AMIETE – ET (NEW SCHEME)   –   Code: AE55

 

Subject: PRINCIPLES OF ELECTRICAL ENGINEERING

Flowchart: Alternate Process: DECEMBER 2009Time: 3 Hours                                                                                                     Max. Marks: 100

 

 

NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q.1 must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

·      Any required data not explicitly given, may be suitably assumed and stated.

 

 

Q.1       Choose the correct or the best alternative in the following:                                  (210)

                

             a.  A coil consisting of 750 turns carries a current of 10 A and gives arise to a magnetic flux of 1200. Inductance of the coil is

 

                  (A) 0.07 H                                           (B) 0.08 H

                  (C) 0.09 H                                           (D) 1 H

 

             b. The flux per pole required for a 4 pole, lap connected generator with 360 conductors and generating 250 V at 1000 rpm is 

 

                  (A) 67.41 mWb                                   (B) 41.67 mWb

                  (C) 16.47 mWb                                   (D) 76.14 mWb

 

             c.  In a 25 kVA 2000/1200 V transformer, the iron and full load copper losses are 350 W and 400 W respectively. Input power at unity pf is

 

                  (A) 25.75 kW                                      (B) 75.25 kW

                  (C) Zero                                               (D) 57.25 kW

 

             d.  A 3 phase, 6 pole induction motor running on a 50 Hz supply has a maximum speed

 

                  (A) 800 rpm                                         (B) 1000 rpm

                  (C) 900 rpm                                         (D) 1200 rpm

 

             e.  A 6 pole alternator is driven at a speed of 500 rpm. Frequency of the generated emf is 

 

                  (A) 100 Hz                                           (B) 75 Hz

                  (C) 50 Hz                                             (D) 25 Hz

 

 

             f.   A 250 V shunt motor takes a total current of 20 A. Resistance of the shunt field is 200  and of the armature is 0.3 . Then the armature current is  

 

                  (A) 15.87 A                                         (B) 18.75 A

                  (C) 81.75 A                                         (D) 51.87 A

 

             g. An emf of 5 V is induced in a coil when the current in adjacent coil varies at a rate of 80 A/sec. The mutual inductance of the two coils are

 

                  (A) 26.5 mH                                        (B) 62.5 mH

                  (C) 56.2 mH                                        (D) 25.6 mH

 

             h.  The drives used for ceiling fan are

 

(A)  Self start                                         (B) Inductive start

                  (C) Capacitor start                               (D) Resistive start

 

             i.   The starter of an alternator is laminated to reduce

 

                  (A) Hysteresis loss                                (B) Copper losses

                  (C) Mechanical loss                              (D) Eddy current loss

 

             j.   The decay constant of radium is 1.3566  10 -11/sec. It’s half life will be

 

                  (A) 5.1090910 10 S                           (B) 5.9090110 5 S

                  (C) 9.09015 10 10 S                          (D) 1.50909 10 10 S

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

 

  Q.2     a.   Compare electric and magnetic circuits.                                                              (4)

                  

             b.   Derive an expression for the energy stored in a magnetic field.                             (4)

 

             c.   A cast steel electromagnet has an air gap of length 2 mm and an iron path of length 30 cm. Find the number of Ampere Turns necessary to produce a flux density of 0.8 Wb/m2.

                   (Assume .                                          (8)

 

  Q.3     a.   Derive an expression for the torque developed in a dc motor. Also enumerate the factors affecting the torque.                                                                                                               (8)

            

             b.   The armature of a 8 pole dc generator has 960 conductors and runs at 400 rpm. The flux per pole is 40 mWb. Calculate the induced emf, if the armature is lap wound. At what speed should it be driven to generate 400 V, if the armature is wave connected?                                                                         (8)

       

  Q.4     a.   What is the all day efficiency of a transformer? Derive the condition for maximum efficiency of a transformer.                                                           (8)

       

             b.   A 25 kVA single phase transformer has 500 turns on the primary and 40 turns on the secondary. The primary is connected to 3000 V, 50 Hz supply. Calculate (i) Primary and secondary currents on full load   (ii) Secondary emf   (iii)  The maximum flux in the core.                                                           (8)

       

       

Q.5       a.    Why the alternators could also be called as synchronous generators? Derive a general expression for the emf generated in an alternator by accounting for winding factor also.                                          (8)

 

             b.   Why does the power angle lag in a synchronous motor?                                      (4)

 

             c.   If the excitation current is held constant and the load on the synchronous motor is increased, what happens to the power factor of the motor?             (4)

 

  Q.6     a.   Reason out why induction motor can never run at synchronous speed. Also derive an expression for the slip in terms of rotor input and rotor copper loss for an induction motor.                                    (8)

 

             b.   A 5 Hp, 400 V, 6 pole, 50 Hz, 3 phase Induction motor operating at full load draws a line current of 7 A at 0.866 pf, with 2% slip. Find the rotor speed and efficiency of the motor.                             (8)

            

  Q.7     a.   Briefly explain the theory of two rotating fields in a single phase Induction motor. Also reason out why such motors are not self starting.                  (8)

 

             b.   Explain the torque-speed characteristics of single phase induction motor.             (8)

                                                                                                                                                

  Q.8     a.   With a neat sketch, explain the working of a Fast Breeder Reactor. Also discuss the criteria for the selection of a site for nuclear power plant.          (8)

 

             b.   Briefly explain the environmental aspects of electric energy generation.                (8)

                                                                             

  Q.9     a.   Discuss the construction and operation of a fuel cell. Comment on the application on the fuel cells.                                                              (8)

 

             b.   Write at least five advantages and disadvantages of HVDC transmission systems.               (8)