Code No: R05220204 Set No. 1 II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007 ELECTRICAL MACHINES-II (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Explain the various types of material used in construction of core of transformer? Briefly explain all the properties. Explain how quality of core material is related with core losses in transformer. (b) A single phase transformer has 400: 1000 turns ratio. The net cross sectional area of the core is 60 cm 2. The primary winding be connected to 50 Hz supply & 500 V. Calculate the peak value of flux density in the core. The voltage induced in the secondary winding. [8+8] 2. (a) Why it is essential to calculate all day efficiency for distribution transformers. How it is different from full load efficiency. (b) A 20 kva transformer has maximum efficiency of 98% when delivering 75% load at 0.95 pf. If during the day transformer is delivering load as follows, find the all day efficiency of transformer. [6+10] 12 hours No load 6 hours 12 kw, 0.8 pf 6 hours 20 kw at upf. 3. (a) Explain the various simple tests conducted on a single transformer to find the approximate equivalent circuit of transformer. (b) OC test is preferred to conduct on LV side & SC test is preferred to conduct on HV side. Explain the reasons. [10+6] 4. (a) What is the three winding transformer? What is the third winding called as? Why third winding is needed? (b) A Scott connected transformer is fed from a 6600 V, 3-Φ network and supplies 2-Φ power at 500 V/Ph. Calculate the line current on 3-Φ system, if the load on the 2-Φ sides are 500 A at 0.8 pf lagging (teaser) and 400 A at 0.9 lagging (main). [6+10] 5. (a) Deduce the expression for (rotor side) starting current, starting power factor, standstill frequency and standstill EMF of squirrel cage IM. (b) A 4-pole IM is fed from 50 Hz supply and has rotor speed of 1425 RPM find slip speed & slip. (c) A 12 pole, 3-Φ alternator driven at a speed of 500 rpm supplies power to an 8-pole, 3-Φ, IM. If the slip of the motor at full load is 3 %, calculate the full load speed of the motor. [8+4+4] 1 of 2
Code No: R05220204 Set No. 1 6. The parameters of a 3-Φ IM are r 1 = 0.8 Ω, x 1 = 1.4 Ω, r 2 = 0.4 Ω, x 2 = 0.7 Ω, X o = 40 Ω, R o = 160 Ω. The supply voltages are V a = 150 at 0 0, V b = 250 at 90 0, V c = 350 at 110 0. If the slip is 0.04 find: (a) air gap power for positive and negative sequence (b) rotor copper loss for positive & negative sequence (c) total mechanical power developed. [16] 7. With neat diagram explain the various tests to be conducted on 3-Φ IM to plot the circle diagram. [16] 8. Explain all the modes of operation of Induction machine. Plot the neat characteristics. [16] 2 of 2
Code No: R05220204 Set No. 2 II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007 ELECTRICAL MACHINES-II (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Explain the various types of material used in construction of core of transformer? Briefly explain all the properties. Explain how quality of core material is related with core losses in transformer. (b) A single phase transformer has 400: 1000 turns ratio. The net cross sectional area of the core is 60 cm 2. The primary winding be connected to 50 Hz supply & 500 V. Calculate the peak value of flux density in the core. The voltage induced in the secondary winding. [8+8] 2. (a) Derive the condition for maximum efficiency of a transformer. (b) A single phase 150 kva transformer has efficiency of 96 % at full load, 0.8 pf and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and corresponding load. [8+8] 3. (a) Explain the working principle of auto transformer and give the constructional features for the same. (b) An auto transformer has input voltage of 9.5 V at its input terminal AC. The output terminals AB are shorted. The ratio of transformer is 2:1. The resistance and leakage reactances of section AB are 0.06 Ω & 0.2 Ω respectively, while those for section BC are 0.04 Ω & 0.1 Ω respectively. Find the input current & power. [8+8] 4. (a) With neat phasor diagram, explain the voltage regulation of three-phase transformer. (b) An ideal 3-Φ step down transformer connected in delta/star delivers power to a balanced 3-Φ load of 120 kva at 0.8 pf. The input line voltage is 11 kv and the turn s ratio of transformer (phase to phase) is 10. Determine the line voltage line currents, phase voltages, phase currents on both primary & secondary sides. [8+8] 5. (a) A four pole/ eight pole multi speed 60 Hz, 10 HP, 240 V, 3-Φ, IM operating with 4 poles runs at 1750 RPM when fully loaded with its rated voltage & frequency. Determine: i. slip speed ii. % slip iii. The synchronous speed if operating in the 8 pole mode and at 20 % rated frequency 1 of 2
Code No: R05220204 Set No. 2 (b) Obtain the direction of rotation of RMF, rotor, rotor MMF. Obtain the relation between them. [8+8] 6. (a) Explain the effect of load on performance of IM. How the operating range of IM can be improved. (b) The power supplied to a 3-Φ IM is 40 kw and the corresponding stator losses are 1.5 kw. Calculate: total mechanical power developed rotor copper loss & efficiency when the slip is 4 %. [8+8] 7. (a) A 4 pole, 3-Φ, 50 Hz, IM has a starting current which is 5 times its full load value when switched on directly. What will be the % reduction in starting torque if the motor is started with i. with star- starter ii. Auto transformer starter with 65 % tapping. (b) What are the specific advantages of rotor resistance starter over any other type of starter? What is the reason that rotor resistance starters are limited in use? [8+8] 8. Compare various methods of speed control of 3-Φ IM. [16] 2 of 2
