DieselShip's Online Content Library

Q1) Fig Q1 represents a ring main system of total length 620 m and resistance (go + return) of 0.002 Ω⁄m.
Calculate EACH of the following:
(a) the cable resistances for AC, CD, DE, and EB; (5)
(b) the currents applied at each end of the distributor cable; (6)
(c) the voltage at each load point. (5)

Q2) When connected to a 20 V d.c supply a relay starts to operate 0.52 ms after swiching on the supply at which time the instantaneous current is 200m mA. The relay coil has a time constant of 5 ms.

(a) Calculate EACH of the following:

(i) the final steady state relay current; (5)

(ii) the resistance of the relay coil; (2)

(iii) the inductance of the relay coil. (2)

(b) To increase the operating time a 40 Ω resistor is connected in series with the relay coil.  Calculate the new operating time for the relay assuming the instantaneous current is 200 mA.

Q3) THREE identical coils are star-connected across a three-phase, 440 V, 60 Hz power supply and consume a total power of 3 kW at a power factor of 0.8 lagging.

(a) Determine the resistance and inductance of each coil. (8)

(b) Calculate the current in each line if one coil is:

(i) short- circuited; (5)

(ii) open-circuited. (3)

Q4) A THREE-phase, star-connected, eight-pole induction motor has the following operation parameters:

Calculate EACH of the following:

(a) the output torque; (2)

(b) the rotor winding loss; (5)

(c) the stator winding loss; (2)

(d) the stator core loss; (5)

(e) the efficiency. (2)

Q5) A THREE-phase, 440 V, shaft driven generator shares the total electric load of a ship with an auxiliary diesel generator. An over-excited synchronous motor is used in the supply system for kVAr compensation.

The ship’s consumer load is 1 MW at 0.83 power factor lagging and the synchronous motor takes 40 kW.

(a) Sketch a single-line diagram of the power system. (3)

(b) The shaft-generator is loaded to its rated output of 650 kW at unity power factor; the diesel generator is operated at a power factor of 0.9 lagging.

Determine EACH of the following:

(i) the kW and kVAr loading of the diesel generator; (5)

(ii) the load current supplied by the diesel generator; (2)

(iii) power factor of the synchronous motor. (6)

Q6) A single-phase, 50 Hz transformer has 144 primary turns and 432 secondary turns, and a maximum flux of 7.5 mWb. The no-load input is 0.24 kVA at 0.26 power factor lagging. The transformer supplies a 1.2 kVA load at a power factor of 0.8 lagging.

Calculate EACH of the following:

(a) the magnetising current; (5)

(b) the primary current; (9)

(c) the primary power factor. (2)

Q7) With reference to a shaft-driven generator:

(a) state TWO advantages; (2)

(b) state ONE disadvantage; (1)

(c) sketch clearly labelled block diagram of a shaft-driven generator system which employs a frequency converter and a synchronous compensator; (8)

(d) describe the operation of the system sketched in Q7( c). (5)

Q8. (a) With reference to the principle of operation of a synchronous motor, explain how it differs from that of an induction motor. (5)

(b) Explain why a synchronous motor is unable to produce starting torque. (5)

(c) State how an electronic converter is used to start a synchronous. (3)

(d) State THREE shipboard applications of synchronous motor. (3)

Q9. With reference to a full-wave bridge rectifier using diodes:

(a) sketch a clearly labelled circuit diagram; (4)

(b) explain the circuit operation; (4)

(c) sketch clearly labelled waveforms to show the relationships between the following:

(i) the bridge input voltage; (2)

(ii) the current through each diode; (4)

(iii) the load currents. (2)