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Q1. For the circuit shown in Fig Q1, calculate EACH of the following:

a) the current in each battery (10)

b) the load voltage

c) the load power

Q2. A 600  capacitor is charged from a 30 V d.c. supply via a 250 Ω resistor. When fully charged the capacitor is disconnected from the supply and connected across a 25 Ω resistor in order to be discharged. Calculate EACH of the following:

(a) The initial charging current; (2)

(b)The capacitor voltage after 137 ms; (4)

(c) The time taken for the capacitor to charge to 22 V; (3)

(d)The initial discharge current; (2)

(e)The discharge current after 30 ms; (3)

(f)The resistor voltage after 30 ms of discharge. (2)

Q3. Three resistive loads of 50 Ω, 20 Ω and 30 Ω are connected respectively in star to the R, S and T phases of a three- phase, 4-wire, 415 V, power supply.

(a) Determine EACH of the following:

(i) The current in each load;(3)

(ii) The current in the neutral wire;(6)

(iii) The total power supplied to the load. (3)

(b)Sketch, approximately to scale, the phasor diagram of the load and neutral currents. (4)

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)

Q7. (a) Sketch and label the V/I characteristics of a three-phase generator operating at constant speed with EACH of the following load power factors:

(i) unity; (2)

(ii) 0.8 lagging; (2)

(iii) 0.8 leading. (2)

(b) Explain why it is recommended that two identical A.C. generators running in parallel should operate at similar power factors. (5)

(c) Explain how the power factor of paralled A.C. generators may be altered independently of the ship's load power factor. (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 transformers. (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)

Q9. Fig Q9 shows a single-transistor amplifier. The voltage between the transistor base and emitter is 0.6 V and the D.C. voltage at the output terminals is 8 V.

(a) Calculate EACH of the following, assuming that the base current is small enough to be neglected:

(i) The voltage between the collector and the emitter of the transistor;(4)

(ii) The power dissipated in the emitter resistor; (3)

(iii) The power dissipated in the transistor. (3)

(b) Sketch the circuit shown in Fig Q9 to show the additional components needed to make the circuit suitable for amplifying A.C. signal. (3)

(c) State the purpose of the additional components shown in Fig Q9(b). (3)