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Q7. A relay coil is connected across 10 V d.c. supply and starts to operate after 0.55 ms when the instantaneous current is 100 mA. The relay coil has a time constant of 5 ms.

Calculate EACH of the following:

(a) The final relay current;

(b) The resistance and inductance of the relay coil;

(c) In order to increase the operating, time a resistance of 40 Ω is connected in series with the relay coil. Calculate the new operating time assuming that the relay starts to operate when the instantaneous current is 100 mA.

Q4. An unestablished d.c power supply whose voltage varies between 15 V and 50 V is connected to a stabilizer circuit comprising a 400 Ω resistor in series with a 8.2 V Zener diode. The Zener diode has a slope resistance of 10 Ω and requires a minimum operating current of 1 mA. The arrangement supplies a load whose current can vary between zero and 30 mA.

(a) Sketch a circuit diagram for the arrangement.

(b) Calculate EACH of the following:

(i) The load voltage when the load current is zero and the supply voltage is 15 V;

(ii) The load voltage when the load current is 30mA and the supply voltage is 40 V;

(iii) The minimum value of input voltage to give a stabilized load voltage when the load current is 30 mA;

(iv) The power dissipated in the Zener diode when the supply voltage is 50V and the output load current is 20 mA.

Q2. A ship’s electrical load is made up as follows:

* motors totalling 2100 kW at a p.f of 0.7 lag

* lighting and heating totalling 1200 kW at U.P.F.

* an overexcited synchronous motor drawing 600 kW at p.f 0.5 lead.

It is supplied by two alternators one of which provides 2000 kW at p.f. 0.95 lag.

Determine EACH of the following:

(a) the kVA supplied by the second alternator;

(b) the power factor of the second alternator;

(c) the overall power factor of the system if the synchronous motor is not connected.