DHANALAKSHMI COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE ELECTRICAL MACHINES II UNIT-I SYNCHRONOUS GENERATOR

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1 1 DHANALAKSHMI COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING Constructional details Types of rotors EE ELECTRICAL MACHINES II UNIT-I SYNCHRONOUS GENERATOR PART A 1. List out the various types of alternator. (N/D-06) 2. What is meant by hunting? (A/M-04) 3. What are the various functions of damper winding provided with alternator? (N/D-11) 4. What is the working principle of alternator? 5. Why are pole and pole shoes laminated? 6. What is meant by exciter? 7. What are the basic parts of an alternator? 8. What are the advantages of salient pole type of construction used for synchronous machines? 9. List out the types of synchronous generator used in a hydro-electric power station. Emf equation 10. Define Distribution Factor (N/D-06) 11. What is meant by single layer winding and double layer winding? (N/D-11) 12. Derive the relationship between the number of poles, frequency and the speed of an alternator. 13. Write the EMF equation of an alternator and identify the various variables of the equation. Synchronous reactance 14. What is meant by synchronous reactance? (A/M-04) 15. What is meant by synchronous impedance? 16. Define Synchronous Speed

2 2 Armature reaction 17. What is meant by armature reaction? (N/D-12) Voltage regulation 18. What are the reasons for the drop in voltage from no-load to full load of an alternator? (N/D-06) 19. List out the various causes of voltage drop in a loaded alternator. (N/D-12) 20. Why are alternators rated in kva rather than in kw? 21. List out the different methods of finding the voltage regulation of an alternator. 22. Define Voltage Regulation EMF, MMF, ZPF and A.S.A methods 23. Differentiate MMF method from EMF method of finding regulation of an alternator. Synchronizing and parallel operation 24. What is the need for parallel operation of alternators? (A/M-04) 25. What are the conditions to be fulfilled for the parallel operation of alternators? 26. What is meant by load angle of an alternator? 27. What is meant by synchronization of alternators? Constructional details Types of rotors PART B 1. Write short notes on hunting in an alternator. (8) (A/M-05) 2. Explain in detail, the two types of rotor constructions used for alternators. (8) (A/M-05) 3. Explain the principle of operation of an alternator. (8) 4. Compare salient pole machine with non-salient pole machine. (8) 5. Explain in detail, the constructional features of salient pole alternator. (16)

3 3 Emf equation 6. A three phase star connected alternator has the following data: Open circuit voltage = 4000 V at 50 Hz Speed = 500 rpm Stator slots/pole/phase = 3 Conductor/slot = 12 Calculate the number of poles and useful flux per pole. Assume all conductors/phase to be connected in series and coil to be full pitched. (16) (A/M-05) 7. A 3 phase, 16 pole alternator has a star connected winding with 144 slots and 10 conductors per slot. The flux per pole is 0.04 Wb and it is sinusoidally distributed. The speed is 375 rpm. Find frequency, phase emf and line emf. The coil span is 160 electrical. (16) (M/J-06) 8. Derive the emf equation of a three phase alternator. (8) (N/D-11) 9. A 3 phase, 8 pole, 750 rpm, star connected alternator has 72 slots on the armature, each slot has 12 conductors and winding is short chorded by 2 slots. Find the induced emf between lines, if the flux per pole is 0.06 Wb. (8) Synchronous reactance 10. Explain in detail, the synchronous impedance of an alternator with suitable phasor diagrams. (M/June-06) Armature reaction Voltage regulation 11. What are the methods available for the determination of voltage regulation of an alternator? (A/M-05) EMF, MMF, ZPF and A.S.A methods 12. Explain any one of the indirect methods to determine voltage regulation. Also comment on the merit and demerits of the method. (16) (A/M-05) 13. Explain in detail, the methods of determining voltage regulation of an alternator. (16) 14. Explain in detail, the mmf method for predetermining the voltage regulation of an alternator. (16) 15. A 3 phase star connected 1000 kva, 11 kv alternator has a rated current of 52.5 A. The AC resistance of winding per phase is 0.45 Ω. The test results are as follows: OC test: Field current = 12.5 A; Voltage between the lines 422 V SC test: Field current = 12.5 A; Line current = 52.5 A

