University of Mumbai 2007-6th Sem B.E Electrical and Electronics Engineering Electrical Machines-III Old - Question Paper
Con. 2427-07. (OLD COURSE) ND-9617
(3 Hours) [Total Marks : 100
N.B. : (1) Question No. 1 is compulsory.
(2) Attempt any four out of remaining.
(3) Assume data wherever necessary and justify the same.
1. (a) Derive expression for distribution factor and coil span factor for 3 phase ac winding. 10 lb) Derive expression for power developed in salient pole synchronous machine. 10
2. A 220 V, 50 Hz, 6 pole star connected alternator with ohmic resistance of 0-06 Q per phase 20 gave following data :
Field current A 0-2 0-4 0-6 0-8 1 0 1-2 1*4 1-8 2 2 2 6 3*0 3*4 Open circuit
Voltage E, volts 29 58 87 116 146 172 194 232 261*5 284 300 310 Short: circuit
current in Amp. 6-6 13-2 20 26*5 32*4 40 46*3 59 - -
Zpf teminat
voltage in V - - 0 29 88 140 1 77 208 230
Find % voltage regulation at F. L current of 40 amps of 0 8 lag by emf, mmf zpf and ASA method.
3. (a) Explain how synchronous motor can be used to improve the power factor with the help 10
of phasor diagram.
{b) Explain starting methods of synchronous motors. 10
4. (a) Whatis necessity of voltage control alongwith frequency control ? Hsnce, explain V/f control 10
of an induction motor.
(b) Explain v/orking of 3 ph invertor circuit with neat diagram and relevant O/P waveforms. 10
5. (a) State the conditions for parallel operation of two alternators. Derive expressions for load 10
shared by each alternator.
(b) Two similar alternators operating in parallel have following data ; 10
Alternator 1 - Capacity 700 kW, frequency drops from 50 Hz at noload to 48-5 Hz at full laod. Alternator 2 - Capacity 700 kW, frequency drops from 50-5 Hz at noload to 48 Hz at full laod Speed regulation If primovers is linear in both case.
Calculate how a total load of 1200 kW is shared by each alternator and find operating frequency at this bus bar.
6. (a) A 10 pole 50 Hz, 3 phase synchronous machine has 4 slots/ph/pole. Each slot has 12 10
conductors. Winding is chorded by one slot. Find distribution factors and pitch factors for fundamental, third arid fifth harmonics.
(b) Explain hunting in synchronous motor. 10
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