EE 2251 : ELECTRICAL MACHINES - I
INDEX EE 2251 : ELECTRICAL MACHINES – I
S.No
UNIT
Page Number From - To
Two marks Q&A
2.3
2.11
1
I
2.12
2.15
2
II
2.15
2.19
3
III
2.120
2.21
4
IV
2.21
2.22
5
V
2.23
2.26
2.26
2.31
Anna University Question Papers
2. 1
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
JEPIAAR ENGINEERING COLLEGE Jeppiaar Nagar, Rajiv Gandhi Salai, Chennai - 600 119. DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
EE 2251 : ELECTRICAL MACHINES – I 1. INTRODUCTION 6 Electrical machine types – Magnetic circuits – Inductance – Statically and Dynamically induced EMF - Torque – Hysteresis- Core losses - AC operation of magnetic circuits. 2. TRANSFORMERS 10 Construction – principle of operation – equivalent circuit – losses – testing – efficiency and voltage regulation – auto transformer – three phase connections – parallel operation of transformers – tap changing. 3. ELECTROMECHANICAL ENERGY CONVERSION 9 Energy in magnetic systems – field energy, coenergy and mechanical force – singly and multiply excited systems. 4. BASIC CONCEPTS IN ROTATING MACHINES 9 Generated voltages in ac and dc machines, mmf of distributed windings – magnetic fields in rotating machines – rotating mmf waves – torque in ac and dc machines. 5. DC MACHINES 11 Construction – EMF and torque – circuit model – armature reaction – commutation – methods of excitation – characteristics of generators – characteristics of motors – starting and speed control – testing and efficiency – parallel operation. TEXT BOOK 1. Nagrath I. J and Kothari D. P. ‘Electric Machines’, Tata McGraw Hill Publishing Company Ltd, 1990. 2. P.S. Bimbhra, ‘Electrical Machinery’, Khanna Publishers, 2003. REFERENCES 1. Fitzgerald.A.E., Charles Kingsely Jr, Stephen D.Umans, ‘Electric Machinery’, McGraw Hill Books Company, 1992. 2. P. C. Sen., ‘Principles of Electrical Machines and Power Electronics’, John Wiley&Sons, 1997. 3. K. Murugesh Kumar, ‘Electric Machines’, Vikas publishing house Pvt Ltd, 2002. 2. 2
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
Two marks 1. Mention the difference between core and shell type transformers? In core type, the windings surrounded the core considerably and in shell type the core surround the windings i.e winding is placed inside the core. 2. What is the purpose of laminating the core in a transformer? To reduce the eddy current loss in the core of the transformer. 3. Give the emf equation of a transformer and define each term? Emf induced in primary coil E1= 4.44fФmN1 volt emf induced in secondary coil E2 =4.44 fФmN2. f--->freq of AC input Фm---->maximum value of flux in the core N1,N2--->Number of primary & secondary turns. 4. Does transformer draw any current when secondary is open? Why? yes, it(primary) will draw the current from the main supply in order to magnetize the core and to supply for 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 PF 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=(V2noload-V2F.L)*100/V2noload %regulationup=(V2noload-V2F.L)*100/V2F.L 6. Full load copper loss in a transformer is 1600W. what will be the loss at half load? If x is the ratio of actual load to full load then copper loss = x2(F.L copper loss) here Wc = (0.5)2 – 1600=400W. 7. Define all day efficiency of a transformer? It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. all day efficiency=output in kWh/input in kWh tor 24 hrs. 8. Why transformers are rated in kVA?
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
Copper loss of a transformer depends on current & iron loss on voltage. Hence total losses depends on Volt-Ampere and not on PF. That is why the rating of transformers are in kVA and not in kW. 9. What are the typical uses of auto transformer? 1. To give small boost to a distribution cable to correct for the voltage drop. 2. As induction motor starter. 10. What are the application of step-up & step-down transformer? Step-up transformers are used in generating stations. Normally the generated voltage will be either 11kV. This voltage(11kV) is stepped up to 110kV or 220kV or 400kV and transmitted through transmission lines(simply called as sending end voltage). Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV are stepped down to 3phase 400V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations are called Power transformers. 11. How transformers are classified according to their construction? 1. core type 2.shell type. In core type, the winding (primary and secondary) surround the core and in shell type, the core surround the winding. 12. Explain on the material used for core construction? The core is constructed by sheet steel laminations assembled to provide a continuous magnetic path with 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 used from each other by light coat of core-plate vanish or by oxide layer on the surface. The thickness of lamination varies from 0.35mm for a frequency of 50Hz and 0.5mm for a frequency of 25Hz. When will a Bucholtz relay operates and give an alarm. 13. How does change in frequency affect the operation of a given transformer?
