R5310205-electrical Machines - Iii

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Code No: R5310205

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-III (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain the integral slot winding and fractional slot winding with all associated calculations. (b) Determine the slot distribution and the pole phase group sequence for a 48 slot, 10 pole, 3-Φ winding. [6+10] 2. (a) Explain the characteristics and nature of harmonics present in generated emf of alternator? (b) The flux density distribution in the air gap of an alternator is B = B1 sin θ + B3 sin 3θ + B5 sin 5θ wb/m2 , where B3 = 0.3B1 & B5 = 0.2B1 . The total flux per pole is 0.08 wb. The coil span is 80% of pole pitch. Find the RMS value of EMF induced in single turn machine. [6+10] 3. (a) Explain the various tests conducted on an alternator to find the voltage regulation of an alternator by Potier triangle method. (b) The no load excitation of an alternator required to give rated voltage is 1 pu. In a short circuit test with full current flowing in the armature, the field excitation was 0.85 pu. Determine the approximate excitation that will be required to give full load current at 0.78 PF leading at the rated terminal voltage. [8+8] 4. A 5 MVA, 10 kV, 1500 RPM, 3-Φ, 50 Hz alternator is running in parallel with other machines. Its synchronous reactance is 20 %.Find synchronizing power & synchronizing torque per degree mechanical displacement, for no load & full load, 0.8 pf lagging. [16] 5. (a) Explain the use of synchronous motor as synchronous condenser. What are the advantages of using synchronous motor as synchronous condenser over capacitor bank? (b) A 440 V, 50 Hz, 3-Φ circuit takes 18 A at a lagging power factor of 0.8. A synchronous motor is used to raise the power factor to unity. Calculate the kVA input to motor and its power factor, when delivering a mechanical load of 6 kW. The motor has an efficiency of 88%. [8+8] 6. (a) Explain the importance of ‘power circles’ in determining the characteristics of a synchronous machine. (b) Explain the phenomenon of Hunting in synchronous machine?

[8+8]

7. A 230 V, 50 Hz, 4 pole single phase Induction motor has following parameters: Stator resistance = 2.51 Ω ,Rotor resistance referred to stator = 7.81 Ω, Magnetising reactance = 150.88 Ω, Stator reactance = 4.62 Ω, Rotor reactance referred to stator = 4.62 Ω Determine the main winding current and power factor when the motor is running at a slip of 5%. [16] 8. (a) Compare AC series motor & DC series motor. What are the operational difficulties of each? (b) Explain the variable reluctance motor principle. ?????

[8+8]

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Code No: R5310205

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-III (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Derive the expression for per phase EMF induced in a 3-Φ alternator? Explain the factors affecting the value of EMF induced. (b) Give the winding details for a 3-Φ armature winding with following details: Pole = 4, Number of slots = 30, Double layer winding. [8+8] 2. (a) Explain the leakage reactance & armature reactance of an alternator. (b) The SC, OC & DC test data (line) for a Delta connected 125 kVA, 480 V, 60 Hz, are: VOC = 480 V, ISC = 150.6 A - - - - For same field current VDC = 24 V, IDC = 15.6 A Determine: Synchronous reactance. [8+8] 3. (a) With proper explanation & diagram, Justify the statement ‘M M F method f or f inding voltage regulation is optimistic and EM F method f or f inding voltage regulation is pessimistic’. (b) The no load excitation of an alternator required to give rated voltage is 160 A. In a short circuit test with full current flowing in the armature, the field excitation was 135 A. Determine the approximate excitation that will be required to give full load current at 0.8 PF lagging at the rated terminal voltage. [8+8] 4. (a) Explain the various methods of synchronization of alternators. (b) Two similar 4 MVA alternators operate in parallel. The governor of first machine is such that frequency drops from 50 Hz at no load to 47.5 Hz at full load. The corresponding drop for second machine is 50 Hz to 48 Hz. [8+8] i. How will they share a load of 6 MW? ii. What is the maximum load they can share at UPF without over loading any generator? 5. (a) Explain, why synchronous motor is not self starting? (b) A 3-Φ, 600 V, star connected SM has effective per phase armature resistance & synchronous reactance of 0.4 Ω & 3.6 Ω respectively. Calculate the induced EMF per phase if the motor works on full load delivering 326 kW. The full load efficiency is 87 % having power factor of 0.8 leading. Also calculate the load angle. [6+10] 6. (a) Explain the importance of ‘power circles’ in determining the characteristics of a synchronous machine. (b) Explain the phenomenon of Hunting in synchronous machine? [8+8] 7. (a) Why single phase motors are not self starting? (b) Explain the necessary arrangements made to make single phase Induction motor self starting & with neat diagram explain the operations of same. [8+8] 8. (a) Explain the phasor diagram of Universal motor. (b) Prove that the speed of AC series motor, when operated on DC is more than when operated on AC. [8+8] ?????

