EX-402 Electrical Machine - I - 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
   aDiscuss about the open-circuit test and short circuit test on single phase transformer.
   How to answer

   • Purpose of OC test: iron/core loss, no-load parameters R0, X0
   • Procedure: LV/HV supply, instruments, observations Vo, Io, Po
   • Purpose of SC test: copper loss, equivalent resistance/reactance
   • Procedure: shorted winding, reduced voltage, Vsc, Isc, Psc and calculations
2. 7 Marks
   bA 6300/210 V, 50 Hz, single phase transformer has per turn e.m.f of about 9 volts and maximum flux density of 1.2 T. Find the number of high voltage and low voltage turns and the net cross sectional area of the core.
   How to answer

   • EMF per turn equation $E = 4.44 f \Phi_m$
   • Calculate $\Phi_m$ from $E_{turn}$ and $f$
   • Number of turns $N = V/E_{turn}$ for HV and LV
   • Core area $A = \Phi_m / B_{max}$
3. 7 Marks
   aIf part of a primary winding of a transformer were accidentally short circuited what would be the immediate effect? Discuss in detail.
   How to answer

   • Effective turns ratio change due to shorted turns
   • Fall in impedance and rise in magnetizing/no-load current
   • Local circulating current, overheating and insulation damage
   • Effect on secondary voltage, regulation and protection
4. 7 Marks
   bA 20 kVA, 2000/200 V single phase transformer has the following parameters. H.V. Winding: $R_1 = 3\Omega, X_1 = 5.3\Omega$, L.V. winding: $R_2 = 0.05\Omega, X_2 = 0.1\Omega$. Find the voltage regulation at

   1. i)Power factor of 0.8 lagging
   2. ii)UPF
   3. iii)0.707 power factor leading
   How to answer

   • Refer R1, X1 to LV or R2, X2 to HV to get total Req, Xeq
   • Full-load current and $I(R_e \cos\phi \pm X_e \sin\phi)$ formula
   • Calculate % regulation for 0.8 lagging and UPF
   • Calculate % regulation for 0.707 leading, note negative possibility
5. 7 Marks
   aExplain the need of parallel operation of single phase transformers. Give the condition to be satisfied for their successful parallel operation.
   How to answer

   • Reasons: increased reliability, continuity, load growth, efficiency at light load
   • Ideal conditions: same voltage ratio, same per-unit impedance, same X/R
   • Additional conditions: same polarity, phase sequence and vector group
   • Effect of mismatch: circulating current and unequal loading
6. 7 Marks
   bTwo transformers A and B of ratings 500 kVA and 250 kVA are supplying a load of 750 kVA at 0.8 pf lagging. Their open-circuit voltages are 405V and 415V respectively. Transformer A has 0.01 p.u. resistance and 0.05 p.u. reactance and transformer B has 0.015 p.u. resistance and 0.04 p.u. reactance. Find

   1. i)Circulating current at no-load
   2. ii)The load current shared by each transformer
   3. iii)kVA and power factor of each transformer. Take the voltage across load as 400 V.
   How to answer

   • Per-unit impedances ZA, ZB and open-circuit voltages EA, EB
   • Circulating current $I_c = (E_A-E_B)/(Z_A+Z_B)$ at no-load
   • Load sharing using equivalent circuit with 400 V load voltage
   • Compute kVA, current and pf for A and B
7. 7 Marks
   aExplain the construction and working principle of a three phase induction motor.
   How to answer

   • Stator, rotor, frame, windings and air-gap construction
   • Rotating magnetic field production by 3-phase supply
   • Faraday induction, rotor emf, current and torque
   • Slip concept and why rotor never reaches synchronous speed
8. 7 Marks
   bA 4-pole, 3-phase 50 Hz induction motor has a star-connected rotor. The rotor has a resistance of $0.1\Omega$ per phase and standstill reactance of $2\Omega$/phase. The induced emf between the slip rings is 100 V. If the full-load speed is 1460 rpm. Calculate:

   1. i)The slip
   2. ii)The emf induced in the rotor in each phase
   3. iii)The rotor reactance per phase
   4. iv)The rotor current
   5. v)Rotor power factor Assume slip rings are short-circuited.
   How to answer

   • Synchronous speed $N_s=120f/P$ and slip $s=(N_s-N)/N_s$
   • Standstill rotor emf per phase from slip-ring voltage
   • Running emf $sE_2$, reactance $sX_2$ and impedance $Z_2$
   • Rotor current $I_2=sE_2/Z_2$ and pf $\cos\phi_2=R_2/Z_2$
9. 7 Marks
   aExplain the Torque-Slip and Torque Speed characteristics of an 3-phase Induction motor.
   How to answer

   • Torque equation $T = k sE_2^2R_2/(R_2^2+s^2X_2^2)$
   • Low-slip, high-slip and maximum torque regions
   • Torque-slip curve shape with stable/unstable regions
   • Torque-speed curve derived from $N=N_s(1-s)$
10. 7 Marks
    bExplain the different speed control methods of squirrel cage induction motor.
    How to answer

    • Stator voltage control, pole changing, frequency control
    • Rotor resistance control for slip-ring, cascade, injection
    • V/f control and inverter drives for squirrel cage
    • Merits, demerits and applications of each method
11. 7 Marks
    aExplain the procedure of drawing the circle diagram of an induction motor. How is the performance characteristics obtained from it?
    How to answer

    • No-load and blocked-rotor tests data for circle diagram
    • Construction steps: current locus, power lines, torque line
    • Marking full-load point, slip, efficiency, pf
    • How losses, output, torque and performance are read
12. 7 Marks
    bHow does using double revolving field theory? Also explain the principle of operation of single phase induction motor.
    How to answer

    • Pulsating stator mmf split into forward and backward fields
    • Each component produces torque-slip curve opposite in direction
    • Net torque zero at standstill, need for starting means
    • Single-phase motor operation once rotated, applications
13. 7 Marks
    aExplain the construction and working principle of a single phase A.C. series motor.
    How to answer

    • Construction: stator, field, armature, commutator, compensating winding
    • AC operation: pulsating torque, reactance voltage, sparking
    • Modifications vs DC series motor for AC working
    • Characteristics and applications
14. 7 Marks
    bExplain the types, construction and working principle of a servomotor.
    How to answer

    • Types: DC, AC, 2-phase and 3-phase servomotors
    • Construction: low inertia rotor, control and reference windings
    • Working: error signal control, torque-speed linearity
    • Applications in position control systems
15. 14 Marks
    Attempt any two:

    1. a)Discuss about the Excitation Phenomenon in Transformer.
    2. b)Why breather is connected with the conservator and not with the main tank of the transformer?
    3. c)Discuss about the three phase induction motor breaking methods.
    4. d)Discuss about the induction motor steady-state performance under varying quality of supply and stator winding short circuit location using negative sequence current.
    How to answer

    • Excitation phenomenon: magnetizing current, harmonics, inrush
    • Breather and conservator function and moisture prevention
    • Braking: plugging, dynamic, regenerative methods
    • Negative sequence current for supply quality and fault location