BE (I year)
Examination April-May 2009
Q.1 (a) Why are electrical machine made of magnetic materials?
(b) State and explain super position theorem.
(c) Determine the equivalent resistance between the terminals X and Y of the circuit shown.
(d) A steel ring of 25 cm in diameter and of circular section 3 cm in diameter has an air gap of 1.5 mm length. If is would uniformly with 750 turns of wire carrying a current of 2.1 A.
(i) Magneto motive force.
(ii) Flux density in air gap.
(iii) Magnetic flux.
(iv) Relative permeability of steel ring.
Assume that iron path takes about 35% of the total miff.
Q.2 (a) Show that power consumed in a 1 – Φ.
Ac circuit is-given by VI coos θ.
(b) Derive the expression for the band width in series RLC circuit.
(c) A circuit consist of R = 35 ohms in series with an unknown impedance Z. For a sinusoidal current of 2 A, the observed voltages are 200 volte across R and z together , 150 volts across impendence Z. Find the impedance Z.
(d) Show that: tan Φ = (W2 – W1)
W1 + W2
Unit – III
Q.3 (a) The efficiency of a transformer is high compared to other electrical machines. Give reasons .
(b) Write short note on no load and short circuit tests.
(c) What do you mean by efficiency of a transformer ? Derive the condition for maximum efficiency.
(d) A 200 kava transformer has an efficiency of 98% at full load. If the maximum efficiency occurs at 75% of lull load, calculate the efficiency at half load. Assume negligible magnetizing current and pf at 0.8 atall load.
Q.4 (a) A series motor should never be started with our load on if Give reasons .
(b) A 120 V decent motor has an armature resistance of 02 ohrr and a field resistance of 60 ohms. It runs at 1800 rpm it is taking a full load current of 40A. Find the speed of the moto when it is operating at half full load.
(c) With usual notations, derive the torque equation of a 0.1 motor.
(d) Derive the expression for developed torque in a 3 phase in diction motor.
Q.5 (a) How do you extend the rate of an ammeter?
(b) What is the principle of a moving iron voltmeter?
(c) Explain the operation of a single phase energy meter.
(d) A single phase K who meter makes 500 revolutions per K Who. It is found in testing as making 40 revolutions in 58.1 seconds at 5 kW full load find out the percentage error.