Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech II FUNDAMENTALS OF ELECTRICAL ENGINEERING Previous Question Paper (September - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
Compare magnetic and electric circuits.
Answer
The comparison between magnetic and electric circuits is as follows:
| Feature | Electric Circuit | Magnetic Circuit |
|---|---|---|
| Flow | Current (I) | Magnetic Flux (Φ) |
| Driving Force | EMF (E) | MMF (F = NI) |
| Opposition | Resistance (R = ρl/A) | Reluctance (S = l/μA) |
| Path | Conductor (e.g., Copper) | Magnetic Material (e.g., Iron) |
| Energy | Energy is dissipated as heat (I²R) | No energy is consumed in maintaining a steady flux |
| Nature | Closed path for electron flow | Closed path for magnetic flux |
Using star-delta conversion, find the equivalent resistance between terminals A and B in the network shown below in the Figure 1.
Answer
To find the equivalent resistance between terminals A and B using star-delta conversion, follow these steps:
- Identify the delta (Δ) network within the circuit that complicates the series-parallel analysis.
- Apply the delta-to-star transformation formulas to convert the delta-connected resistors (R₁, R₂, R₃) into an equivalent star network (Ra, Rb, Rc):Ra = (R₁ × R₂) / (R₁ + R₂ + R₃)Rb = (R₂ × R₃) / (R₁ + R₂ + R₃)Rc = (R₃ × R₁) / (R₁ + R₂ + R₃)
- Replace the delta network with the new star configuration in the original circuit diagram.
- Simplify the resulting circuit by combining resistors that are now in series or parallel branches relative to terminals A and B.
- Calculate the total equivalent resistance (R_AB) by reducing the simplified series and parallel combinations step-by-step until a single resistance value remains.
How do you transform a voltage source into a current source?
Answer
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An iron ring of 100 cm mean circumference is made from a cross section of 10 cm2. Its relative permeability is 500. A saw-cut of 2 mm wide has been made on it. It is wound with 200 turns. Determine the new current required to produce a flux of 0.12 × 10-2 Wb in the air gap, given the leakage factor as 1.24 and the relative permeability of the iron under the new condition µr = 350.
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Explain with the help of diagrams what do you understand by in phase, lagging and leading as applied to sinusoidal quantities.
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Obtain the expression for resonant frequency of series RLC circuit excited by an sinusoidal voltage of variable frequency.
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A coil of resistance 10 Ω and inductance 0.02H is connected in series with another coil of resistance 6 Ω and inductance 15 mH across a 230 V, 50 Hz supply. Calculate i) impedance of the circuit ii) voltage drop across each coil iii) the total power consumed by circuit.
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What do you mean by active power, reactive power and apparent power?
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State and explain Thevenin’s theorem.
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Find the current in 6 Ω resistor using Norton’s theorem for the network shown below in the Figure 2.
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State and explain Tellegen’s and Milliman’s theorems.
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Draw a neat sketch of a d.c machine showing the different parts and explain the principle of operation of d.c generator.
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An 8 pole d.c machine has a wave winding containing 664 conductors. Calculate the generated e.m.f when the flux per pole is 0.08 Wb and the speed is 210 rpm. At what speed should the armature be driven to generate 500 V, if the flux per pole is made 0.06 Wb?
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Derive e.m.f equation of a single phase transformer.
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A 415/220 V transformer takes a no load current of 1 A and operates at 0.19 p.f lagging when the secondary supplies a current of 100 A at 0.8 p.f lagging, find the primary current.
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