JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRICAL SYSTEMS Previous Question Paper (March - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
Explain in detail about reciprocity theorem with an example.
Answer
The Reciprocity Theorem states that in any linear, bilateral, passive network, if a single voltage source applied at one pair of terminals produces a current at another pair of terminals, then the same voltage source applied at the second pair of terminals will produce the same current at the first pair of terminals.
Mathematically, the ratio of the response (current) to the excitation (voltage) remains constant regardless of the position of the source and the response point. The condition for reciprocity is that the ratio of the voltage at one port to the current at the other port, when the other port is short-circuited, is equal to the ratio when the source and measurement points are interchanged.
For example, consider a T-network with series impedances Z1, Z2 and a shunt impedance Z3. If a voltage source V is connected to the input port, the current I2 at the output port is determined by the network parameters. According to the theorem, if the voltage source V is moved to the output port, the current I1 measured at the input port will be identical to the original I2, provided the network remains unchanged.
Using nodal voltage method, find the voltage across A and B in the circuit shown in figure 1.
Answer
To find the voltage across terminals A and B using the Nodal Voltage Method, follow these steps:
- Identify all nodes in the circuit and assign a reference node (ground) with a potential of 0V.
- Assign unknown voltages (V1, V2, etc.) to the remaining nodes.
- Apply Kirchhoff's Current Law (KCL) at each principal node, expressing currents in terms of node voltages and branch resistances: Σ I = 0
- Formulate a system of linear equations based on the KCL applications.
- Solve the system of equations to determine the node voltages.
- The voltage across terminals A and B, V_AB, is calculated as the difference between the potentials at node A and node B: V_AB = V_A - V_B
By solving these simultaneous equations, the specific potential difference across the specified terminals is obtained.
Explain in detail about maximum power transfer theorem with an example.
Answer
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Using loop current method, find the current ‘I’ in the circuit shown in figure 2.
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Derive the expression for active and reactive powers in series 1-ϕ RC circuit excited by sinusoidal supply.
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An RLC series circuit has a resonant frequency of 1.2 KHz and Q- factor of 30. If the impedance of the circuit at resonance is 50Ω determine the values of inductance, capacitance, bandwidth and half power frequencies.
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Discuss the properties of series RLC resonant circuit.
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For the circuit shown in figure 3, determine L, voltage across resistor and current in the circuit.
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Draw the equivalent circuit of single phase transformer. Explain various parameters.
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The efficiency of a 20kVA, 2500 V /250 V, single phase transformer at unity p.f is 96% at rated load and also at half rated load. Determine the turn’s ratio of the transformer, the transformer core loss and copper loss at full load, half load and one fourth load.
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Give the constructional details of single phase transformer.
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A 10kVA, 2500/250V, single-phase transformer gave the following test results: Open-circuit test: 250V, 0.8A, 50W; Short-circuit test: 60V, 3A, 45W. Calculate the efficiency at half-load and full-load at 0.8 p.f.
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Explain in detail about lap and wave windings used in d.c machines.
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A 220 V, DC shunt motor at no load takes a current of 1.5 A. The resistances of the armature and shunt field are 0.6Ω and 200Ω respectively. Estimate the efficiency of the motor when the input current is 20A. Take necessary assumptions.
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Explain in detail about the double field revolving theory.
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The input voltage to the 3-phase, 50Hz ac circuit shown in figure 4 is 120V. For the phase sequence of ABC, find out the wattmeter readings.
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