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
With a change in frequency, iron and copper loss, regulation, efficiency & heating varies so the operation of transformer is highly affected. 14. What is the angle by which no-load current will lag the ideal applied voltage? In an ideal transformer, there are no copper & core loss i.e loss free core. The no load current is only magnetizing current therefore the no load current lags behind by angle 900. However the winding possess resistance and leakage reactance and therefore the no load current lags the applied voltage slightly less than 900. 15. List the arrangement of stepped core arrangement in a transformer? 1. To reduce the space effectively 2. To obtain reduce length of mean turn of the winding 3. To reduce I2R loss. 16. Why are breathers used in transformers? Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases. 17. What is the function of transformer oil in a transformer? 1. It provides good insulation 2. Cooling. 18. Can the voltage regulation go –ive? If so under what condition? Yes, if the load has leading PF. 19. Distinguish power transformers & distribution transformers? Power transformers have very high rating 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 receiving side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed. 20. 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 at high voltages side(low current). 21. What is prime mover? The basic source of mechanical power which drives the armature of the generator is called prime mover. 2. 5
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
22. Give the materials used in machine manufacturing? There are three main materials used in m/c manufacturing they are steel-----to conduct magnetic flux copper----to conduct electric current insulation. 23. What are factors on which hysteresis loss? It depend on magnetic flux density, frequency & volume of the material. 24. 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 losses are called as core loss. It is important in determining heating, temperature rise, rating & efficiency of transformers, machines & other A.C run magnetic devices. 25. What is eddy current loss? When a magnetic core carries a time varying flux, voltages are induced in all possible path enclosing flux. Resulting is the production of circulating flux in core. These circulating current do no useful work are known as eddy current and have power loss known as eddy current loss. 26. How hysteresis and eddy current losses are are 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. 27. How will you find the direction of emf using Fleming’s right hand rule? The thumb, forefinger & middle finger of right hand are held so that these fingers are mutually perpendicular to each other, then forefinger-field thumb-motion middlecurrent. 28. How will you find the direction of force produced using Fleming’s left hand rule? The thumb, forefinger & middle finger of left hand are held so that these fingers are mutually perpendicular to each other, then forefinger-field thumb-motion middle-current. 29. How are armature windings are classified based on placement of coil inside the armature slots? Single and double layer winding. 30. Write down the emf equation for d.c.generator? 2. 6
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
E=(ФNZ/60)(P/A)V. p--->noof poles Z--->Total noof conductor Ф--->flux per pole ,N--->speed in rpm. 31. Why the armature core in d.c machines is constructed with laminated steel sheets instead of solid steel sheets? Lamination highly reduces the eddy current loss and steel sheets provide low reluctance path to magnetic field. 32. Why commutator is employed in d.c.machines? Conduct electricity between rotating armature and fixed brushes, convert alternating emf into unidirectional emf(mechanical rectifier). 33. Distinguish between shunt and series field coil construction? Shunt field coils are wound with wires of small section and have more noof turns. Series field coils are wound with wires of larger cross section and have less noof turns. 34. How does d.c. motor differ from d.c. generator in construction? Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemicals….etc. to protect the motors against these elements, the motor frames are made either partially closed or totally closed or flame proof. 35. How will you change the direction of rotation of d.c.motor? Either the field direction or direction of current through armature conductor is reversed. 36. What is back emf in d.c. motor? 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 conductor is in opposite to current. As this emf always opposes the flow of current in motor operation it is called as back emf. 37. What is the function of no-voltage release coil in d.c. 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 ON position against spring force. When the supply voltage fails or becomes lower than 2. 7
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
a prescribed value then electromagnet may not have enough force to retain so handle will come back to OFF position due to spring force automatically. 38. Enumerate the factors on which speed of a d.c.motor depends? N= (V-IaRa)/Ф so speed depends on air gap flux, resistance of armature, voltage applied to armature. 39. Under What circumstances does a dc shunt generator fails to generate? Absence of residual flux, initial flux setup by field may be opposite in direction to residual flux, shunt field circuit resistance may be higher than its critical field resistance, load circuit resistance may be less than its critical load resistance. 40. Define critical field resistance of 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. 41. Why is the emf not zero when the field current is reduced to zero in dc generator? Even after the field current is reduced to zero, the machine is left out with some flux as residue so emf is available due to residual flux. 42. On what occasion dc generator may not have residual flux? The generator may be put for its operation after its construction, in previous operation, the generator would have been fully demagnetized. 43. What are the conditions to be fulfilled by for a dc shunt generator to build back emf? The generator should have residual flux, the field winding should be connected in such a manner that the flux setup by field in same direction as residual flux, the field resistance should be less than critical field resistance, load circuit resistance should be above critical resistance. 44. Define armature reaction in dc machines? The interaction between the main flux and armature flux cause disturbance called as armature reaction. 45. What are two unwanted effects of armature reactions? Cross magnetizing effect & demagnetizing effect. 46. What is the function of carbon brush used in dc generators? 2. 8
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
The function of the carbon brush is to collect current from commutator and supply to external load circuit and to load. 47. Name the main parts of a dc machine? Mention the material used for its construction. Yoke – cast iron or cast steel Armature- laminated silicon steel Commutator- copper Armature winding – high grade conductor Brushes – carbon Bearings 48. State the methods of speed control D.C. motor? Armature (voltage) control Field (flux) control ward Leonard method of control 49. What is the purpose of brushes in a dc m/c? In the case of generator brushes are used to collect the current from the commutator in the case of motor they are used to give the supply to the armature. 50. Write short notes on the two types of connections of armature winding of in dc machine? There are two types of connection in the armature winding Lap winding : In this case, if the connection is started from the conductor in slot no1 then the connections overlap each other as winding proceeds. This is continued till the starting point is reached. Thus it has the parallel paths as many as the no of poles. Hence it is suitable for higher current rating. Wave winding : In this case the winding always travels ahead and hence there is no overlapping. Thus it has two no of parallel paths always. Hence it is suitable for higher voltage rating machines. 51. What are the two types of connections in dc compound m/c? According to the connections of field windings Long shunt Short shunt According to the directions of the field fluxes Differentially compounded Cumulatively compounded 52. What is meant by back emf?
