)/250 = 2A.

3. B =rx0x H= 1200 x 1.257 x 10-6x 1000 = 1.508 T.

4. - The hysteresis curve encloses a large area.

- The area enclosed by the hysteresis curve gives the energy lost during each

magnetic reversal. A smaller area indicates a material with a lower iron loss.

5. The reluctance is the opposition (resistance) provided by the material to the production

of magnetic flux by an applied magneto motive force.

6. The magnetic flux is the only quantity which is constant in value throughout the circuit.

7.

8. Yes.


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9.

10. - F= B xl xI xz= 1.2 x0.3 x2 x100 = 72 N

- Torque= F xr= 72 x0.2 x14.4 Nm.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

EE027-Electrical Machines 1-Th-Inst.pdf

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