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## BEE 2021 question paper solution

(a) A coil wire when connected to a DC supply of 100 V takes a current of 10 A. When it connected to an AC supply of 100 V at a frequency of 50 Hz, the current is 5 A. Explain the difference and calculate the coefficient of self- inductance of the coil.

(b) The current in a circuit is given by (4-5+12) A when the applied voltage is (100+j150) V. Determine (i) the complex expression for the impedance stating whether it is inductive or capacitive, (ii) the power and (i) the phase angle between the current and voltage.

(c) What is meant by the Q-factor of a series coil and what is the significance?

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Q3. a) If a coil of unknown resistance and reactance is connected in series with a 100 V, 50 Hz supply, the current locus diagram is found to have a diameter of 5 A and when the value of series resistor is 15, the power dissipated is maximum. Calculate the reactance and resistance of the coil and the value of maximum power in the circuit and the maximum current.

(b) Three similar coils, each of resistance 202 and inductance 0.5 H are connected in (i) star and (ii) delta to a 3-0, 50 Hz, 400 V supply. Calculate the line current and the total power consumed.

(c) The admittance of a circuit is 0-03-j0-04 mho. Find the values of the resistance and inductive reactance of the circuit, if they are joined (i) in series and (ii) in parallel.

Q4. (a) Describe the properties of zinc chloride and mercury battery cell types.

(b) Describe the properties of fuse and its limitations.

(c) What are various factors to be considered for the calculations of energy consumption?

Q5. (a)When a transformer is connected to a 1000 V, 50 Hz supply, the core loss is 1000 W of which 650 is hysteresis and 350 is eddy current loss. If the applied voltage is raised to 2000 V and frequency to 100 Hz, what could be the new core losses?

(b) Explain the star-delta connection of three-phase transformer by assuming step-down mode. Draw the phasor diagram of line and phase voltages.

(c) The primary resistance of a transformer is 0-10 and its leakage reactance is 0-80. When the applied voltage is 1000 V, the primary current is 50 A at a lagging power factor of 0-6. Find the primary induced e.m.f.

Q6. (a) Derive the e.m.f. equation of a DC generator.

(b) A 6 pole, 40 Hz, three-phase induction motor running on full-load with 4% slip develops a torque of 149-3 N-m at its shaft. The friction and windage losses. are 200 W and the stator core and copper losses are 1620 W. Calculate-

(i) the total output power;

(ii) the rotor copper losses; (iii) the efficiency at full load

(c) Explain the speed control of separately excited DC motor above and below its rated speed.

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7. (a) What is the necessity of damper winding in an alternator? Describe the constructional details of the damper winding.

(b) Is it possible for the rotor to run at synchronous speed? Explain answer.

(c) Compare the lap winding and wave winding of a DC motor.

(d) Derive an expression for the torque developed by DC motor.

8. (a) An iron ring, cross-sectional area of 2 5 cm and mean length of 100 cm, has an air gap of 2 mm cut in it. Three separate coils having 100, 200 and 300 turns are wound on the ring and carry currents of 1 A, 2-5 A and 3 A, respectively such that they produce additive fluxes in the ring. Relative permeability of the ring material is 1000. Calculate the flux in the air gap.

(b) Explain the nature and direction of force between two parallel current- carrying conductors

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