ROEVER COLLEGE OF ENGINEERING & TECHNOLOGY ELAMBALUR, PERAMBALUR-621220 DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING ELECTRICAL MACHINES I Unit I Introduction 1. What are the three basic types of electrical machines? 2. State Lenz Law. 3. State Ohm s law for magnetic circuit. 4. Define leakage flux 5. Define magnetic reluctance 6. Draw the typical normal magnetization curve of ferromagnetic material. 7. What is fringing? 8. State stacking factor. 9. Mention some magnetic materials 10. What is magnetostriction? 11. Define statically induced emf. 12. What are the two types of statically induced emf? 13. State Faradays law of Electromagnetic Induction: 14. Define dynamically induced emf. 15. State Fleming s right hand rule. 16. State Fleming s Left hand rule. 17. What is meant by core loss? 18. What is meant by Coercivity? 1. Discuss the Magnetic circuits. 2. Describe the dynamically induced emf and statically induced emf? 3. Explain the AC operation of magnetic circuit in electrical machines. 4. A ring has a diameter of 21 cm and a cross-sectional area of 10 cm 2. The ring is made up of semicircular sections of cast iron and cast steel, with each joint having reluctance equal to an air-gap of 0.2 mm. Find the ampere-turns required to produce a flux of 8 10 4 Wb. The relative permeabilities of cast steel and cast iron are 800 and 166 respectively. Neglect fringing and leakage effects. 5. A rectangular iron core is shown in Fig. It has a mean length of magnetic path of 100 cm, cross-section of (2 cm 2 cm), relative permeability of 1400 and an air-gap of 5 mm cut in the core. The three coils carried by the core have number of turns N a = 335, N b = 600 and N c = 600 ; and the respective currents are 1.6 A, 4 A and 3 A. The directions of the currents are as shown. Find the flux in the air-gap.
UNIT II TRANSFORMERS 1. Mention the difference between core and shell type transformers. 2. What is the purpose of laminating the core in transformers? 3. Write the emf equation for a single phase transformer 4. Define voltage regulation of a transformer 5. Define all day efficiency of a transformer. 6. Why transformers are rated in kva? 7. What are the conditions for parallel operation of Transformers? 8. Compare two winding Transformer and autotransformer. 9. What is the angle when no-load current will lag the ideal applied voltage? 10. What are the various types of Connection in three phase transformer? 11. State the condition for maximum efficiency 1. (i)explain the construction and working principle of single phase transformer. (6) (ii) Draw phasor diagram to represent conditions in a single-phase transformer-supplying load at 1. Unity p.f 2.Lagging p.f 3. Leading p.f (10) 2. (i) Derive an emf equation for the single phase transformer.(8) (ii)the maximum flux density in the core of a 250/3000-volts, 50-Hz single-phase transformer is 1.2 Wb/m 2. If the e.m.f. per turn is 8 volt, determine (i) primary and secondary turns (ii) area of the core.(8) 3. (i)explain the equivalent circuit of a transformer and derive its expression.(8) (ii) The 2ransforme of a 2300/230 V, 50-Hz 2ransformer are given below : R 1 = 0.286 Ω, R 2 = 0.319 Ω, R 0 = 250 Ω, X 1 = 0.73 Ω, X 2 = 0.73 Ω, X 0 = 1250 Ω The secondary load impedance Z L = 0.387 + j 0.29. Solve the exact equivalent circuit with normal voltage across the primary.(8) 4. (i) Explain the types of testing of transformer.(8) (ii) Obtain the equivalent circuit of a 200/400-V, 50-Hz, 1-phase transformer from the following test data : O.C test : 200 V, 0.7 A, 70 W on L.V. side S.C. test : 15 V, 10 A, 85 W on H.V. side Calculate the secondary voltage when delivering 5 kw at 0.8 p.f. lagging, the primary voltage being 200V.(8) 5. Explain the Back to back (Sumpner s test) method of testing of two identical single phase transformers. 6. Explain the construction and principle of operation of autotransformer. Also derive its
expression for saving of copper when an auto transformer is used. 7. (i)explain the Construction and operation of 3-phase transformer with various connection diagrams.(8) (ii) A 3 Phase step down transformer is connected to 6.6 KV mains and it takes 10A. Calculate the secondary line voltage, line current, and output for the (i) Delta-Delta (ii) Star- Star (iii) Delta-Star (iv) Star-Delta Connections. The ratio of turns per phase is 12 and neglect no load losses.(8) 8. (i) Describe the voltage regulation of transformer.(8) (ii)the efficiency of a 1000-kVA, 110/220 V, 50-Hz, single-phase transformer, is 98.5 % at half full-load at 0.8 p.f. leading and 98.8 % at full-load unity p.f. Determine (i) iron loss (ii) full-load copper loss and (iii) maximum efficiency at unity p.f.(8) 9. (i) Explain the parallel operation of transformer. (8) (ii)the corrected instrument readings obtained from open and short-circuit tests on 10-kVA, 450/120-V, 50-Hz transformer are : O.C. test : V1 = 120 V; I1 = 4.2 A; W1 = 80 W; V1, W1 and I1 were read on the low-voltage side. S.C. test : V1 = 9.65 V; I1 = 22.2 A ; W1 = 120 W with low-voltage winding shortcircuited Compute : (i) the equivalent circuit (approximate) constants, (ii) efficiency and voltage regulation for an 80% lagging p.f. load, (iii) the efficiency at half full-load and 80% lagging p.f. load.(8) UNIT III ELECTROMECHANICAL ENERGY CONVERSION 1. What do you mean by field energy? 2. What are the requirements of the excitation systems? 3. Draw the power flow diagram for motor and generator operation. 4. Draw an energy flow diagram of an electromechanical energy conversion device when it acts as a motor and Generator. 5. Write the expression for mechanical force for singly excited system. 6. What is meant by co-energy? What is its use? 7. Draw a single excited magnetic system and double excited magnetic system. 8. State the advantages of electromechanical energy conversion principles. 9. Based on the principal of conservation of energy, write an energy balance equation of for a motor. 10. What are the three basic principles for the electromechanical energy conversion? 11. Write an expression for the stored energy in the magnetic field. 1. Obtain an expression for the mechanical force of field origin in a typical attracted armature relay. 2. Find an expression for the magnetic force developed in a multiply excited magnetic system. 3. Discuss the flow of energy in electromechanical devices in detail. 4. The magnetic flux density on the surface of an iron face is 1.8T which is typical saturation level value for ferromagnetic material. Find the force density on the iron face.
