Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech II PRINCIPLES OF ELECTRICAL ENGINEERING Previous Question Paper (September/October - 2023 - R13)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
Define time constant and write its significance.
Answer
The time constant of a first-order linear circuit is defined as the time required for the response (such as voltage or current) to reach approximately 63.2% of its final steady-state value during a transient process. For an RL circuit, the time constant is
Its significance lies in determining the speed of the transient response; a smaller time constant indicates a faster transition to the steady state, while a larger time constant indicates a slower response. It effectively characterizes the duration of the transient period, as the circuit typically reaches a steady state after approximately 5 time constants.
Draw the transient current growth and decay curves for an L-R circuit and find the time at which these currents are equal?
Answer
In an L-R circuit, the growth of current is given by i(t) = (V/R)(1 - e-Rt/L) and the decay of current is given by i(t) = I0e-Rt/L. The growth curve starts at zero and rises exponentially to a steady-state value of V/R, while the decay curve starts at an initial current I0 and falls exponentially toward zero.
To find the time t when these currents are equal, we equate the two expressions:
Assuming the initial current I0 is equal to the steady-state current V/R, the equation simplifies to:
Taking the natural logarithm on both sides:
Thus, the currents are equal at t = 0.693τ, where τ = L/R is the time constant of the circuit.
Find Z and Y parameters if they exist for the two-port network given in Figure 1.
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Define term reciprocity for a given two-port network. Give the expressions reciprocity in case of impedance and hybrid parameters.
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State advantages of m-derived filters.
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What is a band-elimination filter? Write its properties.
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What are the functions of field winding in DC machine?
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List out the differences between a DC generator and a DC motor.
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Why core of the transformer is laminated.
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What is a stepper motor? Write its applications.
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Part B
Derive an expression for voltage across capacitor in a series RC circuit excited by a unit step voltage. Assume that initial voltage across capacitor is zero.
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A 200Ω resistor is in series with an inductor L. The initial value of the inductor current is 5 mA and its value after 5 ms is 3 mA. Find the time constant and the inductance.
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An RLC series circuit having resistance = 20 Ω, inductance = 0.05 H and capacitance = 20 µF are connected in series with a 100V constant source when the switch is closed at t=0. Find the current transient.
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The parameters of a series RC circuit are R=10Ω and C= 0.1 F. A constant voltage of 20V is applied to the circuit at t = 0. Obtain current equation. Determine the voltage across the resistance and the capacitance.
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Derive Z-parameters in terms of Y and ABCD parameters.
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Find the hybrid parameters of the network shown in Figure 2.
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Derive conditions of symmetry and reciprocity for Y-parameters and of given two port network.
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Two 2-port networks A and B are connected in parallel. Each of these networks has their own Y-parameters. Show that resultant Y-parameters of the combined parallel network are sum of Y-parameters of the individual networks A and B.
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What are attenuators? Discuss various types of attenuators briefly.
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Derive the expression for characteristic impedance of a symmetrical bridged T-type network.
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What is the propagation constant of a symmetrical T-section and symmetrical π-section filters? What is its significance?
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A filter is required to pass all frequencies above 30 kHz and to have a nominal impedance of 500 Ω. Design a high-pass, (i) T-section and (ii) π-section, filters to meet these requirements.
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Explain the Swinburne’s test to determine the no load losses of a DC machine. What are the limitations of this test?
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Briefly explain various speed control methods of a DC shunt motor. Discuss their relative merits and demerits of each method.
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With a neat diagram, explain construction details and principle of operation of DC generator.
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A 400V, DC series motor has an armature resistance of 0.12 Ω. When motor takes a current of 80A, its speed is 600 rpm. Determine its speed if current drawn by the motor changes to 40A.
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Explain how the equivalent circuit parameters are determined from OC and SC tests?
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Draw the no load and on load phasor diagram for a single-phase transformer on lagging load taking secondary terminal voltage as the reference phasor and explain.
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Why the primary of the transformer draws a current from the main supply when the secondary is not carrying any load. Explain in detail.
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The emf per turn of a 1-phase, 6.6 kV/440 V, 50Hz transformer is 12 V. Calculate (i) the number of turns in the HV and LV windings (ii) the net cross-sectional area of the core for a maximum flux density of 1.5 T.
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