MAJOR CONSIDERATIONS IN ELECTRICAL MACHINE DESIGN DESIGN OF MACHINES · 2019. 12. 8. · EE 6604...

EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE

MAJOR CONSIDERATIONS IN ELECTRICAL MACHINE DESIGN

DESIGN OF MACHINES

LIMITATIONS IN DESIGN

ELECTRICAL ENGINEERING MATERIALS

ELECTRICAL CONDUCTING MATERIALS

1. High Conductivity Materials

a) Copper

✓ Most electrical machines employ windings of annealed high conductivity copper

b) Aluminium

Table:1 Comparison of copper and aluminium wires

c) Alloys of copper

2. High Resistivity Materials

ELECTRICAL CARBON MATERIALS

MAGNETIC MATERIALS

TYPES OF MAGNETIC MATERIALS

SOFT MAGNETIC MATERIALS

i) Solid core materials

ii) Electrical sheet and strip

iii) Special purpose alloys

INSULATING MATERIALS

ELECTRICAL PROPERTIES OF INSULATING MATERIALS

TEMPERATURE RISE AND INSULATING MATERIALS

CLASSIFICATION OF INSULATING MATERIALS

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APPLICATIONS OF INSULATING MATERIALS

i) Insulating materials for wires

ii) Insulating materials for laminations

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iii) Insulating materials for machines

iv) Insulating materials for transformers

INSULATING ELECTRICAL MATERIALS USED IN MODERN ELECTRIC MACHINES

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SPACE FACTOR

Space factor is defined as the ratio of copper area to total winding area. It is denoted by Sf.

Copper area Total winding area

Space factor S =

It is also defined as the ratio of bare conductor area to total slot area.

fSpace factorTotal slot areaS = Bare conductor area

TOTAL LOADINGS

SPECIFIC MAGNETIC LOADING

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SPECIFIC ELECTRICAL LOADING

The typical values of specific magnetic loading and electric loading for various types of rotating machines are listed in table.

CHOICE OF SPECIFIC MAGNETIC AND ELECTRICAL LOADINGS

a) Choice of specific magnetic loading

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Note: In general small machines have lower specific magnetic loadings.

b) Choice of specific electrical loading

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-----------------------(1)

--------------------------- (2)

-------------------------- (3)

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PROBLEM: 01

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THERMAL CONSIDERATIONS

1. MODES OF HEAT DISSIPATION

The losses appear as heat and therefore the temperature of every affected part of the machine rises above the ambient medium which is normally the surrounding air. The heated parts of an electrical machine dissipate heat into their surroundings by conduction, and convection assisted by radiation.

a) Conduction

This mode of dissipation of heat is important in the case of solid parts of machine like copper, iron and insulation.

Note: The material having a high value of thermal Resistivity will dissipate less amount of heat or alternatively for the dissipation of same heat the temperature rise will be higher.

b) Radiation

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Note: In electrical rotating machines and transformers radiation does not normally occur by itself and in almost every case it is accompanied by convection.

c) Convection

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2. NEWTON’S LAW OF COOLING

------------------------ (1)

Equation (1) represents the Newton’s law of cooling.

3. AMBIENT TEMPERATURE

PROBLEM: 01

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PROBLEM: 02

PROBLEM: 03

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PROBLEM: 04

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HEAT FLOW

CALCULATION OF INTERNAL TEMPERATURE

Fig. Temperature gradient

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----------------------------- 1

TEMPERTURE GRADIENTS IN CORE

Fig. Built up laminated core

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PROBLEM: 01

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PROBLEM: 02

Core loss per unit volume =core loss×stacking factor ×density of steel

HEAT FLOW IN TWO DIMENSIONS

Practically heat does not travel along parallel paths and the dissipating surfaces are not homogenous. In actual practice the heat flow is in different directions and the winding and cores have insulation in addition to copper and iron respectively.

✓ The thermal Resistivity of built up windings and cores depends upon relative thickness of insulation to copper or iron.

Fig. Heat flow in two dimensions

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THERMAL RESISTIVITY OF WINDING

--------------------------- 1

Equation 1 can be written as

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PROBLEM: 01

Volume of coil = Cross section of coil ×length mean turn

total lossvolume of coil

TEMPERATURE GRADIENTS IN CONDUCTORS PLACED IN SLOTS

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PROBLEM: 01

PROBLEM: 02

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THERMAL STATE IN ELECTRICAL MACHINES

HEATING

-------------------------1

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COOLING

------------------------------- 2

It is clear from equation 2, that the cooling curve is also exponential in nature as shown in fig.

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PROBLEM: 01

PROBLEM: 02

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PROBLEM: 03

PROBLEM: 04

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METHODS OF COOLING

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RATING OF MACHINES

TYPES OF DUTIES AND RATINGS

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SELECTION OF MOTOR POWER RATING

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STANDARD SPECIFICATIONS

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ADDITIONAL PROBLEMS IN SPECIFIC MAGNITIC AND ELECTRIC LOADINGS

Example: 01

For a certain d.c. generator the core loss is 1000 W and the armature resistance is0.025 ohm. The core and windings form a cylinder 0.25 m long and 0.25 m in diameter. Specific loss dissipation is 230 W/m2-0C. Calculate the specific electric loading which would result in windings and core having a temperature rise of 400C. The machine is wave wound with 270 armature conductors.

Assume that the heat is dissipated from the cylindrical surface only.

