EE-2355 Design of Electrical Machines.pptx

Basics of machine design

1. EE-2355 Design of Electrical Machines – UNIT -I 11/21/2019 1

2. Major consideration in design Design of machines: Design is 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. The problem of design and manufacture of electric machinery is to build, as economically as possible , a machine which fulfils a certain set of specifications and guarantees. 11/21/2019 2

3. Design considerations The major design considerations to evolve a good design are as follows: Design Base : Matching the existing experience with R & D; bringing in the latest material technology; limitations in design; convenience in production line and transportation; working safety and reliability; maintenance and repair; environmental conditions ; cost economy; optimization. Specification: Meeting with the customer’s requirements, guarantees; satisfy the national and international standards. Design Transfer: Transfer of design to factory foreman i.e. drawings, processes, instructions, job-flow, meeting the delivery schedule. Information updating: Technical journals, R and D papers and reports, interaction in meetings and seminars. 11/21/2019 3

4. It is impossible to design a machine which is cheap and is also durable at the same time. Long life span – high quality materials and advanced manufacturing techniques → high cost Good design – machine has reasonable operating life (20-30 years) and reasonable initial cost. 11/21/2019 4

5. Areas of design Main areas of design: Knowing the characteristics and specifications that a machine has to satisfy, the main areas of design are: The magnetic circuit; core, yoke, airgap etc., The electric circuit; the windings The insulation (Dielectric circuit) Heating and cooling circuit (Thermal circuit) The mechanical construction The designer work lies in suitably allocating the space to frame, core, airgap , winding, insulation and cooling medium in the machine. 11/21/2019 5

6. Limitations Saturation of magnetic parts : Increased core losses and excitation at higher flux density. Temperature rise : increased temperature rise under higher output deteriorates the insulation and affects the life of the machine. Insulation: Breakdown due to high voltage gradient; temperature rise limitations, mechanical damage. Mechanical strength: specially in turbo-machine due to large size and high speed. Efficiency: If high efficiency is the aim, the machine becomes costly, for lower efficiency higher running cost and temperature rise with associated problems. Customer’s specifications: Impose limitations to identify criterion for best design. Commutation: In d.c . machine output is limited because of commutation problem Power Factor: P.F imposes a limitation specially in case of 3-ph induction motor. Because of p.f . limitation higher flux density cannot be taken in airgap which would have otherwise resulted in more compact size motor. 11/21/2019 6

7. Principles of Elec. Machines Basic principles: All electric machines are electromagnetic devices. Among them the rotating machines are electromechanical energy conversion devices but transformer is a static machine where energy conversion does not take place, only the level of voltage changes. All electrical machines are based upon three principles: 1)Induction 2)Interaction 3) Alignment. 11/21/2019 7

8. ELECTRICAL ENGINEERING MATERIALS: Electrical engineering materials used in the construction of all commercial machines are classified as, Conducting material Insulating material Magnetic material Select the above materials properly to improve the efficiency of the machine, reduce the size, weight, cost and increase the reliability of operation. Conducting materials: Conducting materials used in electrical engineering can be divided into two groups, Materials of high conductivity which are used for making conductors for all types of windings used in electrical machines. These materials must posses least possible electrical resistivity. High conductivity materials should meet the following requirements, highest possible conductivity (least resistivity), least possible temperature coefficient of resistance, sufficient mechanical strength, good welding and soldering of joints which ensure high reliability and low resistance, Property of rolling are important in the manufacture of wires. Materials of high resistivity which are normally used for making heating devices, thermo couples, resistance, etc., 11/21/2019 8