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
As soon as the armature starts rotating, dynamically induced EMF is produced in the armature conductors. The direction of this induced EMF is found to be opposite to the applied voltage. Hence it is called Back EMF or counter EMF. It provides necessary means for the energy conversion. 53. Write the torque equation of a dc motor, defining each term involved? Torque developed on the armature Ta=0.159ZIaP/A Where - Flux per pole Z- No of conductors Ia – Current in armature winding P – No of poles A – No of parallel paths 54. State the effect of armature reaction in a dc motor. Since it affects the distribution of magnetic flux under the poles, the MNA axis shifts to a new position which in turn affect the current distribution in the armature winding. Since the brush axis is stationary the sparking is produced. It also reduces the net magnetic field strength of field poles which affects the performance of the machine directly. 55. What is the necessity for starter in a dc motor? Or A D.C. motor should not be started direct on line-why? If the motor is started with rated voltage it leads to the following drawbacks The armature current will be around 100 times the rated current. It produces excessive sparking in the brushes which may destroy the brushes and commutator. Sudden development of large torque causes mechanical shock to the shaft, reducing its life. Such heavy current cannot be permitted to draw from the source of supply. 56. Draw the circuit diagram and the advantages of Wardleonard speed control method? Refer B.L Theraja page no. 966 57. State what is meant by braking in dc motor? It means to bring the motor speed to zero or a desired value quickly. 2.10
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
58. What are stray losses? Friction and windage losses are called stray losses 59. State the applications of dc series motor? It is used in Electric locomotives Cranes Elevators Trolleys and Conveyers 60. Define a). Critical resistance b). Critical speed. Critical resistance :It is the value of field resistance beyond which the generator fails to generate the voltage. Critical speed :It is the speed at which the given value of field resistance becomes the critical resistance of the generator. 61. Mention the reasons for compounding & what is degree of compounding. The compounding is the process of adding the series field AT with the shunt field AT. Degree of compounding gives you the kind of addition and hence how the shunt AT getting affected or altered 62. What is meant by OCC & how saturation affects the OCC? It is the open circuit characteristic which shows the variation of generated voltage with the increase in field current. The saturation of field poles limits the generated voltage ie though the fields flux increases the generated voltage will not increase beyond the saturation. 63. What is residual magnetism and cause for residual magnetism? It the magnetism retained by the field poles even then the supply is disconnected. 64. State the applications of dc shunt motor? Blowers and fans Centrifugal and reciprocating pumps Lathe machines Machine tools Milling and Drilling machines
2.11
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
UNIT – I: INTRODUCTION PART - A 1. What does magnetic circuit mean? 2. Define (i) magnetic flux with unit (ii) flux density with unit. 3. What is meant by magneto-motive force? Give its unit. ( Anna University –Nov’03) 4. What is leakage flux? ( Anna University –April’03) 5. Explain the phenomenon of fringing. 6. What is leakage flux? Explain with diagram. 7. Define leakage coefficient. 8. Explain Faraday’s laws of electromagnetic induction. 9. What is Fleming’s right hand rule? 10. Give the Lenz rule. 11. What is Fleming’s left hand rule? ( Anna University –April’03) 12. What are the 2 types of induced emf? ( Anna University –Nov’03) 13. Define statically and dynamically induced emf. 14. Write the rules for the direction of statically and dynamically induced emf. 15. Give the expression for dynamically induced emf. 16. What are the 2 types of statically induced emf? Explain. 17. Define self-inductance of a coil. 18. Define mutual –inductance of a coil. 19. Give the various expressions for L and M. 20. Define coefficient of coupling. 21. Give the expression for inductances in series if fluxes (a) aid (b) oppose. 22. Give the expression for inductances in parallel if fluxes (a) aid (b) oppose. 23. State the Ohm’s law for magnetic circuit 24. What does magnetising field intensity mean? 25. What is the residual flux and residual flux density? 26. Mention the types of electrical machines used in practice. 27. List the three main materials used for constructing electrical machines. 28. What are the types of rotating machines? 29. Draw the line diagram for a complete electric system. 30. Define Leakage Inductance. (A.U. – ’02 Oct) 2.12
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
31. What is meant by statically induced emf? (A.U. – ’02 Oct) PART - B 1. A wrought iron bar 30cm long and 32cm in diameter is bent into a circular shape with an air gap of 1mm. It is mounted with 600 turns of wire. The main core length is 30cm. Area of cross section is x 10-4 m2. Calculate the current required to produce a flux of 0.5 mWb in the magnetic circuit with the air gap. Assume r = 4000. Refer page 19 - Electric machines by Nagrath and Kothari 2. The flux in magnetic core is alternating sinusoidally at a frequency of 600V, 50Hz.The maximum flux density is 2T and the eddy current is 15 W. Find the eddy current loss in the core if the frequency is raised to 800 HZ and the maximum flux density is reduced to 1.5 T. Refer page 48 – Electric Machines by Nagrath and Kothari 3. A wrought iron bar 30 cm long and 2 cm in diameter is bent into a circular shape. It is then wound with 600 turns of wire. Calculate the current required to produce a flux of 0.5 mWb in the magnetic circuit in the following cases: (i) no air gap; (ii) with an air gap of 1mm: r = 4000 and (iii) with an air gap of 1mm: assume the following data for magnetization of iron : H in AT/m 2500 3000 3500 4000 B in T 1.55 1.59 1.6 1.615 Refer page 19 – Electric Machines by Nagrath and Kothari 4. An electromagnet has a magnetic circuit comprising of three parts in series a) length of 10cm and cross sectional area 80mm2 b) length of 12cm and cross sectional area 100mm2 c) an air gap of 0.5mm and cross sectional area 90mm2. The material used for the parts a) and b) is cast steel. Determine the exciting current necessary for the coil of 4500 turns wound on part a) to produce a flux density of 0.8 T in the air gap. Neglect leakage and fringing. 5. Two similar couple coils have a coupling coefficient of coupling of 0.25. When these are connected in series cumulatively the total inductance is 80mH. Calculate the self-inductance, the total inductance when the coils are connected differentially and when the coils are connected in parallel.
