Third Semester Diploma Examination


Electrical Machines-II

TIME: 3 Hours                                                                                         Max Marks: 100   

                         Note:  i) Section I is compulsory.

                                    ii) Answer any six full questions taking two full questions

    from each of the remaining sections.


 Section I


1. a) Fill in the blanks.                                                                                                 (5X1=5)

      i)       The transformer has K=1/2. It means secondary voltage ____________

               than the primary voltage.

      ii)      The maximum efficiency occurs in a transformer when iron loss is

               _______ to copper loss.

      iii)     All day efficiency is defined for ____ Transformer.

      iv)     Rheostatic control is used with ______ type of induction motors

      v)      The watt meter reading in a blocked rotor test on 3 phase induction

                motor represents ____ losses in motor.


 b) List out the constructional features of a welding transformer.                                         (5)



Section II


2. a) Explain the working principle of transformer with a neat sketch.                                  (5)

    b) List the properties of transformer oil.                                                                             (5)

    c) Derive an emf equation of transformer with usual notations.                                        (5)


3. a) Draw the vector diagram of transformer with lagging PF load.                                     (5)

    b) Draw the exact and approximate equivalent circuit diagram of a transformer.             (6)

    c) A 5 KVA distribution transformer has a full load efficiency at UPF of  95%. The copper  

        and iron losses being equal, calculate all day efficiency if it  is loaded throughout 24 hrs as

        below.  No load for 10 hrs, half load for 5 Hrs, Quarter load for 7 Hrs, Full load for   2 Hrs.

        Assume UPF load.                                                                                                         (4)


4. a) Draw the circuit diagram for star/star and delta/delta banking connections of

         Transformer                                                                                                                    (6)

    b) Explain the necessity of cooling of transformer.                                                            (5)

    c) List the conditions for the parallel operation of the transformers.                                 (4)


Section III


5. a) Compare induction motor with transformer.                                                                  (4)

    b) Write the equation for frequency of the rotor current in terms of slip.                          (2)

    c) With a neat sketch explain the constructional features of a squirrel cage

        type induction motor.                                                                                                     (9)


6. a) Explain the procedure for constructing circle diagram from no load and blocked rotor test 

        on induction motor.                                                                                                        (9)

    b) Write the relation between the following.

         i) Rotor copper loss, rotor input and slip

        ii) Rotor gross output, slip and rotor input                                                                     (4)

    c) List the application of a liner induction motor                                                               (2)


7. a) Draw a neat sketch of a DOL starter and explain its working principle.                       (8)

    b) Explain poll changing method of speed control of induction motor.                             (5)

    c) List the applications of induction generators.                                                                 (2)


                   Section IV  


8. a) Explain the general principle of operation of synchronous motor.                                 (8)

    b) Draw and explain V and inverted V curves of synchronous motor.                              (7)


9. a) List the methods of starting synchronous motor and explain any one

        method in brief.                                                                                                              (7)       

    b) Compare synchronous motor with induction motor.                                                      (5)

    c) State the characteristics of permanent magnet synchronous motor.                               (3)


10. a) Explain the construction and applications of single phase capacitor start

          and capacitor run induction motor.                                                                               (7)

     b) List the different types of single phase motors.                                                            (3)

     c) Write a short note on soft starters                                                                                  (5)

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