Q1. Fig Q1 shows a ring main distributor fed at point at 440 volts. The distances between the various loads are given in metres and the two cables has a go and return resistance of 0.02 Ω per 100 metres.
Determine EACH of the following:
(a) The current in the cable between the 30 A and 70 A loads; (8)
(b) The lowest p.d across any of the loads; (4)
(c) The total power loss in distributor. (4)
Q5. A capacitor of 200 μF is charged from a 120 volt d.c. supply via a 100 kΩ resistor.
(a) The time taken for the p.d. across the capacitor to reach 80 volts;
(b) The charge on the capacitor at this time.
If the capacitor is now disconnected from the supply and joined to another, Uncharged, capacitor of 100 μF determine EACH of the following:
(c) The final p.d. across the pair of capacitors;
(d) The energy stored by the pair of capacitors.
Q3. Fig Q3 shows a simple voltage regulator circuit designed to provide a stable 15 V d.c. output from an unregulated supply which can vary between 20 V and 30 V. The Zener diode has a breakdown potential of 15 V and the slope resistance when conducting is 1Ω.
It requires a minimum reverse current of 1 mA for satisfactory regulation.
(a) The minimum value of the series resistor ‘R’ if the Zener diode does not exceed its power rating of 3 W;
(b) The regulated output voltage when the input voltage is 20 V. and the load current is 30 mA;
(c) The maximum permissible output current for satisfactory regulation when the input voltage is 30 V.
Q3. A coil possessing resistance and inductance is connected to a 120 V variable frequency supply. When the frequency is 50 Hz the current is 4A and when the frequency is raised to 100 Hz the current falls to 3 A. Determine EACH of the following:
(a) The resistance of the coil;
(b) The inductance of the coil;
(c) The power factor of the coil at 50 Hz;
(d) The power dissipated by the coil at 100 Hz.
Q11. Three identical coils are delta connected to a 3 ph, 440 V, 60 Hz supply and consume a total power of 9 kW at a power factor of 0.8 lag.
(a) Calculate the resistance and inductance of EACH coil.
(b) If the same three coils are now connected in star to the same supply, calculate the current in each line if:
(i) One coil is short circuited;
(ii) One coil is open circuited
Q2. A 3ph, 440 V, 60 Hz 8 pole induction motor drives a load of 7 kW and runs at 14.4 rev/min. The power factor is 0.8 lag. The stator loss is 0.6 kW and the rotational losses (windage and friction) are 0.4 kW. Calculate EACH of the following:
(a) The slip;
(b) The frequency of the rotor e.m.f;
(c) The input power to the motor;
(d) The line current.
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