DESIGN OF ELECTRICAL MACHINES EE 6604 FACULY NAME SHRI.N.B.RAJESH M.E. AP-II/EEED VCET, Madurai

1. DESIGN OF ELECTRICAL MACHINES EE 6604 FACULY NAME SHRI.N.B.RAJESH M.E. AP-II/EEED VCET, Madurai

2. DESIGN OF ELECTRICAL MACHINES Aim: • The aim of the design is to completely obtain the dimensions of all the parts of the machine to furnish the data to the manufacturer. • The main aim of carrying out the design is to achieve the following: • Lower cost • Lower weight • Reduced size • Better operating performance.

3. DESIGN OF ELECTRICAL MACHINES Course Objectives: • To provide the learners with an insight into the concepts , procedure and computation of Design aspects of DC and AC Machines. • To provide the learners with an insight into the Design of Armature, Field system and various factors that affect the design such as Heat generation, Temperature rise, Insulation requirements etc.,

4. Objectives:(Alternative) • To design the electrical machines properly, one should be familiar with the following aspects of electrical engineering. • Various electrical materials and their properties. • Properties of magnetic and electric fields. • Laws governing electric circuits. • Laws of electromagnetic induction. • Calculation of magnetic circuits. • Construction of various types of electrical machines. • Behaviour of electrical machines under working conditions

5. Unit –I – Introduction Module –I: • Major considerations of electrical machine design • Name plate specifications of DC machines - factors affecting design and limitations • Choice of specific loadings (Magnetic and Electric) • Temperature rise and cooling of Electrical Machines. • Rating of Machines • Standard Specifications

6. What is meant by Electrical Machine Design? • Design may be defined as a creative physical realization of theoretical concepts. • Engineering design is application of science, technology and invention to produce machines to perform specified tasks with optimum economy and efficiency.

7. Major considerations to develop a design are: • The design should be carried out based on the given specification using available materials economically and to achieve the following: • Lower cost • Durability 3. Conformity with performance criteria as laid down in specifications

8. Limitations in Design • Apart from the availability of suitable materials and facilities to create an electrical machine including transportation, the following are the limitation on designing of the electrical machines: • Saturation •     Temperature Rise •     Insulation •     Efficiency        •    Mechanical Parts •     Commutation •     Power Factor • Consumer’s Specifications •     Standard Specifications

9. SATURATION: In the designing of the electromagnetic machines, use ferromagnetic material. The flux density of the machine is determined by the saturation of the ferromagnetic material used. Higher flux density results in higher cost. • TEMPERATURE RISE: The most important part of the machine is the insulation. It should be according to the maximum temperature in the machine. If the operating temperature is higher than the allowable temperature its life will be drastically reduced. Proper ventilation techniques are used to keep the temperature rise in the safer limits. The coolant will allow the heat from the machine to dissipate. Increased temperature rise under higher output weakens the insulation and affects the life of machine

10. INSULATION: The insulation is the most important part as it should with stand the electrical, mechanical and thermal stress produced by the machine. Transformersare the machines which should have higher insulation where the large axial and radial forces are produced when the secondary winding of the transformer is short-circuited with primary on. It should withstand high mechanical stress due to secondary winding is short circuited. • EFFICIENCY: Efficiency should be high to reduce the operation cost of the machine. So it requires large amount of materials to design. We can reduce the operating cost by increasing the design cost.    If high efficiency is the aim, the machine becomes costly, for lower efficiency higher running cost and temperature rise with associated problems.

11. MECHANICAL PARTS: The construction of the mechanical parts should be economical but it should satisfy the requirements of performance, reliability, and durability. For the high speed machines it is very important because it will be having more mechanical stress at the rotor. The length of the air-gap is reduced to increase the high power factor. • COMMUTATION: This problem only occurs in the Commutator machines. As it decreases the maximum output taken from the machine. In DC machine output is limited because of commutation problem • POWER FACTOR: Poor power factor results in large amount of current in the same power, therefore large conductor sizes have to be used. It mostly affects the induction motor.

12. CONSUMER’s SPECIFICATION: The important in this is it should satisfy the consumer specification with their economic constrains. The design should evolve in this manner.Imposes limitations to identify criterion for best design • STANDARD SPECIFICATIONS: Specification is biggest strain on the design because both the manufacturer as well as the consumer cannot get away from them without satisfying them.

