EX-402 Electrical Machine - I - Jun 2022 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain how the performance of three phase induction motor is predetermined using the circle diagram by conducting the necessary test?
   How to answer

   • Need for no-load and blocked-rotor tests and data obtained
   • Construction steps of circle diagram with current locus
   • Marking of output, torque, slip and efficiency lines
   • Procedure to predetermine performance at any load
2. 7 Marks
   bThe total core loss of a specimen of silicon steel is found to be 1500 W at 50 Hz keeping the flux density constant the loss become 3000 W when the frequency is raised to 75 Hz. Calculate separately the hysteresis and eddy current losses of each of these frequencies.
   How to answer

   • Core loss separation: $P_h \propto f$, $P_e \propto f^2$ at constant $B$
   • Form two equations for 50 Hz and 75 Hz
   • Solve for hysteresis and eddy coefficients
   • Compute $P_h$ and $P_e$ at each frequency
3. 7 Marks
   aWhy single phase induction motor is not self starting? Explain the double revolving field theory.
   How to answer

   • Single phase field as pulsating, zero starting torque
   • Double revolving field: two oppositely rotating components
   • Torque-slip of forward and backward fields
   • Net torque zero at standstill, need for starting means
4. 7 Marks
   bExplain the operation of Scott connections for balanced and unbalanced load with the help of phasor diagram.
   How to answer

   • Scott connection: main and teaser transformer ratings and turns
   • Phasor diagram for 3-phase to 2-phase conversion
   • Operation under balanced load
   • Operation under unbalanced load
5. 7 Marks
   aWrite short notes on:

   1. i)Linear induction motor
   2. ii)Servo motor
   How to answer

   • Linear induction motor: construction, travelling field, applications
   • Servo motor: construction, control characteristics, applications
   • Comparison with conventional rotary machines
6. 7 Marks
   bCalculate the efficiency for half, full load of a 100 kVA transformer for the P.F. of unity and 0.8 the copper loss at full load is 1000 W and iron loss is 1000 W.
   How to answer

   • Efficiency formula $\eta = output/(output+losses)$
   • Copper loss varies as square of load, iron loss constant
   • Calculate for half load and full load
   • Calculate for unity and 0.8 p.f.
7. 7 Marks
   aDescribe the working of self excited and externally excited induction generator.
   How to answer

   • Principle of induction generator with negative slip
   • Externally excited: grid-connected operation and reactive power
   • Self-excited: capacitor excitation and voltage build-up
   • Applications and limitations
8. 7 Marks
   bExplain these terms:

   1. i)Braking
   2. ii)Induction motor
   How to answer

   • Meaning of braking in induction machines
   • Types: regenerative, plugging, dynamic
   • Induction motor principle recap
9. 7 Marks
   aDraw the torque speed characteristics of double cage rotor motor. How these characteristics are different from squirrel cage induction motor?
   How to answer

   • Torque-slip of outer and inner cages separately
   • Combined torque-speed curve sketch
   • High starting torque, low starting current features
   • Difference from single squirrel cage characteristic
10. 7 Marks
    bBring out clearly, with the help of neat sketches the difference between the 3 phase slip ring induction motor and three phase squirrel cage induction motor.
    How to answer

    • Construction: slip rings vs shorted cage rotor
    • Rotor resistance starting and speed control
    • Neat sketches of both rotors
    • Performance comparison
11. 7 Marks
    aWrite short notes on:

    1. i)DC shunt motor
    2. ii)AC Series motor
    How to answer

    • DC shunt motor: connection, characteristics, applications
    • AC series motor: construction, operation on AC, applications
    • Speed-torque behaviour of each
12. 7 Marks
    bDerive the expression for saving of copper in auto transformer.
    How to answer

    • Copper weight proportional to $I^2$ conductor volume
    • Compare two-winding vs autotransformer currents
    • Derive saving $= K \times$ weight of two-winding
    • Effect of transformation ratio $K$
13. 7 Marks
    aWhat is meant by inrush currents in transformer? Specify the nature of inrush current and it's problem during transformer charging.
    How to answer

    • Definition of magnetizing inrush on switching
    • Nature: high peak, asymmetrical, harmonic rich
    • Problems: protection tripping, mechanical stress, voltage dip
14. 7 Marks
    bExplain these terms in short:

    1. i)Cogging and crawling
    2. ii)Double cage and deep cage
    How to answer

    • Cogging: slot harmonics locking at low speed
    • Crawling: stable operation at sub-synchronous speed
    • Double cage vs deep cage construction and purpose
15. 7 Marks
    aWhat do you mean by SPP related to electrical machine? What is it significance?
    How to answer

    • SPP full form: slots per pole per phase
    • Formula and meaning for winding layout
    • Significance for harmonics, cogging and performance
16. 7 Marks
    bExplain the method of speed control of three phase induction motor by:

    1. i)Varying the supply frequency
    2. ii)By varying rotor resistance
    How to answer

    • Speed control by varying supply frequency with V/f
    • Speed control by varying rotor resistance in slip-ring motor
    • Effect on slip, torque and efficiency
    • Advantages and limitations of each