EX-501 Electrical Machine - II - Nov 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
   aWhat are the advantages of Swinburne method used to determine the efficiency of a DC machine?
   How to answer

   • Define Swinburne's (no-load) method as indirect efficiency test
   • No-load test gives constant losses: iron + mechanical + shunt field
   • Efficiency at any load calculated without actual loading
   • Advantages: economical, small supply, suitable for large machines, predetermination at various loads
2. 7 Marks
   bWhat is armature reaction in a DC machine how does it affect commutation what steps are taken to have effective commutation?
   How to answer

   • Define armature reaction: effect of armature mmf on main field
   • Effects: demagnetizing and cross-magnetizing, distortion of flux, sparking at brushes
   • Effect on commutation: reactance voltage, delayed commutation
   • Remedies: compensating winding, interpoles, brush shift, high resistance brushes
3. 7 Marks
   aA 4-pole DC series wound fan motor runs normally at 600 r.p.m., a 250V supply taking 20A. The field coils are connected all in series. Estimate the speed if the coils are reconnected in two parallel groups of two in series. Assumed that the flux is directly proportional to the current and ignore losses. The load torque increases as square of the speed.
   How to answer

   • Given: $V=250V$, $N_1=600$ rpm, $I_1=20A$, series field reconnection
   • Use $E \approx V$, $E \propto \Phi N$, $\Phi \propto I_f$ per pole/coil
   • Relate torque $T \propto \Phi I_a$ and fan load $T \propto N^2$
   • Solve for new current and $N_2$ ignoring losses
4. 7 Marks
   bWhy is the starting current very high in a DC motor? How does the starter reduce the starting current to a safe value?
   How to answer

   • At start $N=0$ so back EMF $E_b=0$, $I_a = V/R_a$ very large
   • Explain small armature resistance and danger to winding/commutator
   • Starter inserts external resistance in armature circuit at start
   • Resistance cut off in steps as speed and $E_b$ build up
5. 7 Marks
   aWhy is DC excitation used in a synchronous alternator instead of AC excitation? Explain it.
   How to answer

   • Need for constant, stationary polarity field for synchronous speed
   • DC gives fixed poles; AC would produce pulsating/alternating field
   • AC excitation would induce transformer EMF, hunting, no net synchronizing torque
   • Slip-rings, brushless exciter supply DC to rotor
6. 7 Marks
   bExplain, why does the voltage regulator of alternator go high for the EMF method?
   How to answer

   • Define EMF method: synchronous impedance from OC and SC tests
   • Assumes $Z_s$ constant, neglects saturation
   • Uses unsaturated $Z_s$ giving larger $E_0$ and regulation
   • Hence called pessimistic method with high regulation value
7. 7 Marks
   aHow can you determine the synchronous impedance and armature resistance of an alternator? Explain them.
   How to answer

   • OC test gives open-circuit voltage vs field current
   • SC test gives short-circuit current vs field current
   • $Z_s = V_{oc}/I_{sc}$ per phase at same $I_f$
   • Determine $R_a$ by DC measurement, $X_s = \sqrt{Z_s^2 - R_a^2}$
8. 7 Marks
   bThe speed regulation of two 1000 kW alternators $x$ and $y$ running in parallel is 100 to 104% and 100 to 105% from full load to no load respectively. How will this machine share a total load of 1600 kW?
   How to answer

   • Speed regulation droop: 100% to 104% and 100% to 105% from full to no load
   • Assume linear frequency-load characteristics for x and y
   • Total load 1600 kW shared inversely to droop
   • Solve simultaneous equations for $P_x$ and $P_y$
9. 7 Marks
   aWhat is Armature reaction? Explain the effect of armature reaction on the terminal voltage of an alternator at ZPF lag and ZPF lead with the help of necessary phasor diagram.
   How to answer

   • Define armature reaction in synchronous machine
   • Draw phasor diagrams for ZPF lag and ZPF lead
   • Lag: demagnetizing, terminal voltage drops; Lead: magnetizing, voltage rises
   • Explain with $E_0 = V \pm I_a$ drops
10. 7 Marks
    bSketch the Power-Angle characteristics of a salient-pole Synchronous Machine, including both the generation and motoring modes. Hence, explain why the salient-pole synchronous generator is preferred to the cylindrical synchronous generator in terms of stability.
    How to answer

    • Sketch $P-\delta$ curves: $P = (VE/X_d)\sin\delta + (V^2/2)(1/X_q-1/X_d)\sin2\delta$
    • Show both generating and motoring modes
    • Second harmonic reluctance term increases peak and stability margin
    • Conclude salient-pole has higher steady-state stability than cylindrical
11. 7 Marks
    aWhat is negative sequence reactance? How negative and zero sequence reactances are measured in the laboratory?
    How to answer

    • Define negative and zero sequence reactances
    • Negative sequence: opposite phase rotation test
    • Zero sequence: single-phase supply with all phases in series
    • Measurement connections and $X_2$, $X_0$ calculation
12. 7 Marks
    bThe results of a 'Slip-Test' for determining direct-axis ($X_d$) and quadrature-axis ($X_q$) reactances of a star-connected, salient-pole alternator are given below: Phase values: $V_{max} = 108V$; $V_{min} = 96V$, $I_{max} = 12A$, $I_{min} = 10A$. Hence the two reactances will be.
    How to answer

    • Slip-test principle: $X_d = V_{max}/I_{min}$, $X_q = V_{min}/I_{max}$ per phase
    • Substitute $V_{max}=108V$, $V_{min}=96V$, $I_{max}=12A$, $I_{min}=10A$
    • Compute $X_d$ and $X_q$ in ohms
    • Interpret direct vs quadrature axis alignment
13. 7 Marks
    aDescribe the three-lamp synchronisation method is used to synchronise an alternator with an infinite bus.
    How to answer

    • Circuit of three lamps between alternator and bus
    • Condition for synchronism: equal voltage, frequency, phase sequence
    • Bright-dark indication and rotation of light
    • Procedure to close synchronizing switch at all dark / middle dark
14. 7 Marks
    bDescribe the construction, working principle and derive the expression of torque of switched reluctance motor. Also draw the characteristics of SRM.
    How to answer

    • Construction: salient stator and rotor poles, concentrated windings, power converter
    • Working: reluctance torque, sequential excitation, $L(\theta)$ variation
    • Derive $T = (1/2)i^2 dL/d\theta$
    • Draw torque-speed / magnetization characteristics of SRM