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
6. The magnetic circuit shown below has a steel core whose dimensions are as follows: Length (ab + cd) = 50 cm cross sectional area = 25 cm2 Length ad = 20 cm cross sectional area = 12.5 cm2 Length dea = 50 cm cross sectional area = 25 cm2 Determine the exciting coil MMF required to establish an air gap flux of 0.75 Wb. H = 200 AT/m for B = 0.3 T, H = 500 AT/m for B = 0.82 T and B = 1.04 T for H = 800 a AT/m. lg = 0.25mm
b
e
c
Air - gap
d Refer page 23 – Electric Machines by Nagrath and Kothari 7. The magnetic circuit has the dimensions of area of cross section for the core has 16cm2 the length of the air gap is 0.06cm, length of the core is 40cm, N = 800 turns and r = 6000 for iron. Find the exciting current for B = 1.2T, flux and flux density. Refer page 18- Electric Machines by Nagrath and Kothari 8. A steel ring has a mean diameter of 20 cm, cross section of 25cm2 and a radial air gap of 0.8mm cut across it. When excited by a current of 1 A through the coil of 1000 turns wound on the ring core, it produces an air gap flux of 1mWb. Neglecting leakage and fringing calculate relative permeability of steel, total reluctance of the magnetic circuit. Refer page 45 – Electric Machines by Nagrath and Kothari 9. For the Magnetic circuit shown, find self and mutual inductances between the two coils. N1=500, N2=1000, r=1600 and thickness = 2cm and I = 1A. Refer page 32 – Electric Machines by Nagrath and Kothari
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
2cm
S1
S2
4 cm
N1
N2
S
1 cm
6 cm
3 cm
10. For the magnetic circuit shown find the flux and flux density in each of the outer limbs and the central limbs. Core thickness is 5cm. The length of the central limb is 10cm. Assume relative permeability for iron of the core to be a) infinity and b) 4500. Refer page 44 – Electric Machines by Nagrath and Kothari A 5cm N=1000 0.5 A 40cm
1mm 30cm
2mm
30cm
B
UNIT – II: TRANSFORMER PART - A 1. What are the losses in a transformer; on what factors do these losses depend. Refer page 76 – Electric Machines by Nagrath and Kothari 2. State the conditions for the operation of 3-phase transformer in parallel. 2.15
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
Refer page 123 - Electric Machines by Nagrath and Kothari 3. Define Voltage Regulation and All-day Efficiency of a single phase transformer. Refer page 93 & 90 – Electric Machines by Nagrath and Kothari 4. Distinguish between ideal and practical transformers. Refer page 61 – Electric Machines by Nagrath and Kothari 5. Mention the difference between the current and voltage transformer. 6. State the principle of working of a transformer. 7. List the types of transformer based on construction. a. core type b. shell type 8. Define transformation ratio and classify the transformers based on this ratio. Refer page 58 – Electric Machines by Nagrath and Kothari. Types: a. step up b. step down 9. Define voltage regulation. ( Anna University –Nov’03) Refer page 93 – Electric Machines by Nagrath and Kothari 10. Draw the complete equivalent circuit diagram of a transformer. ( Anna University –Nov’03) Refer page 67 – Eletric Machines by Nagrath and Kothari 11. List the various losses in a transformer and state the condition for maximum efficiency. Refer page 76 & Electric Machines by Nagrath and Kothari 12. Define all – day efficiency of a transformer. Refer page 90 – Electric Machines by Nagrath and Kothari 13. What are the demerits of an autotransformer and state its applications. 14. Write the emf equation of a transformer and explain the terms involved. 15. Why the rating of transformers is in KVA? ( Anna University – April’03) 16. State the two disadvantages of an autotransformer. 17. What are breathers? 18. Define Stacking Factor.
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
19. What are the types of transformers according to the cooling systems? 20. What material is used in the transformer core? (A.U. – ’02 Oct) 21. Why is the efficiency of transformer more than the machines? ( Anna University –April’03) 22. State the different connections of three phase transformer. (A.U. – ’02 Oct) PART - B 1. a) Derive the expression for emf induced in the transformer. b) A 50 KVA single-phase transformer has iron loss of 400 W and full load copper loss of 800 W. Find (i) The load at which the maximum efficiency occurs and the value of maximum efficiency at unity power factor. (ii) If the maximum efficiency occurs at 80% of full load. Find the new core loss and the full load copper loss assuming that the total full load is a constant. 2. Two three phase 4400/400 V transformer A & B have the following data: Transformer A : % impedance / phase referred to secondary = (1 + j3), Rated KVA = 400. Transformer B : % impedance / phase referred to secondary = (1 + j4), Rated KVA = 800. Determine the load shared by each transformer and its power factor, if the total load is 100 KVA at 0.8 p.f. Lagging. 3. The following data are from the open circuit and short circuit tests conducted on a 10 KVA, 220/110 V single-phase transformer. O.C. Test: 220 V 0.5 A 80 Watts. S.C. Test: 60 V 40 A 400 Watts. In both the cases the measurements were made on the H.V. side. Obtain the equivalent circuit. 4. a)Obtain the ratio of copper used in an autotransformer to that of a two winding transformer for the same KVA ratings. ( Anna University –Nov’03) b)A 200/400 V, 20 KVA, 50 HZ transformer is connected as an autotransformer to transform 600 V to 200 V. Determine the autotransformer ratio & the KVA rating of the autotransformer. 2.17
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
