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Q1) Fig Q1 shows a d.c. ring main supplied by two generators, The cable section resistances are for go and return.

Calculate EACH of the following:

(i) the current in each cable section; (8)

(ii) the power loss in each cable section. (5)

(iii) the voltage at each load point. (3)

Q2) A d.c. relay coil having an inductance of 300 mH and negligible resistance is connected in series with a 5 Ω resistor across a 20 V d.c. supply.

(a) Calculate the value of resistor voltage 30 ms after switch-on. (4)

(b) State the expression for the instantaneous current and determine its value 30 ms after switch-on. (4)

(c) When steady state conditions have been reached, the supply is replaced instantaneously by a short-circuit.

(i) Determine the time taken for the current to fall to 10 % of the steady state value. (6)

(ii) Sketch a clearly labelled graph with approximately scaled axes to show the growth of circuit current after switch-on. (2)

Q3). Three loads of 100 , 0.2 H and 50 Ω are respectively connected in star to the R, S and T phases of a three-phase, 4-wire, 440 V, 60 hz supply.

(a) Determine EACH of the following:

(i) the current in each load; (4)

(ii) the total power supplied to the load.   (2)

(b) Sketch, approximately to scale, a clearly labelled phasor diagram showing the phase voltages and the load currents. (4)

(c) Determine the magnitude of the neutral current and its angle (relative to the R phase). (6)

Q4) An electric load of 800 kW is operating at a power factor of 0.7 lagging. It is desired to improve the power factor to 0.92 lagging by connecting a synchronous motor driving a load of 200 kW with an efficiency of 91 %.

Determine EACH of the following:

(a) the kVA rating of the synchronous motor; (13)

(b) the power factor of the synchronous motor. (3)

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.

Their 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 a core loss of 4 kW, and a full-load winding 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 0.5 power factor. (8)

Q7) With reference to a shaft-driven generator:

(a) state TWO advantages; (2)

(b) state ONE disadvantage; (1)

(c) sketch clearly labelled block diagram of a shaft-driven generator system which employs a frequency converter and a synchronous compensator; (8)

(d) describe the operation of the system sketched in Q7©. (5)

Q8) (a) Sketch a clearly labelled diagram of the power circuit for a star/delta starter. (8)

(b) Describe the sequence of operation of the circuit sketched in Q8(a). (6)

(c) State TWO limitations of the star/delta starting. (2)

Q9) With reference to a full-wave bridge rectifier using diodes:

(a) sketch a clearly labelled circuit diagram; (4)

(b) explain the circuit operation; (4)

(c) sketch clearly labelled waveforms to show the relationships between the following:

(i) the bridge input voltage; (2)

(ii) the current through each diode; (4)

(iii) the load current.  (2)