13. Basic Structure of Electrical Machine 1. Magnetic circuit : Core , Yoke, Air-gap etc. 2.Electric circuit: Stator, Rotor winding and Transformer winding 3. Dielectric circuit : Insulation 4. Thermal circuit :Heating and Cooling medium 5. Mechanical parts : Frame, Bearings and Shaft

14. General Idea Electrical Energy Mechanical Energy Electrical Machine Losses – I2R, friction, etc

15. Types of electrical machines Transformers

16. Rotating machines DC Machines AC Machines Continuation….. DC Generator DC Motor Synchronous Machine Asynchronous Machine Synchronous Generator Synchronous Motor Induction Machines 1 phase 1 phase 1 phase 3 phase 3 phase 3 phase

17. CLASSIFICATION OF DESIGN PROBLEMS • The design of an electrical machine involves solution of many complex and diverse engineering problems. The design problems may be classified under the following four headings. • Electromagnetic design • The electro magnetic design problem in rotating machines involves the design of stator & rotor core dimensions, stator & rotor teeth dimensions, air-gap length , stator and rotor windings. • In transformer it is the problem of designing the core and the windings. • Mechanical design • The mechanical design in rotating machine involves the design of frame (enclosure), shaft and bearings. In transformer it is the design of tank (i.e., housing for core and winding assembly).

18. Thermal design • The thermal design in rotating machine involves the design of cooling ducts in core and cooling fans. • In case of large machines coolants like air or hydrogen may be forced to circulate in the ducts and air-gap. In transformer it involves the design of cooling tubes or radiators. • Dielectric design • Another important design problem, that may require great attention in the design of insulations (Dielectric design). Dielectric materials are used to insulate one conductor from other and also the windings from the core. • The dielectric materials are designed to withstand high voltages stresses. The breakdown of dielectric materials may lead to failure of machine

19. STANDARD SPECIFICATIONS The standard specifications issued for electrical machines includes the followings: • 1. Output : kW (for generators), kW or Hp (for motors) • 2. Voltage : V volt • 3. Speed : N rpm • 4. Rating : Continuous or Short time • 5. Temperature rise: θ0C for an ambient temperature of 400C • 6. Cooling : Natural or forced cooling • 7. Type: Generator or motor, separately excited or self-excited-shunt, series, or compound, if compound type of connection - long or short shunt, type of compounding - cumulative or differential, degree of compounding - over, under or level. With or without inter poles, with or without compensating windings, with or without equalizer rings in case of lap winding. • 8. Voltage regulation ( in case of generators) : Range and method

20. 9. Speed control ( in case of motors ) : range and method of control • 10. Efficiency: must be as for as possible high (As the efficiency increases, cost of the machine also increases). • 11. Type of enclosure: based on the field of application - totally enclosed, screen protected, drip proof, flame proof, etc., • 12. Size of the machine etc.,

21. The Name plate of rotating machine has to bear the following details as per ISI specifications: • kW or kVA rating of machine • Rated working voltage • Operating speed • Full load current • Class of insulation • Frame size • Manufactures name • Serial number of the product

22. Some of Indian standard specifications numbers along with year of issue for electrical machines are listened here. • IS 325-1966 : Specifications for three phase induction motor. • IS 1231-1974: Specification for foot mounted induction motor. • IS 4029-1967 : Guide of testing three phase induction motor. • IS 996-1979: Specifications for single phase AC and universal motor. • IS 1885-1993: Specifications for electric and magnetic circuits. • IS 9499-1980: Conventions concerning electric and magnetic circuits. • IS 7538-1996: Specifications for three phase induction motor for centrifugal pumps and agricultural applications.

23. IS 12615-1986 : Specifications for energy efficient induction motor. • IS 9320-1979 : Guide for testing dc machines. • IS 4722-1992 : Specifications for rotating electrical machines. • IS 12802-1989 : Temperature rise measurement of rotating electrical machines. • IS 4889-1968 : Method of determination of efficiency of rotating electrical machines. • IS 13555-1993 : Guide for selection and application of three phase induction motor for different types of driven equipment. • IS 7132-1973 : Guide for testing synchronous machines. • IS 5422-1996 : Turbine type generators. • IS 7572-1974 : Guide for testing single-phase ac and universal motors.

24. IS 8789-1996 : Values of performance characteristics for three phase induction motors. • IS 12066-1986 : Three phase induction motors for machine tools. • IS 1180-1989 : Specifications for outdoor 3-phase distribution transformer upto 100 kVA.( Sealed and Non-sealed type) • IS 2026-1994 : Specifications for power transformers. • IS 11171-1985 : Dry type power transformers. • IS 5142-1969 : Continuously variable voltage auto transformers. • IS 10028-1985 : Code of practice for selection , installation and maintenance of transformers. • IS 10561-1983 : Application guide for power transformers. • IS 13956-1994 : Testing transformers.

