ELECTROMECHANICAL ENERGY CONVERSION-I


UTTARAKHAND TECHNICAL UNIVERSITY
B.TECH (SEM. 3RD) 
2012
ELECTROMECHANICAL ENERGY CONVERSION-I


time : 3hr]
max marks: 100
Note : Attempt all questions.
SECTION A
Q1:- Attempt any four of the following :
4 x 5=20 

  1. 1.    Show that the energy stored in a magnetic field is equal to the area between the ψ-i curve for the system and the flux linkage (ψ) axis.
    2.    Show that the field energy in a linear magnetic system is given by:-      Wf = ½ Li2 = ½ ψi = ½L ψ²
    3.    Distinguish between singly excited and doubly excited systems.
    4.    For a singly linear magnetic system, derive an expression for the electromagnetic torque.
    5.    Define field energy and coenergy.
    6.    Why most practical energy conversion devices use magnetic field as the coupling medium between electrical and mechanical system?
    Q2:- Attempt any four of the following :
    SECTION B

    4 x 5=20 
    1. What do you understand by demagnetizing and cross magnetizing effects of armature reaction in a d.c machine.
    2. Derive the e.m.f and torque equations for d.c machines.
    3. Explain clearly the functions of the following in d.c machines:
      (a) interpoles (b) compensating windings
    4. What are the various possible cause for d.c shunt generator not building up voltage ?
    5. What are the difference types of d.c generators according to the ways in which fields are excited? Show the connection diagram of each type.
    6. A 4 pole generator with wave wound armature has 51 slots each having 48 conductors. The flux per pole is 7.5 mWb. At what speed must the armature be driven to give an induced emf of 440V
    Q3:- Attempt any two parts of the following :
    SECTION C
      10 x 2=20 
    1. A 4 pole, d.c. shunt motor has a flux per pole of 0.04 Wb and the armature is lap wound with 720 conductors. The shunt field resistance is 240 ohm and the armatureresistance is 0.2 ohm. brush contact drop is 1V per brush. Determine the speed of the machine when running as:
      (a) A motor taking 60 A
      (b) A generator supplying 120 A. The terminal voltage in each case is 480 V.
    2. What are the drawbacks of a 3 point starter? Describe a 4 point starter with a neat sketch.
    3. Describe the test conducted on d.c motors in which the temperature rise and the commutation conditions can be checked under rated load condition such that only losses are being supplied by the power supply.
      Q4:- Attempt any two parts of the following :
    SECTION D
         10 x 2=20   

    1. Explain Sumpner's test for testing two single phase transformers. What is the main advantage of this test over Open circuit and Short circuit tests conductance on single phase transformers?
    2. The HV terminals of a 3 phase bank of three single phase transformers are connected to a 3 wire ,3- ɸ, 33 kV(LL) system. The LV terminals are connected to a 3 wire ,3- load rated 1800 kVA and 2500 V(LL). Specify the voltage,current and 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. Derive an expression for saving in conductor material in an autotransformer over a two winding transformer of equal rating. State its advantages and disadvantage over two winding transformer.

    .Q5:- Attempt any two parts of the following :
    SECTION E 
    10 x 2=20 
      1. In open delta transformer,show that the secondary line voltages from a balanced three phase system of voltages,in case the supply voltage are balanced.
      2. Two 1 phase Scott connected transformers supply a 3 phase, 4 wire system with 231 V between the lines and the neutral. The HV windings are connected to a 2 phase system with a phase voltage of 6000 V. Determine the number of turns in each section of HV and LV windings and the position of the neutral point if the induced voltage per turn is 8 V.
      3. What are the condition for satisfactory operation of 1 phase transformer? Deduce an expression for the load shared by two transformers in parallel when no- load voltages of these transformers are not equal. What will be the load distribution if the voltage ratio is exactly equal ?
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