9. Various conducting materials are, sliver copper aluminium iron and steel super conducting materials 11/21/2019 9

10. 11/21/2019 10 MATERIALS OF HIGH RESISTIVITY: They can be classified into 3 categories: I- GROUP: (MATERIALS USED FOR PRECISION WORKS) Consists of materials used in precision measuring instruments & in making standard resistances and resistance boxes . Important material used is Manganin (composition of Cu86 %, Mn 12%, Ni 2%) II- GROUP: (MATERIALS USED FOR RHEOSTATS) Consists of materials from which resistance elements are made for all kinds of rheostats and similar control devices. Principal alloy is constantan consisting of 60 to 65 % Cu and 40 to 35% Ni). II- GROUP: (MATERIALS USED FOR HEATING DEVICES ) Consists of materials suitable for making high temperature elements for electric furnaces, heating devices and loading rheostats . Alloys of Nickel, Chromium and Iron called as Nichrome . Alloys of Aluminium , Iron and Chromium. ELECTRICAL CARBON MATERIALS: Mmanufactured from graphite and other forms of carbon coal, etc. Carbon brushes are often graphited i.e., heat treated to increase the size of crystals. This raises the conductivity of the brushes and reduces their hardness . Carbon brushes should acquire a mirror smooth surface in order that they does not cause wear of commutator when mounted on them.

11. 11/21/2019 11 MAGNETIC MATERIALS: The magnetic properties of materials are characterized by their relative permeability. 1. Ferromagnetic material : The relative permeability of these materials are much greater than unity. 2. Paramagnetic material: The relative permeability of these materials are slightly greater than unity. 3. Diamagnetic material: The relative permeability of these materials are slightly less than unity. Types of magnetic materials: soft magnetic materials Solid core materials Iron Gray cast iron Cast steel Soft steel Electrical sheet and strip special purpose alloys Hard magnetic materials

12. Type Part Material used Operating flux density Transformer winding Hot rolled silicon steel Cold rolled grain oriented silicon steel 1.1 - 1.4 1.5 – 1.7 core Hot rolled silicon steel Cold rolled grain oriented silicon steel 1.0 – 1.2 1.3 – 1.5 D.C Machines Yoke Cast steel 1.0 – 1.3 Pole Silicon steel, dynamograde 1.4 – 1.6 Air gap Air 0.43 – 0.76 Armature teeth Silicon steel, dynamograde 1.9 – 2.2 Armature core Silicon steel, dynamograde 1.2 – 1.4 Magnetic circuits of electrical machines: 11/21/2019 12

13. Type Part Material used Operating flux density Induction Motor Stator core Silicon steel, dynamograde 1.1 – 1.4 Stator teeth Silicon steel, dynamograde 1.8 – 1.9 Air gap Air 0.35 – 0.65 Rotor teeth Silicon steel, dynamograde 1.8 – 1.9 Rotor core Silicon steel, dynamograde 1.1 – 1.4 Synchronous Machines Stator core Silicon steel, dynamograde 1.1 – 1.4 Stator teeth Silicon steel, dynamograde 1.8 – 1.9 Air gap Air 0.5 - -0.7 Rotor pole Silicon steel(salient), Forged steel (non salient) 1.2 – 1.4 Rotor core 1.1 – 1.4 11/21/2019 13

14. INSULATING MATERIALS: Insulating materials are used to provide an electrical insulation between parts at different potentials. Design of electrical machine is limited by the restriction imposed by the insulating materials. Properties of insulating materials: The proper selection of an insulating material for a particular condition needs the knowledge of their electrical and mechanical properties. The three fundamentals electrical qualities of insulating materials of great importance to the designer of electrical machine or equipment are, insulation resistance or resistivity electric strength or dielectric strength dielectric loss angle In addition to the above, other properties such as mechanical strength, heat resistance, etc., An ideal insulating materials must have the following properties, high insulating resistance high dielectric strength low dielectric loss and low dielectric loss angle no attraction for moisture good heat conductivity sufficient mechanical strength to withstand vibration and bending solid materials should have a high melting or softening point. liquid material should not evaporate. 11/21/2019 14