Code No: R05220204 Set No. 3 II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007 ELECTRICAL MACHINES-II (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Obtain the expression for the EMF induced in the transformer. (b) The cross section of the core of a 4 kva, 5000/500 V, 50 Hz single phase transformer having maximum flux density of 1.5 T is 500 cm 2. Determine the following: i. The number of turns of primary and secondary windings ii. The EMF per turn. Take staking factor = 0.85. [8+8] 2. (a) Derive the condition for maximum efficiency of a transformer. (b) A single phase 150 kva transformer has efficiency of 96 % at full load, 0.8 pf and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and corresponding load. [8+8] 3. (a) Explain how voltage regulation & approximate equivalent circuit of transformer can be obtained from OC & SC test data. (b) A 30 kva, 6000/230 V, 50 Hz, single-phase transformer has the following winding resistances and leakage reactances: R 1 =10 Ω, R 2 (ref to primary) = 10.8 Ω, X 1 = 16 Ω, X 2 (ref to primary) = 18 Ω. Calculate the primary current with SC secondary. [8+8] 4. (a) With neat phasor diagram, explain the voltage regulation of three-phase transformer. (b) An ideal 3-Φ step down transformer connected in delta/star delivers power to a balanced 3-Φ load of 120 kva at 0.8 pf. The input line voltage is 11 kv and the turn s ratio of transformer (phase to phase) is 10. Determine the line voltage line currents, phase voltages, phase currents on both primary & secondary sides. [8+8] 5. A 3-Φ, 50 Hz, 4 pole, 400 V IM has a delta connected stator winding has 240 conductors per phase and rotor winding has 48 conductors/phase. The per phase rotor resistance & standstill rotor reactance are 0.02Ω and 0.08 Ω. Neglect stator impedance.find with slip rings short circuited: (a) Flux per pole (b) Rotor emf/ph (stand still) (c) Rotor current (stand still) 1 of 2
Code No: R05220204 Set No. 3 (d) Rotor power factor (stand still) (e) Rotor EMF/ph (at 4% slip) (f) Rotor current (at 4% slip) (g) Rotor power factor (at 4% slip) (h) Speed at 4 % slip [16] 6. (a) With neat diagram the explain Torque-Slip characteristics of IM. (b) A 3-Φ, 50 Hz, 4 pole slip ring IM gives a reading of 120 V across slip rings on open circuit, when at rest and supplied with normal supply voltage. The rotor impedance per phase is 0.3 + j1.5 Ω. Find the rotor current and torque when machine is running at 5 % slip. [8+8] 7. A 3-Φ, star connected, 3.73 kw, 200 V, 4-pole, 50 Hz IM gave the following test results: No load Test: 200 V, 5 A, 350 W Blocked rotor test: 100 V, 26 A, 1700W All above are the line values. Plot the circle diagram and for full load find: (a) The line current (b) The power factor (c) Slip (d) Torque (e) Efficiency (f) Maximum power factor Given that rotor copper losses are 50% to stator copper losses at stand still. [16] 8. Explain all the modes of operation of Induction machine. Plot the neat characteristics. [16] 2 of 2
Code No: R05220204 Set No. 4 II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007 ELECTRICAL MACHINES-II (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Explain the various types of material used in construction of core of transformer? Briefly explain all the properties. Explain how quality of core material is related with core losses in transformer. (b) A single phase transformer has 400: 1000 turns ratio. The net cross sectional area of the core is 60 cm 2. The primary winding be connected to 50 Hz supply & 500 V. Calculate the peak value of flux density in the core. The voltage induced in the secondary winding. [8+8] 2. (a) Derive the condition for maximum efficiency of a transformer. (b) A single phase 150 kva transformer has efficiency of 96 % at full load, 0.8 pf and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and corresponding load. [8+8] 3. (a) Explain why short circuit test is conducted on transformer. Give related calculations. (b) Obtain the equivalent circuit of a 200/400 V, 4 kva transformer, referred to LV side; with following test data: OC test: 200 V, 0.8 A, 70 W (LV side) SC test: 20 V, 10 A, 60 W (HV side) [6+10] 4. (a) With neat phasor diagram, explain the voltage regulation of three-phase transformer. (b) An ideal 3-Φ step down transformer connected in delta/star delivers power to a balanced 3-Φ load of 120 kva at 0.8 pf. The input line voltage is 11 kv and the turn s ratio of transformer (phase to phase) is 10. Determine the line voltage line currents, phase voltages, phase currents on both primary & secondary sides. [8+8] 5. A 3-Φ, 50 Hz, 4 pole, 400 V IM has a delta connected stator winding has 240 conductors per phase and rotor winding has 48 conductors/phase. The per phase rotor resistance & standstill rotor reactance are 0.02Ω and 0.08 Ω. Neglect stator impedance.find with slip rings short circuited: (a) Flux per pole (b) Rotor emf/ph (stand still) (c) Rotor current (stand still) (d) Rotor power factor (stand still) 1 of 2
Code No: R05220204 Set No. 4 (e) Rotor EMF/ph (at 4% slip) (f) Rotor current (at 4% slip) (g) Rotor power factor (at 4% slip) (h) Speed at 4 % slip [16] 6. (a) A 4 pole, 400 V, 3-Φ IM has a standstill rotor EMF of 100 V per phase. The rotor has resistance of 50 mω/ph and standstill reactance of 0.5 Ω/ph. Calculate the maximum torque & slip at which it occurs. Neglect stator impedance. (b) Explain the various losses taking place in IM. Explain the effect of slip on the performance of IM. [8+8] 7. With neat diagram explain the various tests to be conducted on 3-Φ IM to plot the circle diagram. [16] 8. (a) Give the connections of poles in consequent pole method of speed control. (b) A 6-pole, 3-Φ, 50 Hz, slip ring IM has its rotor resistance of 0.3 Ω/ph and full load speed of 975 rpm. Calculate the external resistance per phase required to be added in rotor circuit to decrease the speed to 800 rpm. The torque remains the same as before. [8+8] 2 of 2