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Q9. Fig shows a. d. c. relay circuit, when the supply is switched on the coil current rises to a final value of 16 mA with a time constant of 1.8 ms. The relay contact close when the current is 11.20mA.

(a) calculate EACH of the following:

(i) the coil resistance; (2)

(ii) the coil inductance; (2)

(iii) the time taken for the relay contact to close. (4)

(b) when the supply is switched off the coil current falls with a time constant of 2.5 ms, and the relay contact opens when the current is 2 mA.

(i) State the purpose of the diode. (2)

(ii) calculate the time taken to open the relay contact, assuming the diode has negligible forward resistance. (4)

(iii) state why a resistor connected in series with the diode will reduce the time taken to open the relay contact.         (2)

Q10. A ship’s electrical load totals 3500 kw made up as follows:

* Induction motors 2000kw, 0.7 lag

* Lighting and heating 1000kw, u.p.f

* Synchronous motor 500 kw, 0.6 lead

The loads are supplied by two, three phase generators operating in parallel.

Generator 1 supplies 2000 kw at 0.9 lag.

Calculate EACH of the following:

(a) the apparent power (kVA) supplied by generator 2; (8)

(b) the power factor of generator 2; (2)

(c) the busbar voltage if the supply current is 3036 A when the synchronous motor is disconnected. (6)

Q11. A 5.1 V, 600 mW Zener diode is used to stabilize the output from a single-phase rectifier circuit which can vary between 9 V and 12 V. The stabilizer circuit has an adjustable resistive load which can be disconnected, and the minimum diode current for stable operation is 2 mA.

(a) Sketch and label the circuit diagram for the Zener stabiliser and load, indicating all currents. (6)

(b) Calculate EACH of the following for stable operation and protection of the diode:

(i) the value of series resistor; (5)

(ii) the maximum load current. (5)