DieselShip's Online Content Library

Q9. Fig Q1 shows a supply system fed with a different voltage at each end. The resistance of the twin supply cable (‘go and return’) is 0.001 Ω/m. Calculate EACH of the following:

(a) The current supplied from each end of the system;

(b) The p.d. at each of the three load points;

(c) The power lost in the supply cables.

Q6. A capacitor of 500µF is charged from a 24 V d.c. supply via a 200 Ω resistor. When fully charged, it is disconnected from the power supply and connected across a 20Ω resistor in order to be discharged. Calculate EACH of the following:

a) the peak charging current;  (2)

(b) the time for the capacitor to become fully charged;  (2)

(c) the time taken for the capacitor to charge to 12 V;  (3)

(d) the voltage across the capacitor after 25 ms of charging;  (3)

(e) the peak discharge current;  (2)

(f) the discharge current after 5 ms;  (2)

(g) the voltage across the capacitor after 5 ms of discharge.  (2)

Q5. A power silicon transistor with the characteristics shown in Worksheet Q3 has a maximum permissible dissipation of 16 Watts

(a) Plot dissipation curve of 16 Watts on the characteristics.

(b) Determine, by superimposing a suitable load line on the characteristics, the minimum safe value of collector load resistance which may be used with a supply voltage of 12 V.

(c) Determine from the characteristics the variation in collector current when a signal of 40 mA is applied to the base if the bias is fixed at 60 mA.

Q4. The circuit shown in Fig Q4 is supplied from a 400 Hz a.c. supply.

Determine EACH of the following:

(a) the supply voltage V;

(b) the value of the resistor R and the capacitor C;

(c) the p.d. across the resistor R;

(d) the p.d. across the capacitor C.

Q6. Three identical coils each of inductance 0.1 H and resistance 30Ω are connected in delta to a three phase three wire supply of 440 V 50 Hz. Three identical capacitors connected in star are now joined to the same supply to raise the overall power factor to 0.9 lag.

Calculate EACH of the following:

(a) The value of each capacitor;

(b) The percentage reduction in line current;

(c) The kVA taken by the capacitors.

Q1. Two 440 V three phase alternators operating in parallel supply the following loads:

- 1200 kVA at 0.8 lag

- 900 kW at U.P.F.

- 450 kW at 0.75 lead

Calculate EACH of the following, if one alternator provides 1200 KW at unity power factor:

(a) The power factor of the second alternator;

(b) The output current of the second alternator;

(c) The total current supplied to the three loads.