36778702-electrical-machines-1.pdf 54326m

N.I COLLEGE OF ENGINEERING KUMARACOIL ELECTRICAL AND ELECTRONICS ENGINEERING THIRD SEMESTER ELECTRICAL MACHINES 1 EE232 1. Mention the difference between core and shell type transformers. In core type , the windings surround the core considerably and in shell type the core surround the winding. 2. What is the purpose of laminating the core in a transformers ? To reduce eddy current loss. 3. Give the emf equation of a transformer and define each term Emf induced in primary coil E1 = 4.44 fΦ mN1 volt Emf induced in secondary coil E2 = 4.44fΦ mN2 volt Where f is the frequency of AC input Φ m is the maximum value of flux in the core N1, N2 are the number of primary and secondary turns. 4. Does the transformer draw any current when secondary is open ? Why ? Yes,it (primary) will draw the current from the main supply in order to magnetise the core and to supply iron and copper losses on no load . There will not be any current in the secondary since secondary is open. 5. Define voltage regulation of a transformer When a transformer is loaded with a constant primary voltage , the secondary voltage decreases for lagging power factor load, and increases for leading pf load because of its internal resistance and leakage reactance . The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation . % regulation down = (0V2-V2) x 100/0V2 % regulation up = (0V2-V2) x 100/V2 6. Full load copper loss in a transformer is 1600 watts. What will be the loss at half load ? If x is the ratio of actual load to full load then copper loss = x2(full load copper loss) Here Wc = (0.5)2 x 1600 = 400 watts 7. Define all day efficiency of a transformer . It is the computed on the basis of energy consumed during a certain period , usually a day of 24 hrs. ηall day = output in kWh /input in kWh for 24 hrs.

8. Why transformers are rated in kVA ? Copper loss of a transformer depends on current and iron loss on voltage . Hence total losses depends on VoltAmpere and not on the power factor. That is why the rating of transformers are in kVA and not in kW. 9. What are the typical uses of auto transformer ? (i)To give small boost to a distribution cable to correct for the voltage drop. (ii)As induction motor starters. (iii)As furnace transformers (iv)As interconnecting transformers (v)In control equipment for single phase and 3 phase elective locomotives. 10. What are the applications of a step-up and step-down transformers ? Step-up transformers are used in generating stations. Normally the generated voltage will be either 11 kV or 22 kV. This voltage is stepped up to 110 kV or 220 kV or 400 kV and transmitted through transmission lines. (In short it may Be called as sending end). Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kV or 22 kV and transmitted through feeders.(In short it may be called as receiving end). Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations and receiving stations are called power transformers. 11. How transformers are classified according to their construction ? Or Mention the difference between “CORE” and “SHELL” type transformers. Or What are the two types of cores used ? Compare them. Transformers are classified according to their construction as , (i)Core type (ii)Shell type (iii)Spirakore type. Spirakore type is a latest transformer and is used in big transformers. In “core” type, the windings(primary and secondary)surround the core and in “shell” type, the core surround the windings. 12. Explain on the material used for core construction. The core is constructed of transformer sheet steel laminations assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities.the eddy current loss is minimized by laminating