4 4 Determine the full load voltage regulation of the alternator by emf method at 0.8 lagging and leading power factor. (16) Synchronizing and parallel operation, Synchronizing torque, Change of excitation and mechanical input 16. Two similar 3 phase, 6600 V, 60 Hz, 1200 kva, star connected alternators has resistance of 1.05 Ω per phase and synchronous reactance of 5 Ω per phase and the no-load saturation curve as follows: Field current, A : Terminal voltage, V : When operating in parallel with a terminal voltage of 6600 V, the first machine supplies 90 A at 0.8 pf lagging. If the total load power factor is lagging and total load is 1600 kw, determine the excitation of the second alternator. (16) (A/M-05) 17. Explain in detail, the conditions for parallel operation of 3 phase alternators with neat diagram. (16) Synchronous reactance using slip test 18. Explain in detail, the two reaction theory of salient pole synchronous machine. Also describe a method to determine direct and quadrature axis reactances of salient pole alternator. (16)

5 5 UNIT II SYNCHRONOUS MOTOR PART A Principle of operation 1. List out the disadvantages of synchronous motor. (N/D-10) 2. What are the main parts of a synchronous motor? 3. Why does a synchronous motor has no starting torque? 4. Why does a synchronous motor always run at synchronous speed? 5. What are the uses of damper windings in a synchronous motor? 6. How does a synchronous machine differ from an induction machine? Torque equation Operation on infinite bus bars 7. What is meant by torque angle? (A/M-11) 8. What is meant by synchronous capacitor? (N/D-11) 9. How is a synchronous motor operated at any desired power factor? 10. What are the various applications of synchronous motor? V-curves 11. What are called V curves? (A/M-11) 12. What is the power factor of a synchronous motor when it is operated at under excited conditions? 13. What are called inverted V curves? 14. Does the change in excitation affect the power factor of a synchronous motor? Power input and power developed equations 15. Write an expression for gross mechanical power developed by synchronous motor. (N/D-06) 16. At what load angle does the power developed in a synchronous motor become maximum? 17. What happens when the load on a synchronous motor is changed?

6 6 Starting methods Current loci for constant power input, constant excitation and constant power developed. 18. What are the different methods of starting a synchronous motor? 19. What are the advantages of 3 phase induction motor over synchronous motor? 20. List out the applications of synchronous motor. Principle of operation Torque equation PART B 1. Draw the equivalent circuit and phasor diagram of synchronous motor. (8) (N/D-10) 2. Explain in detail, the applications of synchronous motor. (8) 3. Explain in detail, the working principle of a synchronous motor with neat diagram. (8) 4. (i) Explain the methods of starting synchronous motor against high-torque loads. (8) (ii) Explain the various torques associated with synchronous motor. (8) Operation on infinite bus bars 5. Explain a synchronous condenser and its usage in power factor correction. (M/J-06) V-curves 6. Explain in detail, the V and inverted V curves of synchronous motor. (8) (N/D-10) 7. How is the power factor of industrial installations improved? (8) 8. (i) Explain the working of a synchronous motor with different excitations. (8) (ii) Explain the main characteristic features of a synchronous motor. (8) Power input and power developed equations 9. A 3300 V, 3 phase, star connected synchronous motor carries a current of 200 A. Armature resistance per phase is 0.2 Ω, field resistance is 1.5 Ω and field current is 30 A. Core losses are 30 kw. Friction, windage and excitation losses are 20 kw. Find the efficiency of the motor, when working at unity power factor and taking rated current. (16) (M/J-06) 10. A 75 kw, 400 V, 4 pole, 3 phase, star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04 Ω and 0.4 Ω respectively. Compute for full load and

