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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; (3)

(c) The load power. (3)

Q2) A d.c. relay coil has an inductance of 400 mH and a time-constant of 1.6 ms. The current rises to 86.5 mA after 3.2 ms from switch on.

Calculate EACH of the following:

(a) the coil resistance; (2)

(b) the maximum current; (3)

(c) the supply voltage; (2)

(d) the current 4 ms after switch-on; (3)

(e) the time taken for the current to reach 70 mA; (4)

(f) the final value of the stored energy. (2)

Q3. A three-phase, 415 V, 4-wire power supply has the following unity power factor loads:

i) R-N: 24 kW

ii) S-N: 18 kW

iii) T-N: 12 kW

Calculate EACH of the following:

(a) The current in each load; (4)

(b) The current in the neutral conductor; (10)

(c) The phase angle of the current in the neutral conductor relative to the red phase voltage. (2)

Q4. A three-phase, star-connected., eight-pole induction motor runs on a 415 V, 50 Hz power supply. The stator resistance is 0.1 Ω per phase, the rotational losses ( windage and friction) are 1 kW, the speed is 12 rev/s and the shaft output power is 24 kW.

If the input current is 57 A at a lagging power factor of 0.707, calculate EACH of the following:

(a) The output torque; (2)

(b) The rotor copper losses; (5)

(c) The stator copper losses; (2)

(d) The stator iron losses; (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) With reference to three-phase transformers, sketch labelled diagrams for EACH of the following connection types indicating line and phase voltages and line currents:

(i) star-star; (4)

(ii) delta-star. (4)

(b) A three-phase, step-down transformer has a turns ratio of 12 and takes 10 A when it is connected to a 6600 V supply. If the transformer is star-delta connected, calculate EACH of the following:

(i) the secondary line voltage; (3)

(ii) the secondary line currents; (3)

(iii) the output kVA. (2)

Q7) With reference to shipboard three-phase generator:

(a) describe, with the aid of a sketch, EACH of the following:

(i) an insulated netural distribution system; (3)

(ii) an earthed netural distribution system. (3)

(b) explain the effect of single earth fault on each of the systems in Q7(a); (6)

(c) state TWO causes of earth faults. (4)

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. An unstabilised d.c. supply voltage varies between 25 V and 35 V. A voltage stabiliser circuit comprising a 12 V zener diode and a series resistor R is connected across the unstabilised supply. The Zener has a slope resistance of 14 Ω and requires a minimum operating current of 1 mA. A 0-80mA variable load is to be supplied by the stabiliser circuit.

(a) When the supply voltage is minimum and the load current demand is a maximum, calculate EACH of the following:

(i) The maximum value for R to give a stable load voltage; (4)

(ii) The load voltage. (2)

(b) Using the value of R determined in Q9(a), calculate EACH of the following.

(i) The load voltage when the supply voltage and load current are both at maximum value; (4)

(ii) The Zener diode current when the supply voltage is minimum, and the load is switched off; (3)

(iii) The load voltage when the supply voltage is 30 V and the load current is 30 mA. (3)