5. Explain the tests conducted on a constant potential transformer to arrive at its equivalent circuit. 6. The following data pertains to open circuit and short circuit tests conducted on a 10KVA, 220 V/110 V single phase transformer. O.C. Test : 220 V 5A 80 W S.C. Test : 60 V 40 A 400 W In both the cases the measurements were made on the HV side. Obtain its equivalent circuit. 7. The emf per turn for a single phase 2200V/220V, 50 Hz transformer is approximately 12 Volts. Calculate the number of primary and secondary turns and net cross sectional area of the core for a maximum flux density of 1.5 T. 8. a) Define regulation of a 3 phase transformer and mention its significance. b) A transformer has its maximum efficiency of 0.98 at 20 KVA at unity power factor. During the day it is loaded as follows: 12 hours : 2 KW at 0.6 pf 6 hours : 10 KW at 0.8 pf 6 hours : 20 KW at 0.9 pf. Find the all – day efficiency 9. A 100KVA, 1000/100 V, single phase transformer gave the following test results: O.C. Test : 400 W, 6.0 A and 100 V S.C. Test : 1800 W, 100 A and 50 V a) Determine the equivalent circuit parameters of the transformer referred to the HV side. b) Determine the voltage regulation of the transformer at full load, 0.6 pf leading. c) Draw the phasor diagram for the condition in (b) 10. A single phase, 10KVA, 2400/240 V, 50 Hz distribution transformer has the following characteristics: Copper loss at full voltage = 100 W and copper loss at half full load = 60 W. Determine the efficiency of the transformer when it delivers full load at 0.8 pf lagging and also the all day efficiency of the transformer for the following load cycle. No load for 6 hours. 70 percent full load for 10 hours at 0.8 pf and 90 percent full load for 8 hours at 0.9 pf. 11. Explain the principle of working of a transformer. 12. Describe the constructional details of a transformer. 13. Derive the emf equation of an ideal transformer. 2.18
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EE 2251 : ELECTRICAL MACHINES - I
14. Draw the equivalent diagram for a transformer. Explain. ( Anna University –April’03) 15. Explain the back to back method of testing of two identical single phase transformers. ( Anna University –April’03) 16. Define voltage regulation and derive the expression for voltage regulation. 17. Explain with neat circuit diagram the O.C. and S.C. tests on a transformer and how the efficiency and equivalent circuit of a transformer can be determined. 18. Discuss about 3 phase transformer and their constructional details. State their advantages. 19. What is an autotransformer? State their merits and demerits and give its applications. 20. Give a brief account of instrument transformers. 21. Write about tap changing transformer. 22. Draw the phasor diagram for a transformer on load for a) noninductive load b) inductive load c) capacitive load. 23. Derive an expression for saving copper in an autotransformer. 24. The voltage/turn of a 1 phase transformer is 1.1 V, when the primary winding is connected to a 220 V, 50 Hz ac supply, the secondary voltage is found to be 660V. Find the primary and secondary currents, the core area if the max flux density is 1.2T. 25. The max flux density in the core of a 250/3000V, 50 Hz single phase transformer is 1.2 Wb/m2. If the emf/turn is 8 V, determine the primary and secondary turns. 26. A 25 KVA transformer has 500 turns on the primary and 100 turns on the secondary winding. The primary is connected to 2000 V. Determine the full load primary and secondary currents and the no-load secondary voltage. 27. A 5 KVA transformer has iron loss and copper loss of 100 and 200 W. Determine the efficiency at half full load and 0.8 pf lagging. 28. The maximum flux density in the core of 250/3000V, 50 Hz single phase transformer is 1.2Wb/m2 if the emf/turn is 8V, determine the number of primary and secondary turns. 29. The required N.L voltage ration in a 150 KVA, 50 Hz single phase transformer is 5000/250 V. Find the number of turns in each winding for a maximum core flux of 0.06 Wb and efficiency at half rated KVA, upf. 2.19
EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
UNIT – III: ELECTROMECHANICAL ENERGY CONVERSION PART - A 1. Express the energy stored in a magnetic field in different forms. 2. Define energy and co energy. 3. Write the energy balance equation of a motor.(AU-Nov’03) 4. Give the examples for singly excited systems. ( AU–Nov’03) 5. Draw the schematic diagram indicating flow of energy in the conversion of mechanical energy to its electrical form. 6. Why the I- relationship of a magnetic circuit is almost linear? (AU – ’02 Oct) 7. Write the Steinmetz empirical formula for hystersis loss. (AU–’02 Oct) 8. Explain how flow of energy takes place in electro mechanical device. (AU – ’02 Oct) 9. Mark the co energy region in i- graph. (AU –April’03) 10. What is multiply excited system? (AU –April’03) PART - B 1. Two coils have self and mutual inductances of L11 = L22 = 2/(1+2x); L12 = (1-2x) The coil resistances may be neglected. a)If the current I1 is maintained constant at 5 A and I2 at -2 A, find the mechanical work done when x increases from 0 to 0.5m. What is the direction of the force developed? b)During the movements in part a), what is the energy supplied by the sources supplying currents I1 and I2? 2. Derive the expressions for the field energy and co-energy in a singly excited electromechanical device operating under saturated condition. 3. A doubly excited magnetic field system has coil self and mutual inductances of L11 = L12 = 2 + cos(2); L12 = cos Where is the angle between the axes of the coils. The coils are connected in series and carry a current of i = 2 I sin t. Derive an expression for the time average torque as function of angle. 4. The stator and rotor coils of an electromechanical device have self and mutual inductances of L11 = 0.4mH, L22 = 0.2mH, L12 = L21 = 0.1 Cos mH. Where is the angle between the coil axes.
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5.
