DieselShip's Online Content Library

Q6. A coil of resistance 10 ohm and inductance 0.1H is connected in series with a capacitor of capacitance 150pF, across a 200 V, 50 Hz supply.

Calculate:

a) the inductive reactance. (3)

b) the capacitive reactance. (3)

c) the circuit impedance. (2)

d) the circuit current. (2)

e) the circuit power factor. (2)

f) the voltage drop across the coil. (2)

g) the voltage drop across the capacitor. (2)

Q6. (a) What are the different types of DC motors? (6)

(b) A 10 H.P. 230 V shunt motor takes an armature current of 6A from 230 V mains at no load runs at 1200 r.p.m. The armature resistance is 0.25Ω. Determine speed and electromagnetic torque when the armature takes 36 amps with the same flux. (10)

Q8. a) Explain how excitation of the rotor is produced and supplied. (6)

b) A 75-kW, 400-V, 4-pole, 3-phase star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04 Ohm and 0.4 Ohm respectively. Compute for full-load 0.8 p.f. lead the open circuit e.m.f. per phase and mechanical power developed. Assume an efficiency of 92.5%. (10)

Q6. (a) Describe the provisions of additional structural strength to withstand pounding. (6)

(b) A vessel travelling at 17 knots turns with a radius of 450 m when the rudder is put hard over. The centre of gravity is 7 m above the keel, the transverse metacentre 7.45 m above the keel and the centre of buoyancy 4 m above the keel. If the centripetal force is assumed to act at the centre of buoyancy, calculate the angle of heel when turning. The rudder force may be ignored. (10)

Q4. a) Why is it important to maintain high efficiency of operation? And low values of voltages regulation for power transformers? (6)

b) 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 Ω. 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. (10)

Q4. (a) Why is a synchronous motor not self-starting? What are the various ways in which it can be started?

(b) Differentiate Ge and Si diode characteristics. The reverse bias saturation current for a P-N junction diode is 1.0 µA at 300 K. Determine its AC resistance at 150 mV Forward bias. (10)

Q6. With reference to Diesel engine driven alternator:

a) Explain the significance of the term “speed droop” during the parallel operation of alternators. Assuming two alternators are running in parallel, outline the frequency vs load characteristics of the alternators while load sharing. (8)

b) Two 3Ø alternators operate in parallel, the rating of A is 1000 kW and B is 800 kW. The droop setting of each generator is 4%, If the load to be shared by both the generators is 1000 kw, calculate the load sharing by generators A and B, if the original frequency at no-load is 62Hz. (8)

Q6. (a) Explain how excitation of the rotor is produced and supplied. (6)

(b) A 25 kVA, single phase transformer has 250 turns on the primary and 40 turns on the secondary winding. The primary is connected to 1500 V, 50 Hz mains calculate: (10)

(i) Secondary emf

(ii) Primary and secondary current on full load

(iii) Maximum flux in the core.

Q10. (A). Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse.

(B) An AC Voltage Of 24 V is connected in series with the silicon diode and load resistance 500 Ohm having forward resistance 1O Ohms. Calculate the peak output voltage.

Q 7. With reference to Synchronous Motors.

A. Draw and explain the principle of operation Synchronous Motors.

B. Find the synchronous impedance reactance of an alternator in which a given field current produces an armature current of 200 A on short circuit and a generated e.m.f. of 50V on open circuit. The armature resistance is 0.1 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100A at a p.f of 0.8 lagging, with a terminal voltage of 200 V.

Q4. A) Calculate the Line and Phase current at full load of a 50 kW, 440 Volt 3 phase deltas connected motor having full load efficiency 90% and power factor O.8 lagging. B) Calculate the starting current if motor is started by star/delta starter and Auto transformer starter which has transformer tapping 65 %

Q8. A. What is Silicon controlled rectifier (SCR)? How is the breakover voltage of the SCR defined? (6)

B. A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2𝛀. The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate: (a) The speed; (b) The gross torque developed by the armature. (10)