Question Paper: Basic Electrical Engineering (BEE) : Question Paper May 2014 - First Year Engineering (Semester 2) | Pune University (PU)
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## Basic Electrical Engineering (BEE) - May 2014

### First Year Engineering (Semester 2)

TOTAL MARKS: 50
TOTAL TIME: 2 HOURS
(1) Solve Q.1 or Q.2, Q.3 or Q.4, Q.5 or Q.6, Q.7 or Q.8
(2) Assume suitable data, if necessary.

### Answer any one question from Q1 and Q2

1 (a) With usual notations: derive the relationship, $$\alpha_2 = \dfrac {a_1}{1+\alpha_1 (t_2-t_1)}$$ And hence obtain $$i) \ \ a_t = \dfrac{a_0}{1+a_0t} \\ i) \ \ a_0 = \dfrac {a_t}{1-a_t t}$$(6 marks) 1 (b) A coil of N turns is wound on a cast iron ring which has mean length of 50 cm and its cross-section is of 4 cm diameter. The current flowing through the coil is 2 A which produces a flux Of 6 nWb in the air-gap of 2mm length. If the relative permeability of iron is 1000, calculate the number of turns N.(6 marks) 2 (a) With reference to a magnetic circuit explain the teRMS-i) magnetic flux, ii) magneto-motive force, iii) magnetic field intensity, iv) magnetic flux density, v) reluctance, vi) permeability of free space. State their units.(6 marks) 2 (b) A pump driven by a DC electric motor lifts 1.5 m3 of water per minute to a height of 40 m. The pump has an efficiency of 90 % and the motor an efficiency of 85%. Determine (i) the power input to the motor, (ii) the current taken from a 480 V supply. Assume the mass of one m3 of water is 1000 kg.(6 marks)

### Answer any one question from Q3 and Q4

3 (a) Draw neat sketches to show core-type and shell-type single phase transformers. State:
i) losses occurring in the transformer on load.
ii) their location, cause, whether constant or variable.
iii) factors on which they depends
(6 marks)
3 (b) A series of combination having R= 2MΩ and C= 0.01 μF is connected across the DC source of 50 V. Determine the capacitor voltage and charging current after 0.02s and 0.06s.(6 marks) 4 (a) Derive an expression for instantaneous current and power consumed when voltage of V= Vm sin (ωt) is applied to pure inductance alone. Also draw the phasor diagram.(6 marks) 4 (b) A transformer is rated at 100 KVA. At full load, its copper loss is 1200 W and its iron loss is 960 W. Calculate:
(i) Efficiency at full load, 0.8 power factor,
ii) Load KVA at which maximum efficiency will occur.
(iii) Maximum efficiency at 0.85 power factor
(6 marks)

### Answer any one question from Q5 and Q6

5 (a) Three identical coils, each having resistance of 15Ω 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
(7 marks)
5 (b) Derive the expression for line voltage in teRMS of phase Voltage for three phase star connected balanced load, with phasor diagram across three phase supply.(6 marks) 6 (a) An emf given by v=100 sin 100πt is impressed across a circuit consisting of resistance of 40π in series with 100 μF capacitor and 0.25 H inductor. Determine:(i) RMS value of the current, (ii) power consumed, (iii) power factor.(7 marks) 6 (b) For a single phase A.C. circuit, the applied voltage is v=Vmsin ωt and current drawn is i=Im sin (ωt-ϕ). Derive expression for average power. Draw waveform of voltage, current and instantaneous power over one cycle of voltage.(6 marks)

### Answer any one question from Q7 and Q8

7 (a) State and explain Thevenin;;s Theorem.(6 marks) 7 (b) Apply Kirchhoff's Law to calculate current drawn by 4Ω resistance for the circuit. (7 marks) 8 (a) Derive formula to convert DELTA connected network into its STAR connected equivalent circuit.(6 marks) 8 (b) Apply superposition theorem to calculate current flowing in 3Ω resistance for the network. (7 marks)