EX-801 Electrical Drives - May 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
   aExplain about the different speed torque characteristics of different machines and give their utility in section for industrial loads.
   How to answer

   • Define speed-torque characteristics of load and motor
   • Describe characteristics of different machines - DC, induction, synchronous
   • Explain selection criteria for industrial loads
   • Give utility examples for constant torque, variable torque loads
2. 7 Marks
   bDifferentiate between two and four quadrant drives.
   How to answer

   • Define two-quadrant operation
   • Define four-quadrant operation
   • Compare on operating quadrants, braking, reversibility
   • Give applications examples
3. 7 Marks
   aExplain briefly about the functions of different converters that are operated in block diagram of electrical Drives.
   How to answer

   • Show general block diagram of electrical drives
   • Function of power converter, control unit, sensing
   • Role of source, motor, load interface
   • Examples of converters used
4. 7 Marks
   bDraw and explain the torque-speed characteristics for dynamic braking operation of D.C series motor. Why torque becomes zero at finale speed?
   How to answer

   • Explain dynamic braking connection for DC series motor
   • Draw torque-speed characteristics in braking mode
   • Show variation of braking torque with speed
   • Explain why torque becomes zero at final low speed due to zero induced emf
5. 7 Marks
   aDefine a DC chopper. Describe the various types of chopper configuration with necessary diagrams.
   How to answer

   • Define DC chopper as DC-DC converter
   • Classify choppers Type A, B, C, D, E
   • Draw circuit diagrams for each configuration
   • Explain quadrant operation and applications
6. 7 Marks
   bSketch the appropriate voltage and current waveforms for firing angle $30^{\circ}$ that is working with a single phase semi converter fed with DC series motor.
   How to answer

   • Show single-phase semi-converter feeding DC series motor circuit
   • Mark firing angle 30 degrees on supply voltage
   • Sketch output voltage waveform with conduction gaps
   • Sketch armature current waveform continuous/ripple nature
7. 7 Marks
   aExplain with suitable block diagrams the various types of VSI controlled induction motor drives.
   How to answer

   • Define VSI fed induction motor drive principle
   • Draw block diagrams of 180-degree, PWM, square-wave VSI drives
   • Explain voltage and frequency control method
   • Compare V/f control and vector control variants
8. 7 Marks
   bA 3-Phase 6-pole, 400 V, 970 rpm, 20 kW, 970 rpm, 50 Hz delta connected induction motor has rotor leakage impedance of $(0.5+j2)\Omega$ stator impedance and rotational losses are assumed as negligible. If this motor is energized from a source of 3-phase, 400 V, 80 Hz, then compute

   1. i)The motor speed at rated torque
   2. ii)The slip at which maximum torque occurs
   How to answer

   • Calculate synchronous speed at 80 Hz and rated torque from 20 kW rating
   • Use approximate equivalent circuit neglecting stator impedance to find slip at rated torque
   • Solve torque-slip equation for motor speed
   • Derive slip for maximum torque as Rr/Xr
9. 7 Marks
   aExplain the conventional Scherbius system with that of a Scherbius drive.
   How to answer

   • Describe slip-power recovery principle
   • Explain conventional Scherbius with auxiliary machines
   • Explain static Scherbius with converter-inverter
   • Compare components, efficiency and control range
10. 7 Marks
    bDraw the block diagram of closed loop synchronous motor drive fed from VSI and explain.
    How to answer

    • Show power circuit with VSI, synchronous motor and position sensor
    • Draw closed-loop speed and current control blocks
    • Explain self-control and commutation logic
    • Explain starting and synchronous operation
11. 7 Marks
    aDescribe self connected load commutated inverter, controlled synchronous motor drive in detail and compare them.
    How to answer

    • Define load commutated inverter principle
    • Describe self-controlled synchronous motor drive circuit
    • Explain commutation by motor emf and leading pf operation
    • Compare features, advantages and limitations
12. 7 Marks
    bA 500 kW, 3-phase, 6.6 kV, 60 Hz, 6 pole Y-connected wound field synchronous motor has the following parameters: $X_m=78\Omega$, $X_{sr}=3\Omega$, p.f=1, n=5, Rs=0 (negligible) the motor speed is controlled by variable frequency control with a constant V/f ratio up to base speed and rated terminal voltage above base speed. Calculate T, $P_m$, $I_m$ and $I_f$ for the motor operation at rated armature current and upf. What is the range of constant power operation, neglect friction, windage and core loss.
    How to answer

    • Calculate base speed, V/f ratio and armature current at rated kVA
    • Use synchronous machine phasor with Rs=0, upf to find T, Pm, Im, If
    • Determine constant power speed range above base speed
    • Neglect friction, windage and core loss assumptions
13. 7 Marks
    aExplain the fundamental of switched reluctance motors.
    How to answer

    • Define switched reluctance motor construction
    • Explain inductance variation and torque production
    • Describe power converter and switching sequence
    • Explain speed control characteristics
14. 7 Marks
    bDiscuss about the digital control of drives.
    How to answer

    • Define digital control in drives
    • Describe microprocessor/DSP based control structure
    • Explain sampling, PWM generation and feedback
    • Discuss advantages like flexibility and accuracy
15. 14 Marks
    Write short notes on any two of the followings:

    1. i)Group and individual drives
    2. ii)CNC
    3. iii)Energy recovery system
    How to answer

    • For each chosen topic give definition and block/circuit
    • Explain working principle and key features
    • Give industrial applications
    • Conclude with advantages/limitations