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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 the 240 V end:

Load 1 10 A at 100 m

Load 2 100 A at 250 m

Load 3 70 A at 450 m

Load 4 75 A at 500 m

The cable resistance (go and return) is 0.16 Ω per 100 m.

Calculate EACH of the followings:

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

(b)The voltage at each load point; (8)

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

Q2. A relay coil has a resistance of 200 Ω and the current required to operate the relay is 150 mA. When the coil is connected to 50 V d.c. it takes 40 ms for the relay to operate

(a) Calculate EACH of the following:

(i)The steady state relay current; (2)

(ii) The time constant for the coil; (4)

(iii) The inductance of the coil. (4)

(b) To increase the operating time for the relay, a 50 Ω resistor is connected in series with the coil. Calculate the new operating time for the relay. (6)

Q3. The p.d. between base and emitter for the transistor shown in Fig Q3 is 0.3 V and the steady state output voltage VC is 6 V.

Determine EACH of the following, assuming that the base current is small enough to be ignored:

(a) The voltage at the base with respect to earth; (3)

(b) The p.d. between emitter and collector; (3)

(c) The value of the load resistor RL ; (3)

(d) The power dissipated in the 200 Ωresistor; (3)

(e) The power dissipated in the transistor. (4)

Q4. (a) A 400 V/110 V transformer has 3468 turns on each primary phase winding. If the volt drop in the windings are negligible, calculate the number of turns of each secondary phase winding for EACH of the following connections:

(i) Delta/delta; (5)

(ii) Delta/star; (5)

(b) Explain why the two transformers described in Q4(i) and Q4(ii) cannot be operated with their primaries and secondaries connected in parallel. (6)

Q3. A balanced, star connected, three phase load has a coil of inductance 0.2 H and resistance of 50 Ω in each phase. It is supplied at 415 V, 50 Hz. Calculate EACH of the following:

(a) The line current; (5)

(b) The power factor; (2)

(c) The value of each of the three identical delta connected capacitors to be connected to the same supply to raise the overall power factor to 0.9 lag; (7)

(d) The value of new line current. (2)

Q6. A 6 pole 3 phase squirrel cage induction motor runs on 380 V 60 Hz supply.

It draws a line current of 80 A at a power factor of 0.8 lag.

The shaft speed is 19 rev/sec.

If 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 as a per unit value;(3)

(b) The rotor copper loss; (5)

(c) The shaft output power; (5)

(d) The efficiency. (3)

Q7. (a)Sketch the reverse voltage/current characteristic for a low power Zener diode with a break down voltage of 10 V. (5)

(b) Sketch a simple voltage regulator circuit using a Zener diode. (5)

(c) State which factors determine the value of series resistor used in the circuit described in Q7(b). (3)

(d) State which factors determine the power rating of the Zener diode in the circuit described in Q7(b). (3)

Q7) (a) Explain the meaning of the term power factor. (3)

(b) State TWO advantages of power factor correction. (4)

(c) Explain, with the aid of a circuit diagram, how power factor correction can be achieved in a three-phase circuit using capacitors. (5)

(d) Explain ONE method, other than the use of capacitors, by means of which power factor correction can be achieved. (4)

Q9. (a) Explain how torque is produced in a 3-phase squirrel cage induction motor. (5)

(b) State why the starting current is several times higher than the full load current. (3)

(c) State why the power factor is very low on starting. (3)

(d) Describe ONE method of construction by means of which the starting power factor may be raised, the starting current lowered and the starting torque improved. (5)