Code No: R05220204 Set No. 1

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007ELECTRICAL MACHINES-II

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Explain the various types of material used in construction of core of trans-former? Briefly explain all the properties. Explain how quality of core materialis related with core losses in transformer.

(b) A single phase transformer has 400: 1000 turns ratio. The net cross sectionalarea 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 voltageinduced 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, findthe all day efficiency of transformer. [6+10]

12 hours No load6 hours 12 kW, 0.8 pf6 hours 20 kW at upf.

3. (a) Explain the various simple tests conducted on a single transformer to find theapproximate equivalent circuit of transformer.

(b) OC test is preferred to conduct on LV side & SC test is preferred to conducton 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 supplies2-Φ power at 500 V/Ph. Calculate the line current on 3-Φ system, if the loadon 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 findslip speed & slip.

(c) A 12 pole, 3-Φ alternator driven at a speed of 500 rpm supplies power to an8-pole, 3-Φ, IM. If the slip of the motor at full load is 3 %, calculate the fullload 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 r1 = 0.8 Ω, x1 = 1.4 Ω, r2′ = 0.4 Ω, x2′ = 0.7 Ω,Xo = 40 Ω, Ro = 160 Ω. The supply voltages are Va = 150 at 00 , Vb = 250 at−900, Vc = 350 at 1100. 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 thecircle diagram. [16]

8. Explain all the modes of operation of Induction machine. Plot the neat character-istics. [16]

⋆ ⋆ ⋆ ⋆ ⋆

2 of 2

Code No: R05220204 Set No. 2

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007ELECTRICAL MACHINES-II

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Explain the various types of material used in construction of core of trans-former? Briefly explain all the properties. Explain how quality of core materialis related with core losses in transformer.

(b) A single phase transformer has 400: 1000 turns ratio. The net cross sectionalarea 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 voltageinduced 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 pfand at half load, 0.8 pf lagging. Find maximum efficiency of transformer andcorresponding load. [8+8]

3. (a) Explain the working principle of auto transformer and give the constructionalfeatures 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. Theresistance 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 inputcurrent & power. [8+8]

4. (a) With neat phasor diagram, explain the voltage regulation of three-phase trans-former.

(b) An ideal 3-Φ step down transformer connected in delta/star delivers powerto a balanced 3-Φ load of 120 kVA at 0.8 pf. The input line voltage is 11kV and the turn’s ratio of transformer (phase to phase) is 10. Determine theline 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 operatingwith 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 % ratedfrequency

1 of 2

Code No: R05220204 Set No. 2

(b) Obtain the direction of rotation of RMF, rotor, rotor MMF. Obtain the rela-tion between them. [8+8]

6. (a) Explain the effect of load on performance of IM. How the operating range ofIM can be improved.

(b) The power supplied to a 3-Φ IM is 40 kW and the corresponding stator lossesare 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 loadvalue when switched on directly. What will be the % reduction in startingtorque 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 othertype of starter? What is the reason that rotor resistance starters are limitedin 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 2007ELECTRICAL MACHINES-II

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll 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 phasetransformer having maximum flux density of 1.5 T is 500 cm

2. Determine thefollowing:

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 pfand at half load, 0.8 pf lagging. Find maximum efficiency of transformer andcorresponding load. [8+8]

3. (a) Explain how voltage regulation & approximate equivalent circuit of trans-former can be obtained from OC & SC test data.

(b) A 30 kVA, 6000/230 V, 50 Hz, single-phase transformer has the followingwinding resistances and leakage reactances:

R1=10 Ω, R2 (ref to primary) = 10.8 Ω,X1= 16 Ω, X2(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 trans-former.

(b) An ideal 3-Φ step down transformer connected in delta/star delivers powerto a balanced 3-Φ load of 120 kVA at 0.8 pf. The input line voltage is 11kV and the turn’s ratio of transformer (phase to phase) is 10. Determine theline 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 240conductors per phase and rotor winding has 48 conductors/phase. The per phaserotor resistance & standstill rotor reactance are 0.02Ω and 0.08 Ω. Neglect statorimpedance.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 ringson open circuit, when at rest and supplied with normal supply voltage. Therotor impedance per phase is 0.3 + j1.5 Ω. Find the rotor current and torquewhen 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 testresults:

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 character-istics. [16]

⋆ ⋆ ⋆ ⋆ ⋆

2 of 2

Code No: R05220204 Set No. 4

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2007ELECTRICAL MACHINES-II

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Explain the various types of material used in construction of core of trans-former? Briefly explain all the properties. Explain how quality of core materialis related with core losses in transformer.

(b) A single phase transformer has 400: 1000 turns ratio. The net cross sectionalarea 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 voltageinduced 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 pfand at half load, 0.8 pf lagging. Find maximum efficiency of transformer andcorresponding load. [8+8]

3. (a) Explain why short circuit test is conducted on transformer. Give relatedcalculations.

(b) Obtain the equivalent circuit of a 200/400 V, 4 kVA transformer, referred toLV 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 trans-former.

(b) An ideal 3-Φ step down transformer connected in delta/star delivers powerto a balanced 3-Φ load of 120 kVA at 0.8 pf. The input line voltage is 11kV and the turn’s ratio of transformer (phase to phase) is 10. Determine theline 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 240conductors per phase and rotor winding has 48 conductors/phase. The per phaserotor resistance & standstill rotor reactance are 0.02Ω and 0.08 Ω. Neglect statorimpedance.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 statorimpedance.

(b) Explain the various losses taking place in IM. Explain the effect of slip on theperformance of IM. [8+8]

7. With neat diagram explain the various tests to be conducted on 3-Φ IM to plot thecircle 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 fullload speed of 975 rpm. Calculate the external resistance per phase requiredto be added in rotor circuit to decrease the speed to 800 rpm. The torqueremains the same as before. [8+8]

⋆ ⋆ ⋆ ⋆ ⋆

2 of 2