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Code No: R5310205

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-III (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Compare i. full pitch winding and short pitch winding ii. concentrated winding and distributed winding. (b) A 3-Φ, star connected, 2 pole, 3000 RPM alternator has 72 slots on its periphery. Each slot has 8 conductors per slot and winding is short pitched by 4 slots. Find the pitch factor and distribution factor of the winding. Also, calculate the induced EMF between lines, if the fluxes of 10 mwb are distributed sinusoidaly. All the conductors in a phase are connected in series. [8+8] 2. (a) What are harmonics? Explain how harmonics are present in generated e.m.f of alternator? What are the effects of hormonics an generated emf? (b) A star connected 3-Φ alternator has an induced EMF of 400 V between the lines. Due to presence of third harmonic component, the phase voltage is 244 V. Find the value of i. Find the value of third harmonic voltage.

[10+6]

3. (a) Explain the various tests conducted on an alternator to find the voltage regulation of an alternator by Potier triangle method. (b) The no load excitation of an alternator required to give rated voltage is 1 pu. In a short circuit test with full current flowing in the armature, the field excitation was 0.85 pu. Determine the approximate excitation that will be required to give full load current at 0.78 PF leading at the rated terminal voltage. [8+8] 4. (a) Explain the ‘two bright one dark’ & ‘all dark’ method of synchronization of alternators. (b) The EMFs of two alternators are 30006 200 & 29006 00 V. Their synchronous impedances are 2 + j20 Ω/ph & 2.5 + j30 Ω/ph. The load impedance is 10 + j4 Ω/ph. Find the circulating current. [8+8] 5. A 500 V, 3-Φ, star connected synchronous motor has resistance & Synchronous reactance of 0.4 Ω/ph & 3.6 Ω/ph respectively. The OC voltage is 600 V. If friction and core losses are 1 kW, calculate the line current & power factor when the motor output is 62 kW. [16] 6. (a) What is damper winding? What is the need of providing damper winding for synchronous machines? (b) Give the applications of Synchronous motor & Synchronous Induction motor. (c) Give the limitations of Synchronous Induction motor.

[8+4+4]

7. (a) Explain the construction & operation of repulsion motor. (b) List the applications of single phase Induction motor. What are the limitations of single phase Induction motor? [8+8] 8. (a) Compare AC series motor & Universal motor. (b) Compare variable reluctance stepper motor & permanent magnet stepper motor. ?????

[8+8]

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Code No: R5310205

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ELECTRICAL MACHINES-III (Electrical & Electronics Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain the differences between stationary armature and rotating armature. What are the advantages of rotating armature over stationary armature? (b) A 4 pole alternator has an armature with 25 slots and 8 conductors per slot and rotates at 1500 rpm and the flux per pole is 0.05 wb. Calculate the EMF generated, if winding factor is 0.96 and all the conductors in a phase are in series. [8+8] 2. A 3-Φ star connected alternator has 81 slots, 6 poles and a double layer, narrow spread winding, having a coil span of 13 slot pitches. The flux density distribution in the air gap is B(θ) = sinθ + 0.4 sin3θ + 0.25sin5θ. Find (a) RMS value of third & fifth harmonic phase voltages in terms of fundamental frequency phase voltage. (b) Ratio of line voltage to phase voltage. [12+4] 3. (a) Explain the voltage regulation method of an alternator by which the armature reaction & leakage reactance can be separated. (b) A 3-Φ, star connected salient pole synchronous generator is driven at a speed near synchronous with the field circuit open and the stator is supplied from a balanced 3-Φ supply. Voltmeter connected across the line gave minimum and maximum readings of 1196 V & 1217 Volts. The line current fluctuated between 120 & 225 Amp. Find the direct and quadrature axis reactances per phase. Neglect armature resistances. [8+8] 4. (a) A 2 MVA, 8 pole, 3-Φ, alternator is connected to 6000 V, 50 Hz bus bars & has a synchronous reactance of 4 Ω/ph. Calculate the synchronizing power & synchronizing torque per mechanical degree of rotor displacement at no load. Assume normal excitation. (b) Explain the effect of damper winding & field winding on the transient behavior of an alternator. How the effect of these two can be minimised? [8+8] 5. (a) Compare (all 3-Φ) synchronous motor, Induction motor & transformer. (b) A synchronous motor absorbing 50 kW is connected in parallel with a factory load of 200 kW at 0.8 lagging pf. If the resultant power factor after connecting SM is 0.9 lagging, how much leading kVAR are supplied by synchronous motor. At what power factor is it working? [8+8] 6. (a) Explain the construction of damper winding. Clearly show the location of damper winding. (b) With neat diagram & explanation, show how damper winding prevents oscillations. [8+8] 7. (a) Why single phase motors are not self starting? (b) Explain the necessary arrangements made to make single phase Induction motor self starting & with neat diagram explain the operations of same. [8+8] 8. (a) Explain how the speed of Universal motor can be controlled. (b) Explain ‘conductively compensated’ AC series motor & ‘inductively compensated’ AC series motor. [8+8] ?????