EX-501 Electrical Machine-II - Dec 2025 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 the construction and principle of operation of DC machines.
   How to answer

   • Main parts: yoke, poles, field winding, armature core and winding, commutator, brushes
   • Working as generator: Faraday's law and commutator action; as motor: Lorentz force
   • Function of each component and energy conversion
   • Neat labelled cross-sectional sketch
2. 7 Marks
   bDerive the EMF equation of a DC generator.
   How to answer

   • Flux per pole, conductors, parallel paths definitions
   • Derivation steps: flux cut per revolution and time
   • Formula $E_g = \Phi Z N P/(60A)$
   • Lap vs wave winding value of A
3. 7 Marks
   aExplain armature reaction and methods of improving commutation.
   How to answer

   • Definition of armature reaction: cross-magnetizing and demagnetizing effects
   • Effects: flux distortion, MNA shift, demagnetization, sparking
   • Commutation: reactance voltage and sparking cause
   • Improvement methods: brushes shift, interpoles, compensating winding, high resistance brushes
4. 7 Marks
   bExplain the construction and applications of brushless DC motors.
   How to answer

   • Construction: permanent magnet rotor, stator winding, electronic commutator and Hall sensors
   • Working principle with switching sequence
   • Advantages over brushed DC motor
   • Applications: drives, fans, EVs, robotics
5. 7 Marks
   aExplain torque equation and operating characteristics of DC motors.
   How to answer

   • Torque derivation $T_a = 0.159 \Phi Z I_a P/A$
   • Relation $E_b$, $N$, $T \propto \Phi I_a$
   • Characteristics of shunt, series and compound motors
   • Speed-torque and speed-armature current curves
6. 7 Marks
   bDescribe 3-point and 4-point starters used for DC motors.
   How to answer

   • Need for starter to limit starting current
   • 3-point starter construction, working and limitation
   • 4-point starter difference and advantage for field control
   • Connection diagrams and operation steps
7. 7 Marks
   aExplain speed control methods of DC motors.
   How to answer

   • Speed relation $N \propto (V-I_aR_a)/\Phi$
   • Armature voltage control and field flux control methods
   • Ward-Leonard and chopper control concept
   • Advantages, limitations and applications of each
8. 7 Marks
   bExplain Swinburne's test and Hopkinson's test on DC machines.
   How to answer

   • Swinburne's test: no-load test, constant losses and efficiency of shunt machines
   • Hopkinson's test: back-to-back test for two identical machines
   • Circuit diagrams and power flow
   • Advantages, limitations and calculations
9. 7 Marks
   aExplain construction and EMF equation of an alternator.
   How to answer

   • Construction: stator, salient vs cylindrical rotor, exciter
   • EMF generation principle and winding factors $K_c, K_d$
   • Derivation $E_{ph}=4.44 f \Phi T_{ph} K_c K_d$
   • Frequency-speed relation
10. 7 Marks
    bExplain voltage regulation of alternator by synchronous impedance method.
    How to answer

    • Definition of voltage regulation
    • OC and SC tests to find $Z_s = R_a + jX_s$
    • Phasor diagram for lagging, leading, unity pf and $E_0$ calculation
    • Regulation formula $(E_0-V)/V$ and nature of method
11. 7 Marks
    aExplain two-reaction theory of salient pole alternator with phasor diagram.
    How to answer

    • Need for two-reaction: unequal air-gap in salient pole
    • Resolution of $I_a$ into $I_d$ and $I_q$ components
    • Phasor diagram with $X_d$, $X_q$ voltage drops
    • Power-angle expression and reluctance power
12. 7 Marks
    bExplain synchronizing of alternator with infinite busbar.
    How to answer

    • Conditions: equal voltage, frequency, phase sequence and phase angle
    • Dark and bright lamp methods and synchroscope
    • Procedure of paralleling to infinite busbar
    • Effects of incorrect synchronizing
13. 7 Marks
    aExplain synchronous moter operation and V-curves.
    How to answer

    • Principle: magnetic locking at synchronous speed, not self-starting
    • Starting methods and phasor diagram
    • Effect of excitation on armature current and power factor
    • V-curves and inverted V-curves explanation
14. 7 Marks
    bExplain hunting in synchronous motors and the use of damper winding.
    How to answer

    • Definition and causes of hunting: load changes, faults
    • Effects: power swings, resonance, loss of synchronism
    • Role of damper winding in damping oscillations
    • Other remedies: flywheel, design measures
15. 7 Marks
    aExplain transient and sub-transient reactances of synchronous machine.
    How to answer

    • Short-circuit transient current components
    • Definitions of $X_d''$, $X_d'$, $X_d$ and corresponding time constants
    • Oscillogram and decrement of current
    • Physical significance for protection
16. 7 Marks
    bExplain construction and working of hysteresis and reluctance motors.
    How to answer

    • Hysteresis motor: cylindrical rotor, hysteresis torque principle
    • Reluctance motor: salient rotor, reluctance torque principle
    • Construction differences and starting
    • Characteristics and applications