Basic Electrical Engineering-Pune-Winter-2015 - Grad Plus

# Basic Electrical Engineering-Pune-Winter-2015

## B.E. Second Semester All Branches (C.B.S.)

Time: 2 hours
Maximum marks: 50

Notes :
1. Attempt Q. No. 1 or Q. No. 2, Q. No. 3 or Q. No. 4 and Q. No. 5 or Q. No. 6.
2. Neat diagram must be drawn wherever necessary.
3. Figures to the right indicate full marks.
4. Assume suitable data, if necessary and clearly state.
5. Use of cell phone is prohibited in the examination hall.

1. (a) Obtain the equation for coefficient of coupling between two coils placed on common magnetic field. [6M]

(b) A water pump lifts 50 m3 of water  per hour to a height of 20 m. The overall efficiency of water pump set is 0.60. Find monthly bill, when the pump is used is used 4 hours a day for a month of 30 days, at the rate of Rs. 5/kWh. [6M]

OR

2. (a) Explain the effect of temperature on resistance of : [6M]
(i) metal conductors
(ii) insulators
(iii) alloys.

(b) A steel ring of mean diameter of 50 cm is wound with 500 turns on it. Flux density of 1 Tesla is produced in the ring by mmf of 4000 AT/m. Calculate magnetising current. Also find the current when an air gap of 1 mm is cut in it, keeping B = 1 Tesla in the ring. [6M]

3. (a) Obtain the expression for composite capacitor having 3-dielectric materials. [7M]

(b) A 50 Hz sinusoidal voltages has runs value of 200 V. At t = 0, the instantaneous value is positive and half of its maximum value. Write down the expression for voltage and sketch the wave form. [6M]

OR

4. (a) Derive an equation for average value of sinusoidal current in terms of its peak value. [6M]

(b) A 40 kVA, 220/220 V, 50 Hz 1-ph transformer an iron loss of 250 W. The resistance 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. [7M]

5. (a) Obtain the expression for power, if x = vm sin ωt is applied to R-L series circuit. [6M]

(b) Three identical impedance each of 6 + j8 Ω are connected in star across 3-ph, 400 V, 50 Hz a.c. supply. Calcuate : [7M]
(i) line current
(ii) active power
(iii) reactive power.

OR

6. (a) What is series resonance ? Obtain the equation for resonant frequency. [6M]

(b) Two circuits having impedances Z1 = 8 + j6 Ω and Z2 = 5 + j10 Ω are connected in parallel across 200 V, 50 Hz, 1-ph a.c. supply. Calculate : [7M]
(i) current drawn by each circuit
(ii) total current
(iii) p.f. of whole circuit.

7. (a) Using simple series-parallel combination, find RBC for Fig. 1. [6M]

(b) Obtain the equations to convert Δ-connected resistances into equivalent star circuit. [6M]

OR

8. (a) State the Thevenin’s theorem and explain the steps to find Rth, Vth and Ith. [6M]

(b) Using Mesh/Loop analysis, calculate current flowing through RBC for Fig 2. [6M]

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