M GROUP DESIGN OF ELECTRICAL APPARATUS 1. What are the main dimensions of a rotating machine? The main dimensions of a rotating machine are armature diameter and stator core length. 2. Define specific magnetic loading. It is defined as the average flux density over the air gap of a machine. Specific magnetic loading=total flux around the air gap/area of flux path at the air gap 3. Define specific electric loading. It is defined as the number of armature conductors per meter of armature periphery at the air gap. Specific electric loading=total number ampere conductors/armature periphery at air gap. 4. What is output equation? The equation describing the relation between the output and main dimensions, specific loadings and speed of a machine is known as output equation. 5. Write the expressions for output coefficients For dc machine, output coefficient.C0=П2Bav acx10-3, in KW/m3-rps For ac machine, output coefficient.C0=П2Bav ac Kwsx10-3, in KVA/m3-rps 6. List the factors that influence the separation of D and L of a dc machine. 1. Pole proportions 2.moment of inertia 3. Peripheral speed 4. Voltage between adjacent segments. 7. What is square pole criterion? The square pole criterion states that for a given flux and cross section area of pole, the length of mean turn of field winding is minimum, when the periphery forms a square. This implies that the length must be approximately equal to pole arc. 8. In a dc machine what are the limiting values of armature peripheral speed and voltage between adjacent commutator segments Maximum armature peripheral speed=Vamax=30m/sec Maximum voltage between commutator segments Ecm=30Volts
9.List the various L/٦ used for separation of D and L in induction motor.
For minimum cost, 1.5 to 2 For good power factor, 1.0 to 1.25 For good efficiency, 1.5 For overall length, 1.0 10. What are the factors that affect the size of rotating machines? The factors affecting the size of the rotating machines are speed, specific electric and magnetic loadings. 11. What are the factors that decide the choice of specific magnetic loading? Maximum flux density in iron parts of machine Magnetizing current Core losses 12. .What is the factors that decide the choice of specific electric loading. Permissible temperature rise Voltage rating of machine Size of machine Current density. 13. What is magnetic circuit. It is the path of magnetic flux. The mmf of the circuit creates flux in the path by overcoming the reluctance of the path. The equation which relates flux, mmf and reluctance are given by Flux=mmf/reluctance 14. What are the constituents of magnetic circuit in rotating machines. The magnetic circuit of rotating machine consists of airgap,stator and rotor cores, stator and rotor teeth. 15. What is magnetization curve? It is a graph showing the relation between the magnetic field intensity and the flux density, B of a magnetic material ,it is used to estimate the mmf required for flux path in the magnetic material and it is supplied by the manufacturers of stampings of laminations. 16. What is leakage flux? The leakage flux is the flux passing through unwanted path. The leakage flux will not help either for transfer or conversion of energy. 17. What is leakage coefficient? It is defined as the ratio of total flux to useful flux. Leakage coefficient,Cl=total flux/useful flux 18. What is fringing flux.
The bulging of magnetic path at the air gap is called fringing. The fluxes in the bulged portion are called fringing effect. 19. What are the differences between leakage flux and fringing flux? The leakage flux is not useful for energy transfer. But the fringing flux is useful flux. The leakage flux flows in unwanted path but fringing flux flows in magnetic path. 20. what are the factors that modify the reluctance of air gap It is modified by slots, radial ventilating ducts and non uniform air gaps. 21. Define gap contraction factor for slots. It is defined as the ratio of reluctance of air gap of slotted armature to reluctance of air gap of smooth armature. Kgs=reluctance of air gap with slotted armature/ reluctance of air gap with smooth armature 22. Define gap contraction factor for ducts. It is defined as the ratio of reluctance of air gap of with ducts to reluctance of air gap without ducts. Kgs=reluctance of air gap with ducts / reluctance of air gap without ducts. 23. Define total gap contraction factor. It is defined as the ratio of reluctance of slotted armature with ducts and the reluctance of smooth armature without ducts. 24. Define field form factor. It is defined as the ratio of average gap density over the pole pitch to maximum flux density in the air gap. 25. What are the problems encountered in estimating the mmf for teeth. The flux density in different section of a tooth is non uniform. The slot provides another parallel path for the flux, shunting the tooth. 26. List the methods used for estimating the mmf for teeth. Graphical method Three ordinate method Bt1/3 method. 27. What is real and apparent flux density? The real flux density is due to the actual flux through a tooth. The apparent flux density is due to total flux that has to be pass through the tooth. Since some of the fluxes passes through slot the real flux density is always less than the apparent flux density.
