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Q1.  A 525 m, two core distributor cable is fed at one end with 240 V d.c and at the other end with 250 V d.c. The following loads are applied at distances measured from 240 V end:

i) Load 1 10 A at 100 m

ii) Load 2 100 A at 250 m

iii) Load 3 70 A at 450 m

iv) Load 4 25 A at 500 m

The cable resistance (go and return) is 0.08 Ω per 100 m.

Calculate EACH of the following:

(a) The current supplied at each end of the cable distributor; (6)

(b) The voltage at each end; (8)

(c) The power delivered at each end of the cable distributor. (2)

Q2. A 120  capacitor is charged through a 4.7 kΩ resistor from a 12 V d.c. power supply.

(a) Calculate the instantaneous charging current at switch on. (2)

(b) State the expression for the capacitor charging voltage and determine its value 2 seconds after switch on. (4)

(c) Calculate the energy stored in the capacitor 2 seconds after switch on. (2)

(d) After 2 seconds of charging the supply is switched off and the capacitor is discharged through a 1.2  resistor.

(i) Determine the time taken during discharge for the capacitor voltage to fall to 5 V.  (4)

(ii) Sketch a graph with approximately scaled axes to show the capacitor voltage changes over its charge/discharge cycle. (4)

Q3. A three phase, 4 wire. Unbalanced load draws the following currents with reference to :

I_RN=5 A ∠ 0^o
I_YN=8 A ∠-150^o
I_BN=3 A ∠85^o

Calculate EACH of the following:

(a) The current in the neutral line; (7)

(b) The angular position of the neutral current with reference to ; (2)

(c) The total power dissipated by the load if it is connected to a 415 V supply. (7)

Q4. A three phase, 6 pole, 380 V, 60 Hz induction motor draws a line current of 80 A at a power factor of 0.8 lag with a shaft speed of 19 rev/s. The core losses are 2 kW, the stator winding loss is 1 kW and the windage and friction loss is 1.5 kW.

(a)  Calculate EACH of the following;

(i) The slip;(2)

(ii) The rotor copper loss;(4)

(iii) The shaft output power; (2)

(iv) The efficiency. (3)

(b) Sketch a clearly labelled power-flow diagram for the motor indicating power at each stage. (5)

Q5. A three phase, 440 V a.c. generator supplies the following loads:

i) A star connected load of 33 kVA and power factor 0.9 leading

ii) A delta connected load of 40 kW and power factor 0.85 lagging

iii) Miscellaneous loads of 23 kVA and power factor 0.8 lagging

Calculate EACH of the following:

(a) The kVA supplied by the generator; (10)

(b) The generator current;(2)

(c) The phase currents for the star and delta connected loads. (4)

Q6. A single phase, 50 Hz transformer has primary 144 turnsand 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 magnetizing current; (5)

(b) The primary current; (8)

(c) The primary power factor. (3)

Q7. With reference to shipboard three-phase generators:

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

(i) An insulated neutral distribution system; (3)

(ii) An earthed neutral distribution system; (3)

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

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

Q8.  With reference to a three-phase brushless generator system:

(a) Sketch a clearly labelled diagram showing the essential features;(8)

(b) Describe the system sketched in Q8(a); (8)