EX-501 Electrical Machine - II - 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
   aA shunt generator delivers $450\,A$ at $230\,V$ and the resistance of the shunt field and armature are $50\,\Omega$ and $0.03\,\Omega$ respectively. Calculate the generated EMF.
   How to answer

   • Shunt field current Ish = V / Rsh
   • Armature current Ia = IL + Ish
   • Generated EMF Eg = V + Ia*Ra
   • Substitute values and compute Eg in volts
2. 7 Marks
   bDescribe the basic construction of DC machines with all its parts.
   How to answer

   • Yoke, poles, field winding, armature core and winding
   • Commutator, brushes, bearings and shaft
   • Function of each part briefly
   • Neat labelled sketch of construction
3. 7 Marks
   aExplain the commutation process in DC machines.
   How to answer

   • Definition of commutation and commutator action
   • Current reversal in coil undergoing short by brush
   • Time of commutation, linear vs delayed commutation
   • Role of reactance voltage and sparking
4. 7 Marks
   bDescribe the working of Brush less DC Motor.
   How to answer

   • Construction: permanent magnet rotor, stator, electronic commutator
   • Position sensing and inverter switching sequence
   • Working with phase energization
   • Advantages and applications
5. 7 Marks
   aExplain the working of 2 point starter used in DC Motor.
   How to answer

   • Application for series motor and circuit arrangement
   • Operation of 2-point starter, NVC connection
   • Starting steps and protection function
   • Difference from 3-point starter
6. 7 Marks
   bWrite down the methods used to control the speed of DC Motor. Explain one of them with a diagram.
   How to answer

   • List speed control methods: flux, armature voltage, resistance
   • Select one method e.g. field control or armature control
   • Circuit diagram and working
   • Speed relation N proportional to (V-IaRa)/phi
7. 7 Marks
   aExplain the Swinburne's Test for determination of efficiency of DC shunt machines.
   How to answer

   • No-load test principle, constant losses
   • Circuit and procedure to find stray losses
   • Efficiency formula for motor and generator
   • Assumptions and limitations
8. 7 Marks
   bDescribe the types of losses in DC motors and methods to reduce the losses.
   How to answer

   • Copper, iron, mechanical and stray losses
   • Where each loss occurs in DC motor
   • Methods to reduce each loss: design, materials
   • Effect on efficiency
9. 7 Marks
   aDerive emf equation for an alternator. Explain the meaning of distribution factor.
   How to answer

   • EMF per conductor and per turn derivation
   • Pitch factor Kp and distribution factor Kd
   • Final EMF equation E = 4.44 f phi Tph Kw
   • Meaning of distribution factor for spread winding
10. 7 Marks
    bThe speed regulations of two $800\,kW$ alternators, A and B running in parallel are $100\%$ to $104\%$ and $100\%$ to $105\%$ from full load to no load respectively. How will the two alternators share a load of $1000\,kW$?
    How to answer

    • Speed-load characteristics of A and B from regulation
    • Common frequency/speed condition in parallel
    • Solve for load sharing with total 1000 kW
    • Load on A and load on B in kW
11. 7 Marks
    aDraw the phasor diagram of salient - pole synchronous generator supplying lagging, leading and unity power factor load.
    How to answer

    • Two-reaction theory basis for salient-pole machine
    • Phasor diagrams for lagging, unity, leading pf
    • Position of IaXd, IaXq, Ef components
    • Effect of saliency on excitation
12. 7 Marks
    bExplain the armature reaction and the effect of armature reaction in the terminal voltage of alternator at unity power factor load.
    How to answer

    • Definition of armature reaction in alternator
    • Nature at unity pf: cross-magnetizing/distorting
    • Effect on terminal voltage and regulation
    • Phasor explanation
13. 7 Marks
    aDescribe the hunting phenomenon in synchronous machines. Explain its cause and means to reduce them.
    How to answer

    • Definition of hunting and rotor oscillation
    • Causes: load change, faults, supply variation
    • Effects on synchronism and power
    • Damper winding and other remedies
14. 7 Marks
    bWrite a short note on the V-curve in synchronous machines.
    How to answer

    • V-curve: armature current vs field current
    • Shape and minimum at unity pf
    • Inverted V-curve for power factor
    • Use for synchronous motor control
15. 7 Marks
    aExplain transient and subtransient reactance in synchronous machines.
    How to answer

    • Definitions of transient and subtransient periods
    • Reactances Xd', Xd'' and time constants
    • Fault current decay explanation
    • Equivalent circuits
16. 7 Marks
    bDescribe the construction and working of hysteresis Motor.
    How to answer

    • Construction: stator and hysteresis rotor material
    • Production of hysteresis torque
    • Synchronous running principle
    • Characteristics and applications