the core, the laminations being insulated from each other by light coat of core-plate vanish or by an oxide layer on the surface .the thickness of laminations varies from 0.35 mm for a frequency of 59 Hz and 0.5 mm for a frequency of 25 Hz. 13. When will a Bucholz relay operate in a transformer ? Bucholz rely is a protective device in a transformer. If the temperature of the coil exceeds its limit, Bucholz relay operates and gives an alarm. 14. How does change in frequency affect the operation of a given transformer ? With a change in frequency, iron loss, copper loss, regulation, efficiency and heating varies and thereby the Operation of the transformer is affected. 15. What is the angle by which no-load current will lag the ideal applied voltage ? In an ideal transformer , there are no copper loss and no core loss,(i.e. loss free core). The no load current is only magnetizing current. Therefore the no-load current lags behind by an angke of 90˚. However the windings possess resistance and leakage reactance and therefore the no-load current lags the applied voltage slightly less than 90˚. 16. List the advantages of stepped core arrangement in a transformer . (i) To reduce the space effectively. (ii) To obtain reduced length of mean turn of the windings. (iii) To reduce I2R loss. 17. Why are breathers used in transformers ? Breathers are used to entrap the atmospheric moisture and thereby not allowing it to on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases. Also to avoid sledging of oil i.e. decomposition of oil. Addition of 8 parts of water in 1000000 reduces the insulations quantity of oil. Normally silica gel is filled in the breather having pink colour. This colour will be changed to white due to continuous use, which is an indication of bad silica gel, it is normally heated and reused. 18. What is the function of transformer oil in a transformer ? Nowadays instead of natural mineral oil, synthetic oils known as ASKRELS (trade name ) are used. They are noninflammable, under an electric arc do not decompose to produce inflammable gases. PYROCOLOR oil possess high dielectric strength. Hence it can be said that transformer oil provides , (i)good insulation and (ii)cooling .

19. A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary. We know V1 / V2 = k = N2 / N1 Substituting 400/1100 = 100/N1 N1 = 100/400 x 1100 = 275 turns. 20. What are the functions of no-load current in a transformer ? No-load current produces flux and supplies iron loss and copper loss on no-load. 21. How will you transfer the quantities from one circuit to another circuit in a transformer ? 1.Secondary to primary 2.Primary to secondary Symbol Value Symbol Value V2 ̉ V2/k VL ̉ kV1 I 2̉ kI2 IL ̉ I1 /k R2 ̉ R2/k2 RL ̉ k2R1 2 X2 ̉ X2/k XL’ k2X1 ZL ̉ ZL/k2 22. Define voltage regulation of a transformer. When a transformer is loaded with a constant primary voltage , the secondary voltage decreases for lagging factor load, and increases for leading power factor load because of its internal resistance and leakage reactance. Let oV2 = secondary terminal voltage at no load = E2 = kE = kV because of no load the impedence drop is negligible V2 = secondary terminal voltage on full load. The change in secondary terminal voltage from no load to full load is = oV 2 – V2.This change divided by oV2 is known regulation down. If this chane is divided by V(i.e full load secondary terminal voltage ) then it is called as regulation up. %regulation down =[(oV2 – V2)/oV2 ]*100 and %regulation up =[(oV2 – V2)/V2]*100 23. Can the voltage regulation of a transformer go to negative? If so under what condition? Yes. If the load has leading power factor. 24. Distinguish between power transformer and distribution transformer . Power transformers have very high power ratings in the order of mVa. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in

consumer side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed. 25. What is the purpose of providing ‘taps’ in transformer and where these are provided? In order to attain the required voltage , ‘taps’ are provided. Normally it will be provided at low voltage side. 26. What is prime mover? The basic source of mechanical power which drives the armature of the generator is called prime mover. 27. Give the materials used in machine manufacturing Three materials are used in machine manufacturing. (i)steel – to conduct magnetic flux (ii)copper – to conduct electric current (iii)Insulation 28. What is MMF? Give it’s expression MMF is the work done in moving a unit magnetic pole once around the magnetic circuit. MMF=N I ampere turns Where N=no of turns I =current in amp 29. Define magnetic field intensity . It is the MMF per unit length. Ampturn/metre H=N I/l where N=no of turns;I=current in amp;l=length in metre 30. What is reluctance? It is the property of the magnetic material to oppose the magnetic lines of flux S=F/Φ where F=MMF ; Φ=flux ;S=reluctance in amp turns per weber 31. What is stacking factor? It is the ratio between the net cross sectional area of the core to the cross section occupied by the magnetic material 32. What is quasi static field? It is the field pattern which is fixed in space but field intencity at every point varies as a replica of time variation of current. 33. What is leakage flux? The flux which takes a path which is not intended for it is called leakage flux.. 34. What is fringing effect? While ing through the non magnetic medium the magnetic lines of force try to bulge out because the lines of force repel each other.this is fringing effect. 35. Compare electric and magnetic circuits Magnetic circuit Electric circuit 1. Closed path of magnetic flux Closed path for electric current 2. Flux = mmf/reluctance current=emf/resistance 3. Reluctance=1/aμ0 μr ResistanceR=ρl/a 4. Magnetic flux dencity B=φ/a current dencity J=I/a 5. MMF drop=φ S Voltage drop= I R 6. Magnetic field intencity H=N I/l Electric field intencity E=V/d