15. Classification of insulation: Insulating materials are classified according to their heat resisting properties as per IS 1271 and 1281. Class Y – maximum temperature 90 C, this includes paper, cotton, silk, wood, fiber cellulose, etc., These materials have a tendency to absorb moisture and used for electrical machines. Class A – maximum temperature 105 C Class E – maximum temperature 120 C Class B – maximum temperature 130 C Class F – maximum temperature 155 C Class H – maximum temperature 180 C Class C – maximum temperature greater than 180 C i . Class Y - cotton, silk, paper, cellulose, wood, etc neither impregnated nor immersed in oil. ii. Class – materials of class Y suitably impregnated. iii. Class E- Synthetic resin enamels, cotton, paper laminated with formaldehyde bonding, etc. iv. Class B- mica, glass fibre , asbestos, etc, with suitable bonding materials . v. Class F-materials of class B with suitable bonding materials of higher thermal stability(25°C higher than class B). vi. Class H- glass fibre , asbestos & built up mica, with suitable bonding materials like silicon resins . Class C- Mica, ceramics, glass, quartz without binders or with silicon resins of higher thermal stability. 11/21/2019 15

16. 11/21/2019 16 INSULATING MATERIALS USED IN MODERN ELECTRIC MACHINES: Mica, Micafolium , Fibrous glass, Asbestos, Cotton fibre , Polyamides, Synthetic-resin enamels, Slot-lining materials, Wood, Silicones, Epoxide thermosetting resins, Synthetic resin, Petroleum based mineral oils, Askarels . APPLICATIONS: a. Wires for magnetic coils and windings of machines. b. Laminations c. Machines& Transformers.

17. MAGNETIC CIRCUITS: The path of the magnetic flux is called a magnetic circuit. Laws of magnetic circuits is Φ – magnetic flux in Wb A - area of the magnetic path in m 2 . l – length of magnetic path in m B – flux density in Wb/m 2 (Tesla) H – at – magnetising force in A/m AT – total mmf in A μ - μ r μ o – absolute permeability of the magnetic materials in H/m μ r – relative permeability μ o – permeability of free space = 4 π *10 -7 H/m S – reluctance in A/Wb In an electric circuit ohm’s law expresses a relation between current, emf and resistance. while in magnetic circuits relates flux, mmf and reluctance. Magneto Motive Force(MMF): The mmf creates flux in a closed path. If N is the number of coils and i be the exciting current in amperes, then mmf is MMF, F=Ni in AT Reluctance (S): Reluctance is the property of a magnetic material which opposes the flow of flux through it. S=mmf/flux=AT/ Φ AT=S Φ Permeance=1/S 11/21/2019 17

18. Φ =AT/S=AT*permeance H=at=mmf per unit length=flux*reluctance per unit length For the case of a material of length l and carrying a uniform flux, the total mmf (AT) is AT=H*l=at*l In a series magnetic circuit, the total reluctance is the sum of reluctance of individual parts, S=S 1 +S 2 +S 3 +….. The total mmf acting around a complete magnetic circuit is then given by MMF T =AT= Φ S= Φ [S 1 +S 2 +S 3 +….] =AT 1 +AT 2 +AT 3 +… =at 1 l 1 +at 2 l 2 +at 3 l 3 +… In parallel circuits, the same mmf is applied to each of the parallel paths and the total flux divides between the paths in inverse proportion to their reluctance as in corresponding electric circuits or Φ = Φ 1 + Φ 2 + Φ 3 +…. 11/21/2019 18

19. Dividing by AT, the applied mmf, we get Φ/AT= Φ 1 /AT+ Φ 2 /AT+… or 1/S=1/S 1 +1/S 2 +…. Magnetising curves (B-H or B- ‘at’ curves) : Fig. f shows the B-H curves of commonly used magnetic materials. Two methods of mathematical relationship, Fig.f 2.13 2.14 Eq. 2.13 gives reasonable and approximate values while eq. 2.14 gives better overall value, odd powers of H is used for alternating magnetisation. Non-magnetic materials(air) have a constant value of permeability and so the B- ‘at’ curve is straight line passing through origin. H=B/ μ =B/(4 π *10 -3 )=800000B 2.15 11/21/2019 19

20. Fig.g. B-H curve for electrical steel-dynamo grade Fig.h. B-H curve for electrical steel-cold rolled grain oriented 11/21/2019 20

