Electrical Technology — DU B. Sc. (Hons) Electronics Previous Year Question Papers

Uniquepaper code : B.Sc.al) Electronics (Core) Electrical Technology Instruction for Candidates (a) What are the advantages of polyphase system over single phase system? a) Name the parameters measured by the open circuit and short circuit test for transformer. (c) State different kind of losses in a dc generator. (d) A series motor should not be started without I.oad. Justify. (e) A three phase llKV, 50Hz synchronous generator runs at 1500rpm.Determine the number of poles. (I) What is Slip? What is its value at synchronous speed? 2. (a) Show that the total power in both delta and star three phase system for balanced load conditions is given by the same expression. (b) With the help of phasor diagram, explain the operation of single phase transformer under full load condition. (c) A 25-kvA transformer has 500 turns on the primary and 50 turns on the secondary winding. The primary is connected to 3000-V, 50-Hz supply. Find the full-load primary and secondary currents, the secondary e.in.f. and the maximum flux in the core. Neglect leakage drops and no-load prinary current. 3. (a) Derive the voltage equation of a dc motor. What is the significance of back e.in.fin dc motor? a) A shunt generator delivers 195 A at terminal p.d. of 250 V. The amature resistance and shunt field resistance are 0.02 a and 50 a respectively. The iron and friction losses 4. 5. 6. 7. equal 950 W. Find (a) e.in.f. generated a) Cu losses (c) output of the prime mover (d) commercial, mechanical and electrical efficiencies. (c) What are the various methods of controlling the speed of a dc shunt motor? Explain one method in detail. (a) A 50-kvA, 4,400/220-V transformerhas R1 = 3.45 a, R2 = 0.009 a. The values of reactances are X1 = 5.2 a and X2 = 0.015 a. Calculate for the transformer (i) equivalent resistance as referred to primary (ii) equivalent resistance as referred to secondary (iii) equivalent reactance as referred to both primary and secondary (iv) equivalent .impedance as referred to both primary and secondary. (b) Give the characteristics of DC shunt motor. 6 (c) A short-shunt compound generator delivers a load current of 30 A at 220 V, and has arinature, series-field and shunt-field resistances of 0.05 f2, 0.30 a and 200 f2 respectively. Calculate the generated etm.f. and the armature curent. Allow 1.0 V per brush for contact drop. (a) Derive the expression for starting torque of three-phase induction motor. (b) Define regulation of a transformer. Derive the condition for maximum efficiency of a transformer. (c) Write a short note on universal motor. 6 (a) Obtain the relationship between torque and rotor power factor for a polyphase induction motor. Discuss the two cases (i) when the rotor is inductive and (ii) when the rotor is non-inductive. (b) A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50 (i) the speed at which the magnetic field of the stator is rotating. (ii) the speed of the rotor when the slip is 0.04.