ELECTROMECHANICAL ENERGY CONVERSION-I B.Tech previous year qus paper 2nd year (III sem)

UTTARAKHAND TECHNICAL UNIVERSITY

B.TECH (SEM. 3RD) 

2011-2012

sub: ELECTROMECHANICAL ENERGY CONVERSION-I

time : 3hr]

Total marks :100

Note: Attempt all questions. Be precise in your answer. 

SECTION A

Q1:- Attempt any two of the following :

10x2=20 

1. Describe the principle of energy conversion. For singly excited magnetic system, derive the relation for the magnetic stored energy in terms of reluctance.
2. Define field energy and co-energy. Give the significance of co-energy in the derivation of torque or force in an electromechanical energy conversion device.
3. State and briefly explain the various phenomenon useful for the electromechanical energy conversion in rotating machines. Also derive an expression for reluctance torque in a rotating electrical machine.

SECTION B

Q2:- Attempt any two of the following : 

10x2=20

1. Explain "armature reaction" in a dc machine, indicating remedies to overcome its adverse effects.
2. Derive the emf and torque equation for d.c machines. A 6 pole dc machine has 300 conductors and each conductor is capable of carrying 80 A without excessive temperature rise. The flux per pole is 0.015 Wb and the machine is driven at 1800 rpm. Compute the total current, e.m.f power developed in the armature and electromagnetic torque, if the armature conductors are:
   (a) wave connected
   (b) lap connected
3. What is commutation in case of dc machines? What is the use of compensating windings and effect of brush shifting in dc machines?

SECTION C

Q3:- Attempt any two of the following:

10x2=20

1. (a) Hopkinson's test on two dc machines gave the following results for the full load; line voltage 250 V, line current excluding field current 50 A; motor armature current 380 A; field currents 5 and  4.2 A . Calculate the efficiency of each machine. The armature resistance of each machine is 0.01 ohm. State the assumptions made.(b) Why is starter is necessary for starting a dc motor? Explain briefly working principle of a 3 point starter with the help of neat diagram. 
2.  Explain with the help of neat diagram the Ward Leonard method used for speed control of dc motor. What are its advantage and disadvantages?
3.  In a 230 V, 10 kW dc shunt motor, it is required that the starting armature current should not exceed twice its rated value. During the starting of the motor, the starting resistance is cut out in steps, as soon as the armature current drops to its rated value. The field resistance is 115 ohm and the total armature circuit resistance is 0.348 ohm. Neglect the armature reaction.Find:
   (a) external resistance required at the time of starting the motor
   (b) the external resistance to be cut out in first step
   (c) the total number of steps required and their resistance values. 

SECTION D

Q4:- Attempt any four of the following :

4x5=20

1. Two transformers each rated 250 kVA, 11/2 kV and 50 Hz are connected in open delta on both the primary and secondary. Find the load kVA that can be supplied from this transformer connection.
2. The HV terminals of a 3 phase bank of three single phase transformers are connected to a 3 wire, 3 phase, 11 kV (LL) system. The LV terminals are connected to Δ a 3 wire, 3 phase, load rated of 1000 kVA ratings of each transformer (both HV and LV windings) for the following connections:
   (a) Y - Y
   (b) Δ - Δ
   (c) Y – Δ
   (d) Δ - Y
3. Explain Scott connection with the help of neat diagram.
4. Explain the difference phasor groups of transformers and draw connections for delta-zigzag and star-zigzag connections
5. Explain the advantages and disadvantages of a bank of three single phase transformers over a single three phase transformer.
6. Explain the role of Buchholz relay, conservator, breather, and transformer oil in case of a transformer oil in case of a transformer. Mention the types of cooling methods used in case of transformers.

SECTION E

Q5:- Attempt any four of the following :

4x5=20

1. Explain Sumpner's test with suitable diagram for finding efficiency of a transformer.
2. Derive an expression for the saving of copper in auto-transformer as compared with an equivalent two winding transformer.
3. A transformer has its maximum efficiency of 0.98 at 20 kVA at unity power factor. During the day it is loaded as follows:
   12 hours; 2 kW at power factor 0.6
   6 hours; 10 kW at power factor 0.8
   6 hours; 20 kW at power factor 0.9
   Find the all-day efficiency of the transformer.
4. Explain with the help of diagrams the phenomenon of harmonics in case of single phase transformers.
5. What are the condition required to be satisfied for the parallel operation of a single phase and three phase transformers? Derive the expression for the circulating current flowing through the secondary windings of transformers connected in parallel.
6. What are the purpose of using three winding transformers? What are the advantages and disadvantages of auto transformer?

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Just practice it !!!