28. Define specific permeance.
It is defined as the permeance per unit length of field. 29. What is unbalanced magnetic pull. It is the radial force acting on the rotor doe to non uniform air gap around armature periphery. 30. List the various techniques employed to over come unbalanced magnetic pull. The length of the rotor is small and diameter is made higher. Ball bearings are employed and rotor is dynamically balanced. The combinations of rotor and stator slots which produce vibrations are avoided. 31. What factor decides the number of turns in a winding? The number of turns in a winding is decided by the emf/turn and flux density. The emf/turn depends on the type of insulation employed. 32. How the area of cross-section of a conductor is estimated. The area of cross-section of a conductor is estimated based on temperature rise,resistivity and cooling methods. Assume a current density based on the above factors. For copper conductors the normal range of current density is 2.5to5A/mm2. Area of cross-section of a conductor=current per conductor/current density. 33. What is the range of specific magnetic loading in dc machine? The range of specific magnetic loading in dc machine is 0.4 to 0.8wb/m2. 34. What is the range of specific electric loading in dc machine? The range of specific electric loading in dc machine is 15000to50000amp.cond/m. 35. What are the factors to be considered for the selection of number of poles in a dc machine? The factors to be considered for the selection of number of poles in a dc machine are frequency of flux reversals, current/brush arm and armature mmf per pole. 36. What are the parameters that are affected by the number of poles? Weight of iron and copper, length of commutator and dimension of brushes are the parameters affected by number of poles. 37. List the advantages and disadvantages of large number of poles. Advantages. Weight of armature core and yoke. Cost of armature and field conductors. Overall length and diameter of machine. Length of commutator. Distortion of field form under load condition. Disadvantages: Frequency of flux reversals.
Labour charges. Possibility of flash over between brush arms. 38. Why square pole is preferred. If the cross section of the pole body is square then the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross section is preferred for the poles of dc machine. 39.What factors decides the minimum number of armature coils. The maximum voltage between adjacent commutator segments decides the minimum number of coils. 40. State the difference between the armature winding of dc machine and the stator winding of ac machine. The armature winding of dc machine has closed coils but the stator winding of ac machine has open coils 41. Define winding pitch. It is defined as the distance between the starts of two consecutive coils measured in terms of coil sides. 42. What is back pitch? The distance between top and bottom coil sides of a measured around back of the armature is called back pitch. 43. What is front pitch? It is the distance between two coil sides connected to the same commutator segment. 44. Define commutator pitch. It is defined as the distance between the two commutator segments to which the two ends of a coil are connected. 45. What is equalizer connection? The equalizer connections are low resistance copper conductors employed in lap winding to equalize the induced emfs in parallel paths. The difference in induced emf in parallel paths is due to slight unsymmetrical in flux per pole in dc machines. 46. What are the factors to be considered for estimating the length of air gap in dc machines? The factors to be considered for estimating the length of air gap are armature reaction, cooling, iron losses, distortion of field form and noise. 47. What are the effects of armature reaction? The various effects of armature reaction are reduction in induced emf, increase in iron losses, delayed commutation, sparking and ring firing. 48. How the polarities of inter pole are decided.
The polarity of the interpole must be that of the main pole just ahead for the generator and just behind for a motor. 49. What is the effect of interpole on main pole? In case of generator the interpole will magnetize the leading edge and demagnetizing the trailing edge of main pole. In case of motor the interpoles will demagnetize the leading edge and magnetize the trailing edge main pole. 50.What are standard periods for short time rating of a machine. The standard periods for short time ratings are 10, 30,60 and 90mts. 51. Why silicon content in electrical sheet steel is limited to 4-5%. Silicon content in sheet steel exceeds 5%acts brittle and creates difficulty in punching. 52. What is the fundamental requirement of a good insulating material? High dielectric strength, high insulating resistance with low dielectric loss, good mechanical strength, good thermal conductivity, high degree of thermal stability. 53. What is the importance of temperature as a factor in the life of insulating materials? The life of an insulating material is closely related with the maximum temperature rise to which it is subjected. The life is halved or doubled for each 8oC rise or fall in temperature from its specified permissible temperature. 54. Why mineral insulating oils are the most widely used liquid insulation. Because of its easy availability, cheap and good electrical properties as well as heat transfer capability. 55. Why large size machines have large rating time constant. As the size of the machine increases the volume and hence weight increases in proportion to third power of linear dimensions and surface area in proportion to second power giving large heating time constant for large size machines. 56. Why the real flux density is always less than the apparent flux density with reference to armature teeth of a dc machine. It is because the actual flux passing through the tooth is always less than the total or apparent flux making the real flux density less than apparent flux density. 57. Why are ac armature winding is always made short-pitched. It helps in reducing the effect of harmonics in the voltage wave and also gives a saving in the amount of copper in the overhang.
