Q3. Differentiate between half and full wave rectification. State where half wave rectification may be used and the purpose for which it is not well adapted. Sketch a bridge connection by which full wave rectification may be obtained.
Q4. A. Describe with the aid of a simple sketch the arrangement of the three-phase winding of an alternator showing the neutral point.
B. Explain why for most ships the neutral point is insulated.
C. Explain why in some installation the neutral point is earthed?
Q4. Explain the effect of reduced voltage on standard squirrel cage motors with respect to EACH of the following:
(i) Burn out; (ii) Starting current; (iii) Starting torque; (iv) Speed.
Q4. A. what is intrinsic electric safety? B. can live maintenance be done on intrinsically safe circuits? C. Describe intrinsically safe equipment used on board ship.
Q5. A. (i) Sketch a diagrammatic arrangement of a static or self-excited alternator.; (ii) Describe the operation of the self-excited alternator; B. state why the voltage dip is less in the self-excited alternator than in brushless or conventional alternators.
Q6. A. What are factors on which the speed of a motor depends? Discuss them for series and shunt motors. A shunt motor supplied at 230 V runs at 900 rpm. When the armature current is 30 A, the resistance of the armature circuit is 0.4 , calculate the resistance required in series with the armature circuit to reduce the speed to 500 rpm. Assume that the armature current is 25 Amps.
Q7. A. Derive an expression for the e.m.f induced in an a.c. generator. B. A 3000 KVA, 6 pole alternators runs at 1000 r.p.m in parallel with other machines on 3300 V bus-bars. The synchronous reactance is 25%. Calculate the synchronizing power for one mechanical degree of displacement and the corresponding synchronizing torque.
Q8. A. Explain the purpose of interlopes and state their magnetic polarity relative to the main poles of both generators and motors.
A 200V, long-shunt compound-wound generator has a full-load output of 20kW. The various resistances are as follows; armature (including brush contact) 0.15 ohm, series field 0.025 ohm, interpole field 0.028 ohm, shunt field (including the field-regulator resistance) 115 ohm. The iron losses at full load are 780W, and the friction and wind age losses 590W. Calculate the efficiency at full load.
Q6. A 72 KVA transformer supplies a heating and lighting load of 12 KW at unity power factor and a motor load of 70 kVA at 0.766 (lagging) power factor; Calculate the minimum rating of the power-factor improvement capacitors which must be connected in the circuit the ensure that the transformer does not become overloaded.
Q10. Evaluate for a frequency of 15 kHz, the amplification and the phase difference between input and output signals of a voltage amplifier using a triode having an amplification factor of 48 and a mutual conductance of 1.2 m A/V with an anode-load resistant of 160 k . The output p.d. is fed by a coupling capacitor of negligible reactance to a subsequent circuit of resistance 480 k and the total shunt capacitance is 90µ F.
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