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

(a) The current through the 10 Ω resistor; (10)

(b) The p.d. across EACH of the following:

(i) The 0.1 Ω resistor; (2)

(ii) The 0.2 Ω resistor; (2)

(iii) The 10 Ω resistor. (2)

Q2) A capacitor of 100  is charged from a 120 V supply via a 100  resistor for 4 seconds.

(a) Calculate EACH of the following:

(i) the time constant of the circuit; (2)

(ii) the voltage to which the capacitor has charged after 4 seconds. (4)

(b) A second capacitor of 80  is now charged for 4 seconds from the same supply via the same 100  resistor. Both charged capacitors are now disconnected from the supply and then connected in parallel.

Calculate EACH of the following:

(i) the final steady state voltage across the pair of capacitors; (8)

(ii) the total energy stored in the two capacitors. (2)

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 has a line current of 80 A at a power factor of 0.8 lag with a shaft speed of 19 rev/s. The iron losses are 2 kW, the stator copper loss is 1 kW and the windage and friction loss is 1.5 kW.

Calculate EACH of the following;

(a) The slip; (3)

(b) The rotor copper loss; (5)

(c) The shaft output power; (5)

(d) The efficiency. (3)

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 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 a shaft-driven generator:

(a)  State TWO advantages; (2)

(b) State ONE disadvantage; (1)

(c) Sketch a labelled diagram of a shaft-driven generator system which employs a frequency converter; (8)

(d) Describe the operation of the system sketched in Q7(c). (5)

Q8. (a) State the conditions necessary to turn on and turn off a thyristor (‘SCR’). (4)

(b) Describe the operation of the circuit shown in Fig Q9. (8)

(c) Sketch a clearly labelled load-voltage waveform for EACH of the following trigger delay angles:

(i) 60 o. (2)

(ii) 120o. (2)

Q9) A transistor with the characteristics shown on Worksheet Q9 is used in a common emitter amplifier circuit with a 14 V d.c. supply and a 2 k  resistive load. The base bias current is 80 mA for a sinusoidal input signal current of 60 mA.

(a) Draw the load line on Worksheet Q9. (2)

(b) Determine the values of collector d.c. voltage and current. (4)

(c) Determine the R.M.S. values of EACH of the following:

(i) the a.c. load current; (4)

(ii) the a.c. output voltage. (4)

(d) Calculate the a.c. signal current gain of the amplifier. (2)