EX-501 Electrical Machine - II - Dec 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
   aThe armature of a d.c. generator has 81 slots and the commutator has 243 segments. It is wound to give lap winding having 1 turn per coil. If the flux per pole is 30 mWb, calculate the generated emf at a speed of 1200 r.p.m. Number of poles is 6.
   How to answer

   • Lap winding: A=P=6, total conductors Z calculation from slots/segments/turns
   • EMF formula $E_g = \Phi Z N P/(60A)$
   • Substitute $\Phi=30\,mWb, N=1200\,rpm$
   • Compute generated emf value
2. 7 Marks
   bWhat do you understand by demagnetizing and cross magnetizing effects of armature reaction in a d.c. machine? Derive the expression for calculations of demagnetizing and cross magnetizing ampere-turns when brushes are given a shift from geometrical neutral axis.
   How to answer

   • Define armature reaction and brush shift from GNA
   • Explain demagnetizing vs cross-magnetizing components
   • Derive AT expressions using brush shift angle
   • Give formulae for demagnetizing and cross-magnetizing ampere-turns per pole
3. 7 Marks
   aA commutator having a diameter of 76 cm rotates at a speed of 600 r.p.m. Find the time of commutation if the brush width is 1.5 cm. Neglect mica thickness.
   How to answer

   • Commutator peripheral velocity $v=\pi D N/60$
   • Commutation period $T_c$ = brush width / velocity
   • Neglect mica thickness assumption
   • Substitute D=76 cm, N=600 rpm, brush width 1.5 cm
4. 7 Marks
   bSketch and the speed-current, speed-torque and torque-current characteristics of a shunt motor, series motor and compound motor.
   How to answer

   • Circuit connections of shunt, series, compound motors
   • Sketch speed-current characteristics
   • Sketch speed-torque characteristics
   • Sketch torque-current characteristics with explanation
5. 7 Marks
   aA 220 V d.c. shunt motor runs at 500 r.p.m. when the armature current is 50A. Calculate the speed if the torque is doubled. The armature resistance of the motor is $0.2 \, \Omega$.
   How to answer

   • Back EMF equation $V = E_b + I_a R_a$
   • Torque $T \propto \Phi I_a$ with constant flux for shunt motor
   • Find new $I_a$ when torque doubled
   • Find new $E_b$ and speed $N \propto E_b$
6. 7 Marks
   bWhich method is preferred when speed of a d.c. shunt motor is to be controlled above rated value?
   How to answer

   • Speed control methods: armature voltage vs field flux control
   • Limitation of armature control above rated speed
   • Field weakening for above rated speed
   • Preferred method: field control with reason
7. 7 Marks
   aExplain the potier-triangle method of determining the voltage regulation of an alternator.
   How to answer

   • Need for separation of leakage reactance and armature reaction
   • Open-circuit and zero power factor tests
   • Construction of Potier triangle
   • Steps to find voltage regulation from triangle
8. 7 Marks
   bCalculate the distribution factor for a single-phase alternator having 6 slots/pole (i) when all the slots are wound and (ii) when only four adjacent slots per pole are wound, the remaining slots being unwound.
   How to answer

   • Distribution factor formula $K_d = \sin(m\gamma/2)/[m\sin(\gamma/2)]$
   • Case (i) all 6 slots/pole wound: compute spread and Kd
   • Case (ii) only 4 adjacent slots wound: effective distribution
   • Compare values
9. 7 Marks
   aA 3-phase, 16-pole synchronous generator has a star-connected winding with 144 slots and 10 conductors per slot. The flux per pole is 0.03 Wb, sinusoidally distributed and the speed is 375 r.p.m Calculate

   1. i)The frequency
   2. ii)Line induced e.m.f.
   How to answer

   • Frequency $f = PN/120$ with P=16, N=375 rpm
   • Winding factors Kd, Kp and total conductors
   • EMF per phase $E_{ph}=4.44 f \Phi T_{ph} K_w$
   • Line EMF for star: $E_L=\sqrt{3}E_{ph}$
10. 7 Marks
    bA 500 kVA, 3-phase, star-connected alternator has a rated line-to-line voltage of 3300 V. The resistance and synchronous reactance per phase are $0.3 \Omega$ and $4 \Omega$ respectively. Calculate the line value of the emf generated at full-load, 0.8 p.f. lagging.
    How to answer

    • Full-load current from $S=\sqrt{3}VI$
    • Synchronous impedance $Z_s=R_a+jX_s$ per phase
    • Phasor equation $E = V + I Z_s$ at 0.8 lagging
    • Convert phase EMF to line value
11. 7 Marks
    aDiscuss Blondel's two reaction theory of salient pole synchronous machines.
    How to answer

    • Salient pole: unequal air-gap along d-q axes
    • Resolution of armature current into Id and Iq
    • Phasor diagram and voltage equations
    • Power-angle expression with reluctance term
12. 7 Marks
    bWhat do you mean by synchronizing of alternators? Describe different method of synchronizing.
    How to answer

    • Meaning of synchronizing to bus/infinite bus
    • Conditions: voltage, frequency, phase sequence, phase angle
    • Methods: lamps, synchroscope, synchronizer
    • Procedure description
13. 7 Marks
    aConsider a 3300 V delta connected synchronous motor having a synchronous reactance per phase of $18 \Omega$. It operates at a leading p.f. of 0.707 when drawing 800 kW from mains. Calculate its excitation e.m.f. and load angle.
    How to answer

    • Delta connection: $V_{ph}=V_L$, $I$ from $P=\sqrt{3}VI\cos\phi$
    • Phasor $E = V \pm jI X_s$ for leading pf motor
    • Magnitude gives excitation EMF per phase
    • Load angle $\delta$ from phasor angle
14. 7 Marks
    bExplain construction and working of repulsion motor.
    How to answer

    • Stator and compensated commutated rotor construction
    • Brush position and repulsion principle
    • Torque production and starting
    • Speed-torque behaviour and applications
15. 14 Marks
    Write a short notes on (any two)

    1. i)Permanent magnet DC motors
    2. ii)Losses and efficiency of DC machines
    3. iii)Armature reaction of alternators
    4. iv)Hysteresis motors
    How to answer

    • Select any two topics from list
    • Give construction/principle/definitions for each
    • Include key formulae/diagrams and characteristics
    • Mention applications and losses/effects where relevant