DieselShip's Online Content Library

Q4. a) What design factor limits the maximum torque of a d.c. motor? (6)

(b) A shunt motor runs on no load at 700 r/min off a 440 V supply. The resistance of the shunt circuit is 240Ω. The following table gives the relationship between the flux and the shunt current:

Shunt current (A):      0.5   0.75   1.0   1.25   1.5   1.75   2.0

Flux per pole (mWb): 6.0   8.0   9.4   10.2   10.8   11.2   11.5

Calculate the additional resistance required in the shunt circuit to raise the no-load speed to 1000r/min (10)

Q5. a) Describe the effects of changes in speed, rotor current and torque as load is applied to an induction motor. How does the motor adjust its stator current with changes in mechanical load? (6)
b) A 20 KVA, 2000/220V, single-phase transformer has a primary resistance of 2.1 Ω and a secondary resistance of 0.026 Ω. The corresponding leakage reactance’s are 2.5 Ω and 0.03 Ω. Estimate the regulation at full load under power-factor conditions of:
(i) Unity

(ii) 0.5 (lagging)

(iii) 0.5 (Leading) (10)

Q4. a) Explain the preference for a 60 Hz system. Describe the dangers of running a 50 Hz system from a 60 Hz supply. (6)
b) A transistor amplifier stage comprises a transistor of parameters hie = 800  hfe = 5O and hoe = 20µ S, and bias components and coupling capacitors of negligible effect. The input signal consists of an e.m.f. of 60 mV from a source of internal resistance 2.2 k , and the total load on the stage output is 4 k . Determine the current, voltage and power gains of the amplifier stage. (10)

Q4. A) With reference to voltage variation profiles caused by load changes imposed on alternating current generators when starting large motors online:

a) Sketch a voltage dip, showing an acceptable recovery time. (2)

b) State salient factors that cause the variation in part (a) (3)

c) Outline salient factors that assist recovery from the deviation shown in part (3)

B) A three-phase induction motor is wound for four poles and is supplied from a 50 Hz System calculate:

a) The synchronous speed

b) The speed of rotor when the slip is 4 per cent.

c) The rotor frequency when the speed of the rotor is 600 r/min (8)

Q4. (a) Explain the significance of the root- mean- square value of an alternating current or voltage waveform. Define the form factor of such a wave form.  (6)

(b) A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000 kW at 0.9 power factor. Find the kVA rating of the other alternator and the power factor. (10)

Q9 a) Explain the preference for a 60 Hz system. Describe the dangers of running a 50 Hz system from a 60 Hz supply. (6)

b) A ring-main, 900m long, is supplied at a point A at a p.d. of 220V. At a point B, 240m from A, a load of 45A is drawn from the main, and at a point C, 580m from A, measured in same direction, a load of 78A is taken from the main. If the resistance of the main (lead and return) is 0.25 ohm per kilometre, calculate the current which will flow in each direction round the main from the supply point A and the potential difference across the main, at the load where it is lowest. (10)

Q8. A. Why is it important to maintain high efficiency of operation?  And low values of voltages regulation for power transformers?

B. A 100 KVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 and 0.01 respectively, and the corresponding leakage reactance’s are 1.1 and 0.035 respectively. The supply voltage is 2200 V. Calculate:

(i) The equivalent impedance referred to the primary circuit;

(ii) The voltage regulation and secondary terminal voltage for full load having a power factor of (i) 0.8 lagging and (ii) 0.8 leading.

Q7. Two 10 MVA 3 phase Alternator operate in parallel to supply at 0.8 power factor with lagging load of 15 MVA. If the output of one Alternator is 8 MVA at 0.9 lagging. A. Calculate the output of second Alternator. B. Calculate the value of Power factor of second Alternator.

Q7. A. Describe the characteristic of d.c. generator.

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

Q10. a) Describe the various resistances acting against the motion of the ship?

The following data are available from the hydrostatic curves of a vessel.

Calculate the TPC at a draught of 5.05m.

Q9. A. Describe measures which may be taken to improve the stability or trim of a damaged ships.

B. A ship 85m long displaces 8100 tonne when floating in seawater at draughts of 5.25m forward and 5.55m aft. TPC 9.0, GML 96m, LCF 2m aft of midships. It is decided to introduce water ballast to completely submerge the propeller and a draught aft of 5.85m is required. A ballast tank 33m aft of midships is available. Find the least amount of water required and the final draught forward.

Q10. A. Explain what is meant by singing of propeller.

B. A propeller 6m diameter has a pitch ratio of 0.9, BAR 0.48 and, when turning at 110 rev/min, has a real slip of 25% and wake faction 10.30. If the propeller delivers a thrust of 300 KN and the propeller efficiency is 0.65 calculate - i Blade area; ii Ship speed; iii. Thrust power; iv Shaft power v. Torque