Jawaharlal Nehru Technological University
Jawaharlal Nehru Technological University B.Tech II Year ELECTRICAL ENGINEERING Previous Question Paper (April/May - 2018 - R15)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
State Lenz’s law and give its significance.
Answer
Lenz's law states that the direction of an induced electromotive force (EMF) and the resulting current is always such that it opposes the change in magnetic flux that produced it. Its significance is that it ensures the conservation of energy in electromagnetic induction.
State Kirchhoff’s voltage and current law?
Answer
Kirchhoff's Current Law (KCL) states that the algebraic sum of currents meeting at a junction or node in an electrical circuit is zero. Kirchhoff's Voltage Law (KVL) states that the algebraic sum of all voltages around any closed loop in a circuit is zero.
Define active power and reactive power.
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What are the characteristics of a series resonant circuit?
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What is the principle of DC generator?
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What are the disadvantages of three point starter?
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Write the basic principle of a transformer?
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Draw the slip torque characteristics of an induction motor.
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What is meant by synchronous impedance of an alternator?
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Why synchronous impedance method is called pessimistic method.
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Part B
State and explain Faraday’s laws of electromagnetic induction.
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A 50 ohm resistor is in parallel with a 100 ohm resistor. The current in 50 ohm resistor is 7.2A. What is the value of third resistance to be added in parallel to make the line current as 12.1A?
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Explain the operation of moving iron instrument. Discuss its merits over moving coil instrument.
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Three resistors of 50 ohms, 100 ohms, and 150 ohms are joined in parallel. If the current in 100 ohms resistor is 5 A, what is the current in other resistors and total current?
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Derive relationship between line and phase voltages and currents in three phase balanced delta connected system.
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Two circuits, the impedance of which are given by Z1 = (12 + j15) ohms and Z2 = (8 - j4) ohms are connected in parallel. If the potential difference across one of the impedances is 230 volts, compute: (i) total current and current supplied to each branch (ii) total power consumed (iii) overall power factor.
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Derive expression for R.M.S. and average value of a sinusoidal alternating quantity?
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A balanced delta connected load of (2+j3) Ω per phase is connected to a balanced three phase 440 V supply. The phase current is 10 A. Find (i) total active power (ii) reactive power (iii) apparent power?
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Explain load characteristics of DC generators?
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A 400V DC shunt motor takes 5A on no load. The armature and field resistance are 0.5 ohms and 200 ohms respectively. Find the efficiency of the motor when running on full load and taking a current of 50A.
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Derive torque equation of a DC motor from fundamentals.
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A 4-pole DC generator is delivering 20 A to load of 10 Ω. If armature resistance is 0.5 Ω and shunt field resistance is 50 Ω, calculate induced e.m.f and efficiency of machine. Allow a drop of 1V/brush.
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Explain how OC and SC tests are to be performed on a single-phase transformer to find efficiency at any load.
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Explain constructional details and principle of operation of a single phase transformer?
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Compare slip ring Induction motor with squirrel cage Induction Motor.
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A 5kVA, 500/250 V, 50 Hz single-phase Transformer has the following test results: OC Test: 250 V, 1.1 A, 50 W; SC Test: 15 V, 6 A, 21.6 W. Determine the efficiency for Full load output at 0.8 pf (lag).
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Explain procedure to conduct OC and SC tests on an alternator.
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A 25 kVA, 400 V, 3-phase, 50 Hz star connected alternator gives rated current on short circuit with a field current of 2 A. The same field current gives 200 V on open circuit. The effective armature resistance per phase is 1.0 Ω. Calculate full load voltage regulation at a power factor of (a) 0.8 lag and (b) 0.8 lead.
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