58. Why equalizer connections are necessary for the armature winding of a dc machine with lap winding. The emfs developed in the various parallel paths of lap winding are not equal giving rise to circulating current and increased heating. the equalizer connections help in by passing the circulating current through the winding and avoid over heating. 58. Why the voltage wave form of an ac armature winding contains harmonics. It is because of the field flux and variation in the reluctance of air gap due to slotting in the stator and rotor core. 59. Why the conductor eddy current loss increases of embedded deeper in the slot. The conductor faces increased amount of leakage flux if embedded deeper in the slot, thereby increasing the conductor eddy loss. 60. Why short time rating of an electrical machine is much higher than the continuous rating. To reach the maximum permissible temperature rise in a short duration, the machine can be loaded to higher than the continuous rating. 61. What is the cause of noise in transformer? The cause of noise in the transformer is mainly because of magneto-striction effect and also loosening of stampings and mechanical forces produced during working. 62. Why is the area of yoke of a transformer usually kept 15-20% more than that of core. By keeping yoke area 15-20% higher the flux density in the yoke is reduced, resulting into reduction in iron losses for yoke. higher 63. Why does induction motor designed with specific electric loadings have smaller over load capacity. A machine having higher value of specific electric loading results in large number of turns/phase , hence high leakage reactance resulting in to a circle diagram of small diameter, hence lesser over load capacity. 64.Why is the harmonic leakage flux in squirrel cage induction ,rotor is not present. In squirrel cage induction motor, the current in the rotor balances the stator current at every point; hence there is no harmonic leakage flux. 65. Why the length of air gap in induction motor is kept minimum possible. The air gap of a poly phase motor is kept small to reduce the possibility of crawling, reduce the noise, reduce the magnetizing current, to obtain high starting torque.
66. Why do die cast rotors is extensively used in making 3 phase cage induction motor. Because it gives quiet operation it is economical and ease in mass production, it improves the starting torque, it avoids the problem of harmonics. 67. Why do 3 phase squirrel cage induction motor finds wide application in industry. Because the robust construction and vary little maintenance, its high starting torque, its high power factor, its wide range of speed control. 68.What is hot spot temperature. Hot spot temperature is the temperature of that part in which heat generated is having highest value. 69. Mention the magnetic material used for the magnetic frames. Hot rolled silicon steel, cold rolled grain oriented silicon steel 70. What are the important properties of transformer steel. High permeability, high resistivity, low coercive force. 71.Why stepped core are generally used for transformer. LV and HV coils are circular ,for better utilization of apace, for reducing the mean length of LV and HV turns, resulting in saving of copper material. 72. What are the advantages of using higher flux density in the core? Reduction in core and yoke section for same output. reduction in mean length of LV and HV turns, resulting in saving of copper material, reduced over all size and weight of transformer. 73. List various disadvantages of using higher flux density in design of core. Increased magnetizing current and iron losses, saturation of magnetic material, lower efficiency, because of higher no load losses, higher temperature rise of transformer. 74. Why the cross section of yoke is taken greater than core section. In order to reduce flux density in the yoke, there by reducing iron losses and no load current in the yoke section. 75. What are the types of windings, commonly used for lv winding. Cylindrical winding with rectangular conductors and helical winding. 76. What are the draw backs of sand width winding? Requires more lab our in its maintenance, more difficult to insulate different coils from each other and from yoke. 77. Name a few insulating materials that are used in transformers. Press board, cable paper, varnished silk, transformer oil, porcelain, insulating varnish. 78. How iron losses occurring in transformers are minimized.