6. 7. 8.
a)Derive the expression for the time average torque when the coils are connected in series and carry a current i = 2 I sin t b)For I = 10 A, = 45, Calculate the value of the average torque. c)The rotor is now restrained from movement by a spring torque which produces a torque Ts = 0.001 Nm/ of rotor coil displacement from 90 position. Calculate the steady value of for I = 5, 52, 10 A. What conclusions can you draw from these values of regarding the use of the device as a measuring instrument? An electromechanical device has the following stator and rotor coil inductances L11 = L22 = 1.6 + Cos 2 H, L12 = L21 = 1 + Cos H. Neglecting the coil resistances find the torque developed as a function of angle when both the coils are connected in parallel to a voltage source 160 sin 314t V. Find the value of Tf(av) for = 30. Explain about energy stored in magnetic system. (AU –April’03) Give a brief note on the flow of energy in electromechanical devices. (AU –April’03) Derive the expression for co energy in a multiple excited magnetic field system. (AU –April’03)
UNIT – IV: BASIC CONCEPTS IN ROTATING MACHINES PART - A 1. Distinguish between electrical and mechanical angle in an electric machine. ( Anna University –April’03) 2. Define coil span, coil pitch and short pitching. 3. Define phase spread, distribution factor and breadth factor. 4. What is meant by SPP and what is its significance? 5. Which type of MMF wave does a distributed winding have? Give reasons. 6. Explain the terms slip and slip frequency with reference to an induction motor. 7. Define the pole pitch. (AU –Nov’03) 8. Define the term synchronous speed. (AU –Nov’03) 9. Define the distribution factor. (AU –April’03)
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
PART – B 1. Derive the expression of torque developed in a D.C. motor. In a doubly salient and singly excited electromagnetic system express the stator inductance as a function of rotor position, when there is no winding on the rotor. a) Obtain an expression for the torque acting on the rotor when the stator current is Imsin t. b) Let = mt + , where m is the angular velocity of the rotor and is the rotor position at t = 0. Find the condition for the non-zero average torque and obtain an expression for the average torque. 2. Explain the operation of a D.C. shunt generator and describe the process of emf build up. If the generator fails to self excite discuss the most likely reasons for this and state how you would proceed to remedy the fault. 3. A 50 Hz synchronous salient pole generator is driven by a hydroelectric turbine at a speed of 125rpm. There are 576 stator slots with two conductors per slot. The air gap diameter is 6.1m and stator length is 1.2m the sinusoidally distributed flux density has a peak value of 1.1T. a)Calculate the maximum rms single phase voltage that can be produced by suitably connecting all the conductors. b)Find the per phase emf if the conductors are connected in balanced 3 phase winding. 4. A dc armature is built up of laminations having an external diameter of 80cm and internal diameter of 42cm. The length of the armature is 32cm. The flux density in the armature core is 0.85T. The armature is wave connected with 72 sots with 3 conductors per slot. If the number of poles is 6, find the emf induced when the armature is rotated at a speed of 600rpm. 5. Trace out the variations in MMF due to a belt carrying conductors representing one phase pf a 2 pole, 3 phase winding. The belt may be assumed to be assumed to be a current sheet with uniform current density. What is the peak amplitude of the MMF wave if the total current in the belt is A amperes? 6. Explain the constructional features of synchronous motor. Derive the expression of RMS voltage of induction motors.
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
UNIT – V: DC MACHINES PART - A 1. Mention the applications of D.C. Series Motor and D.C. Shunt Motor. 2. What is meant by Commutation in D.C. machines? State the different types of Commutations. 3. What does Armature Reaction mean in D.C. machines? State the effect of Armature Reaction. 4. Define Critical Speed and Critical Resistance of D.C. Shunt generator. 5. Explain the process of commutation in a D.C. generator. 6. Mention the applications a Series Generator. 7. State the working principle of a dc generator. 8. State the principle of the working of a dc motor. 9. State the various types of dc machines based on excitation. 10. Explain the function of communtator in dc generator and dc motor. 11. What are the effects of armature reaction? 12. What are the methods of neutralizing the effects of armature reaction? ( Anna University –Nov’03) 13. What are the various methods improve commutation? 14. List the various losses in dc machines. 15. State the applications of dc series, shunt and compound generators. 16. State the applications of dc series, shunt and compound motors. 17. State the methods of speed control of dc motors. 18. How can you reverse the direction of rotation of a dc motor? 19. State the condition for maximum efficiency of a dc generator. 20. What is meant by back emf? 21. Write the torque equation of a dc motor, defining each term involved? 22. What is the necessity for a starter in a dc motor? PART – B 1. An 8-pole dc shunt generator with 788 wave connected conductors and running at 500 rpm supplies a load of 12.5 ohms resistance at a terminal voltage of 250 V. The armature resistance is 0.24 ohms and
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EE 2251 : ELECTRICAL MACHINES - I
the field resistance is 250 ohms. Find the armature current, induced emf and flux/pole. 2. Draw the load characteristics of dc shunt, series and compound motor and compare them. 3. A 500 V dc shunt motor takes 4A on no-load. The armature resistance including that of the brushes is 0.2 ohms and the field current is 1 A. Estimate the output and efficiency when the input current is 20 A. 4. Draw the load characteristics of a dc shunt generator and explain the reasons for such a characteristics. 5. A 4 pole 500 V shunt motor takes a 7 A current on no-load, the noload speed being 750 rpm. It has a shunt field current of 2 A. Calculate the full load speed of the motor if it takes 122 A at full load, armature resistance = 0.2 ohms. Contact drop/brush = 1 Volt. Armature reaction weakens the field by 4 % on full load. 6. a) Explain the necessity for D.C. starter. State the approximate value of the starter resistance. b) Explain how the efficiency of a D.C. machine can be predetermined using Swinburne's Test. c) State and explain WARD LEONARD speed control of DC Shunt motor. 7. Explain the effects of armature reaction in D.C. generators. 8. A 250 V shunt motor on no – load runs at 1000rpm and takes 5 A. The total armature and shunt resistance are respectively 0.2 and 250 . Calculate the speed when loaded and taking a current of 50 A, if armature reaction making the field by 3%. 9. A 500 V shunt motor takes 4 A on no-load. The armature resistance including that of the brushes is 0.2 and field current is 1 A. Estimate the output and the efficiency when input current is 20 A. 10. A 220 V D.C. shunt generator has the following open circuit characteristics, when run at 1000 rpm. If in amps : 0 0.5 1.0 1.5 2.0 2.5 3.0 4.0 Eg in volts : 10 60 120 170 210 240 260 285 a) Determine its critical resistance when the generator is to run at 1000 rpm. b) Determine the field current required to generate 200 V at 800 rpm under no load condition.
