EX-802-B Special Machine - May 2024 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 working principle of hybrid stepper motor with neat diagrams.
   How to answer

   • Construction: stator with teeth, rotor with permanent magnet, toothed poles
   • Winding arrangement and excitation sequence
   • Working principle: alignment of rotor teeth with stator field, stepping action
   • Neat labelled diagrams of cross-section and operation
2. 7 Marks
   bDraw and explain in detail the static and dynamic characteristics of stepper motor.
   How to answer

   • Definition of static characteristics: torque-angle, torque-current
   • Dynamic characteristics: pull-in, pull-out, slewing range
   • Torque-speed curves explanation
   • Neat sketches of characteristics
3. 7 Marks
   aExplain dual voltage driver circuit for two phase on drive of a four phase stepper motor and explain the nature of current build up in dual voltage drive.
   How to answer

   • Circuit diagram of dual voltage driver for two-phase-on drive
   • High voltage for turn-on, low voltage for holding
   • Current build-up waveform nature: fast rise then limited
   • Advantages for four-phase stepper motor
4. 7 Marks
   bDraw the cross sectional view of switched reluctance motor and explain the principle of operation. State the advantages of switched reluctance motor.
   How to answer

   • Cross-sectional view of SRM showing stator and rotor poles
   • Principle: minimum reluctance alignment, sequential excitation
   • Switching sequence and torque production
   • Advantages: simple construction, high speed, fault tolerance
5. 14 Marks
   A SRM with 6 stator poles and 4 rotor poles has a stator pole arc of $30°$ and rotor pole arc is $32°$. The aligned inductance is $10.7\text{ mH}$ and unaligned inductance is $1.5\text{ mH}$. Saturation can be neglected. Calculate the instantaneous torque when the rotor is $30°$ before the aligned position and phase current is $6\text{A}$. What is the maximum energy conversion for one stroke, if the current is limited to $7\text{A}$? Determine the average torque corresponding to this energy conversion.
   How to answer

   • Inductance slope from aligned to unaligned values over overlap angle
   • Formula for instantaneous torque $T = (1/2)i^2 dL/d\theta$
   • Energy conversion per stroke $W = (1/2)(L_a-L_u)I^2$
   • Average torque from energy and stroke angle
6. 7 Marks
   aDerive an expression for permeance coefficient of PMBLDC motor.
   How to answer

   • Air-gap permeance model for PMBLDC motor
   • Flux path and reluctance components
   • Derivation steps for permeance coefficient
   • Final expression and its significance
7. 7 Marks
   bState the advantages of BLMP DC motor over conventional DC motor.
   How to answer

   • No brushes, commutator maintenance free operation
   • Higher efficiency, power density and speed range
   • Better thermal and dynamic performance
   • Comparison points with conventional DC motor
8. 7 Marks
   aDiscuss the use of Hall sensors for position sensing in PMBLDC motor with necessary block diagram.
   How to answer

   • Role of Hall sensors for rotor position detection
   • Three-sensor 120° placement and signals
   • Block diagram: sensors, decoder, inverter, motor
   • Commutation logic based on Hall output
9. 7 Marks
   bWith a neat sketch explain the torque-speed characteristics of AC servomotor.
   How to answer

   • Construction and control of AC servomotor briefly
   • Torque-speed curves for different control voltages
   • Stable operating region and linearity
   • Neat sketch of characteristics
10. 7 Marks
    aExplain the methods of energy conversion via Electric field with examples of Electrical Machines.
    How to answer

    • Energy conversion via electric field fundamentals
    • Force and energy density expressions
    • Examples: capacitor, electrostatic machines
    • Comparison with magnetic field systems
11. 7 Marks
    bDiscuss in detail the following:

    1. i)B-H relationship
    2. ii)Leakage flux
    3. iii)Fringing
    4. iv)Stacking factor
    How to answer

    • B-H relationship: magnetization curve, hysteresis
    • Leakage flux: definition and effects
    • Fringing: air-gap flux spreading correction
    • Stacking factor: lamination packing effect
12. 7 Marks
    aDerive the EMF equation of Permanent Magnet Synchronous Motor (PMSM).
    How to answer

    • Flux linkage and winding assumptions
    • Derivation of induced EMF per phase
    • Final EMF equation with constants
    • Dependence on speed, flux and turns
13. 7 Marks
    bWith necessary phasor diagram and circle diagram, describe the torque speed characteristics of PMSM.
    How to answer

    • Phasor diagram of PMSM under load
    • Circle diagram construction
    • Torque-speed characteristics description
    • Effect of load angle and pull-out torque
14. 14 Marks
    Write a short note on any two:

    1. i)Torque equation of stepper motor.
    2. ii)Applications of switched reluctance motors
    3. iii)Permanent magnet materials used in PMBLDC motors
    4. iv)Magnetic reluctance
    How to answer

    • Select any two topics with definition and key points
    • Relevant formulas/diagrams for chosen topics
    • Applications or significance
    • Concise structured short notes