By laminating the magnetic cores and yokes, by using lamination of low thickness, 0.35mm, by using good quality steal like cold rolled grain oriented silicon steel. 79. Mention clearly the condition for maximum efficiency. Efficiency is maximum at a load, at which copper losses are equal to iron losses. 80. Why the efficiency of a transformer is so high. I. Mechanical losses are zero, which forms great part of total losses. ii. Iron losses are comparatively less, because of better magnetic material used for magnetic frame. 81. Mention the main function of cooling medium used in transformers. i. To transfer the heat by convection from the heated surface to the tank surface. ii. To create good level of insulation between various conducting parts. 82. How much heat is dissipated by radiation and convection. 6w/m2/oC by radiation and 6.5w/m2/oC by convection. 83. Why salient pole construction is rejected for high speed alternators. i. The rotating part are subjected to very high mechanical stress. ii. Excessive wind age loss iii. The machine would be very noisy. 84. What material user for the construction of turbo alternator. Chromium nickel steel is generally used for the rotor of turbo alternator. 85. What is run away speed, in case of hydraulic turbine used as prime movers for hydro generators. The speed attained by the hydraulic turbine under suddenly unloaded conditions is called run away speed. 86. What is approximately the run away speed of Kaplan turbine. The run away speed of Kaplan turbine is around 2.4 to 2.7 times the rated speed. 87. Write the expression for the output coefficient of synchrouns machine. Output coefficient,K=11Bavqkw10-3. Where Bav=average flux density in the air gap,Tesla q= specific electric loading, amp-cond/m kw=stator winding factor. 88. What are the advantages of designing the alternators with higher flux density? I. Reduced size of the machine ii. Higher stability limit. iii. Satisfactory parallel operation. iv. Reduced cost of machine.
89. What are the disadvantages of designing the alternators with higher gap flux density? i. Higher iron losses. ii. Increased transient short circuit current. iii. decreased efficiency. iv. Higher temperature rise. 90. What are the various types of 3phase induction motor. i. squirrel cage induction motor. ii. Slip ring induction motor. 91. Why semi- closed slots are generally preferred for the stator of induction motors. Use, of semi-closed slots reduces the magnetizing current drawn by the motor; as a result power factor is improved. Moreover, noise is also reduced by using semi closed slots. 92. What is the effect of specific magnetic loading on the size of the machine? Size of the machines is reduced with increase in specific magnetic loading. 93. What is the effect of specific electric loading on the copper losses. Copper losses increases with increase in specific electric loading. 94. Write down the main consideration in the selection of specific loadings for the design of induction motor. Main considerations are,i.size of the machine .ii.cost of the machine.iii.power factor. iv.efficiency. v. temperature rise. vi. Noise and over load capacity. 95. What is critical speed of alternator? A rotor is a structure with mass and velocity, and so has a natural frequency of vibration. The speed at which this takes place is known as critical speed of turbo-alternator. 96. What are the functions of damper winding. Damper windings are provided to damp out the oscillations. These also help in starting as they form a cage winding. 97. Why SCR is one of the most important parameters in synchronous machine design. SCR value affects the machine performance like stability, voltage regulation etc. Hence it is important. 98. What is the effect of SCR on synchronous machine performance? SCR affects the vital performance like stability, voltage regulation, short circuit current and parallel operation. 99. Why copper is the most widely used conducting material in making an electrical machine? Copper offers excellent electrical and mechanical properties so most widely used conducting material. 100. Why it is necessary to cool an electrical machine. The energy losses in the machine generate heat and temperature rise has to keep within permissible limit to avoid insulation deterioration. Thus it is necessary to cool an electrical machine.
16 MARKS QUESTIONS 1. Explain how to select the number of poles for a DC machine. 2. Discuss the requirements of high conductivity materials. 3. Write notes on temperature gradient in conductors placed in slots, with help of equations. 4. Explain the heat flow in two dimensions. 5. Writes notes on classification of insulating materials. 6. Derive the voltage per turn equation for a single phase transformer. 7. Derive an expression to find the specific slot permeance of s fully opened rectangular slot. 8. What are the various types of synchronous machines based on rotor construction? Bring out the constructional differences between them. 9. Discuss the effects of short circuit ratio on the performance of a synchronous machine. 10. Explain the concept of determining the temperature gradients in conductors placed in slots. 11. What are the limitations of design of electrical apparatus? Explain them. 12. Explain the various factors that affected by the selection of number of poles in DC machines. 13. Explain the design of rotor bars and slots. 14. Explain the choice of specific magnetic and electric loadings of synchronous machines. 15. Explain the design of induction motors using circle diagram. 16.i Discuss the factors that influence the choice of number of poles of a dc machine ii.Design a shunt field coil of a dc motor from the following data. Field ampere turns/pole=9000 Mean length of turn=1.4m Depth of coil=35*10-3m Voltage across field coil=40V Receptivity of wire=0.021 ohm/m and mm2. Thickness of insulating varnish=0.2mm Power dissipation from total surface of the coil <700w/m2 Check your design for power dissipation 17. i.. How do you estimate the ampere turns required for an interpole with compensating winding ii.A 4 poles 25hp, 500V, 600rpm series motor have an efficiency of 82%. The pole faces are square and ratio of pole arc to pole pitch is 0.67. Take Bav=0.55wb/m2 and ampere conductors are 17000 amp.cond/m. obtain the main dimensions of the core and particulars of a suitable armature winding.