21. Fig.i. B-H curve for electrical steel-cold rolled transformer steel 11/21/2019 21

22. S.No . Electric Circuits Magnetic circuits 1 Current flow in the circuit Flux is assumed to flow 2 Path of current is called electric circuit Path of flux is called magnetic circuit 3 Current flow due to emf Flux flow due to mmf 4 Flow of current is restricted by resistance of the circuit Flow of flux is restricted by reluctance of ciruit 5 Current= emf /resistance Flux= mmf /reluctance 6 R=l/ σ A S=l/A μ 7 Current actually flows in the circuit Flux does not flow assumed to flow 8 Energy is needed till the current flow Energy is needed only to create a magnetic flux 9 Resistance of circuit is independent of current Reluctance of circuit is change with magnetic flux 11/21/2019 22

23. Magnetic leakage: It is impossible to confine (lock) all the magnetic flux to given path , therefore the designer providing a path of low reluctance to supply large mmf to compensate for the flux leakage. For the operation of electric machinery, some air gap is necessary in the magnetic path but this air gap should be kept to a minimum of length and maximum of cross-section so as to reduce their reluctance. Some of the flux flows away the path is called leakage flux. For magnetic flux calculation, a term leakage co-efficient is introduced in order to take into account the leakage flux. The value of leakage co-efficient is defined as CALCULATION OF TOTAL MMF IN A MAGNETIC CIRUCIT: The calculation of total mmf required to establish the requisite flux in a magnetic circuit involves the knowledge of dimensions and configuration of magnetic circuit. Mmf of air gap: The iron surface is not smooth, the calculation of mmf for the air gap by ordinary method gives wrong results because of slotted, radial ventilation, for salient pole machine the air gap dimension is not uniform. L-length of core; l g -gap length; y s -slot pitch; W s -width of slot; W t -width of tooth; W -slot opening; n d -number of radial ducts; W d -width of each slot 11/21/2019 23

24. In smooth armature as in fig.j, the reluctance of air gap for one slot pitch is In slotted armature with small gap length as in fig.k, the reluctance of air gap for one slot pitch is Fig.j Fig.k 2.16 11/21/2019 24

25. Fig.l Fig.m δ W s Where, K cs – Carter’s gap co-efficient which depends upon the ratio of slot width / gap length If the slot present in armature, it increase the reluctance of air gap. 2.17 11/21/2019 25

26. Carter’s co-efficient Fig.n. Carter’s air gap co-efficient Carter’s air gap co-efficient Carter’s co-efficient for parallel sided open slots is The reluctance of air gap with slotted armature 2.18 The ratio of reluctance of air gap slotted armature(eq.2.18) into smooth armature (eq.2.16) be K gs . Where, y=W s /2l g 11/21/2019 26

27. K gs is called gap contraction factor for slots, its value is greater than 1. The provision of radial ventilating ducts results in contraction of flux in the axial direction as shown in fig.o. Contracted or effective axial length is L’=L-K cd n d W d . Where, K cd -Carter’s co-efficient for ducts. Let the ratio of reluctance of air gap with ducts to reluctance of air gap without ducts is K gd . Fig. o 2.19 2.20 11/21/2019 27

28. When two ventilating ducts, one on stator and the other on rotor are exactly opposite to each other, the Carter’s co-efficient must then be based upon ratio of duct width into half of (1/2) gap length. The reluctance of air gap of a smooth armature without ducts=l g /( μ Ly s ) The reluctance of air gap of a slotted armature with ducts=l g /( μ L’y’ s ) K g – total gap contraction factor for slots and ducts. Modification of eqn. 2.17 &2.19 for semi closed slots as 2.21 2.22 2.23 Fig. p 11/21/2019 28

29. Fromeqn.2.15 mmf per metre for air gap=800000B Therefore, mmf required for air gap having a length l g metre, with smooth armatures, AT g =mmf/(m)*length=800000Bl g Therefore, mmf required for air gap having a length l g metre, with slotted armatures, AT g =mmf/(m)*length=800000K g Bl g AT g =800000K g ( Φ /A g )l g =800000 [ Φ /K g A g ] l g =800000[ Φ /A’ g ]l g . 2.24 2.25 Where, Φ -flux per pole in wb; A g ’=A g /K g ; A g -actual area of air gap per pole in m 2 . Area of gap per pole, A g =slot per pole*slots pitch*core length=S/p*y s *L Where, S-total number of slots; p-number of poles Contracted or effective gap area per pole, The mmf required for a smooth armature is AT g = 800000Bl g , While for a slotted armature the air gap mmf is AT g =800000K g Bl g The above relation may be interpreted as that the length of air gap is increased K g times due to provision of slots and ducts. The effective gap length is l gs =K g l g . Where, K g in this case is called gap expansion factor. 11/21/2019 29

