EX-802-B Special Machine - Jun 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
   aWrite down the different types of stepper motors. Draw a schematic diagram of any one of them and explain its working.
   How to answer

   • List types: variable reluctance, permanent magnet, hybrid stepper motor
   • Draw schematic diagram of any one type with stator, rotor, windings
   • Explain construction and step-by-step working
   • Mention step angle and excitation sequence
2. 7 Marks
   bWhat are the different ways in which stepper motors can be used?
   How to answer

   • Define stepper motor positioning use
   • List different modes of use: open-loop positioning, closed-loop, etc.
   • Give application examples: printers, CNC, robotics
   • Summarize advantages for incremental motion
3. 7 Marks
   aWhat are the different driver circuits used in stepper motors? Explain any one driver circuit with a schematic diagram.
   How to answer

   • List driver circuits: unipolar, bipolar, chopper, L/R, PWM drive
   • Select one driver for detailed explanation
   • Draw schematic diagram with transistors, windings, supply
   • Explain current switching sequence and operation
4. 7 Marks
   bExplain micro stepping in stepper motors.
   How to answer

   • Define microstepping as subdivision of full step
   • Explain sinusoidal current proportioning in two phases
   • Describe improved resolution and reduced resonance
   • Mention driver requirement and applications
5. 7 Marks
   aDescribe the principle of operation of a switched reluctance motor.
   How to answer

   • State reluctance torque principle: rotor aligns to minimum reluctance
   • Describe stator phases, salient rotor construction
   • Explain sequential excitation and continuous rotation
   • Mention doubly salient structure
6. 7 Marks
   bHow the torque is produced in a switched reluctance motor, explain in brief with the help of expression. Also explain shaft position sensing.
   How to answer

   • Give torque expression based on co-energy / inductance variation
   • Explain torque production when inductance changes with position
   • Describe need for rotor position synchronization
   • Explain shaft position sensing methods: optical, Hall, sensorless
7. 7 Marks
   aExplain construction and principle of a brushless DC motor.
   How to answer

   • Describe construction: permanent magnet rotor, stator windings, electronic commutator
   • Explain Hall sensor based commutation principle
   • Describe trapezoidal / sinusoidal excitation
   • Draw block / schematic of BLDC system
8. 7 Marks
   bExplain the torque production in brushless DC motor.
   How to answer

   • State Lorentz force / interaction of stator field and rotor magnets
   • Explain commutation sequence producing rotating field
   • Give torque expression $T \propto \Phi I$
   • Mention torque-speed characteristics
9. 7 Marks
   aExplain how the speed of a brushless DC motor is controlled?
   How to answer

   • List speed control methods: DC bus voltage, PWM duty, chopper control
   • Explain closed-loop control with position/speed feedback
   • Describe role of inverter switching frequency
   • Mention speed range and applications
10. 7 Marks
    bWhat are the different applications of permanent magnet DC motors?
    How to answer

    • Define permanent magnet DC motor
    • List applications: automotive, toys, drives, appliances
    • Explain advantages for each application
    • Conclude with low power portable uses
11. 7 Marks
    aCompare between soft and hard ferromagnetic materials.
    How to answer

    • Define soft and hard ferromagnetic materials with hysteresis
    • Compare on basis of coercivity, retentivity, hysteresis loss
    • Give examples: silicon steel vs Alnico, ferrites
    • Mention motor applications of each
12. 7 Marks
    bDraw and explain the constructional details of a permanent magnet DC motor.
    How to answer

    • Draw cross-section showing yoke, poles, PM, armature, brushes
    • Label stator permanent magnets and rotor conductors
    • Explain flux path and commutation
    • Describe working of PMDC motor
13. 7 Marks
    aExplain the PM synchronous motor construction and operation.
    How to answer

    • Describe stator with 3-phase winding and PM rotor types
    • Explain synchronous operation at synchronous speed
    • Describe starting and vector control / V/f
    • Mention rotor configurations: surface and interior PM
14. 7 Marks
    bDifferentiate between permanent magnet BLDC motor and permanent magnet synchronous motor.
    How to answer

    • Compare back-EMF: trapezoidal vs sinusoidal
    • Compare commutation: six-step vs vector control
    • Compare torque ripple, control and applications
    • Tabulate differences
15. 14 Marks
    Write short notes (Any Two)

    1. a)Motors are suitable for electric vehicles
    2. b)Motors used for PV water pumping system
    3. c)Advantage and disadvantage of solid rotors
    How to answer

    • For EV motors: types like BLDC, PMSM, SRM with requirements
    • For PV pumping: induction, BLDC with MPPT and solar features
    • For solid rotors: advantages like robustness and disadvantages like losses
    • Conclude with suitability for chosen two topics