18.i.. Derive the relation ship between real and apparent flux densities ii. Determine the mmf required for the air gap of a dc machine having open slots, given the following particulars. Slot pitch=4.3cm; gross core length=48cm; air gap length=0.6cm; slot opening=2.1cm; pole arc=18cm; flux per pole=0.056wb. There are 8 ventilating ducts each 1.2cm wide. The data given below is applicable for slots and ducts. Ratio slot opening/gap length Carter’s coefficient
1
2
3
3.5
4.0
0.15
0.28
0.37
0.41
0.43
19..i.. Derive the expression for temperature rise –time curve for an electrical machine ii. A single phase transformer is on full load for 1 ½ hrs, no load for 1 hour and 25% over load for 1 hour. Calculate the temperature rise at the end of the period if the temperature rise s of 20oC and 35oC occur at 1 hour and 2 hours respectively on full load. The temperature starts from cold in both the cases. Take full load copper loss as 2.5 times the core loss. 20.i... For a transformer show that the emf per turn Et=K√KVA. Where KVA= rating of transformer What are the factors on which the value of K depends on ii. Determine the dimensions of the core, the number of turns and the core sectional area of conductors in the primary and secondary windings of a 100 KVA, 2200/480V, single phase core type transformer to operate a frequency of 50 Hz, approximate voltage per turn=7.5V, maximum flux density=1.2wb/m2,ratio of effective cross-sectional area of core is square of diameter of circumscribing circle=0.6, ratio of height to width of window=2, window space factor 0.28, current density 2.5A/mm2 21.i.. Obtain the expression for leakage reactance of a single phase core type transformer ii. Calculate the no load current of a 400V,50Hz single phase core type transformer , the of which are follow as; Length of mean magnetic path =200cm Gross core section=100cm2, joints equivalent to 0.1mmair gap. Maximum flux density 0.7 tesla, specific core loss at 50 Hz and 0.7 tesla, stacking factor 0.9, density of core material 7.5*103Kg/m3
22i.What are the factors to be considered in the selection of magnetic and electric loading in a 3 ph induction motor. ii.Estimate the main dimensions, air gap length, No. of stator slots, stator turns/phase and cross sectional area of stator conductors for a 3 phase 20 hp, 400V,6 pole, 50 Hz,970 rpm induction motor suitable for a star delta starting. Assume magnetic and electric loading as 0.45 wb/m2and 23000 ac/m respectively, ratio of core length to pole pitch 0.85, full load efficiency 0.88 and power factor 0.89. 23. i.Explain the dispersion coefficient and how does it affect the performance of induction motor. ii. Calculate the equivalent resistance of rotor per phase referred to stator for the following data of a 400V,3 phase, 4 pole, 50 Hz cage motor. Stator slots 48, conductors per slot 30, rotor slots 53, one bar per rotor slot, length of each bar 12 cm, area of bar 0.6 cm2, mean diameter of end rings 18cm, area of ring 1.5cm2. full pitch winding with 600 phase spread for stator. Specific resistance is 0.021Ω/m length per mm2 area. 24. i.Prove that with usual notations, that the KVA rating of a synchronous generator is given by the following relation. KVA=1.11 Kw Bav ac L Va2*10-3. Where Va= peripheral speed in m/sec ii. Design suitable values of diameter and length of a 75 MVA, 11KV, 50 Hz,3000 rpm 3 phase star connected alternator. Also determine the value of flux, conductor/slot, number of turns/phase and size of alternator conductor. Given: Average gap density=0.6 Tesla Ampere cond/m=50000 Peripheral speed=180m/sec Winding factor=0.95 Current density=6A/ 25.i. How do you estimate the length of air gap in a salient pole alternator? What are the advantages of having large air gap OR ii. A 1250 KVA, 3 phase, 6600V salient pole alternator has the following data. Air gap diameter= 1.6m, length of core=0.45m, number of poles=20, armature ampere conductors per meter=28000, ratio of pole pitch=28mm, current density in damper bars 3 A/mm2. Design a suitable damper winding for the machine