EX-501 Electrical Machine - II - May 2023 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 phenomenon of commutation in dc machine. State the methods adopted for minimizing the sparking at the brushes. Explain the cross magnetization and demagnetization effects of armature reaction in a dc machine.
   How to answer

   • Define commutation and commutation period in DC machine
   • Explain sparking causes: reactance voltage, delayed commutation
   • Methods to minimize sparking: brush shifting, interpoles, compensating winding
   • Explain armature reaction cross-magnetizing and demagnetizing effects
2. 7 Marks
   bExplain the operation of a three point starter with the help of a neat diagram.
   How to answer

   • Show construction of three-point starter: rheostat studs, NVC, OLC
   • Explain starting sequence: arm movement, resistance cut-out
   • Role of no-volt and overload coils
   • Provide neat labelled diagram
3. 7 Marks
   aExplain how the efficiency of a dc machine is calculated from Swinburne's test.
   How to answer

   • State principle: no-load test to find constant losses
   • Circuit connection and procedure at rated speed and voltage
   • Separation of iron, friction, windage and shunt copper loss
   • Formula for efficiency as motor and as generator
4. 7 Marks
   bWhat are the methods of speed control of a dc motor?
   How to answer

   • Relation $N \propto (V-I_aR_a)/\Phi$
   • Armature voltage control method
   • Field flux control method
   • Armature resistance / Ward-Leonard method
5. 7 Marks
   aExplain the different types of braking of dc machines.
   How to answer

   • Define braking need in DC machines
   • Explain rheostatic / dynamic braking
   • Explain plugging / reverse current braking
   • Explain regenerative braking
6. 7 Marks
   bWhat do you mean by voltage regulation of an alternator? Describe some method for determining voltage regulation of an alternator.
   How to answer

   • Define voltage regulation: $(E_0-V)/V$ at constant excitation and speed
   • Need for predetermination by open-circuit and short-circuit tests
   • Describe one method: synchronous impedance / MMF / Potier method
   • Steps to obtain $E_0$ from phasor and compute regulation
7. 7 Marks
   aExplain in brief how slip test is performed to determine $X_d$ and $X_q$ of a salient synchronous machine
   How to answer

   • Principle: small slip between rotor and rotating field
   • Test connections: low voltage 3-phase to armature, field open
   • Record max-min voltage and current with slip
   • Compute $X_d = V_{max}/I_{min}$, $X_q = V_{min}/I_{max}$ per phase
8. 7 Marks
   bA 3-phase, 4-pole, star-connected turbo-alternator has a smooth cylindrical type rotor. The effective resistance and synchronous reactance per phase are $0.15\Omega$ and $2.5\Omega$. Calculate the voltage regulation when delivering 250 A at 6.6 kV and at 0.6 p.f lagging.
   How to answer

   • Compute phase voltage $V_{ph}=6600/\sqrt{3}$
   • Compute $E_{0ph}=\sqrt{(V\cos\phi+I R_a)^2+(V\sin\phi+I X_s)^2}$ for lagging pf
   • Calculate regulation $=(E_0-V)/V \times 100\%$
   • Insert $R_a=0.15\Omega$, $X_s=2.5\Omega$, $I=250\,A$, $\cos\phi=0.6$ lag
9. 7 Marks
   aExplain the effect of damper winding in an alternators:
   How to answer

   • Location of damper bars in pole faces
   • Damping of hunting / oscillations
   • Provision of starting torque as induction motor
   • Improvement of stability and parallel operation
10. 7 Marks
    bA three-phase 6.6 KV, 50 Hz, 500 KVA, star connected alternator has an effective resistance of $.5\Omega$/phase and synchronous reactance of $5\Omega$/phase. Determine the voltage regulator at full load at unity power factor.
    How to answer

    • Find full-load current from $S=\sqrt{3}VI$
    • Compute phase voltage and $E_0$ at unity pf
    • Apply regulation formula $(E_0-V)/V$
    • Use $R_a=0.5\Omega$/phase, $X_s=5\Omega$/phase
11. 7 Marks
    aDescribe the various excitation systems arrangements of synchronous alternators. https://www.rgpvonline.com
    How to answer

    • Need for DC excitation on rotor
    • DC exciter, static and brushless excitation
    • Pilot and main exciter arrangement
    • AVR control for voltage regulation
12. 7 Marks
    bDraw the phasor diagrams of salient-pole synchronous generator supplying lagging, leading and unity power factor loads.
    How to answer

    • Draw two-reaction phasor with $I_d$, $I_q$ components
    • Show $E_0$ position for lagging load
    • Show leading and unity pf variations
    • Label $V$, $I_aR_a$, $jI_dX_d$, $jI_qX_q$
13. 7 Marks
    aExplain the working principle of reluctance motor. Draw and explain the torque-speed characteristics of reluctance motor.
    How to answer

    • Principle: minimum reluctance alignment
    • Construction: stator and salient reluctance rotor
    • Starting as induction motor to synchronism
    • Torque-speed characteristics with diagram
14. 7 Marks
    bWhy is synchronous motor non-self starting? Discuss the various starting methods of synchronous motors.
    How to answer

    • Reason: inertia prevents instant lock with fast rotating field
    • Average torque zero at standstill
    • Starting methods: damper winding, pony motor, V/f
    • Synchronizing after near-synchronous speed
15. 14 Marks
    Explain the following any two:

    1. a)Stepper motor
    2. b)Hysteresis motor
    3. c)Repulsion motor
    How to answer

    • For each chosen motor: construction and principle
    • Operation with diagram / characteristics
    • Applications and limitations