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Q1) Fig Q1 shows a ring main of total length 1000 m and resistance (go + return) of 0.002 Ω/m. The ring main is supplied with 240 V D.C. and the following loads are connected to the ring at distances measured clockwise from the supply point:

i) 60 A at 200 m

ii) 90 A at 500 m

iii) 150 A at 700 m

Calculate EACH of the following:

(a) The currents in sections AB and AD; (6)

(b) The lowest voltage across any of the three loads; (5)

(c) The total power loss in the ring main. (5)

Q2.  A relay has a time constant of 5 ms and starts to operate 0.52 ms after connection to a 20 V D.C. supply.

(a) If the instantaneous current is 200 mA, calculate EACH of the following:

(i) The final steady state relay current; (6)

(ii) The resistance and inductance of the relay coil. (4)

(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. (6)

Q3) A balanced three-phase, star connected load of 200 kW takes a leading current of 160 A from a 1.1 kV, 60 Hz supply.

Calculate EACH of the following:

(a) The resistance per phase; (6)

(b) The capacitance per phase; (3)

(c) The power factor; (2)

(d) The power consumed if the same load is connected in delta. (5)

Q4.  A three-phase, 440 V, 60 Hz, 6 pole induction motor runs at a power factor of 0.82 lag and drives a load of 11 kW at a speed of 19.6 rev/s. The stator loss is 1.3 kW and the rotational losses (windage and friction) amount to 1.0 kW.

Calculate EACH of the following:

(a) The synchronous speed; (3)

(b) The rotor copper loss; (6)

(c) The input power to the motor; (4)

(d) The motor current. (3)

Q5.  A three-phase, 440 V, 60 Hz shaft-generator supplies the following loads:

i) incandescent lighting and heating 80 kW at unity pf

ii) fluorescent lighting 60 kW at 0.9 pf lagging

iii) navigation aids and miscellaneous 45 A at 0.85 pf lagging

iv) induction motors 240 kW at 0.8 pf lagging

(a) Determine the total kW, kVAr, kVA and the overall power factor of the ship’s load. (10)

(b) A three-phase synchronous motor which takes 70 kW is now connected to the power system.

(c) Determine EACH of the following:

(i) The required power factor of this motor to cause the shaft generator to operate at unity power factor; (3)

(ii) The current taken by the synchronous motor. (3)

Q6. A 250 kVA single-phase transformer has iron losses of 1.8 kW. The full load copper loss is 2 kW.

Calculate EACH of the following:

(a) Efficiency at full load, 0.8 lagging p.f.; (6)

(b) kVA supplies at maximum efficiency; (4)

(c) maximum efficiency at 0.7 lagging p.f. (6)

Q7.  With reference to shipboard electrical distribution systems:

(a) describe the meaning of the term earth fault; (2)

(b) explain the term earth bonding of electrical equipment, stating how it is achieved; (3)

(c) sketch a circuit diagram of one arrangement for detecting phase to earth faults in a shipboard high voltage three-phase system: (6)

(d) calculate the value of a neutral earthing resistor (NER) to limit the earth fault current to the full load rating of a 2 MW, 0.8 p.f., 3.3 kV, three-phase neutral earthed A.C. generator (5)

Q8. (a) With reference to the principle of operation of a synchronous motor, explain how it differs from that of an induction motor. (4)

(b) Explain why a synchronous motor is unable to produce a starting torque.  (6)

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

(d) State THREE shipboard applications of a synchronous motors. (3)

Q9. With reference to a single-phase, full-wave bridge rectifier:

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

(b) explain the circuit operation; (4)

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

(i) the bridge input voltage; (2)

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

(iii) the load current. (2)