EX-402 Electrical Machine - I - Dec 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
   aDiscuss the constructional features of transformers. Draw neat diagrams.
   How to answer

   • Core, windings, insulation, tank and cooling arrangements
   • Types of cores - core type vs shell type construction
   • Winding types - concentric, sandwich, LV/HV placement
   • Neat labelled diagrams of core and shell type
2. 7 Marks
   bWhat is the SC test on transformer is conducted at rated Current and also draw circuit diagram.
   How to answer

   • Purpose of SC test - determine copper loss and equivalent impedance
   • Test conditions - LV shorted, rated current on HV side, low voltage applied
   • Circuit diagram with supply, ammeter, voltmeter, wattmeter
   • Procedure and observations
3. 7 Marks
   aWith relevant phasor diagrams, explain the operation of a practical single phase transformer under no load condition.
   How to answer

   • No-load equivalent circuit with shunt branch R0, X0
   • Phasor diagram with V1, I0, Iw, Imu, phi0
   • Working vs magnetizing component explanation
   • Effect of core loss and magnetizing reactance
4. 7 Marks
   bA $230/2300V$ transformer takes a no load current of $6.5A$ and absorbs $187W$. If the resistance of primary is $0.06\Omega$, find (a) Core loss (b) no load power factor (c) active component of current and (d) magnetizing current.
   How to answer

   • No-load input $W_0 = V_0 I_0 \cos\phi_0$ to find $\cos\phi_0$
   • Subtract primary $I_0^2 R_1$ to get core loss
   • Calculate $I_w = I_0 \cos\phi_0$ and $I_m = I_0 \sin\phi_0$
   • Numerical values for all four quantities
5. 7 Marks
   aIn a transformer, derive the condition for maximum efficiency and thus find the load current at which the efficiency is maximum.
   How to answer

   • Efficiency expression $\eta = V_2 I_2 \cos\phi / (V_2 I_2 \cos\phi + P_i + I_2^2 R_{e2})$
   • Condition $d\eta/dI_2 = 0$ gives $P_i = I_2^2 R_{e2}$
   • Copper loss equals iron loss for maximum efficiency
   • Load current $I_{2m} = \sqrt{P_i/R_{e2}}$
6. 7 Marks
   bA $20KVA$, $2000/200V$ 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)pp. of 0.8 lagging
   2. ii)UPF
   3. iii)0.707 p.f leading
   How to answer

   • Refer $R_1, X_1$ to LV or $R_2, X_2$ to HV to get $R_e, X_e$
   • Voltage regulation formula $(I_e R_e \cos\phi \pm I_e X_e \sin\phi)/V$
   • Calculate for 0.8 lagging, UPF and 0.707 leading
   • Comment on sign for leading pf
7. 7 Marks
   aExplain the Scott connection of two single phase transformers with neat circuit diagram.
   How to answer

   • Purpose - 3-phase to 2-phase / 2-phase to 3-phase conversion
   • Two transformers - main and teaser with 86.6% tapping
   • Neat circuit diagram with connections
   • Voltage relationships explanation
8. 7 Marks
   bDescribe the parallel operation of three phase transformers with unequal voltage ratios.
   How to answer

   • Conditions for parallel operation with unequal ratios
   • Circulating current due to voltage difference
   • Load sharing and equivalent circuit
   • Effects on currents and losses
9. 7 Marks
   aDraw and explain the equivalent circuit of a three-phase induction motor.
   How to answer

   • Stator and rotor per-phase equivalent circuit
   • Parameters R1, X1, R2/s, X2, Xm, Rc
   • Explanation of slip-dependent load resistance
   • Diagram with all components labelled
10. 7 Marks
    bA three phase induction motor is wound for 4 poles and is supplied from 50 Hz system. Calculate:

    1. i)Synchronous speed
    2. ii)Speed of the motor when slip is 4% and
    3. iii)Rotor current frequency when the motor runs at 600 rpm.
    How to answer

    • Synchronous speed $N_s = 120f/P$
    • Rotor speed $N_r = N_s(1-s)$ for 4% slip
    • Slip from 600 rpm to find rotor frequency $f_r = s f$
    • Numerical values
11. 7 Marks
    aFrom fundamentals, deduce a relationship between Rotor power input, rotor copper loss and mechanical power developed in case of Induction motor.
    How to answer

    • Air-gap power $P_g$, rotor copper loss $sP_g$
    • Mechanical power $P_m = (1-s)P_g$
    • Ratio $P_g : P_{cu2} : P_m = 1 : s : (1-s)$
    • Derivation from rotor equivalent circuit
12. 7 Marks
    bA 6-pole, 50HZ, 3-phase induction motor runs at 960 rpm when the torque on the shaft is 200 Nm. If the stator losses are 1500 W and the friction and windage losses are 500 W. Find;

    1. i)Rotor copper loss
    2. ii)The efficiency of the motor
    How to answer

    • Synchronous speed and slip from 960 rpm
    • Shaft power $P_{sh} = 2\pi N T/60$
    • Air-gap power and rotor copper loss $sP_g$
    • Efficiency $\eta = P_{sh}/P_{in}$ with given losses
13. 7 Marks
    aExplain the various schemes of starting squirrel cage induction motor.
    How to answer

    • Need for reduced voltage starting
    • DOL, star-delta, auto-transformer starters
    • Rotor resistance and soft starter methods
    • Circuit diagrams and comparison
14. 7 Marks
    bExplain the various techniques of speed control of induction motor from rotor side control.
    How to answer

    • Speed control by rotor resistance, slip power recovery
    • Kramer and Scherbius schemes
    • Rotor voltage injection principle
    • Advantages, limitations and applications
15. 5 Marks
    aGive the classification of single phase motors. Explain any two types of single phase induction motors.
    How to answer

    • Classification - split phase, capacitor, shaded pole, etc.
    • Construction and working of any two types
    • Torque-speed characteristics
    • Applications of each type
16. 5 Marks
    bDevelop equivalent circuit of a single phase induction motor ignoring core losses.
    How to answer

    • Double revolving field theory basis
    • Forward and backward equivalent circuits
    • Combining to get total impedance neglecting Rc
    • Final equivalent circuit diagram
17. 4 Marks
    cExplain the principle of operation and applications of Linear Induction motor.
    How to answer

    • Construction - primary and secondary separated by air gap
    • Travelling magnetic field and thrust production
    • Slip concept in linear motor
    • Applications - traction, conveyors, actuators