Q1. Fig Q1 shows a d.c. ring main supplied by two 230 V generators. The cable section resistances are for go and return. Calculate EACH of the following:
(a) The current in each cable section; (8)
(b) The power loss in each cable section; (5)
(c) The p.d. at each load. (3)
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) THREE identical coils are star-connected across a three-phase, 440 V, 60 Hz power supply and consume a total power of 3 kW at a power factor of 0.8 lagging.
(a) Determine the resistance and inductance of each coil. (8)
(b) Calculate the current in each line if one coil is:
(i) short- circuited; (5)
(ii) open-circuited. (3)
Q4. A three-phase electrical load of 800 Kw is operating at a power factor of 0.7 lagging. It is desired to improve the supply 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 of the synchronous motor; (12)
(b) The power factor of the synchronous motor. (4)
Q5. Two, three-phase, four-pole generators operating in parallel share a total load of 900 kW at 0.75 power factor lagging. Test results are shown in Table Q5 and the load characteristics are linear.
(a) The bus-bar frequency; (5)
(b) The bus-bar voltage;(5)
(c) The operating power factor of each generator. (6)
Q6. With reference to a single- phase power transformer with natural air cooling:
(a) Sketch labelled diagram of the basic construction; (3)
(b) Describe the principle of operation; (5)
(c) Explain why it is rated in kVA; (2)
(d) Explain why it may overheat if operated at reduced frequency; (3)
(e) State how operation at reduced frequency may be compensated to avoid overheating. (3)
Q7. (a) State the function of an AVR. (2)
(b) State FOUR conditions external to an alternator that causes its terminal voltage to drop. (4)
(c) Sketch and label a typical AVR response curve, identifying the application of load, recovery time, maximum acceptable voltage dip, and the permissible steady-state voltage regulation. (6)
(d) State the function of an AVR trimming potentiometer for EACH of the following cases:
(i) When a generator operates in standalone mode;(2)
(ii) When a generator operates in parallel. (2)
Q8. With reference to shipboard electrical distribution systems:
(a) Describe the meaning of the term earth fault;(2)
(b) Explain the meaning of the term earth-bonding and why it is required;(3)
(c) Sketch a circuit diagram of one arrangement for detecting phase to earth faults in an earthed neutral system; (6)
(d) Calculate the value of a neutral earthing resistor (NER) to limit the earth fault current to the full load rating of a 2 MW, 0.8 p.f., 3.3 kV, three-phase generator. (5)
Q9. (a) Sketch a labelled circuit diagram of a half-wave rectifier with a capacitor filter and resistive load. (4)
(b) Describe the operation of the circuit sketched in Q9(a) (9)
(c) Sketch the load voltage waveform for the circuit sketched in Q9(a) and identify the peak-peak ripple voltage. (4)
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