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Q4. A two-core feeder cable 1200 m long has a resistance (‘go and return’) of 0.005 Ω /100 m and is fed at each end with 440 V. It supplies the following loads at the distances given, all measured from the same end of the feeder.

(i) 200 A at 400 m

(ii) 150 A at 700 m

(iii) 250 A at 1000 m

Calculate EACH of the following:

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

(b) The p.d. across the 150A load;

(c) The power loss in the feeder cable.

Q2. A capacitor of 200 µF is charged from a 120V d.c. supply via a 100 kΩ resistor for 10 secs. It is now disconnected from the supply and a second capacitor of 150 µF is charged from the same supply via the same resistor for 15 secs. The two charged capacitors are now connected in parallel.

Calculate EACH of the following:

(a) The potential to which each of the capacitors has been charged;

(b) The energy stored in each capacitor at the end of the charging period;

(c) The final potential across the parallel combination of two capacitors.

Q4. The NPN transistor shown in Fig Q3 has a volt drop between base and emitter of 0.4 V and the base current is small enough to be ignored. The volt drop across the emitter resistor is 3.6 V.

Calculate EACH of the following:

(a) The value of the resistor RB in the bias network;

(b) The value of the collector-emitter current;

(c) The value of the load resistor RL if the steady state voltage at the collector is 12 V as shown.

Q4. A coil having inductance and resistance is connected to a supply of 120 V 50 Hz and draws a current of 2.5 A at a power factor of 0.75. Calculate EACH of the following:

(a) The resistance of the coil;

(b) The inductance of the coil.

(c) A capacitor is now connected in series with the coil and the current rises to 3 A and the power factor is still lagging.

Calculate the value of the capacitor.

(d) Calculate the value of capacitor which would have to be connected in series with the coil to raise the current to 3 A with a leading power factor.

Q3. Fig Q5 shows three identical loads, each consisting of a 100 μF capacitor in series with a 50 Ω resistor, connected in star to a 415 V, 3ph, 50 Hz supply. Calculate EACH of the following:

(a) The current drawn from the supply;

(b) The power factor;

(c) The power consumed by the three phase load;

(d) The p.d. between points A and B.

Q1. A six pole three phase induction motor operates from a 380 V 60 Hz supply. It draws a current of 40 A at power factor 0.7. The frequency of the e.m.f. in the rotor is 2.4 Hz. If the stator loss is 4 kW and the rotational losses (windage and friction) total 3 kW, calculate EACH of the following:

(a) The power input;

(b) The slip;

(c) The rotor copper loss;

(d) The shaft output power.