JNTU-B.Tech-Utilization of Electrical Energy-Supplementary Examinations-Aug/Sep 2008-Set-II

JNTU B.Tech Supplementary Examinations, Aug/Sep 2008

UTILIZATION OF ELECTRICAL ENERGY

(Electrical & Electronic Engineering)

SET-II

1. (a) “ If a high degree of speed control is required, d.c. is preferable to a.c. for an electric drive” -Justify.

(b) A 200 V shunt motor has an armature resistance of 0.5 Ohm. It takes a current of 16 amps on full load and runs at 600 r.p.m. If a resistance of 0.5 Ohm is placed in the armature circuit, find the ratio of the stalling torque to the full load torque.And derive the expression used.

 

2. (a) Discuss the various modes of heat dissipation.

(b) A motor driving a load has to deliver a load rising uniformly from zero to a maximum of 2000 h.p. in 20 sec during the acceleration period, 1000 h.p. for 40 sec during the full speed period and during the deceleration period of 10sec when regenerating braking is taking place the h.p. returned to the supply falls from 330 to zero. The interval for decking before the next load cycle starts is 20 sec. Estimate the horse power rating of the motor.

 

3. (a) What are the causes of failure in heating elements?

(b) Six resistances each of 40 ohms are used as heating elements in furnace. Find the power of the furnace for various connections to a three phase 230V supply.

 

4. (a) What are various types of electric braking used?

(b) Explain how rheostatic braking is done in D.C. shunt motors and series motors.

 

5. (a) State and Explain cos3; law.

(b) A lamp of 500 candle power is placed at the centre of a room, 20m×10m×5m. Calculate the illumination in each corner of the floor and a point in the middle of a 10m wall at a height of 2m from floor.

 

6. Give the construction and working of the following types of lamps.

(a) Arc lamp

(b) Neon lamp

(c) Sodium lamp

 

7. (a) Review the various systems of traction work.

(b) Write short notes on sub-traction for single-phase A.C systems.

 

8. (a) An electric locomotive is required to haul a train of 12 coaches each weighing 30 tonnes on the main line service requiring an initial acceleration of 0.8km/hr/sec up a gradient of 1 in 100. Estimate the adhesive weight and hence the number of driving axles the locomotive must have, if the permissible axle loading is 20 tonnes per axle. Assuming for rotational inertia to be 4%, for the coaches and 15% for the locomotive. Maximum coefficient of adhesion is 0.2 and the tractive resistance 5kg/tonne.

(b) An electric train weighing 200 tonnes has 8 motors geared to driving wheels, each wheel is of 80cms diameter. Determine the torque developed by each motor to accelerate the train to a speed of 48km/hr in 30seconds up a gradient of 1 in 200. The tractive resistance of 50newtons/tonne, the effect of rotational inertia is 10% of the train weight, the gear ratio is 4 in 1 and gearing efficiency is 80%.

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