7 7 0.8 pf lead, the open circuit emf per phase and gross mechanical power developed. Assume an efficiency as 92.5%. (8) 11. A 6600 V, 3 phase, star connected synchronous motor draws a full load current of 80 A at 0.8 pf leading. The armature resistance is 2.2 Ω per phase and reactance is 22 Ω per phase. If the stray losses of the machine are 3200 W, determine the following: a) Emf induced b) Output power c) Efficiency of the machine (16) Starting methods Current loci for constant power input, constant excitation and constant power developed. 1. Explain the methods used for starting of synchronous motor (N/D-10) 2. Explain the effect of changing the field current of a synchronous motor at constant load. (N/D-10) 3. Explain the effect of change in load on load angle and power factor of a three phase synchronous motor operating on infinite bus bar with constant excitation. (16) (N / D 2010) 4. Explain in detail, the various methods of staring of synchronous motor. (16) 5. Explain in detail, the effect of excitation on armature current and power factor of a synchronous motor. (16) 6. Explain the following: a) Constant excitation circle b) Constant power circle (16)

8 8 Constructional details Types of rotors Unit III THREE PHASE INDUCTION MOTOR PART A 1. Write the advantages of slip ring induction motor. (N/D-06) 2. List out the advantages of double squirrel cage motor. (N/D-06) 3. What are the advantages of slip-ring induction motor over squirrel cage induction motor? 4. List out the types of rotors of an induction motor. 5. What are the advantages of a cage induction motor? Principle of operation 6. What are the two fundamental characteristics of rotating magnetic field? (N/D-10) 7. Why are the rotor slots skewed in squirrel cage induction motor? (A/M-11) 8. Why should not the high power induction motors be started directly? 9. What is the principle of operation of an induction motor? 10. Why is an induction motor called as rotating transformer? 11. Define Crawling 12. Define Cogging Slip 13. When will the slip of the induction motor be negative? (N/D-10) 14. Find the slip of a 3 phase, 6 pole, 50 Hz induction motor running at 970 rpm. (A/M-11) 15. Define Slip (N/D-11) 16. A 3 phase induction motor has 2 poles and is connected to 400 V, 50 Hz supply. Calculate the actual rotor speed and rotor frequency, when the slip is 4%. Equivalent circuit circuit 17. Draw the equivalent circuit of three phase squirrel cage induction motor. (N/D-06)

9 9 18. What is meant by synchronous watt? Slip-torque characteristics -Condition for maximum torque Losses and efficiency Load test 19. State the condition at which the maximum torque is developed in a 3 phase induction motor. 20. What are the various losses in an induction motor? Synchronous induction motor 21. What is meant by synchronous induction motor? PART B Constructional details Types of rotors Principle of operation 1. Explain in detail, the principle of working of a three phase induction motor. (8) (N/D-11) 2. Explain in detail, the constructional details of a squirrel cage 3 phase induction motor with neat diagram. (16) Slip 3. The power input to the rotor of 400 V, 50 Hz, 6 pole, 3 phase induction motor is 75 kw. The rotor electromotive force is observed to make 100 complete alternations per minute. Calculate slip, rotor speed, rotor copper loss per phase and mechanical power developed. (16) 4. A 6 pole induction motor is fed from 50 Hz supply. If the frequency of rotor emf at full load is 2 Hz, find full load speed and slip. (6) 5. The power input to the rotor of a 3 phase, 50 Hz, 6 pole induction motor is 80 kw. The rotor emf makes 100 complete alternations per minute. Determine the following: a) Slip b) Motor speed c) Mechanical power developed d) Rotor copper loss per phase e) Rotor resistance per phase if rotor current is 65 A f) Torque developed Equivalent circuit 6. Develop the equivalent circuit for a three phase slip ring induction motor. (16) (N/D-11)