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EE 2251 : ELECTRICAL MACHINES - I
11. A coil wound on a toroidal core of circular cross section has 200 turns. The inner and outer radii are 20 and 25 cm. The magnetic flux density within the core is uniform and has a value of 1.25 T. For a coil current of 3.5 A, determine the inductance of the coil. 12. A toroidal core with a radial saw cut is to be used to provide an inductance of 0.5 H. The flux density is low, so that the relative permeability may be taken as 2500. The core has a mean radius of 15 cm and its area of cross section is 5 cm2. If a coil of 1200 turns can be wound on the core, determine the width of the radial saw cut. 13. Explain the various methods of speed control of D.C. shunt motors. 14. Determine the change in speed of a 220 V D.C. shunt motor when the flux is reduced by 10%. Prior to field weakening the motor was running at 960 rpm carrying an armature current of 50 A. The motor has an armature resistance of 0.2. The total torque may be assumed to remain constant and the losses may be neglected. 15. With a neat diagram explain the construction of a dc machine. 16. Derive the expression for emf generated in a dc generator and explain the difference between lap and wave winding. 17. Explain the armature reaction in dc generator and suggest ways for neutralizing the effects of armature reaction. 18. With the help of a neat diagram of magnetization curve explain the process for voltage built up in a dc shunt generator and test the factors affecting the built up process. 19. Draw the internal and external characteristics of a dc shunt generator and explain. 20. Deduce the expression for torque developed in a dc motor. What is the back emf and state its importance. 21. Deduce the equation for speed of a dc motor and hence suggest various methods of speed control and compare the merits and demerits. 22. Draw the characteristics of dc shunt motor and from the nature of the curve explain the applications of dc shunt motor. 23. Draw the characteristics of dc shunt motor and from the nature of the curve explain the applications of dc series motor. 24. Explain the principle of operation of dc motor. 25. What is the need for starters? With the help of a neat diagram explain the working of a 3 point starter and state its limitations.
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EEE DEPARTMENT
EE 2251 : ELECTRICAL MACHINES - I
26. Draw the external characteristics of a dc shunt, series and compound generators and explain. 27. Explain how torque is developed in a dc motor and discuss the various methods of speed control of dc series motor. 28. Discuss the process of self excitation in a dc machine and explain the term commutation with suitable illustrative figures. 29. The emf generated by a four pole dc generator is 400 V, when the armature is driven at 1200 rpm. Calculate the flux per pole if the wave wound armature has 39 slots having 16 conductors in each slot. 30. A 50 KW, 250 V dc shunt generator has a field circuit resistance of 60 ohm and an armature resistance of 0.02 ohm. Calculate the load current, armature current, the generator armature voltage when delivering rated current at rated speed and voltage. 31. Derive the emf equation of generator. (AU 32. –Nov’03) 33. Draw the diagram of a 3 point starter and explain. (AU –Nov’03) 34. Explain the speed control of a DC motor. (AU –Nov’03)
B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2008 Third Semester (Regulation 2004) Electrical and Electronics Engineering EE 1202-ELECTRICAL MACHINES-I (Common to B.E.(Part time) Second semester Regulation 2005) Time: 3 hours Maximum: 100 Marks Answer ALL questions. PART-A(10x2=20 Marks) 1. State the principle of electromechanical conversation. 2. Distinguish between statically induced and dynamically induced e.m.fs. 3. What are the condition should be fulfilled for the D.C. shunt generator to build up emf? 4. What is meant by armature reaction? 5. Write down the significance of back emf in the D.C.motor. 6. Mention speed control methods of a D.C.shunt motors. 2.26
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7. Why transformers rating is expressed in terms of kVA? 8. What are the advantages of auto-transformer over two-winding transformer? 9. What are the different losses occurring in a transformer? 10. What are the advantages of Swinburne's test? PART_B(5x16= 80marks) 11. (a) (i) Represent pictorially the flow of energy in electromechanical devices for both generating and motoring action. (10) (ii) The magnetic flux density on the surface of an iron face is 1.6 T which is a typical saturation level value for ferromagnetic material.Find the force density on the iron surface. (6) Or (b) (i) Draw the developed diagram of MMF space wave of a single coil distributed ac winding and explain. (8) (ii) Derive an expression for torque developed in a 2-pole round rotor machine and state the assumption made. (8) 12. (a) (i) Derive an expression for emf generated in aDC generator. (8) (ii) A separately excited dc generator,when running at 1000rpm, supplied 200 A at 125 V. What will be the load current when the speed drops to 800 rpm, if I(f) is unchanged? Given that the armature resistance = 0.004ohm and brush drop= 2V. (8) Or (b) (i) Draw and explain the important characteristics of separately excited DC generator. (ii) Two shunt generator operating at parallel deliver a total current of 250A. One of the generated is rated 50kW and the other is 100kW.he voltage rating of both machine is 500 V and have regulations of 6% (smaller one) and 4%.Assuming linear characteristics,determine (1) the current deliverd by each machine and(2)terminal voltage. (8)
