major considerations in electrical machine design design of machines · 2019. 12. 8. · ee 6604...
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
MAJOR CONSIDERATIONS IN ELECTRICAL MACHINE DESIGN
DESIGN OF MACHINES
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
LIMITATIONS IN DESIGN
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
ELECTRICAL ENGINEERING MATERIALS
ELECTRICAL CONDUCTING MATERIALS
1. High Conductivity Materials
a) Copper
✓ Most electrical machines employ windings of annealed high conductivity copper
b) Aluminium
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
Table:1 Comparison of copper and aluminium wires
c) Alloys of copper
2. High Resistivity Materials
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
ELECTRICAL CARBON MATERIALS
MAGNETIC MATERIALS
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
TYPES OF MAGNETIC MATERIALS
SOFT MAGNETIC MATERIALS
i) Solid core materials
ii) Electrical sheet and strip
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iii) Special purpose alloys
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INSULATING MATERIALS
ELECTRICAL PROPERTIES OF INSULATING MATERIALS
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TEMPERATURE RISE AND INSULATING MATERIALS
CLASSIFICATION OF INSULATING MATERIALS
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
APPLICATIONS OF INSULATING MATERIALS
i) Insulating materials for wires
ii) Insulating materials for laminations
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
iii) Insulating materials for machines
iv) Insulating materials for transformers
INSULATING ELECTRICAL MATERIALS USED IN MODERN ELECTRIC MACHINES
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
SPACE FACTOR
Space factor is defined as the ratio of copper area to total winding area. It is denoted by Sf.
f
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
-----------------------(1)
--------------------------- (2)
-------------------------- (3)
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PROBLEM: 01
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
PROBLEM: 01
Volume of coil = Cross section of coil ×length mean turn
total lossvolume of coil
q=
TEMPERATURE GRADIENTS IN CONDUCTORS PLACED IN SLOTS
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THERMAL STATE IN ELECTRICAL MACHINES
HEATING
-------------------------1
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
PROBLEM: 01
PROBLEM: 02
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METHODS OF COOLING
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RATING OF MACHINES
TYPES OF DUTIES AND RATINGS
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
SELECTION OF MOTOR POWER RATING
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STANDARD SPECIFICATIONS
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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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.
EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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
19
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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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.
28
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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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EE 6604 – DESIGN OF ELECTRICAL MACHINES UNIT – I: INTRODUCTIOND. RAJASEKARAN-ASP/EEE
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.
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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