EX-402 Electrical Machine - I - Jun 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
   aWhat is voltage regulation of a transformer? Derive the conditions for maximum and zero voltage regulation in a transformer.
   How to answer

   • Define voltage regulation as (V2nl - V2fl)/V2fl.
   • Phasor / approximate equivalent circuit and regulation expression in terms of R, X, pf.
   • Condition for maximum regulation and derivation.
   • Condition for zero regulation and derivation.
2. 7 Marks
   bIn a 25 kVA, 2000/200 V transformer the iron and copper losses are 350 and 400 W respectively. Calculate the efficiency on UPF at (i) full load (ii) half load. (iii) Determine the load for maximum efficiency and the copper loss in this case.
   How to answer

   • Efficiency formula using output, iron loss and copper loss.
   • Calculate efficiency at full load UPF.
   • Calculate efficiency at half load UPF.
   • Condition Pi = x^2 Pcu for maximum efficiency, find load and copper loss.
3. 7 Marks
   aDraw the exact equivalent circuit of a transformer and describe briefly the various parameters involved in it.
   How to answer

   • Draw exact equivalent circuit referred to primary/secondary.
   • Identify R1, X1, R2, X2, Rc, Xm and load.
   • Explain no-load branch and series leakage impedances.
   • Mention transformation ratio and referral relations.
4. 7 Marks
   bExplain scott connection with neat diagrams. With necessary phasor diagrams, prove that a three-phase system can established from a two-phase system using T-T connection.
   How to answer

   • Connection diagram of Scott / T-T connection with teaser and main transformer.
   • Phasor diagrams for 3-phase to 2-phase.
   • Voltage/current relations with 86.6% tapping.
   • Proof that balanced 3-phase obtained from 2-phase using T-T.
5. 7 Marks
   aA Scott connected transformer supplies two single phase furnaces at 200V, each taking 200 kW. The load on the leading phase is at unity power factor and that on the other phase is 0.8 power factor. The three-phase input line voltage is 6600 V. Calculate the values of line currents on the 3-phase side. Neglect the magnetizing and core loss currents in the transformers.
   How to answer

   • Compute two-phase secondary currents from kW, V and pf.
   • Refer currents to 3-phase side using Scott turns ratio.
   • Apply phasor addition for three line currents.
   • Neglect magnetizing and core-loss currents as stated.
6. 7 Marks
   bExplain about conservator and breather in transformers.
   How to answer

   • Function and location of conservator tank.
   • Function and working of breather with silica gel.
   • Oil expansion and moisture prevention.
   • Diagram of arrangement on transformer tank.
7. 7 Marks
   aExplain the construction and working principle of three phase induction motor.
   How to answer

   • Stator, rotor, frame, windings construction.
   • Rotating magnetic field production.
   • Induced rotor emf and torque production.
   • Slip concept and working principle.
8. 7 Marks
   bA three phase, 15 kW, 400 V, 50 Hz, 4-pole, delta connected squirrel cage induction motor has the following data: No-load: 400 V, 5.0 A, p.f. 0.2 Blocked rotor: 120 V, 20.0 A p.f. 0.6 The ratio of stator to rotor copper losses on short circuit is assumed to be unity. Draw the circle diagram and determine (i) the full load current and power factor (ii) the maximum power developed (iii) starting torque.
   How to answer

   • Use no-load and blocked-rotor data to find circle diagram constants.
   • Draw circle diagram to scale with current locus.
   • Determine full-load current and power factor from output line.
   • Determine maximum power developed and starting torque.
9. 7 Marks
   aDerive the torque-slip equation for a 3-phase induction motor and also the equation for the slip at which maximum torque occurs?
   How to answer

   • Equivalent circuit and rotor current expression.
   • Torque expression T = k * sR2/(R2^2 + s^2 X2^2).
   • Differentiate for maximum torque condition.
   • Slip at maximum torque sm = R2/X2.
10. 7 Marks
    bExplain the phenomenon of crawling and cogging?
    How to answer

    • Define crawling due to harmonics, 7th harmonic synchronous speed.
    • Define cogging due to slot harmonics and magnetic locking.
    • Causes: stator mmf harmonics, slot combinations.
    • Remedies: skewing, proper slot selection.
11. 7 Marks
    aThe power input to the rotor of 440V, 50 Hz, 6 pole, 3-phase and induction motor is 80 kW. The rotor electromotive force is observed to make 100 complete alterations per minute. Calculate (i) the slip (ii) the rotor speed (iii) rotor copper losses per phase.
    How to answer

    • Synchronous speed Ns = 120f/P.
    • Rotor frequency from alterations per minute gives slip.
    • Rotor speed N = Ns(1-s).
    • Rotor copper loss = s * rotor input divided per phase.
12. 7 Marks
    bExplain the working of y-$\Delta$ starter used in 3-$\phi$ induction motor.
    How to answer

    • Star connection at start to reduce voltage to 1/√3.
    • Changeover to delta for running at full voltage.
    • Circuit diagram with contactors/timer.
    • Starting current and torque reduction to 1/3.
13. 7 Marks
    aDiscuss the procedure to determine the parameters of equivalent circuit of a Single - phase induction motor.
    How to answer

    • No-load, blocked-rotor and DC tests.
    • Determination of R1, X1, R2, X2, R0, X0.
    • Equivalent circuit of single-phase induction motor.
    • Double revolving field basis for parameters.
14. 7 Marks
    bGive the construction and working principle of single phase A.C. Series motors.
    How to answer

    • Stator, commutator, brushes, field construction.
    • AC operation with pulsating field.
    • Torque production and commutation.
    • Characteristics and applications.
15. 14 Marks
    Write short notes on any two.

    1. a)Power and distribution transformer
    2. b)Pulse and high frequency transformer
    3. c)Cooling of transformer
    4. d)Linear induction motor
    How to answer

    • Select any two topics from a-d.
    • Give definition, construction/features for each.
    • Explain working/cooling/types with diagram.
    • Mention applications and advantages.