10 10 Slip-torque characteristics -Condition for maximum torque 7. In a 3 phase induction motor, the maximum torque is 2 times the full load torque and starting torque is equal to the full load torque. Calculate the full load speed and speed at which maximum torque occurs. (16) (A/M-05) 8. Explain in detail, the slip-torque characteristic of three phase induction motor. (8) (N/D-11) 9. Explain the effect of varying rotor resistance of an induction motor on its speed-torque characteristics. (8) 10. Derive the condition for maximum starting torque of a three phase induction motor. (8) 11. Derive the various torque equations of a three phase induction motor. (16) 12. A 3300 V, 10 pole, 50 Hz three phase star connected induction motor has slip ring rotor resistance per phase = Ω and standstill reactance per phase = 0.25 Ω. If the motor runs at 2.5 percent slip on full load, determine the following: a) Speed of the motor b) Speed at which the torque will be maximum c) The ratio of maximum torque to full load torque (16) Losses and efficiency 13. Explain the power stages on a 3 phase induction motor and also their relationships with gross torque T g and overall efficiency. (8) (A/M-05) 14. A 220 V, 3 phase, 4 pole, 50 Hz, star connected induction motor is rated at 3.73 kw. The equivalent circuit parameters are, R 1 = 0.45 Ω, X 1 = 0.8 Ω, R 2 = 0.4 Ω, X 2 = 0.8 Ω and B 0 = 1/30 mho. The stator core loss is 50 W and rotational losses are 150 W. For a slip of 4 %, find the input current, power factor, air gap power, mechanical power, electromagnetic torque, output power and efficiency. Also draw the equivalent circuit and mark the given parameters. (16) (N/D-06) Load test -No load and blocked rotor tests -Circle diagram 15. A 3 phase, 400 V induction motor gave the following test readings: No load test : 400 V; 1250 W; 9 A Blocked rotor test : 150 V; 4 kw; 38 A Draw the circle diagram. If the normal rating is kw, calculate the full load current and slip from the circle diagram. (16) (A/M-05)

11 In a 6 pole, 3 phase 50 Hz motor with star connection, the rotor resistance per phase is 0.03 Ω, reactance at standstill is 1.5 Ω per phase and emf between the slip rings on O.C. is 175 V. Calculate the following: a) Slip at speed of 950 rpm b) Rotor emf per phase c) Rotor frequency d) Reactance at a speed of 950 rpm (16) 17. Draw the circle diagram for a 20 HP, 50 Hz, 3 phase star connected induction motor with the following data: No load test : 400 V; 9 A and pf 0.2 lagging Block rotor test : 200 V; 50 A and pf 0.4 lagging Determine the line current and efficiency at full load. Assume stator and rotor copper losses on blocked rotor to be equal. (16) Separation of no load losses Double cage rotors Induction generator Synchronous induction motor 18. Explain in detail, the construction and working principle of a double cage induction motor.

12 12 UNIT IV STARTING AND SPEED CONTROL OF THREE PHASE INDUCTION MOTOR PART-A Need for starting 1. What are the starting methods used for three phase induction motor? (A/M-11) 2. What is the necessity of a starter? 3. How does the slip vary with the load? Types of starters Rotor resistance, Autotransformerand Star-delta Starters 4. List out the disadvantages of speed control of an induction motor using rotor resistance. (N/D-06) 5. Define Crawling (A/M-11) 6. State the effect of rotor resistance on the starting torque of an induction motor. (N/D-11) 7. Why is the rotor resistance starter not used in a squirrel cage induction motor? 8. What are the various types of starters used for squirrel cage induction motor? 9. List out the disadvantages of autotransformer starter. 10. What are the advantages of DOL starter? 11. What are the advantages of star-delta starter? Speed control Change of voltage, torque, number of poles and slip 12. List out the various methods of speed control of three phase induction motor. (N/D-11) 13. How is super-synchronous speed achieved in an induction motor? 14. What are the possible methods available to control the speed of a 3 phase induction motor? 15. How can the direction of rotation of a 3-phase induction motor be reversed? Cascaded connection Slip power recovery scheme 16. What are the significances of cascaded connection of induction motors? 17. What is slip power recovery scheme?

