SUBJECT : PHYSICS , CLASS : XI,. PSA Questions. Stand Alone Questions. 1. In
a game of tug of wars, two opposite teams are pulling the rope with equal ...
TAGORE INTERNATIONAL SCHOOL EAST OF KAILASH, NEW DELHI
SUBJECT : PHYSICS , CLASS : XI, PSA Questions Stand Alone Questions 1. In a game of tug of wars, two opposite teams are pulling the rope with equal (magnitude) but opposite forces of thousands kg wt at each end of the rope. The tension in the rope if a condition of equilibrium exists will be (a) equal to 1000 kg wt (b) less than 1000 kg wt (c) more than 1000 kg wt (d) equal to 980N. 2. A ball 'x' is dropped vertically and another 'y' is thrown horizontally from the same point. Which of the two balls will reach the ground earlier ? (i) x (ii) y (iii) both will reach the ground simultaneously (iv) ball 'y' will never reach the ground. 3. Let water and oil be the two liquids in the two limbs of a U- tube. Suppose water stands 16cm above the interface and oil stands 20cm above the interface. Which of the following is correct ? Given density of water = 1000 kg /cu.m (i) density of oil is 800 kg/cu.m (ii) specific gravity of oil is 800. (iii) specific gravity of oil is 0.8 (iv) both (i) and (ii) 4.If the masses of all the molecules of a gas are halved and their speeds are doubled, then the ratio of initial and final pressures will be (i) 2 : 1 (ii) 1 : 2 (iii) 4 : 1 (iv) 1 : 4 ________
Passage Based Questions To encourage “Energy Conservation & Environment Protection” in the industrial sector PCRA has been continuously upgrading the energy conservation knowledge through energy audits and sharing the experience of energy efficiency improvement and achievement of industries. The case study detailed below is one of the modification carried out by Visakhapatnam Steel Plant, Rashtriya Ispat Nigam Ltd. Rashtriya Ispat Nigam Ltd. is the corporate entity of Visakhapatnam Steel Plant,which has a capacity to produce 2.656 Mt of saleable steel of which 2.410 Mt is finished steel.Steel making is energy intensive in nature. There are 11nos. of 1700KW and 9 nos. of 1000KW main drive DC motors installed to run the mill stands. They are run at different speeds as required for the profile being rolled. During normal operation,the motors run at lesser field currents. However, when the mill is stopped and the drive is kept ready for rolling, the field current remain at 100%. It was observed that operating at 100% field currents during non rolling period is leading to energy wastage as the duration of rolling is 18Hrs. and non-rolling period is 6Hrs. per day.
So it is proposed to keep the field current at 25% of rated value during non rolling and increase it to 100% field when speed reference is released. This can be implemented by modifying the thyristor converter control circuit of the DC drives. First field is allowed to raise from 25% to 100%, certain time delay only. After field goes to 100% only, computer reference is allowed to run the drive. Then drive starts picking up speed and goes to field weakening mode and settles at the required RPM. In this way the energy saving is achieved during non-rolling period. The field currents are tabulated as follows:
Parameter
1000 KW Motor
1700 KW Motor
No of Motors
9 Nos.
11 Nos.
Field Current and Voltage during Rolling
Less than 112 Amp & 150V
Less than 202 Amp & 150V
Field Current and Voltage during Non Rolling before Modification
112 Amp, 150V
202 Amp 150V
Field Current and Voltage during Non-Rolling for 1000 KW/ 1700KW DC Motors and Voltage after modification)
28 Amp, 150V
50.5 Amp 150V
Reduction in Field Current
84 Amp
151.5 Amp
Field resistance
1.34 O
0.7425 O
Reduction in Energy loss per Motor (I2R)
9.45 KW
17.4 KW
Total Loss (Energy Saved)
85 KW
187.5 KW
1.In the above passage , the word 'field current ' means (i) magnetic field (ii) current produced by a magnetic field (iii) current required to run the DC motors (iv) current from the main supply 2. The percentage reduction in the value of field current for 1000KW motor during non rolling after modification is (i) 75% (ii) 25% (iii) 30% (iv) 100% 3.Energy saving is done at the non rolling period by (i) introducing a certain time delay (ii) thyristor converter control circuit (iii) allowing the computer reference run the drive (iv) all of the above.
