EX-501 Electrical Machine - II - 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
   aWhy is Swinburne's test considered a convenient and economical method for testing DC shunt machines?
   How to answer

   • No-load test at rated voltage, measures no-load losses without loading machine
   • Constant losses separated from copper losses, efficiency at any load by calculation
   • Requires only small power supply, economical for large shunt machines with constant flux
2. 7 Marks
   bWith a neat diagram, explain armature reaction and its effects in a DC machine. Give possible solutions to decrease armature reaction.
   How to answer

   • Definition of armature reaction and cross-magnetising / demagnetising effect
   • Diagram showing main field, armature MMF, resultant flux distortion and MNA shift
   • Effects: flux weakening, sparking, reduced EMF
   • Remedies: compensating winding, interpoles, brush shift, high reluctance pole tips
3. 7 Marks
   aA DC series motor of total resistance $0.5\ \Omega$ runs at a speed of $800\ \text{rpm}$ when taking $60\ \text{A}$ from $500\ \text{V}$ DC mains. Now another identical series motor is mechanically coupled, but connected in series with the first motor. If the set runs at $400\ \text{rpm}$ with load torque equal to $2.5$ times the original torque. Find the current drawn from $500\ \text{V}$ DC source.
   How to answer

   • Calculate original back EMF $E_{b1}=V-I_1R$ and flux-torque relation $T \propto \Phi I$
   • For series motor $\Phi \propto I$ unsaturated, use $T_2/T_1$ and $N$ to relate $E_b \propto N\Phi$
   • For two motors in series: $2E_{b2}+2I_2R = V$ with common current
   • Solve for new current $I_2$ drawn from 500 V source
4. 7 Marks
   bWhat are the drawbacks of three-point starter? Describe a four-point starter with a neat sketch.
   How to answer

   • Drawback of three-point starter: NVC in series with shunt field, fails with field weakening / speed control
   • Construction and connection of four-point starter with separate NVC circuit
   • Neat sketch with studs, resistances, NVC and OLC
   • Working during starting and running
5. 7 Marks
   aExplain the speed control of a DC series motor using the series parallel method.
   How to answer

   • Principle of series-parallel: two identical series motors or field sections
   • Series connection gives half voltage, low speed; parallel gives full voltage, high speed
   • Circuit diagrams for both connections with changeover
   • Torque-speed characteristics and applications
6. 7 Marks
   bWhat are brushless DC motors? Explain three phase three pulse (half wave) brushless DC motor with diagram and basic operations principle. Also draw torque-angle characteristics of it and write their applications.
   How to answer

   • Definition of brushless DC motor: electronic commutation with permanent magnet rotor
   • Three-phase three-pulse half-wave circuit, position sensors and switching sequence
   • Diagram and torque-angle characteristics
   • Operating principle and applications
7. 7 Marks
   aWhy DC excitation is given to the rotor winding why not on stator winding of a synchronous alternator? Explain it.
   How to answer

   • Need for rotating DC field to produce rotating MMF with stationary AC output
   • Advantages: low current slip-rings, easy insulation of high voltage stator, better cooling
   • Problems if DC on stator: stationary field, need rotating AC pickup with high voltage slip-rings
   • Conclusion with construction of salient-pole alternator
8. 7 Marks
   bWhat is the Losses that occur in D.C. machines? Derive the condition for maximum efficiency of a D.C. generator.
   How to answer

   • List losses: copper, iron, mechanical, stray in D.C. machines
   • Power flow diagram for D.C. generator
   • Condition variable loss = constant loss for maximum efficiency
   • Derivation of efficiency expression and condition
9. 7 Marks
   aWhy is the MMF method of finding alternator voltage regulation optimistic? Explain them.
   How to answer

   • MMF method combines armature leakage and reaction MMFs, uses saturated OCC
   • Assumes armature reaction is purely demagnetising and replaces with equivalent field MMF
   • Neglects saturation under load and uses unsaturated synchronous impedance concept
   • Result gives higher EMF than actual hence optimistic regulation
10. 7 Marks
    bThe two alternators I and II having a full-load capacity of $500\ \text{kW}$ each are operating in parallel. Their speed regulation is $100\%$ to $104\%$ and $100\%$ to $105\%$ from full-load to no-load, respectively. How they will share a load of $600\ \text{kW}$.
    How to answer

    • Express frequency / speed vs load droop for each alternator from no-load to full-load
    • At common frequency, sum of loads = 600 kW
    • Solve two equations for $P_1$ and $P_2$ shared
    • Interpretation of sharing due to different regulations
11. 7 Marks
    aShow that a synchronous motor has no net starting torque. Explain different methods of starting synchronous motor.
    How to answer

    • Torque pulsates at double frequency with zero average due to rotating stator field vs stationary rotor
    • Proof with $T \propto \sin(\omega t)$ averaging to zero
    • Starting methods: pony motor, damper winding, slip-ring induction start, VFD
    • Synchronization after near-synchronous speed
12. 7 Marks
    bWhy synchronous machines are normally operated at the power angles much less than $90$ degrees? What happens when this angle crosses $90$ degrees?
    How to answer

    • Power-angle relation $P = (VE/X_s)\sin\delta$
    • Maximum at 90 degrees, stability margin requires operation well below
    • Beyond 90 degrees synchronism lost, pole slipping and hunting
    • Practical load angle 20-30 degrees for transient stability
13. 7 Marks
    aExplain, why is back EMF greater than supply voltage in a synchronous motor in an over excited condition?
    How to answer

    • Phasor $E_b = V - I_aZ_s$ for motor, over-excitation gives leading current
    • Reactive drop $I_aX_s$ adds to V to make $E_b > V$
    • Phasor diagram for over-excited synchronous motor
    • Effect on power factor and V-curves
14. 7 Marks
    bThe direct axis and quadrature axis reactance of a salient pole alternator are $1.2\ \text{p.u}$ and $1.0\ \text{p.u}$ respectively. The armature resistance is negligible. If the alternator is delivering rated kVA at upf and at rated voltage, then its power angle is.
    How to answer

    • Rated kVA at upf means $I=1$ p.u., $V=1$ p.u., $\phi=0$
    • Use two-reaction equations neglecting $R_a$ to find $I_d, I_q$
    • Power angle formula $\tan\delta = I_qX_q/(V+I_d$ terms)
    • Compute $\delta$ value in degrees
15. 7 Marks
    aWhy, in the three bright lamp method for generator synchronization, the switch has to be closed when the lamps are "brightest"? How does it ensure no phase difference between bus bar and alternator?
    How to answer

    • Connections of three lamps across synchronizing switch, bright vs dark lamp principle
    • Brightest indicates voltage across lamps maximum due to phase opposition or direct connection
    • Closing at brightest ensures same phase sequence and zero phase difference in this connection
    • Ensures bus and incoming voltages in phase, no circulating current
16. 7 Marks
    bDescribe two reaction theory of salient pole synchronous machine, draw its equivalent circuit model and phasor diagram for lagging power factor load.
    How to answer

    • Resolution of armature MMF into d-axis and q-axis components
    • Equivalent circuit with $X_d, X_q$ and phasor diagram for lagging load
    • Voltage equations $V + I_d jX_d + I_q jX_q$
    • Power expression and reluctance torque