36. How is the direction of induced e.m.f determined? Direction of induced e.m.f is determined by using (i) right hand gripprng rule and (ii) Right hand cork screw rule. 37. State Lenze’s law. Any induced e.m.f will circulate a current in such a direction as to oppose the cause producing it. e= -N dΦ/dt 38. What is self inductance? The e.m.f induced in a coil due to change of flux in the same coil is known as self inductance. 39. Give the expression for magnetizing force F = B I l sin θ Newton, Where I is the current carried by the conductor l is the effective length of the conductor B is the flux density in Wb/m2 40. Derive an equation for energy density for a magnetic circuit. Energy stored in the magnetic circuit = ½ LI2 L = N2 aμ0 μr / l Energy Density = ½ I2 N2 aμ0 μr / l = ½ B2 H2 aμ0 μr since H =NI/l 2 = ½ B la / μ0 μr = ½ B2a2 /S = ½ Φ2 /R Joules 41. What are the factors on which hysteresis loss depends ? The hysteresis loss depends on the magnetic flux density , frequency f and the volume of the material V. 42. What is core loss? What is its significance in electric machines? When a magnetic material undergoes cyclic magnetization , two kinds of power losses occur on it – hysteresis and eddy current loss which together are known as core loss. It is important in determining heating, temparature rise , rating and efficiency of transformers, machines and other a.c run magnetic devices. 43. What is eddy current loss? When a magnetic core carries a time varying flux voltages are induced in all possible paths enclosing flux. Result is the production of circulating current in core. These induced currents do no useful work are known as eddy current and have power loss known as eddy current loss. 44. How are hysteresis and eddy current losses minimized? Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel alloys with low hysteresis coefficient and electrical resistivity. Eddy current losses are minimized by laminating the core. 45. What is coupling coefficient? k = M/√L1 L2

Coupling coefficient is defined as the ratio of mutual inductance to the square root of the product of two self inductances. 46. Write an expression for energy stored in magnetic field. Wf =B1 ∫ B2 HcdB, Where Hc = Magnetic field intensity in the core. 47. How does electromechanical energy conversion occurs? It occurs through the medium of the magnetic stored energy. 48. Write the equation for energy transfer. Energy input from electric sources = mechanical energy output + increase in energy stored in magnetic field + energy converted into heat. For motors : electric and mechanical energy have positive values For generators: they have negative values. 49. What is the energy conversion medium in a singly-excited magnetic field system ? The magnetic circuits have air gap between stationary and moving in which considerable energy is stored in the magnetic field. This field acts as the energy conversion medium and its energy is the reservoir between electric and mechanical system. 50. What is the basic feature of an electro magnetic energy conversion devices? They contain air gaps in their magnetic circuits in their moving parts. 51. Why does the energy storage in a magnetic material occurs mainly in the air gap? The reluctance of the air gap is much larger than the magetic material . hence the predominant energy storage occurs in the air gap and the properties of the air gap are determined by the dimension of the air gap. 52. What is multiply excited magnetic field system? If the electromechanical devices have more than one set of exciting system it is called multiply excited magnetic field system. 53. Give the relation between energy and co-energy for linear system. They are numerically equal in linear system. 54. Why is a magnetic energy storage system conservative? Since it is loss less. 55. How is voltage generated in rotating machines? In rotating machines voltage is generated in windings or group of coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by deg the magnetic circuit so that the reluctance varies with rotation of the rotor. 56. Why are magnetic coils wound on iron cores? To maximize the coupling between the coils, to increase the magnetic energy density associated with the electromechanical interaction and to shape and distribute the magnetic fields according to the machine design. 57. Why salient pole construction is a characteristic of hydroelectric generators ? Because hydraulic turbines operate at relatively low speeds and a large number of poles are required to produce the

