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Posted Date: 01 May 2009 Posted By: Sunil Reddy Member Level: Platinum
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2007 Jawaharlal Nehru Technological University Electronics and Instrumentation Engineering II B.Tech. I-Semester Examinations , August 2007 ELECTRICAL TECHNOLOGY Question paper
1. a) Explain the constructional features of a D.C. Machine with a help of a neat sketch.
b) Name the main parts of a D.C. Machine and state the materials of which each part is made.
c) Explain different methods of excitation of D.C. Generators with suitable diagrams.
2. a) What are all the various losses in a D.C. Machine?
b) Derive an expression for efficiency of a D.C. Machine.
c) A 6-pole, 250V series motor is wave-connected. There are 240 slots and each slot has 4 conductors. The flux per pole is 1.75 X 10-2 Wb when the motor is taking 80A. The filed resistance is 0.05?, the armature resistance is 0 .1? and the iron and frictional loss is 0.1kw. Calculate i) Speed, ii) bhp and iii) shaft torque.
3. a) Discuss in detail the effects of resistance and leakage reactance of the winding on the performance of a Transformer.
b) A 50 KVA, 2000/200v, 50 HZ single phase Transformer has impedance drop of 8 % and resistance drop of 4%. Calculate the regulation of the transformer at F.L when the load P.F. is 0.8 lag. Also determine the P.F. at which the regulation will be maximum.
4. a) Draw the vector diagram for a Transformer when it is loaded with (i) Inductive load (ii) Capacitive load.
b) A 30 KVA, 2400/120v, 50HZ, transformer has a high voltage winding resistance of 0.1 ohms and leakage reactance of 0.22 ohms. The low voltage winding resistance is 0.035ohms and the leakage reactance of 0.012 ohms. Find the equivalent winding resistance, reactance and impedance referred to the (i) High voltage side (ii) Low voltage side.
5. a) With usual notations deduce the expression for running torque of a 3-phase induction motor.
b) A 3 phase, 50Hz, 6-pole induction motor has a shaft output of 10 kW at 930 r.p.m. Friction and windage loss amount to 1% of output. Total stator losses are 600 W. (i) Determine the rotor input and stator input. (ii) If maximum torque is developed at 800 r.p.m. compute the starting torque with rated-voltage starting.
6. a) Explain why an induction motor at no load operates at a very low power factor.
b) A 50 kVA, 440 V, 3-phase, 50 Hz induction motor is provided with a 3-phase step-down autotransformer starter which steps down the voltage to 60% of the input. Given that the starting current of the motor on rated voltage is 6 times the rated full-load current, estimate the current drawn by the autotransformer from the mains at starting. What is the starting kVA drawn by he autotransformer.
7. a) The open circuit and short circuit tests data on a 3 phase, 1MVA, 3.6KV, star connected synchronous generator is given below:
If(A) : 60 70 80 90 100 110
Voc(line) (V): 2560 3000 3360 3600 3800 3960
Isc(A) : 180
Find the regulation at full load, 0.8 p.f. lagging.
b) Explain why a synchronous motor doesn’t have self-starting torque. Explain one method of starting a synchronous motor.
8. a) Describe the construction and working principle of shaded pole induction motor.
b) Give typical applications of capacitor star, split, phase, permenant capacitor, shaded pole induction motors.
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