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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 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 a 50 V d.c. supply 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.

Determine the new operating time for the relay. (6)

Q3. A star connected three phase load has a coil of resistance 50 Ω and inductance of 0.1 H in each phase. The load is connected to a three phase, 440 V, 60 Hz supply.

Calculate EACH of the following:

(a) The line current; (5)

(b) The power factor of the load; (4)

(c) The value of each of three identical delta connected capacitors which if connected in parallel with this load will raise the overall power factor to unity. (7)

Q4. A three phase, four pole induction motor runs on a 440 V, 50 Hz supply. It delivers a shaft output power of 50 kW. The rotational losses (windage and friction) amount to 4 kW and the speed is 24 rev/s.

If the input current is 120 A at a lagging power factor of 0.7 and the stator copper loss is 3 kW, calculate EACH of the following:

(a) The rotor copper loss; (6)

(b) The stator iron loss; (6)

(c) The efficiency. (4)

Q5) Two, six-pole, three-phase a.c. generators operating in parallel supply a total load of 2000 kVA at a power factor of 0.8 lagging.

The generator load characteristics are linear with the test results given in Table Q5.

Determine EACH of the following:

(a) the supply frequency; (6)

(b) the bus-bar voltage; (6)

(c) the kVA output of each generator; (2)

(d) the operating power factor of each generator. (2)

Q6. A 60 kVA, 440 V/110 V single phase transformer has iron loss of 4 kW, and a full load copper loss of 6 kW.

Calculate EACH of the following:

(a) The kVA output at which maximum efficiency will be achieved;(8)

(b) The efficiency at 50 kW output and 0.85 power factor;(8)

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)

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

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

(b) describe the system sketched in Q8(a). (8)

(c) State ONE advantage and ONE disadvantage. (2)

Q9) A single-phase, 230 V, 50 Hz, 3:1 step-down transformer has a secondary winding resistance of 1 Ω and supplies a half-wave rectifier circuit. The rectifier circuit has a resistive load of 680  and diode has a forward resistance of 14 Ω.

(a) Sketch EACH of the following:

(i) the circuit diagram; (3)

(ii) the load voltage waveform indicating maximum and average voltage levels. (3)

(b) Calculate EACH of the following load values:

(i) the maximum current; (4)

(ii) the average current; (3)

(iii) the average voltage. (3)