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Example:02

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TWO MARKS QUESTION AND ANSWERS

01 Name the basic structural parts of an electromagnetic rotating machine.

The basic structural parts of an electromagnetic rotating machine area) Magnetic circuit b) Electric circuit c) Dielectric circuit d) Thermal circuit e) Mechanical parts

02 What are the factors those limit the design of a machine?

a) Saturation b) Temperature rise c) Insulation d) Efficiency e) Mechanical partsf) Commutation g) Power factor

03 Classify electrical machines from the view point of manufacturing process.

a) Small size machines b) Medium size machines c) Larger size machines

04 What are the major considerations to evolve a good design of electrical machines?

a) Cost b) Durability c) Compliance with performance criteria as laid down in specifications

05 How design problems of an electrical machine can be classified?

The design problems of electrical machines can be classified asa) Electromagnetic design b) Mechanical design c)Thermal design d) Dielectric design

06 What are the fundamental requirements of high conductivity materials?

The fundamental requirements to be met by high conductivity materials area) Highest possible conductivityb) Least possible temperature co-efficient of resistancec) Adequate mechanical strengthd) Rollability and Drawabilitye) Good weldability and Solderabilityf) Adequate resistance to corrosion

07 Where conductors of high resistance are used?

Conductors of high resistance are used where it is actually desired to dissipate electrical energy as heat i.e., in starting and regulating devices for motors etc.

08 Which materials are used for precision device, rheostats and heating devices?

Materials used for precision device: Manganin, Nickel Material used for rheostats : Constantan Material used for heating devices : Nichrome

09 Why does a brush carbon often graphited?

Brush carbons are often graphited i.e. heat treated to increase the size of crystals. This raises the conductivity of the brushes and reduces their hardness.

10 Name the types of magnetic materials based on hysterisis loops? Give examples.

a) Soft magnetic materialsExamples: cast iron, cast steel, Electrical steel sheets, Cold Rolled Grain Oriented Steel

b) Hard magnetic materialsExamples: Aluminium, Nickel, Cobalt

11 What do you mean by ageing?

Ageing is the term used to denote the deterioration of magnetic performance in service, caused by increase in coercive force and Hysteresis loss which in turn caused cumulative overheating and subsequent breakdown.

12 What is dynamo grade steel?

Sheet steels possessing low silicon content (0.5% silicon) are termed as dynamo grade steels are mainly used in rotating electrical machines.

13 What is transformer grade steel?

Sheet steels possessing higher silicon content (4-5% silicon) are termed as transformer grade steel are mainly used in transformers as not much importance is attached to the magnetizing current.

14 What is CRGO steel? What are its uses?

The Cold Rolled Grain Oriented Steel is manufactured by a series of cold reductions and intermediated annealings. This cold reduced material has strong directional magnetic properties, the rolling direction being the direction of highest permeability. This direction is also the direction of lowest Hysteresis loss.

This type of material is suitable for use in transformers and large turbo-alternators.

15 What are the classifications of insulating materials?

16 Which insulating materials are used in modern electric machines?

Mica, Mica folium, Fibrous glass, Asbestos, Polyamides , wood, silicones, Synthetic resin, askarels

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17 What are the fundamental requirements of insulating materials?

a) High dielectric strengthb) High Resistivityc) Low dielectric Hysteresisd) Good thermal conductivitye) High degree of thermal stability

18 Define space factor?

Space factor is defined as the ratio of copper area to total winding area. It is denoted by Sf.

Space factor S = Copper area

f Total winding area

Define specific magnetic loading?

20 Define specific electric loading?

21 What are the factors that decide the choice of specific magnetic loading?

22 What are the factors that decide the choice of specific electric loading?

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23 What are the different modes of heat dissipation in electric machines?

a) Conduction b) Convection c) Radiation

24 State Newton’s law of cooling?

------------------------ (1)

Equation (1) represents the Newton’s law of cooling.

25 What is rating?

Rating of a motor is the power output or the designated operating power limit based upon certain definite conditions assigned to it by the manufacturer.

26 Mention the different types of duties of a machine.

27 Mention the standard specifications issued for electrical machines.

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What are the common methods for measurement of temperature rise in electrical machines?

a) Thermometer methodb) Resistance methodc) Embedded temperature detector method

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29 Name the methods used for determination of motor rating for variable load drives?

a) Method of average lossesb) Equivalent current methodc) Equivalent torque methodd) Equivalent power method

30 What is super conductivity? List few of the metals and compounds that exhibit the property of super conductivity.

Materials exhibiting zero value of resistivity are known as superconductors.

Metals:1. Aluminium 2. Cadmium 3. Indium 4. Lead 5. Mercury 6. Tin 7. Uranium 8. Zinc

Compounds:1. SnSb 2. Cu2 S 3. MoN 4. ZrC

31 Explain the difference between hot rolled steel sheets and cold steel sheets.

Hot rolled steel sheets Cold rolled steel sheetsThe constituent crystals are disposed in a random fashion

The constituent crystals are disposed in a particular fashion i.e. to the direction of rolling.

Low value of permeability High value of permeabilityHigh loss Low loss

32 Why cooling time constant is larger than heating time constant?

The cooling time constant is usually larger owing to poorer ventilation conditions when the machine cools. In self cooled motors the cooling time constant is about 2-3 times greater than the heating time constant because cooling conditions are worse at stand still.

State any two methods of cooling turbo alternators.1. Hydrogen cooling 2. Air cooling 3. Oil cooling 4. Water cooling

Distinguish between continuous rating and short time rating of an electrical machine.

Continuous rating Short time ratingLoad may be carried by the machine for an indefinite time

Load may be carried by the machine for certain specified time

Motor attains its maximum final steady temperature

Motor not attains its maximum final steady temperature

Ex. Fans, pumps Ex. Railway turntable, Navigation lock gates

35 List the advantages of direct cooling of rotating electrical machines.

1. Increased efficiency 2. Increase in rating 3. Increase in life

MAJOR CONSIDERATIONS IN ELECTRICAL MACHINE DESIGN DESIGN OF MACHINES · 2019. 12. 8. · EE 6604...

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