EX-801 Electrical Drives - May 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
   aA motor equipped with a flywheel is to supply a load torque of 1000 N-m for 10 sec. followed by a light load period of 300 N-m long enough for the flywheel to regain its steady state speed. It is desired to limit the motor torque to 700 N-m. What should be the moment of inertia of flywheel? Motor has an inertia of 10 Kg-m$^2$. Its no load speed is 500 rpm and the slip at a torque of 500 N-m is 5%. Assume speed torque characteristic of motor to be a straight line in the region of interest.
   How to answer

   • Load cycle: 1000 N-m for 10 s, light load 300 N-m; motor torque limited to 700 N-m
   • Use linear speed-torque characteristic from no-load 500 rpm and 5% slip at 500 N-m to get speed drop
   • Energy balance: excess load energy supplied by flywheel kinetic energy loss
   • Solve for total inertia then subtract motor inertia 10 Kg-m2 to get flywheel inertia
2. 7 Marks
   bExplain that the steady-state stability of a drive depends on relative characteristics of the motor and load and not just on motor (or load) characteristic.
   How to answer

   • Steady-state stability condition: $dT/d\omega - dT_l/d\omega < 0$ at equilibrium
   • Small perturbation argument for speed deviation
   • Show stable vs unstable intersection of motor and load curves with diagram
   • Conclude relative slope matters, not individual characteristic alone
3. 7 Marks
   aA drive has following equations for motor and load torques. $T = (1+2\omega_m)$ and $T_l = 3\sqrt{\omega_m}$ Obtain the equilibrium points and determine their steady-state stability.
   How to answer

   • Set $T = T_l$ i.e. $(1+2\omega_m)=3\sqrt{\omega_m}$
   • Solve quadratic in $\sqrt{\omega_m}$ to get equilibrium speeds
   • Evaluate stability criterion $d(T-T_l)/d\omega_m < 0$ at each point
   • Classify stable and unstable equilibrium points
4. 7 Marks
   bA 200V, 10.5 A, 2000 rpm shunt motor has the armature and field resistances of 0.5 $\Omega$ and 400 $\Omega$ respectively. It drives a load whose torque is constant at rated motor torque Calculate motor speed if the source voltage drops to 175 V.
   How to answer

   • Find rated $I_a$, $E_b$ from 200 V, $I$, $R_a$, $R_{sh}$
   • Constant load torque implies $\Phi I_a$ = constant; flux proportional to voltage for shunt field
   • Write $E_b = k\Phi N$ for both 200 V and 175 V conditions
   • Solve for new speed N at 175 V
5. 7 Marks
   aField control is employed for getting speeds higher than rated and armature voltage control is employed for getting speeds less than rated. Why?
   How to answer

   • Armature voltage limited by rated voltage; field weakening allows speed > base
   • Below base speed flux kept rated for full torque capability
   • Field control reduces flux causing commutation / power limits if used below rated
   • Armature control reduces voltage keeping flux constant for constant-torque operation
6. 7 Marks
   bA 220 V, 1500 rpm separately excited dc motor has rated armature current of A. The armature resistance is 3 ohm. It is fed through fully controlled converter from a 230 V 50 Hz supply. Final firing angle of converter for half the rated torque at 600 rpm. Assume that torque is proportional to armature current.
   How to answer

   • Rated $E_b$ and $I_a$ from 220 V, 1500 rpm, $R_a=3$ ohm data
   • Half rated torque implies half rated $I_a$
   • Converter output $V_a = (2V_m/\pi)\cos\alpha$ for fully controlled bridge
   • Solve for firing angle $\alpha$ at 600 rpm
7. 7 Marks
   aA wound rotor induction motor has full load speed of 288 rpm. The supply frequency in 50 Hz. Find

   1. i)Number of poles
   2. ii)Slip
   3. iii)Slip for full load if rotor resistance is doubled
   4. iv)New value of r copper loss if initial rotor copper loss is 275 W.
   How to answer