13 13 PART-B Need for starting 1. Explain in detail, the cogging and crawling. (8) (M/J-06) Types of starters Rotor resistance, Auto-transformer and Star-delta starters 2. Explain the rotor rheostat control of a three phase slip ring induction motor. (8) (N/D-06) 3. The rotor of a 4 pole, 50 Hz slip ring induction motor has a resistance of 0.3 Ω per phase and runs at 1440 rpm at full load. Calculate the external resistance per phase which must be added to lower the speed to 1320 rpm when the torque is the same. (8) (N/D-06) 4. A 15 HP, 3 phase, 6 pole, 50 Hz, 400 V, delta connected induction motor runs at 960 rpm on full load. If it takes a current of 86.4 A at starting, find the ratio of starting torque to full load torque with star-delta starter. Full load efficiency and power factor are 88% and 0.85 lagging respectively. (8) (A/M-11) 5. Explain with a neat sketch, the principle and working of a star-delta starter and an autotransformer starter. (16) (N/D-11) 6. Explain in detail, the various schemes of starting of squirrel cage induction motor. (16) 7. Explain in detail, the working of rotor resistance starter with a neat diagram. (16) Speed control Change of voltage, torque, number of poles and slip Cascaded connection 8. Explain the various speed control schemes used for induction motor. (16) (M/J-06) 9. Explain the cascaded operation of induction motors to obtain variable speed. (8) 10. Explain the following speed control schemes of three phase induction motor. a) Voltage control method b) Changing the number of poles Slip power recovery scheme. 11. Explain with a neat sketch, the slip power recovery scheme of three phase induction motor. (N/D-11)

14 14 UNIT V SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES PART-A Constructional details of single phase induction motor Double revolving field theory and operation Equivalent circuit No load and blocked rotor test 1. Draw the equivalent circuit of single phase induction motor without core loss. (N/D-06) 2. List out the types of single phase induction motors. 3. Why are centrifugal switches provided in single phase induction motors? 4. How can the direction of rotation of a capacitor start induction motor be reversed? 5. What is meant by split phase induction motor? 6. What is meant by capacitor start capacitor run motor? Performance analysis Starting methods of single-phase induction motors - 7. List out the applications of single phase induction motor. (A/M-11) Shaded pole induction motor Linear reluctance motor Repulsion motor Hysteresis motor AC series motor 8. List out the applications of linear induction motor. (A/M-04) 9. List out the applications of shaded pole induction motor. (A/M-05) 10. What are the demerits of repulsion motor? (N/D-06) 11. What are the different types of repulsion motors? 12. What is meant by universal motor? 13. How does a universal motor differ from a DC series motor? PART-B Constructional details of single phase induction motor Double revolving field theory and operation 1. Explain in detail, the double revolving field theory. (8) (M/J-06)

15 15 2. Explain in detail, the following types of induction motors: a) Split phase induction motor b) Capacitor run induction motor Equivalent circuit No load and blocked rotor test Performance analysis Starting methods of single-phase induction motors 3. Explain in detail, the split phase capacitor start induction run motor with slip-torque characteristics. (8) (N/D-11) 4. Derive the equivalent circuit of a single phase induction motor with the help of double revolving field theory. (16) 5. A 230 V, 50 Hz, 380 W, 4 pole, single phase induction motor gave the following results: No load test : 230 V; 84 W; 2.8 A Blocked rotor test : 110 V; 460 W; 6.2 A The stator winding resistance is 4.6 Ω and during the blocked rotor test the auxiliary winding is open. Determine the equivalent circuit parameters. Shaded pole induction motor Linear reluctance motor Repulsion motor Hysteresis motor AC series motor. 6. Write short notes on the following: a) Induction generator b) AC series motor c) Permanent magnet motor (16) (A/M-05) 7. What is a stepper motor? Explain the different types of stepper motors. (16) 8. Explain in detail, the construction and working principle of linear induction motor and hysteresis motor. (16) (A/M-11) 9. Explain in detail, the working of a repulsion motor with neat sketch. (16) 10. Explain in detail, the operation of permanent magnet DC motor. Also list out its advantages. (16)

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