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Q1. When connected to a 20 V d.c. supply a relay starts to operate 0.52 ms after switching on the supply, at which time the instantaneous current is 200 mA. The relay coil has a time constant of 5 ms. (a) Calculate EACH of the following: 1. The final steady state relay current; 2. The resistance and inductance of the relay coil. (b) To increase the operating time a 40 Q resistor is connected in series with the relay coil.  Calculate the new time delay assuming the instantaneous current is 200 mA.

Q2. A 440 V/ 110 V single phase transformer takes a no-load current of 5 A at power factor 0.25 lag. On load the transformer supplies 7.5 KVLA at power factor 0.8 lag. Calculate EACH of the following: (a) The transformer secondary current; (b) The transformer primary current; (c) The primary power factor; (d) The efficiency of the transformer at this load.

Q3. Two 415 V three phase alternators supply a ship’s load made up as follows: 1. Lighting totalling 900 kW at U.P.F. 2. Motors totalling 2100 kVA at power factor 0.7 lag; One alternator supplies 1600 kVA at power factor 0.75 lag; (a) Calculate for the other alternator EACH of the following: (i) The kVA output; (ii) The power factor; (iii) The line current. (b) An over excited synchronous motor is now installed to raise the overall power factor to 0.9 lag. Calculate the power factor of the synchronous motor if it takes 400 kW.

Q4. A balanced three phase delta connected load comprises a coil of resistance 50Ω and inductance 0.1 H in each phase. It is connected to a 440 V 50 Hz supply.  Calculate EACH of the following: (a) The line current for the load; (b) The power factor of the load; (c) The power dissipated by the load; (d) The component values in each leg of a star connected load which would draw the same line current at a leading power factor numerically equal to that found in Q4(b).

Q5. Fig Q3 shows a single stage transistor amplifier. The voltage between base and emitter is 0.3 V and the d.c. voltage at the output terminals is 8 V. (a) Calculate EACH of the following, assuming the base current is small enough to be neglected: (i) The voltage between emitter and collector; (ii) The power developed in the 150Ωresistor; (iii) The power dissipated in the transistor. (b) Sketch the circuit diagram and show the additional components needed to make the amplifier suitable for amplifying small a.c. signals.

Q6. For the network shown in Fig.Q1 calculate each of the following: (a) The current drawn from each battery; (b) The potential difference across the 40Ωresistor and across the 50Ωresistor; (c) The power dissipated in the 60Ωresistor.

Q7. A. Sketch a circuit diagram illustrating the auto transformer method of starting a large induction motor. B. State the sequence of events in starting an induction motor by the auto transformer method; C. State one advantage and one disadvantage of the auto transformer starter.

Q8. With reference to a ward Leonard motor control system for powering a DC capstan motor from a 3 phase AC supply

A. Sketch a circuit diagram for the arrangement; B. Explain how the speed of the capstan motor is varied; C. State the advantages and disadvantages of the ward Leonard system of motor control.

Q9. A. Explain the term power factor correction; B. State advantages of PF correction; C. Explain, with the aid of a circuit diagram, how power factor correction can be effected using capacitors in a 3-phase circuit; D. state one method, other than the use of capacitors, by which power factor correction can be effected.