University of Mumbai 2007-6th Sem B.E Electrical and Electronics Engineering Electrical Machines-II - exam paper
Liabrnory t e C$le.i+ ) (Sen
CD- 5535
(Total Marks *nn
B Con. 5034-07.
(REVISED COURSE) ( 3 Hours)
PJ/DJLPOi-OlCOT-l
N. B.; (1) Question No. 1 is compulsory.
(2) Attempt any four questions out of remaining.
(3) Assume data wherever necessary.
1. (a) Derive torque equation for three phase induction motor and hence explain torque
speed characteristics in motoring zone. (Use Thevenin's model approach.)
(b) Explain need of starter for three phase induction motor and explain star delta starter, compare its starting torque with torque of DOL at starting.
2. (a) Explain split phase and capacitor start capacitor run single phase induction motors
in detail. Draw phasor diagrams and speed torque diagrams.
(b) A 230V, 4 pole, 50Hz single phase induction motor has the following constants and
losses.
r, = 2.3 Q x, * 3.2 Q r2 = 4.2 U x2 = 3.2 fi xm = 74 Q
Ifl
Core loss = 98 watts.
Friction and windage loss = 30 watts.
If this motor is running with a slip 0.05 at rated voltage and frequency, then compute the stator current, pf, power output, torque and efficiency with its auxiliary winding open.
3. (a) Explain Blondel's two reaction theory. Draw phasor diagram.
10
10
(b) A 3 phase 6 pole 1000 rpm alternator has 4 slots / ph I pole with 8 conductors per slot coils are chorded by one slot. The fundamental flux is 18 7 mwb, 3* harmonic flux is 1.72 mwb and fifth harmonic. Flux is 0.6 mwb. Find distribution and pitch factors for fundamental, third and fifth harmonic. Find resultant ph voltage on no toad if alternator is star connected.
4. (a) Explain speed control methods for 3 phase sq. cage induction motor. Explain only
10
10
to
10
necessi:y of V/f control.
(b) Derive expression for distribution factor and coil span factor.
6. (a) A 220V, 50Hz, 6 pole star connected altemLle with ohmic resistance of
0.06 O per phase gave following data
Field current 0.2 04 0.6 0.8 1.0 1.2 1.4 1.8 2.2 2.6 3.0 3.4
I, In A
Open Circuit Voltage 29 28 87 116 146 172 194 232 261.5 284 300 310 E( in Volts
During short circuit test, a full load current of 40A was circulated when field current was 1.2A. A field current of 3.2A was required to circulate a full load current at rated voltage under zero power factor. Find percentage regulation at full load current of 40A at power factor of 0.8 lag by zero para factor method and new ASA method.
<b) Explain crawling in induction motor.
7. (a) Explain starting of synchronous motor.
(b) A three phase sq. cage induction motor has a short circuit current of 5 times the full load current. Its full load slip is 5%. Calculate the starting torque as a percentage of full load torque if motor is started by
(a) Direct on line starter
(b) Star Delta Starter
(c) Auto transformer starter, limiting the motor starting current to twice the motor full load current. Also find starting current drawn from the supply in terms of motor full load current. What is the percentage auto transformer tapping in this case.
(d) Auto transformer starter limiting the supply line starling current to twice the full load current. Find auto transformer tapping in this case also.
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| Earning: Approval pending. |