5. Explain the i-λ characteristic of a magnetic system. Also derive expression for co-energy density assumed the i-λ relationship of the magnetic circuit is linear. 6. Two coupled coils have self and mutual inductance of L 11 = 2+ (1/2x), L 22 = 1+ (1/2x), L 12 =L 21 = 1/2x, over a certain range of linear displacement x. The first coil is excited by a constant current of 20A and the second by a constant current of -10A. Find (i) Mechanical work done if x changes from 0.5 to 1m. (ii) Energy supplied by each electrical source in part (i). (iii)change in field energy. UNIT IV BASIC CONCEPTS in ROTATING MACHINES 1. Explain the following terms with respect to rotating electrical machines. a. Pole pitch. b. Chording angle. 2. State advantages of short- pitched coils. 3. Define rotating magnetic field 4. Define pitch factor. 5. Give the equation for emf generated in D.C.machine. 6. Define winding factor. 7. What are distributed windings? 8. How is voltage generated in rotating machines? 9. Write the torque equation for round rotor machine. 10. Which type of winding is suitable for ac machines? 11. State the assumptions made in deriving the torque equation for round rotor machine. 12. State the torque equation for round rotor machine. 1. (a) Derive an expression for the generated emf in synchronous machines.(8) (b) A 50 Hz synchronous salient pole generator is driven at 125 rpm.there are 576 stator slots with two conductors per slot.air gap diameter is 6.1 m and stator length is 1.2m. sinusoidal flux density has a peak of 1.14T.Calculate the line voltage induced for star connection.(8) 2. Derive the torque equation of a round rotor machine. Also clearly state what are the assumptions made? (16) 3. Write in detail about the MMF space wave of three phase distributed winding. (16) 4. Explain in detail about construction of synchronous machine (16) 5. Write in detail about magnetic fields in rotating machines. (16) 6. (a) Explain with neat diagram the concept of MMF space wave of single coil (8) (b) A 3-phase, 400 kva, 50Hz star connected alternator (synchronous generator) running at 300 rpm is designed to develop 3300V between terminals. The armature consists of 180 slots, each slot having one coil side with 8 conductors. Determine the peak value of the fundamental mmf in AT/pole when the machine is delivering full load current. (8) 7. (a) A 3-phase 50 kw, 4-pole, 50Hz induction motor has a winding (ac) designed for delta connection. The winding has 24 conductors per slot arranged in 60 slots. The rms value of the line current is 48A. Find the fundamental of the mmf wave of phase -A when the current is passing through its maximum value. What is the speed and peak value of the resultant mmf/pole? (8)
(b) Explain in detail about torque -production process of synchronous machine (motoring) (8) 8. (a)explain in detail about torque -production process of synchronous machine (generating) (8) 9. (b)a 50 Hz, 400V, 4-pole cylindrical synchronous generator has 36slots, two -layer winding with full pitch coils of 8 turns each. The mean air -gap diameter is 0.16m, axial length 0.12m and a uniform air gap of 2mm. calculate the value of the resultant AT/pole and the peak air gap flux density. The machine is developing an electromagnetic torque of 60 Nm as a generator at a torque angle of 26 0. What should be the rotor AT/pole? What is the stator AT and the angle it makes with the resultant AT? Also find the stator current. (8) UNIT V DC MACHINES 1. How will you change the direction of rotation of a d.c motor? 2. What is back emf in d.c motors? 3. Under what condition the mechanical power developed in a dc motor will be maximum? 4. What is the function of a no-voltage release coil provided in a dc motor starter? 5. Name the two types of automatic starters used for dc motors. 6. List the different methods of speed control employed for dc series motor 7. Name the different methods of electrical breaking of dc motors. 8. To what polarity the inter poles excited in dc motors? 9. Name any four applications of DC series motor. 10. Name the starters used for series motors. 11. What type of DC motors is suitable for various torque operations? 12. Define speed regulation. 13. Why are carbon brushes preferred for dc machines? 14. What are the various types of commutation? 15. Name the two methods for improving commutation. 16. How and why the compensating winding in dc machine excited? 1. Explain the principle of operation of DC motor. 2. Explain the characteristic of DC series motor and shunt motor. 3. Draw and explain the characteristic of DC compound motor. 4. Draw the diagram of a 3 point starter and explain? 5. Explain the 4-point starter with neat diagram. 6. Explain the different methods in speed control of shunt motor? 7. Explain the different methods of speed control in series motor? 8. Explain the armature reaction of the DC machine