University of Mumbai 2007-6th Sem B.E Electrical and Electronics Engineering Electrical Machines-II - Question Paper
N.B.: (1) Question No. 1 Is compulsory. |\T
(2) Attempt any four out of remaining.
{3) Assume additional data if necessary and justify the same.
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1. Write short noles on : s 20
(a) Cogging and crawling in induction motor
(b) Hunting in synchronous motor
(c) Slip test to determine Xd and Xq.
2. (a) The 110 V, 150 W, 50 Hz, 6 pole IM gave following data 10
NL Test 110 V 63 W 2-7 A BR Test 55 V 212 W 5-8 A
Stator winding resistance 2-5 fi and starting winding is open during BR test. Determine eq. circuit parameters and care, friction and windage losses.
(b) Explain split phase 1* IM. How to improve starting torque of split phase IM. Compare 10 their torqje speed characteristics.
3. (a) Derive an expression for mechanical power developed In case of salient pole 10 synchrorous machine.
(b) A 3300 V star connected synchronous motor has synchronous inpedance 0-4 + j5 O. per 10 phase for an excitation emf of 4000 V and motor Input power of 1000 kW at rated voltage, compute line current and power factor.
4. (a) Explain how synchronous motor is used to improve powerfactor with phasor diagram In 10 details hence explain V and inverted V curves.
(b) A 3 phase induction motor has 150% starting torque and 250% max. torque of Its full load 10 torque. Neglect stater resistance and assume const rotor resist. Compute slip at max. torquo, full load slip, rotor current at starting In terms of full load current.
5. (a) A 20 MVA, 3 phase star connected 11 kV, 12 pole, 50 Hz salient pole synchronous motor 10 with negligible armature resistance has reactances of X d = 5 Cl (five) and X = 3 2. At full load, upf and rated voltage compute excitation voltage, powe', synchronising power per electrical degree and corresponding torque.
(b) Explain auto transformer starting and star delta starting for three phase induction,motor 10 and compare their torques with DOL starting.
0. (a) Explain with the help of neat diagram, torque-speed characteristics of a phase induction 10 motor. What is effect of adding resistance to rotor circuit.
(b) A 15 kW, 400 V, 4 pole 50 Hz three phase star connected Induction motor gave following 10 test results.
Line Vtg Line Current power l/p NL test 400 V 9 A 1310 W
BR lest 200 V 50 A 7100 W
Assume stator and rotor ohmic losses equal at standstill. Draw circle diagram and find line current, power factor, slip torque and efficiency at full load.
7. (a) A 6-6 kV 3 phase 50 Hz star connected alternator gave following test data : 15
lr in A 3-2 5-0 7-5 10 0 14-00
E, ir kV 3*1 4-9 6-6 7-5 8-24
lirA 500 778 1170
Zp, terminal voltage in KC - 1-85 4-24 5-76 7*0 Per phase armature resist is 0-2 O, Calculate voltage regulation at full load current of 500 A at 0-8 pf tag by ZPF and ASA method.
(b) Derive expression for distribution factor. 5
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