Chhattisgarh Swami Vivekanand Technical University 2009-4th Sem B.E Electronics & Tele-Communication Engineering - Question Paper
ELECTRICAL MACHINES
(BE, Electronics and telecommunication, fourth semester, MAY 2009)
B. E. (Fourth Semester) Examination, April-May, 2009
(AEI, El, Et & T Engg. Branch)
ELECTRICAL MACHINES
Time Allowed : Three hours
Maximum Marks : 80
Minimum Pass Marks : 28
Note : All questions are compulsory and carry equal marks. Part (a) is compulsory & solve any two from (b), (c), (d). Draw neat and well labelled diagram whenever necessary.
(a) What.is the function of commutator? 2
(b) Explain how EMF is build-up in self excited DC generator? 7
(c) The magnetization characteristic for a 4 pole, 110 V, 1000 rpm shunt generator is as follows :
Field current: 0 0-5 1 1-5 2 0 2-5 3 0 A
OC voltage : 5 50 85 102 112 116 120 V
Armature is lap connected with 144 conductors. Field resistance is 45 ohms. Determine :
(i) Voltage of the machine will build-up at no load
(ii) The critical resistance
(iii) The machine at which the machine just fails to excite
(iv) Residual (lux per pole
(d) A 250 V, 20 kW shunt motor running at 1500 rpm has a maximum efficiency of 85% when delivering 80% of its rated output. The resistance of the shunt field
winding is 125 Q. Determine the efficiency and speed
of the motor when it draws 100 A from the mains.
(a) How will you minimize the Eddy current loss?
(b) What is the condition for maximum efficiency of the transformer? Derive the condition. Also derive the expression of kVA corresponding to maximum efficiency.
(c) Draw the phasor diagram of single phase transformer for 0-8 lagging power factor, and explain it in detail.
(d) A 3-phase step down transformer is connected to 6600 volt mains and it takes 10 A. Calculate the secondary line voltage, line current and output of the following connections :
(i) Slar-Dclta
(ii) Delta-star
The primary to secondary turns ratio is 12.
328412(24)
A
3. (a) Define Pole-pitch. - 2
(b) What is Distribution Factor? Derive the expression for the same. 7
(c) A 4-pole, 3-phase, 50 Hz* star connected, alternator has 60 slots, with 2 conductor per slot and hanng armature winding of two layer type. Coils air short pitched in such a way that if one coil side lies- m slot number 1, the other lies in slot number 15. Determine the useful flux per pole required to generate a line voltage of 6600 V.
Also derive necessary relation, 7
(d) The following OCC is obtained in case of a 3~phase 6000 V alternator running at nonnal speed :
If (A) : 14 18 23 30 43 VT (V) : 4000 5000 6000 7000 8000
The field current is 17 A when armature is short-circuited and the full load current is flowing. I'sing synchronous impedance method determine the \oltage regulation of the alternator supplying the full load of 200 kVA at 0-707 power factor lagging. Neglect armature resistance.
4. (a) What is the condition for maximum torque at starting
in a 3-phase Induction motor?
(b) Draw and explain equivalent circuit of a 5-phasc Induction motor
vw
328412(24)
Determine approximate the starting torque of a 3-phsse Induction motor in terms of full load torque Vnen started by :
; Star-Deita starter
.. An auto-transformer starter with 50% tapping. The short circuit current of motor is 5 times the fuli load current and full load slip is 5%.
The input power of a 6-pole, 3-phase, 50 Hz. Induction motor is 42 kW. The speed is 970 rpm. The stator losses are 1-2 kW and the friction and windage losses are 1-8 kW. Find :
(r The rotor copper loss
ui' Efficiency of the motor
What is the use of the synchronous condenser?
Explain ihe effect of excitation on power factor with phasor diagrams.
Explain the starting methods used in single phase induction motor.
A ?-phase star connected 8-pole, 11 kV, 50 Hz synchronous motor drawns 60 amp. at 0-8 pf bading. The armature resistance and synchronous
reactance per phase are 2 Q. and 30 Q respectively.
Find ihe back emf and angle of retard.
|
Attachment: |
| Earning: Approval pending. |