30. Fig.q. Effect of saliency: In the case of salient pole machines, the length of air gap is not constant over the whole pole pitch. Fig.q, shows a typical flux distribution curve for a salient pole machine. To know the reluctance of one flux tube(line) and the flux flowing through it in order to find the mmf required for air gap. Therefore, mmf required for air gap of a salient pole machine, AT g =flux in one flux tube*reluctance of flux tube Let us consider a flux tube at the centre of the pole. Flux in the flux tube at the centre =flux density*area of flux tube =B g *area of flux tube Reluctance of flux tube at the centre 11/21/2019 30

31. Where, K g -gap contraction factor for a gap length at the centre of pole. The length of flux tube(path) at the centre of pole is equal to length of air gap. Therefore, the value of K g is based upon the air gap length l g . The field form factor (K f ) is defined as Fig.q.c shows the approximate fux distribution curve for a salient pole machine. K f =(average flux density)/(maximum flux density)= Φ The assumption K f = Φ is correct for machines with normal proportion and a reasonable degree of saturation. With long air gap the pole arc may be considered as 4l g to take account of fringing. 11/21/2019 31

32. Net Length of Iron : The core is made up of laminated steel plates insulated by paper and for effective cooling provide vent spacer(duct) about 40 to 80 mm width. So, the whole length of core is not fully occupied by iron some part of length is iron, insulation , ventilating ducts and air space created by irregularities in thickness of lamination. It is usual to define iron space factor called stacking factor, as the ratio of actual length of iron in a stack of assembled core plates into total axial length of stack. Gross iron length L s =length of conductor in slot portion =core length-length of ventilating ducts =L- n d W d Net iron length L i = K i (L- n d W d ) Where, K i – stacking factor which depends upon thickness of plate and type of insulation. An average value of 0.9 may be assumed for all practical purpose. Mmf for teeth: The calculation of mmf necessary to maintain the flux in teeth is difficult for the following reason, Teeth are wedge-shaped or tapered when parallel sided slots are used. Then the area presented to the flux path is not constant and gives different values of flux density over the length of teeth. The slot provides the another parallel path for flux, shunting the tooth. The teeth are normally worked in saturation region and therefore their permeability is low, as results a part of flux goes down the depth of slot. The presence of two parallel paths, the reluctance of one part depending upon the degree of saturation in the other, makes the problem . There are three methods available to calculate mmf in teeth, Graphical method(used for tapered teeth) , 2.Three ordinate method (Simpson’s rule) and 3. B t1/3 Method 11/21/2019 32

33. 1 . Graphical method(used for tapered teeth ): The mmf per metre (H or at) for the whole length of tooth is not uniform as the flux density is not same everywhere. Therefore to obtain correct value of mmf , it is necessary to construct a graph shows the variation of ‘H’ or ‘at’ over the length of tooth. The mean value of this graph gives the equivalent ‘at’ or ‘H’ for the whole tooth. Therefore, total mmf required for the tooth, AT t =mean value*height of tooth =at mean *l t =at mean *d s . Where, height of tooth l t is equal to d s the depth of slot. Fig.2.23 B-at curve of tapered tooth and calculation of at mean by graphical method Fig.2.24 B-at curve of tapered tooth and calculation of at mean by graphical method 2. Three ordinate method(Simpson’s rule): This method can be applied to teeth of very simple form and a small taper and is based upon the assumption that the curve relating ‘at’ with flux density is a parabola. The mean value of ‘at’ is given by 11/21/2019 33