25. IS 9678-1980 : Methods of measuring temperature rise of electrical equipment. • IS 12063-1987 : Classification of degree of protections provided by enclosures of electrical equipment. • IS 3855-1966 : Standard dimensions of enameled round copper conductor. • IS 449-1962 : Standard dimensions of enameled round copper conductor. • IS 1595-1960 : Standard dimensions of enameled round copper conductor. (synthetic enamel) • IS 1897-1962 : Standard dimensions of bare copper strip. • IS 1666-1961 : Standard dimensions of paper covered rectangular copper conductor for transformer windings. • IS 2068-1962 : Standard dimensions of cotton covered rectangular copper conductor windings.

26. IS 3454-1966 : Standard dimensions of paper covered round conductors used for transformer windings. • IS 450-1964 : Standard dimensions of cotton covered round conductors used for transformer windings.

27. Classification of Engineering Materials The engineering materials can be classified based on the branch of engineering as below- • Mechanical Engineering materials – i.e. Iron, Steel etc. • Electrical Engineering materials–i.e. Conductors, Semiconductors, Insulators, Magnetic materials etc. • Civil Engineering materials – i.e. Cements, Iron, Stones, Sans etc. • Electronic engineering – i.e. Semiconducting materials

28. Materials for Electrical Machines • The main material characteristics of relevance to electrical machines are those associated with conductors for electric circuit, the insulation system necessary to isolate the circuits, and with the specialized steels and permanent magnets used for the magnetic circuit. Conducting materials Commonly used conducting materials are copper and aluminum. Some of the desirable properties a good conductor should possess are listed below. • 1. Low value of resistivity or high conductivity • 2. Low value of temperature coefficient of resistance • 3. High tensile strength • 4. High melting point • 5. High resistance to corrosion

29. 6. Allow brazing, soldering or welding so that the joints are reliable • 7. Highly malleable and ductile • 8. Durable and cheap by cost

30. Magnetic materials The magnetic properties of a magnetic material depend on the orientation of the crystals of the material and decide the size of the machine or equipment for a given rating, excitation required, efficiency of operation etc. The some of the properties that a good magnetic material should possess are listed below. • 1. Low reluctance or should be highly permeable or should have a high value of relative permeability μr. • 2. High saturation induction (to minimize weight and volume of iron parts) • 3. High electrical resistivity so that the eddy emf and the hence eddy current loss is less • 4. Narrow hysteresis loop or low Coercivity so that hysteresis loss is less and efficiency of operation is high

31. 5. A high curie point. (Above Curie point or temperature the material loses the magnetic property or becomes paramagnetic, that is effectively non-magnetic) • 6. Should have a high value of energy product (expressed in joules / m3). • Magnetic materials can broadly be classified as Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic and Ferri-magnetic materials. • Only ferromagnetic materials have properties that are well suitable for electrical machines. • Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt and their alloys. The only exceptions are some alloys of manganese and some of the rare earth elements.

32. The relative permeability μr of ferromagnetic material is far greater than 1.0. When ferromagnetic materials are subjected to the magnetic field, the dipoles align themselves in the direction of the applied field and get strongly magnetized. Insulating materials To avoid any electrical activity between parts at different potentials, insulation is used. An ideal insulating material should possess the following properties. • 1) Should have high dielectric strength. • 2) Should with stand high temperature. • 3) Should have good thermal conductivity • 4) Should not undergo thermal oxidation • 5) Should not deteriorate due to higher temperature and repeated heat cycle • 6) Should have high value of resistivity ( like 1018 Ω-cm)

33. 7) Should not consume any power or should have a low dielectric loss angle δ • 8) Should withstand stresses due to centrifugal forces ( as in rotating machines), electro dynamic or mechanical forces ( as in transformers) • 9) Should withstand vibration, abrasion, bending • 10) Should not absorb moisture • 11) Should be flexible and cheap • 12) Liquid insulators should not evaporate or volatilize. Insulating materials can be classified as Solid, Liquid and Gas, and vacuum. The term insulting material is sometimes used in a broader sense to designate also insulating liquids, gas and vacuum.

34. Classification of insulating materials based on thermal consideration:

35. Based on the properties and applications the electrical engineering materials can be categorized as below- • Conductors – i.e. Silver, Copper, Gold, Aluminum etc. • Semiconductors – i.e. Germanium, Silicon, GaAs etc. • Insulators - Plastics, Rubbers, Mica, Insulating Papers etc. • Magnetic materials – Iron, Silicon steel, Alnico, ferrites etc.

