Basic Electrical Engg. - Dec 2014

First Year Engineering (P Cycle) (Semester 1)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks. 1 (a) Find the values of current in all the branches of the network shown in Fig Q1 (a) (6 marks) 1 (b) A current of 20 A flows through two ammeters A and B in series. The potential difference across A is 0.2 V and across B is 0.3. Find the how the same current will devide between A and B when they are in parallel.(6 marks) 1 (c) Coils A and B in magnetic circuit have 600 and 500 turns respectively. A current of 8 A in coil A produces a flux of 0.04 Wb. If co-efficient of coupling is0.2 calculate
i) Self inductance of coil A with B open circuited.
ii) Flux linking with the coil B.
iii) The average emf induced in coil B when the flux with it changes from zero to full value in 0.02 second.
iv) Mutual inductance.(8 marks) 10 (a) A 600 KVA transformer has an efficiency of 92% at full load, unity p.f. and half full load, 0.9 p.f. Determine its efficiency at 75% of full load 0.9 p.f.(8 marks) 10 (b) An 8-pole altenator runs at 750 rpm and supplies power to a 4-pole induction motor. The frequency of motor current is 1.5 Hz. Determine the speed of the motor.(6 marks) 10 (c) Derive the condition for which the efficiency of a transformer is maximum.(6 marks) 2 (a) A circuit consists of two parallel resistors having resistance of 20 ? and 30 ? respectively, connected in series with a 15 ? resistor. If the current through 30 ? resistor is 1.2 A, find
i) Current in 20 ? and 15 ? resistors.
ii) The volages across the whole circuit
iii) Voltage across 15 ? resistor and 20 ? resistor
iv) Total power consumed in the circuit.(8 marks) 2 (b) Obtain the relation between self inductances, mutual inductance and co-efficient of coupling.(6 marks) 2 (c) A coil consist of 600 turns and a current of 10 A in the coil gives rise to a magnetic flux of 1m weber. Calculate (i) self inductance (ii) Induced emf and and (iii) Energy stored when the current is reversed in 0.01 second.(6 marks) 3 (a) Derive the expression for armature torque developed in a d.c. motor.(6 marks) 3 (b) Explain with a neat diagram, the constructioal features and operation of an induction type single phase energy meter.(6 marks) 3 (c) A 30 kW, 300 V DC shunt generator has armature and field resistances of 0.05 ohm and 100 ohm respectively. Calculated the total power developed by the armature when it delivers full output power.(8 marks) 4 (a) Derive the emf equation for a dc generator(6 marks) 4 (b) With a neat diagram explain the construction and working of dynamometer type wattmeter.(6 marks) 4 (c) A 200 V lap wound dc shuntmotor has 800 conductors on its armature. The resistance of the armature winding is 0.5 ? and that of field winding is 200 ?. The motor takes a current of 21 A, the flux per pole is 30 mwb. Find the speed and torque develiped in the motor.(8 marks) 5 (a) Obtain expression for the current through the pure inductor, if the across it is v=v_msin ?t.(6 marks) 5 (b) A voltage v=100 sin 314t is applied to a circuit consisting of a 25 ohm resistor and an 80 ?F capacitor in seris. Determine (i) peak value of current (ii) power factor (iii) Total power consumed by the circuit.(8 marks) 5 (c) Write a short notes on:
i) Necessity of carthing.
ii) Precautions to be takesn to prevent electric shock.(6 marks) 6 (a) A voltage of 200 V is applied to a series circuit consisting of a resistor an inductor and a capacitor. The respective voltages across these components are 170 V, 150 V and 100 V and rectance and capacitive rectance.(8 marks) 6 (b) Explain the necessity and the operation of earth leakage circuit breaker.(6 marks) 6 (c) Two independence z₁=(6-j8) ohms and z₂=(16+j12) ohms are connected in parallel. If the total current of the combinations is (20+ j10) amperes, find
i) Voltage across the combination
ii) Current in the two branches(6 marks) 7 (a) Obtain the relationship between line and phase, voltages and current in a three phase balanced star connected system.(6 marks) 7 (b) A 3-phase delta connected balanced load consumer a power of 60 k.w. taking a lagging current of 200 A at a line voltage of 400 V, 50 Hz. Find the parameters of each phase.(6 marks) 7 (c) A 12 pole 500 rpm star connected alternator has 48 slots with 15 conductors per slot. The flux per pole is 0.02 webers. The winding factors is 0.97 and pitch factors is 0.98. Calculate the phase emf and line emf.(8 marks) 8 (a) Define phase sequence and list out the advantages of three phase system as compared to single phase system.(6 marks) 8 (b) A 4-pole 1500 rpm star connected alernator has 9 slot pole and 8 conductors per slot. Determine the flux per pole to give a terminal voltage of 3300 v. Take winding factor and pitch factors as unity.(8 marks) 8 (c) The input power to a 3-phase induction motor running on 400 V, 50 Hz supply was measured by two wattmeter method and readings were 3000 W and 1000 W. Calculate (i) Total power input (ii) Power factor (iii) Line current.(6 marks) 9 (a) Derive emf equation of a transformer(6 marks) 9 (b) In a 25 KVA, 2000/200 V single phase transformer, the iron and full load copper losses are 350 watts and 400 watts respectively. Calculate the efficiency at unity power factor on (i) full load (ii) half full load.(8 marks)