34. 3. B t1/3 Method: This method is applied to teeth of small taper and is based upon the assumption that value of ‘at’ obtained for flux density at a section 1/3 of tooth height from the narrow end is the mean of ‘at’ for whole of the tooth. This method is the most simple of all the methods and results are sufficiently accurate if the teeth are worked at low saturation. Let, B t1/3 =flux density at 1/3 height from narrow end at 1/3 =value of mmf per metre for B t1/3 as obtained from B-at curve Therefore, total mmf for teeth, AT t =at 1/3 * l t =at 1/3 * d s . Real and apparent flux densities: Due to high density (saturation densities) in teeth, the flux passing through the slots become high and cannot be neglected and any calculation based upon no slot flux leads to wrong results. This means that the real flux passing through the teeth is always less than the total or apparent flux. As a result the real flux density in the teeth is always less than the apparent flux density. The apparent flux density is defined as The real flux density is defined as Fig.2.25 slot flux due to saturation in teeth 11/21/2019 34

35. In an actual machine, taking the flux over one slot pitch, there are two parallel path Iron path: Area of iron path (A i )=tooth width*net iron length=W t *L i . Air path: Area of air path (A a )=total area-iron area=(slot pitch*core length) – (tooth width*net iron length) =y s *L-W t *L i . If Φ s is the flux over one slot pitch, we have Φ s = Φ i + Φ a . Where, Φ i –flux passing through iron over one slot pitch Φ a - flux passing through air(slot) over one slot pitch Where, B a = flux density in air= μ H=4 π *10 -7 ‘ at’ real . 2.26 11/21/2019 35

36. K s =0.5 K s =1.0 K s =1.5 K s =2.0 K s =2.5 K s =3.0 Fig.2.26 Magnetisation curve for various values of K s Fig.2.27 Determination of B real . 4 π When ‘at’ real =0, B=B app ,this corresponding to point A. When B real =0, at real =B app /4 π *10 -7 (K s -1) this corresponding to point A’. 11/21/2019 36

37. LEAKAGE FLUX IN ROTATING MACHINE Content: 11/21/2019 37 Introduction to leakage flux. Different types of leakage flux and steps to minimize the leakage fluxes. References

38. 11/21/2019 38 INTRODUCTION In machines, there are 2 parts to which leakage fluxes i) Main Poles ii) Armature flux Flux linkages in main pole- D.C motor- stator Synchronous machines- rotor Flux is generated by main poles entering the armature passing through the airgap is called useful flux

39. POLE LEAKAGE FLUX 11/21/2019 39

40. 11/21/2019 40 LEAKAGE FLUX IN ARMATURE Armature carries the distributed winding and its slotted hence the airgap is not uniform. Many types of leakage fluxes associated with an armature.

41. SLOT LEAKAGE 11/21/2019 41

42. 11/21/2019 42 SLOT LEAKAGE Leakage flux associated with stator and rotor conductors flowing across the slots having path from tooth to tooth. Flux path is perpendicular to path of the main flux Leakage depends on the shape of the slot

43. Tooth-tip leakage 11/21/2019 43

44. 11/21/2019 44 Tooth-tip leakage Leakage flux flows from tip of the one tooth to the adjacent tooth tip, surrounding all the conductors. This type of flux more in machines where airgap is larger. Airgap is very narrow hence tooth-tip flux is low. eg. Induction machine

45. ZIG-ZAG LEAKAGE 11/21/2019 45

46. 11/21/2019 46 ZIG-ZAG LEAKAGE Wound rotor machines, both stator and rotor are slotted for accommodating the windings. Flux takes path which alternates between stator teeth and rotor teeth. This path zig-zag in nature.

47. Over hang leakage 11/21/2019 47

48. Over hang leakage Design the stator and rotor windings, the end conductors are necessary. Flux associated with end conductors is overhang leakage flux. No overhang- SQIM 12 11/21/2019 48

49. Rotor bar leakage 11/21/2019 49

50. 11/21/2019 50 Rotor bar leakage Leakage flux exists across the rotor bar circuit. Contributes to decide the starting performance of the machine. Mainly responsible for leakage reactance A.C machines affects steady state and starting performance must be considered at the time of designing the machine.