   • Determine synchronous speed just above 288 rpm for 50 Hz to get pole number
   • Calculate slip $s=(N_s-N)/N_s$
   • Slip proportional to rotor resistance for same torque
   • Rotor copper loss $= sP_g$; use slip ratio for new loss
8. 7 Marks
   bFor variable frequency control of induction motor explain the following points

   1. i)For speeds below base speed (V/f) ratio is maintained constant, why?
   2. ii)For speeds above base speed, the terminal voltage is maintained constant, why?
   How to answer

   • Constant flux (V/f constant) below base speed to maintain torque capability
   • Voltage limit and field weakening above base speed
   • Effect on maximum torque and magnetizing current
   • Constant-torque vs constant-power regions
9. 7 Marks
   aA 3-phase induction motor takes 4 time full-load current on direct starting and develops 1.5 times full-load torque. What must be the applied voltage to obtain full-load torque at starting and what will be the starting current?
   How to answer

   • Starting torque $\propto V^2$; scale voltage to get full-load torque from 1.5 times value
   • Starting current $\propto V$; compute reduced starting current
   • Calculate voltage ratio $\sqrt{1/1.5}$ times rated
   • Resulting starting current = 4 times full-load times voltage ratio
10. 7 Marks
    bWhen rotor speed is close to synchronous speed, application of dc field leads to pull-in of the rotor into synchronism. However, application of dc field at a speed considerably lower than synchronous speed does not lead to pull-in of the rotor into synchronism, why?
    How to answer

    • Synchronizing torque pulsates at slip frequency when DC field applied
    • At near-synchronous speed average pull-in torque can accelerate rotor to synchronism
    • At low speed torque alternates too fast, average value zero, rotor inertia cannot follow
    • Hunting / inability to lock due to large slip
11. 7 Marks
    aA 3-phase, 400V, 50Hz, 4 pole star-connected reluctance motor with negligible armature resistance has $X_d = 8\Omega$ and $X_q = 2\Omega$ For a load torque of 80 Nm. Calculate: Neglect rotational losses.

    1. i)The load angle
    2. ii)The line current
    3. iii)The input power factor
    How to answer

    • Reluctance torque formula $T = (V^2/2)(1/X_q-1/X_d)\sin2\delta /\omega_s$
    • Solve for load angle $\delta$ at 80 Nm
    • Find $I_d, I_q$ and line current magnitude
    • Compute input power factor from angle between V and I
12. 7 Marks
    bExplain working of Voltage Source Inverter (VSI) fed synchronous motor drives.
    How to answer

    • Power circuit of VSI: DC link, 6-switch bridge, control
    • Voltage and frequency control by PWM
    • Synchronous motor operation: load angle, leading pf with field control
    • Advantages, applications and closed-loop control
13. 7 Marks
    aBriefly describe how to control load angle in Stepper Motor.
    How to answer

    • Definition of load angle / torque angle in stepper motor
    • Stator excitation sequence controls equilibrium position
    • Micro-stepping / current control to adjust angle
    • Closed-loop vs open-loop positioning
14. 7 Marks
    bDraw and explain digital control of drives.
    How to answer

    • Block diagram of digital/microprocessor control: sensing, ADC, controller, firing/gating
    • Sampling, speed/current feedback loops
    • Advantages: flexibility, protection, accuracy
    • Example application in DC/AC drives
15. 14 Marks
    Write a short notes on (any two) :

    1. i)Constant power and constant torque drive
    2. ii)Transient and steady state analysis of dc drives
    3. iii)Scherbius induction drives
    4. iv)Electric traction drives
    How to answer

    • Select any two topics with definition and key features
    • Relevant diagrams / equations / applications for each
    • Comparison where applicable (e.g., constant power vs constant torque)
    • Industrial uses e.g., traction, Scherbius slip-power recovery