Q5. A 110 V battery with internal resistance of 0.3 Ω, is connected in parallel with a 120 V d.c. generator with internal resistance 0.5 Ω. This parallel combination
supplies a 5 Ω load.
(a) Sketch a labelled circuit diagram.
(b) Calculate EACH of the following:
(i) the battery current;
(ii) the load current.
(c) State whether the battery is charging or discharging.
Q6. (a) A 33 μF capacitor is charged from a 100 V d.c. supply via a 47 kΩ resistor for 3 seconds. This capacitor is then disconnected from the supply and a 68 μF capacitor is charged from the same supply via the same resistor for 5 seconds.
Calculate EACH of the following:
(i) the voltage to which each capacitor has charged. (8)
(ii) the charge stored in each capacitor. (3)
(b) The charged capacitors in Q2(a) are connected in parallel.
Calculate the final voltage across the parallel circuit. (5)
Q7. An unbalanced, three-phase, star connected load is supplied from a four-wire, 440 V, 50 Hz supply. The current in EACH phase is:
• IR= 7 A lagging VR by 30°
• Is = 5 A in phase with Vs
• Ig = 9 A leading VT by 60°
(a) Sketch, approximately to scale, a labelled phasor diagram showing phase voltages and currents. (3)
(i) the neutral current; (5)
(ii) the phase angle of the neutral current with respect to the S phase voltage; (2)
(iii) the total power dissipated. (6)
Q8. (a) Sketch a labelled circuit diagram for a 440V, step-down, star connected, three-phase autotransformer with 50%, 65% and 80% tappings. Indicate primary and secondary phase turns, voltages and currents. (5)
(b) State TWO advantages and the main disadvantage of an autotransformer compared to a double wound transformer. (3)
(c) A three-phase, 440 V induction motor having a full-load current of 75 A is connected to the 65% tappings of the autotransformer in Q6(a). The motor starting current is 6 x full-load when started D.O.L.
Calculate EACH of the following at start:
(i) the current in each section of the transformer phases. (4)
(ii) the voltage across each section of the transformer phases. (3)
(iii) the motor line voltage. (1)
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