EX-402 Electrical Machine - I - Jun 2025 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
   aDerive and explain necessary condition for zero and negative regulation of a transformer?
   How to answer

   • Define voltage regulation formula in terms of R_e, X_e, load current and power factor
   • Phasor diagram for lagging, unity and leading power factor
   • Derive condition for zero regulation: leading pf with tan(phi) = -R_e/X_e approx
   • Derive condition for negative regulation: leading pf beyond zero-regulation condition
   • Explain physical meaning with secondary voltage rise on load
2. 7 Marks
   bWhat is Auto-transformer? Compare auto-transformer with two winding transformer and write the advantages of auto-transformer?
   How to answer

   • Define auto-transformer with single winding common to primary and secondary
   • Compare on basis of construction, copper saving, impedance, isolation, size
   • List advantages: less copper, higher efficiency, smaller size, better regulation
   • Mention disadvantages/limitations briefly
   • Give typical applications
3. 7 Marks
   aA 4400 V, 50-Hz transformer has a hysteresis loss of 1200 W, eddy current loss of 1800 W and full-load copper loss of 4000 W. If the transformer is supplied at 6600 V, 75-Hz. What will be the losses.
   How to answer

   • Note V/f remains constant so maximum flux density Bm unchanged
   • Use hysteresis loss proportional to f and eddy loss proportional to f^2 at constant Bm
   • Calculate new hysteresis and eddy losses at 75 Hz
   • Show full-load copper loss depends on current and remains 4000 W
   • State total losses
4. 7 Marks
   bExplain about conservator and breather in transformers?
   How to answer

   • Define location and construction of conservator tank
   • Explain function: accommodate oil expansion, maintain oil level
   • Define breather and silica-gel arrangement
   • Explain function: absorb moisture from air entering conservator
   • Explain combined working during loading cycles
5. 7 Marks
   aA 400 kVA load at 0.7 power factor lagging is supplied by three 'single-phase' transformers connected in $\Delta$-$\Delta$. Each of the $\Delta$-$\Delta$ transformer is rated at 2000 kVA, 2300/230 V. If one defective transformer is removed from the service, calculate for the $\Delta$-$\Delta$ connection: (i) the kVA load carried by each transformer (ii) percent rated load carried by each transformer (iii) total kVA rating of the transformer bank in $\Delta$-$\Delta$ (iv) ratio of V - V to $\Delta$-$\Delta$ bank transformer ratings.
   How to answer

   • Calculate total delta-delta bank rating from single-phase ratings
   • After removal one unit, bank operates in V-V (open-delta) with 57.7% capacity
   • Compute kVA carried by each remaining transformer for 400 kVA load
   • Compute percent rated load on each transformer
   • Compute ratio of V-V to delta-delta bank ratings
6. 7 Marks
   bUnder what condition will there be no circulating current when two transformers are Operated in parallel at no-load?
   How to answer

   • Write no-load parallel operation equivalent circuit with two secondary emfs
   • Derive circulating current expression in terms of voltage difference and impedances
   • State condition E1 = E2 in magnitude and phase for zero circulating current
   • Translate to same voltage ratio, same per-unit impedance, same polarity
   • Mention effect of tap changer and turns ratio mismatch
7. 7 Marks
   aExplain the principle of production of rotating magnetic field in a 3-phase induction motor
   How to answer

   • State 3-phase balanced supply displaced by 120 degrees
   • Show each phase produces pulsating mmf and sum gives rotating mmf
   • Derive magnitude 1.5 times maximum and constant speed Ns = 120f/P
   • Explain direction reversal with phase sequence
   • Sketch resultant flux vector rotation
8. 7 Marks
   bObtain the relation between rotor input, rotor copper losses and rotor output in terms of slip(s).
   How to answer

   • Define slip s and rotor input Pg = power transferred across air-gap
   • Express rotor copper loss as s*Pg
   • Express mechanical output as (1-s)*Pg
   • Show ratio Pg : Pcu : Pm = 1 : s : (1-s)
   • Mention assumptions and power flow diagram
9. 7 Marks
   aExplain the torque speed characteristics of 3-$\phi$ IM.
   How to answer

   • Sketch torque-slip/speed curve marking starting, maximum, full-load points
   • Explain low-slip stable region and high-slip unstable region
   • Explain effect of rotor resistance on starting and maximum torque
   • Distinguish motoring, generating and braking modes
   • Mention synchronous speed and slip at maximum torque
10. 7 Marks
    bAn 8 pole 50 Hz 3 phase slip ring induction motor has effective rotor resistance of 0.08 ohms per phase. Starting speed is 650 rpm. How much resistance must be inserted in the rotor phase to obtain the maximum torque at starting? Ignore the magnetizing current and stator leakage impedance.
    How to answer

    • Calculate synchronous speed Ns = 120f/P
    • Calculate slip at given starting speed 650 rpm
    • Use condition for maximum torque at start: s_m = 1 => R2_total / X2 = 1
    • Find required total rotor resistance and subtract existing 0.08 ohm
    • State external resistance per phase to be inserted
11. 7 Marks
    aExplain how starting performance of 3-phase squirrel cage type induction motor is improved by means of double cage rotor windings.
    How to answer

    • Explain poor starting torque and high starting current of single cage
    • Describe double-cage construction: high-resistance outer, low-resistance inner cage
    • Explain frequency-dependent leakage reactance at start vs running
    • Show improved starting torque with acceptable running efficiency via characteristics
    • Mention equivalent circuit with two rotor branches
12. 7 Marks
    bA 10 kW, 400 V, 3-phase, 4 pole, 50 Hz delta connected induction motor is running at no load with a line current of 8 A and an input power of 660 W. At full load, line current is 18 A and input power is 11.20 kW. Stator effective resistance per phase is 1.2 $\Omega$ and friction and windage loss is 420 W. For negligible rotor ohmic losses at no load, calculate (i) stator core loss (ii) total rotor losses at full load (iii) total rotor ohmic losses at full load (iv) full load speed.
    How to answer

    • Separate no-load input into stator copper, core and friction-windage components
    • Calculate stator copper loss at no-load and full-load using delta connection
    • Obtain stator core loss and air-gap power at full load
    • Estimate rotor copper and total rotor losses from power balance
    • Calculate full-load slip and speed Ns(1-s)
13. 7 Marks
    aExplain the role of compensating winding in the operation of AC series motor.
    How to answer

    • Define cross-field and armature reaction problem in AC series motor
    • Explain compensating winding placement and connection
    • Explain neutralization of transformer emf and improved commutation
    • Mention effect on power factor and performance
    • Give diagram of winding arrangement
14. 7 Marks
    bExplain the constructional features and working principle of single-phase induction motor.
    How to answer

    • Describe stator, auxiliary winding, capacitor/ shaded pole and squirrel cage rotor
    • Explain double revolving field theory or cross-field principle
    • Explain why single-phase alone gives no starting torque
    • Explain starting methods: split-phase, capacitor, shaded pole
    • Mention applications
15. 14 Marks
    Write short notes (Any two)

    1. a)Tap changer
    2. b)Applications of AC Servo motors
    3. c)Linear induction motor
    4. d)Impact of harmonics on performance of induction motor
    How to answer

    • For chosen two topics give definition/construction and working
    • Give neat sketch or block diagram where applicable
    • List key functions, types and applications
    • Mention advantages/limitations
    • Conclude with practical relevance to machines