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Q20. For the network shown in Fig, calculate EACH of the following:

(a) The value of Rx so that the total current drawn from the 12 V supply shall be 2 A as shown;

(b) The potential difference across Rx;

(c) The value to which the 5 Ω resistor must be changed to give zero potential difference across Rx.

Q11. 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.

Q5. A star connected three phase Load has a coil of resistance 50 Ω and inductance 0.1 H in each phase. It is connected to a three-phase, three-wire supply of 415V, 50Hz.

Calculate EACH of the following:

(a) The Line current;

(b) The power factor of the Load;

(c) The value of EACH of three identical delta connected capacitors, which if connected in parallel with this Load, will raise the overall power factor to unity.

Q5. A three phase, four pole induction motor runs off a 440 V, 50 Hz supply. It delivers a shaft output power of 50 kW. The rotational losses (windage and friction) amount to 4 kW and the speed is 24 rev/sec. if the input current is 120 A at a lagging power factor of 0.7 and the stator copper loss is 3 kW, calculate EACH of the following:

(a) The rotor copper loss:

(b) The stator iron loss;

(c) The efficiency.