Basic Electrical Engineering
Unit 4 A): Poly phase circuits 1. Derive an expression for line current in terms of phase current for 3 phase delta connected balanced load , with phasor diagram, connected across 3 phase supply. (6M) 2. Derive the relation between line & Phase values of currents & voltages for balanced 3 Phase star connected load connected across 3 phase A.C. supply. Derive also power consumed by the load. (6M) 3. Prove that a 3 Phase balanced load draws 3 times as much power when connected in delta, as it would draws when connected in star. (6M) 4. State advantages of three phase system over single phase system. (3 or 4M) Three identical impedances each of 6 + j8 Ω are connected in star across 3-ph , 400V, 50Hz a.c. supply. Calculate : line currentactive powerreactive power (6M)6. Three coils each having a resistance of 20 Ω and an inductance of 15 Ω are connected in star to a 400V , 3- phase 50Hz supply.Calculate :1. The line current2. Power factor3. Power supplied (6M)7. A delta connected balanced load across a 400V 3- phase supply consist of three identical impedances, each equal to (15+j12) Ω . Find the line current, active power and reactive power. (6M)8. Three identical coils, each having resistance of 15 Ω and inductance of 0.03 H, are connected in delta across a 3 phase ,400 V,50 Hz supply. Calculate : (i) the phase current, (ii) the line current, (iii) the power consumed. (6M)9. A 3 phase Delta connected load having an impedance(30+j40) Ω/ phase is fed from a star connected transformer, which has a phase voltage of 231 volts. Draw the ciruit diagram of the system & calculate,Magnitude of each current in each phase of the load. The potential difference across each phase of load Magnitude of current in transformer winding. Total power consumed by the load & its P.F. (6M) 10. A 3 phase Delta connected load having an impedance(15+j20) Ω/ phase is fed from a star connected transformer, which has a phase voltage of 200 volts. Draw the ciruit diagram of the system & calculate,1)Magnitude of each current in each phase of the load. 2)The potential difference across each phase of load 3)Magnitude of current in transformer winding. 4)Total power consumed by the load & its P.F. (6M) B)Single phase transformer 1. Derive the e.m.f equation of a single phase transformer. (6M) 2. Compare core type and shell type transformer. (3 or 4M) 3. State different losses occurring in transformer and how they can be reduced? (6M) 4. Explain direct loading method with circuit diagram for 1- phase 230/110 V, 2 KVA transformer. How to calculate range of meters.? (6M) 5. Write short note on auto transformer. (6M) 6. A 40 KVA , 2200/220 V, 50Hz , 1-ph transformer has an iron loss of 250W. The resistances of low and high voltage windings are 0.005 Ω and 0.5 Ω respectively. Calculate efficiency at full load and load p.f. = 0.8 lagging. (6M)7. A 10 KVA , 3300/220 V 1-Ph 50 Hz transformer has emf per turn equal to 10. Calculate :Turns of prmary side Turns of secondary side Maximum flux in core Primary and secondary full load current. (6M) 500 KVA transformer has iron losses of 2 KW & full load copper losses of 5KW. Calculate: A transformer with 800 primary turns & 200 secondary turns is supplied from a 100V a.c. supply. Calculate the secondary voltage & the volts per turn (6M) A 125 KVA transformer having primary voltage of 2000V at 50Hz has 182 primary turns & 40 secondary turns. Neglecting losses, calculate i) the full load primary & secondary current ii) no load secondary induced emf iii) Max. flux in the core. (6M) A Single phase, 50Hz transformer has 30 turn on primary & 350 turns on secondary side. The net cross-sectional area of the core is 250 cm2. If the primary winding is connected to 230V,50Hz,supply Calculate i) The peak value of flux density in the core ii) the voltage induced in the secondary winding iii) primary current when secondary current is 100A. (6M) A 100 KVA, 50Hz, 440/1100V, 1-ph transformer has an efficiency of 98.5% when supplying current (full load) at 0.8 p.f. lagging & efficiency 99% when supplying half full load current at unity p.f. Find the core losses & copper losses corresponding to full load current. (6M) A single phase transformer working at unity p.f. supplies full load o/p of 500 Watt. It has efficiency of 90% at half load & full load. Determine the efficiency at 75% of full load. (6M) 14. The iron losses of 80 KVA, 1000/250v, 1-PH, 50Hz, transformer are 500 Watt. Copper loss when primary carries 50A is 400 Watt. Find i) Area of cross-section of limb if working flux density is 1T, and there are 1000 turns on primary. Ii) primary & secondary full load current iii) efficiency at full load 0.8 p.f. lagging (6M) 15. The readings of direct loading tests on a 1-ph transformer are given as At no load V1 = 220V, I1 = 1A W1 = 80W V2 = 110V, I2 = 0A W2 = 0W At load V1 = 220V, I1 = 4.5A W1 = 1000W 2. V2 = 106V, I2 = 9.1A W2 = 964.6W Find at given load, its efficiency & voltage regulation. (6M)
- 1)Efficiency at 75% of full load, unity P.F
- 2) Efficiency at full load 0.8 P.F. lag. (6M)
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