36. GENERAL DESIGN PROCEDURE • In general any electrical machine has two windings. • The DC machine and synchronous machine has armature and field winding. • The induction machine has stator and rotor winding. The basic principle of operation of all electrical machine is governed by Faraday’s law of induction. • Also in every electrical machine the energy is transferred through the magnetic field. Hence a general design procedure can be developed for the design of electrical machines.

37. The general design procedure is to relate the main dimensions of the machine to its rated power output. • An electrical machine is designed to deliver a certain amount of power called rated power. • The rated power output of a machine is defined as the maximum power that can be delivered by the machine safely. • In DC machine the power rating is expressed in kW and in AC machine in kVA. In case of motor the output power is expressed in HP.

38. In electrical machine the core and winding of the machine are together called active part. ( Because the energy conversion takes place only in the active part of the machine). • A general output equation can be developed for DC machine which relate the power output to volume of active part (D2 L), speed, magnetic and electric loading. • Similarly a general output equation can be developed for AC machine which relates kVA rating to volume of active part (D2 L), speed, magnetic and electric loading.

39. In rotating machine the active part is cylindrical in shape. The volume of the cylinder is given by the product of area of cross section and length. If D is the diameter and L is the length of cylinder, then the volume is given by πD2 L/4. Therefore D and l are specified as main dimension. In case of DC machine, D represent the diameter of armature and L represent the length of armature. In case of AC machine, D represent the inner diameter of stator and L represent the length of stator core. HereDr = Diameter of rotor lg = Length of air-gap Main Dimensions of a Rotating Machine:

40. Main Dimensions of a Rotating Machine: L D Rotor Stator Air Gap

41. Different Loadings to be Considered while Designing a Rotating Machine: Total Magnetic Loading: “It is the total flux around the armature periphery at the air gap.” Total Electrical Loading: “It is the total no. of ampere conductors around the armature periphery.”

42. SPECIFIC MAGNETIC LOADING Each unit area of armature surface is capable of receiving a certain magnetic flux. Hence the flux per unit area is an important parameter to estimate the intensity of magnetic loading and it is also criterion to decide the volume of active material. This flux per unit area is expressed as the average value of the flux density at the armature surface or specific magnetic loading (by the assuming that the armature is smooth). It is denoted by Bav The average flux density, Bavis given by the ratio of flux per pole and area under a pole. Bav= Flux per pole = Flux per pole . Area under a pole Pole Pitch x Length of armature = ф =Pф. πD x L πDL P Pole pitch, τ = πD/p

43. Specific Loadings: Specific Magnetic Loading: “It is the average flux density over the air gap of a machine.” (Maximum Flux density in iron parts of machine, magnetizing Current and Core losses)

44. SPECIFIC ELECTRIC LOADING • Every section of armature is capable carrying certain amount of current. Hence ampere-turn per unit section of armature periphery (circumference) is an important parameter to estimate the intensity of electric loading and it is also a criterion to decide the volume of active material. • This ampere –turn per unit section of armature periphery is expressed as the specific electric loading . It is denoted by ac.

45. Specific Loadings: Specific Electric Loading: “It is the no. of armature ampere conductors per meter of armature periphery at the air gap.” (Current density, Applied voltage, Size of machine and permissible temperature rise)

46. Advantages due to Higher Specific Loadings: • Reduction in the Volume of the machine. • Reduction in the size of the machine. • Lower cost of the material required. • Lower weight of the machine. • Lower over all cost of the machine. To produce a cheaper machine with reduction in its size, the values of specific loadings must be pushed to the largest possible.

47. Disadvantage due to Higher Specific Magnetic Loading, Bav • Increased iron losses. • Larger requirement of m.m.f. • Higher field copper losses (D.C. Machine, Syn. machines) • Higher tooth density. • Tendency of saturation of magnetic parts. • Increased magnetizing current and poorer power factor (Induction Motor) • Reduced leakage reactance and larger initial current on sudden short circuit (syn. m/c). • Increased temperature rise due to higher losses. • Increased noise.

48. Disadvantage due to Higher Specific Electric Loading, ac: • Increased armature copper losses. • Increased leakage reactance because of larger turns per phase (Ind. & Sy. Machine) • Increased temperature rise because of higher copper losses. • Increased reactance voltage and inferior commutation (DC m/c) • Increased field excitation causing more field copper losses (D.C. m/c) • Poorer regulation and stability impaired (syn. m/c) • Reduction in over load capacity.

49. Specific magnetic and electric loadings

50. Choice of Specific Magnetic Loading: (SML) • Common factors affect the choice of SML are; • Maximum flux density in iron parts of a machine. • Magnetizing current. • Core losses.