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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 100 capacitor is charged from 120 V d.c. supply via a 10 kΩ resistor.

(a) Calculate EACH of the following:

(i) the time taken for the capacitor voltage to reach 80 V. (6)

(ii) the energy stored in the capacitor when its voltage has reached 80 V. (3)

(b) If the supply is now removed and replaced by a second 10 kΩ resistor, calculate EACH of the following:

(i) The time taken for the capacitor to fall to 50 V. (5)

(ii) The circuit current when the capacitor voltage has fallen to 50 V. (2)

Q3. Three identical coils each of inductance 0.1 H and resistance 30 Ω are connected in delta to a three phase, 440 V, 50 Hz supply. Three identical star connected capacitors are connected in parallel with the delta load to raise the power factor to 0. 9 lagging.

Calculate EACH of the following:

(a) the value of each capacitor; (10)

(b the percentage reduction in line current; (3)

(c) the kVAR taken by the three capacitors (3)

Q4. A three phase, 440 V, 60 Hz, 8 pole induction motor drives a load of 7 kW and runs at 14.4 rev./sec. The power factor is 0.8 lag. The stator loss is 0.6 kW and the rotational losses (windage+ friction) are 0.4 kW. Calculate EACH of the following.

(a) The slip; (3)

(b) The frequency of rotor e.m.f; (2)

(c) The input power to the motor; (8)

(d) The line current. (3)

Q5. Two three phase, 3.3 kV alternators operating in parallel supply the following three loads:

à A lighting load of 600 kW at unity power factor

à Motors totalling 2500 kW at p.f. 0.7 lag

à A synchronous motor driving a bow thruster

One alternator supplies 350 A at p.f. 0.9 lag and the other supplies 330 A at p.f. 0.95 lag.

Determine EACH of the following:

(a) The kW supplied to the synchronous motor;(11)

(b) The p.f. of the synchronous motor; (3)

(c) The overall p.f. of the system. (2)

Q6. A 50 kVA transformer has an efficiency of 98 % at full load, 0.8 p.f. and 97 % at half load, 0.8 p.f. lag. Calculate EACH of the following:

(a) The full load copper loss; (8)

(b) Load at which maximum efficiency occurs; (4)

(c) The maximum efficiency for 0.8 p.f. load. (4)

Q7) With reference to shipboard electrical distribution systems:

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

(b) explain why insulated neutral is preferred for low voltage systems; (3)

(c) sketch a circuit diagram of one arrangement for detecting phase to earth faults in   a three-phase system supplied by a star connected generator with earthed neutral resistor (NER); (6)

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

Q8) (a) State THREE reasons why switchboard instruments are supplied via instrument transformers from the power circuits which they monitor. (3)

(b) Explain why it is hazardous to open circuit a current transformer whilst its primary is still energized. (4)

(c) Sketch a circuit diagram showing an ammeter, a voltmeter and a wattmeter fed from a single-phase supply via current and voltage transfomers. (5)

(d) An ammeter, a voltmeter and a wattmeter monitoring a single phase supply read 40 A, 240 V and 8 kW respectively.

Calculate the power factor of the circuit. (4).

(9) 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)