BE Computer Engineering (IOE, TU) Basic Electrical Engineering (IOE, EE 451) Question Paper 2078

(a) State and explain Thevenin's theorem. (b) For the circuit shown, a 20 V battery (with internal resistance of 2 Ω) is connected across a network consisting of a 6 Ω resistor in series with a parallel combination of 12 Ω and 4 Ω resistors. Using Thevenin's theorem, determine the current that flows through a 5 Ω load resistor connected across the parallel combination. Draw the Thevenin equivalent circuit clearly indicating $V_{TH}$ and $R_{TH}$.

(a) Derive the EMF equation of a single-phase transformer and explain why a transformer cannot operate on a DC supply. (b) A 25 kVA, 2000/200 V, 50 Hz single-phase transformer has primary and secondary winding resistances of 1.5 Ω and 0.015 Ω respectively, and corresponding leakage reactances of 2.5 Ω and 0.025 Ω. If the iron loss is 350 W, calculate the efficiency of the transformer at full load and 0.8 power factor lagging.

(a) For a series RLC circuit excited by a sinusoidal voltage, derive expressions for the resonant frequency, quality factor and bandwidth. (b) A coil of resistance 10 Ω and inductance 0.1 H is connected in series with a 150 µF capacitor across a 200 V, 50 Hz supply. Calculate (i) the impedance, (ii) the current drawn, (iii) the power factor, and (iv) the power consumed. Draw the phasor diagram.

(a) With a neat sketch, explain the construction and working principle of a DC shunt motor, and derive the expression for its back EMF. (b) A 220 V DC shunt motor draws an armature current of 20 A at a speed of 1000 rpm. The armature resistance is 0.5 Ω. If the flux is reduced by 10% while the load torque remains constant, determine the new speed of the motor.

Three identical impedances, each of $(8 + j6)\,\Omega$, are connected in star across a balanced three-phase, 400 V, 50 Hz supply. Calculate the line current, the power factor, and the total power consumed. What would the total power be if the same impedances were reconnected in delta across the same supply?

An iron ring of mean circumference 60 cm and cross-sectional area 5 cm² has an air gap of 2 mm cut in it. The ring is wound with 500 turns. If the relative permeability of iron is 600, calculate the current required to produce a flux of 0.5 mWb in the air gap. Neglect leakage and fringing.

Using the superposition theorem, find the current through the 10 Ω resistor in a circuit where a 12 V source and a 6 V source share a common branch. The 12 V source is in series with 4 Ω, the 6 V source is in series with 2 Ω, and both branches join the 10 Ω resistor at a common node. Show the contribution of each source separately.

(a) Explain the working principle of a permanent magnet moving coil (PMMC) instrument and state why it is unsuitable for AC measurements. (b) A PMMC ammeter has a full-scale deflection of 1 mA and a coil resistance of 100 Ω. Calculate the value of the shunt resistance required to extend its range to read up to 5 A.

(a) Explain the production of a rotating magnetic field in a three-phase induction motor. (b) A 4-pole, 3-phase induction motor is supplied from a 50 Hz source. If the motor runs at 1440 rpm, calculate the synchronous speed, the slip, and the frequency of the rotor currents.

Define RMS value and average value of an alternating quantity. For a sinusoidal current $i(t) = 50\sin(314t)$ A, determine its RMS value, average value over a half cycle, form factor, peak factor, and the frequency of the supply.

Explain how the open-circuit and short-circuit tests are performed on a single-phase transformer. State clearly which test gives the iron loss and which gives the copper loss, and explain how the parameters of the equivalent circuit are obtained from these tests.

A DC shunt generator delivers 50 kW at a terminal voltage of 250 V. The armature resistance is 0.02 Ω and the shunt field resistance is 50 Ω. Calculate the generated EMF, neglecting brush contact drop. Also sketch and briefly explain the external characteristic of a DC shunt generator.