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EE 2251 : ELECTRICAL MACHINES - I
13. (a) (i) Deliver an expression for the torque developed in the armature of a DC motor. (8)(ii) Determined develpoed torque and shaft torque of 220 V, 4-pole series motor with 800 conductors wave-connected supplying a load of 8.2 kW by taking 45 A from the mains.The flux per pole is 25 mWb and its armature circuit resistance is 0.6 Ohm. Or (b) (i) Explain the flux conmtrol and armature control methods of speed control of DC shunt motors. (8) (ii) A 220 V shunt motor has an armature resistance of 0.5 Ohm and takes an armature current of 40 A on a certain load.BY how much must the main flux be reduced to raised speed by 50% if the developed speed is constant? Neglect saturation and armature reaction. (8) 14. (a) (i) Draw and explain no-load phasor diagram of a single phase transformer. (8) (ii) A single phase transformer takes 10 A on no-load at 0.2 p.f. lagging.The turn ratio is 4:1 (step down).If the load on the secondary is 200 A at apf 0.85 lagging,find the primary current and power factor. Neglect the voltage drop in the winding. Also draw the phasor diagram. (8) Or (b) (i) With the help of circuit diagrams, explain any two types of three phase transformer connections. (6) (ii) Obtain the equivalent circuit of a 200/400 V, 50 hz, 1-phase transformer from the following test data: (10) O.C.Test : 200 V, 0.7 A, 70W - on L.V. side S.C.Test : 15 V, 10 A, 85 W- on H.V. side 15. (a) (i) Derive the condition for maximum efficiency in a transformer. (8) (ii) A 11000/230 V,150kVA,1-phase, 50 Hz transformer has core loss of 1.4kW and F.L.cu. loss of 1.6kW.determine .The kVA load
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EE 2251 : ELECTRICAL MACHINES - I
for maximum efficiency and the value of maximum efficiency at unity p.f. The efficiency at half F.L. 0.8 p.f. leading. (8) Or (b) With the help of neat circuit diagram, explain Swinburne's test and derive the relations for efficiency (both for generator and motor).Also state the merits and demerits of this method. (16)
B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2003. Third Semester Electrical and Electronics Engineering EE 232 — ELECTRICAL MACHINES – I Time : Three hours Maximum : 100 marks Answer ALL questions. PART A — (10 * 2 = 20 marks) 1. What is meant by leakage flux? 2. State Fleming's left hand rule. 3. Why transformers are rated in kVA instead of kW? 4. Why is the efficiency of transformers more than that of other rotatingmachines? 5. Mark the co energy region in the i λ graph. 6. What is a multiply excited magnetic field system? 7. Write the relation between electrical and mechanical degree. 8. Define distribution factor. 9. What is the function of yoke in a D.C. machine? 10. Give two reasons for failure of build up of emf in d.c. shunt generator. PART B — (5 * 16 = 80 marks) 11. (i) Explain the various methods of commutation. (8) (ii) With neat sketch, explain the function of 3 point starter. (8)
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12. (a) (i) What is a magnetization curve and why is it used in magnetic circuit calculations? (8) (ii) An iron ring of circular cross–section 10 cm2 and mean circumference 30 cm has an air–gap of 2 mm. If the ring is wound with 500 turns, find the exciting current to establish a flux of 0.4 mWb in the air gap. The relative permeability of iron may be assumed to be 2500. (8) Or (b) (i) Explain about a.c. operation of magnetic circuits. (8) (ii) A straight conductor of 1.5 m length carries a current of 40 A. It is lying at right angles to a uniform magnetic flux density of 0.8 T. Find : (1) the force developed on the conductor (2) the power required to drive the conductor at a uniform speed of 25 m/s and (3)the emf induced in the conductor. (8) 13. (a) (i) What is meant by equivalent circuit of a transformer? What is its use? (8) (ii) A 600 kVA, single phase transformer when working at u.p.f. has an efficiency of 92% at full load and also at half load. Determine its efficiency when it operates at unity p.f. and 60% of full load. (8) Or (b) (i) Explain the back to back method of testing of two identical single phase transformers. (8) (ii) State and explain the necessary conditions for satisfactory parallel operation of two three phase transformers. (8) 14. (a) (i) Explain about energy stored in magnetic system. (8) (ii) Give a brief note on flow of energy in electromechanical devices. (8) Or
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EE 2251 : ELECTRICAL MACHINES - I
(b) (i) Derive expression for coenergy in a multiply–excited magnetic field system. (8) (ii) The magnetic flux density on the surface of an iron face is 1.8 T which is a typical saturation level value for ferromagnetic material. Find the force density on the iron face. (8) 15. (a) (i) Explain the constructional features of elementary synchronous machines. (8) (ii) A 3–phase 440 kVA, 50 Hz, star connected synchronous generator running at 400 rpm is designed to develop 3600 V between terminals. The armature consists of 180 slots, each slot having one coil side with eight conductors. Determine the peak value of thefundamental mmf in AT/pole when the machine is delivering full load current. (8) Or (b) (i) Explain about rotating MMF waves in A.C. machines. (8) (ii) Derive an expression for induced emf in a synchronous machine. (8)
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