required frequency. 58. What are distributed windings? Windings which are spread over a number of slots around the air gap periphery. 59. Write the equation for total co-energy density. W’fld = Average co-energy density x volume of air gap = μ0 / 4 [Fsr / g]2 Π Dlg = μ0Π Dl / 4 g Where D = Average diameter of air gap. l = Axial length of air gap. μ0= Permeability of free space. 60. What is the expression for torque in of the resultant mmf wave Fsr? T = -P/2 . Π/2. μ0 Dl / g . Fr Fsr sinδ 61. What is torque proportional to? Torque is proportional to the interacting fields and to the sine of the electrical space angle between their magnetic axes. 62. How will you find the direction of emf using Fleming’s Right Hand Rule? The thumb, the forefinger and the middle finger of the right hand are held so that these fingers are mutually perpendicular, then Forefinger Field ThuMb Motion Middle fingerI,current 63. How will you find the direction of force produced using Fleming’s Left Hand Rule? The thumb , forefinger and middle finger of the left hand are held so that those fingers are mutually perpendicular then Forefinger Field ThuMb Motion(due to force) Middle finger I,current 64. Which type of d.c armature winding requires equalizer rings? Lap winding 65. How are armature windings classified based on the placement of the coil inside the armature slots? Single layer winding and Double layer winding 66. Write down the emf equation for d.c generator. E = (фNZ / 60)(P/A) V Where P= number of poles Z= Total number of conductors A= number of parallel paths Ф= flux per pole N= speed in rpm 67. Why the armature core in d.c machines is constructed with laminated steel sheets instead of solid steel steel? Steel sheets offer low relutance path for the magnetic field , laminated sheets reduce eddy current loss. 68. Why is commutator employed in d.c machines? • Conduct electricity between armature and fixed brushes

• Converts altenating emf into unidirectional emf and vice versa 69. What is meant by selective commutation? The use of more than one pair of brushes in wave winding does not divide the armature coil sides into more tha two parallel paths, but current collected from the armature i divided between the brushes of like polarity. In case of slight differences in resistance the current collected by individual brushes may be different and is called selective commutation. 70. Distinguish between shunt and series field coil constructions. Shunt field coils are wound with wires of small cross section and have more number of turns. Series field coils are wound with wires of larger cross section and have less number of turns. 71. How does a d.c motor differ from d.c generator in construction? Generators are normally placed in closed room , accessible only to skilled operators. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors on the other hand , have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemical fumes etc . To protect the motors against these elements , the motor frames are made either partly closed or totally closed or flame proof etc. 72. How will you change the direction of rotation of a d.c motor? Either the direction of the main field or the direction of current through the armature conductors is to be reserved. 73. What is back emf in d.c motors ? As the motor armature rotates , the system of conductor come across alternate North and South pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors . The direction of the emf induced is in the direction opposite to the current .As this emf always opposes the flow of current in motor operation it is called back emf. 74. Under what condition the mechanical power developed in a dc motor will be maximum? Condition for mechanical power developed to be maximum is Eb = Ua /2 or I a= Ua / 2Ra 75. What is the function of a no-voltage release coil provided in a dc motor starter? As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in the ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus a no-voltage or under voltage protections given to the motor. 76. Name the two types of automatic starters used for dc motors. • Back emf type starter • Time delay type starter 77. Enumerate the factors on which the speed of a dc motor depends. N = 1/CE (Ua-IaRm)/ф The speed of dc motor depends on three factors. • Flux in the air gap

• Resistance of the armature circuit • Voltage applied to the armature 78. List the different methods of speed control employed for dc series moto • Field diverter method • Regrouping of field coild\s • Tapped field control • Armature resistance control • Armature voltage control for single motor • Series parallel control for multiple identical motors 79. Draw the N Vs Eb characteristics of a dc motor for two different field currents.

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Ehn 80. Name the different methods of electrical breaking of dc motors. (i) Dynamic braking (ii) Regenerating braking (iii) Counter current braking or plugging 81. Under what circumstances does a dc shunt generator fail to build up? • Absence of residual flux. • Initial flux set up by the field winding may be in opposite direction to residual flux • Shunt filed circuit resistance may be higher than its critical field resistance • Load circuit resistance may be less than its critical load resistance. 82. Define critical field resistance in dc shunt generator Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specified field. 82. How can one differentiate between long shunt compound generator and short shunt compound generator? In a short shunt compound generator the shunt field circuit is shorter i.e. across the armature terminals. In a long shunt compound generator the shunt field circuit is connected across the load terminals. 83. Why is the emf not zero when the field current is reduced to zero in a dc generator ? Even after the field current/magnetizing force is reduced to zero the machine is left out with some flux as residue. Emf due to this residual flux is available when field current is zero. 84. Define the term ‘critical speed’ in dc shunt generator. Critical sped is defined as the speed at which the generator is to be driven to cause selfexcited generator to

Build up its emf for the given field circuit resistance. 85. On what occasions dc generators may not have residual flux? • The generator may be put for its first operation after its construction. • In previous operation the generator would have been fully demagnetized. 86. What are the conditions to be fulfilled for a dc shunt generator to build up emf? • The generator should have residual flux • The field winding should be connected in such a manner that the flux set up by the field winding should be in the same direction as that of residual flux • The field circuit resistance should be less than critical field resistance • Load circuit resistance should be above its critical load resistance 87. How the critical field resistance of a dc shunt generator is estimated from its OCC? Critical field resistance can be obtained from OCC by drawing a straight line ing through the origin and tangent to the initial straight line portion of OCC. The slope of this line gives the value of critical field resistance for the given speed at which OCC is obtained. 88. Define the term armature reaction in dc machines. The interaction between the flux set up by the current carrying armature conductors with the main field flux is defined as armature reaction. 89. What are the two unwanted effects of armature reaction? • Cross magnetizing effect / Distorting effect • Demagnetising effect 90. Differentiate between geometric neutral axis (GNA) and magnetic neutral axis (MNA). GNA is the axis which is situated geometrically or physically in the mid way between adjacent main poles. MNA is the axis which es through the zero crossing of the resultant magnetic field waveform in the air gap. 91. In which part of the dc machine is the compensating winding situated? In the slots provided in the main pole phases . 92. What are the various types of commutation? • Linear commutation • Sinusoidal commutation 93. Name the two methods of improving commutation. (i) Emf commutation. (ii) Resistance commutation. 94. What is reactance emf in dc machine? The self induced emf in the coil undergoing commutation which opposes the reversal of current is known as reactance emf. 95. Define the term commutation in dc machines. The changes that take place in winding elements during the period of short circuit by a brush is called commutation. 96. How and why the compensating winding in dc machine excited? As the compensation required is proportional to the armature current the compensating winding is excited by the armature current.

97. How is the interpole winding in dc machine excited? Interpole winding is connected in series with the armature circuit and is excited by the armature current. 98. To what polarity are the interpoles excited in dc generators? The polarity of the interpoles must be that of the next main pole along the direction of rotation in the case of generator. 99. To what polarity the interpoles excited in dc motors? For motor operation the polarity of the interpoles must be that of the previous main pole along the direction of Rotation. 100. Why are carbon brushes preferred for dc machines? The high resistance carbon brushes help the current in the coil undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.