(SEM VII) THEORY EXAMINATION 2024-25 ELECTRIC DRIVES

KEE075 – ELECTRIC DRIVES

Complete Solved Question Paper

SECTION A

(Attempt all questions – 2 × 10 = 20 marks)

Q1(a) Components of an electric drive

Power source                                          Power modulator (converter)

Electric motor                                         Control unit

Load

Q1(b) Role of power electronic converters

Control voltage, current, and frequency  Enable speed and torque control

Improve efficiency and performance

Q1(c) Steady-state stability

Steady-state stability is the ability of an electric drive to maintain its speed under small load disturbances.

Q1(d) Types of mechanical loads

Constant torque load                            Variable torque load

Constant power load                             Impact load

Q1(e) Electric vs mechanical braking

Electric braking is preferred because:  Faster response

Less wear and tear                               Energy recovery possible

Higher efficiency

Q1(f) Dynamic braking

Dynamic braking is the process in which the motor acts as a generator and dissipates energy in an external resistor.

Q1(g) Firing & extinction angles

Firing angle (α): Delay angle for triggering thyristor

Extinction angle (β): Angle at which current falls to zero

Q1(h) Power factor & distortion factor

Power Factor=Displacement Factor×Distortion Factor\text{Power Factor} = \text{Displacement Factor} \times \text{Distortion Factor}Power Factor=Displacement Factor×Distortion Factor

Distortion factor accounts for waveform harmonics.

Q1(i) VSI vs CSI

Q1(j) Voltage control & speed of IM

Reducing stator voltage reduces air-gap flux, which reduces torque and hence speed of an induction motor.

SECTION B

(Attempt any three – 10 × 3 = 30 marks)

Q2(a) Active & passive load

Active load: Supplies energy (e.g., generators)

Passive load: Consumes energy (e.g., motors, fans)

Q2(b) Load equalization

Load equalization smoothens peak loads by storing energy during low demand.

Importance:                                                   Reduces peak power

Improves efficiency                                         Protects power supply

Q2(c) Plugging in DC motor

Plugging is reversal of armature connections while supply remains unchanged, producing braking torque.

Precautions:                                                  Limit current using resistors

Avoid prolonged operation

Q2(d) Effect of firing angle

Increase in firing angle → Reduced output voltage

Lower voltage → Reduced torque & speed    Used for smooth speed control.

Q2(e) BLDC motor drive                              Uses electronic commutation

Hall sensors detect rotor position                  Inverter supplies controlled DC

High efficiency & low maintenance

SECTION C

Q3(a) Speed-torque conventions

Forward motoring: Speed & torque positive    Forward braking: Speed positive, torque negative

Reverse motoring: Speed & torque negative    Reverse braking: Speed negative, torque positive

Q3(b) Load torque classification

Q4(a) Effect of inertia on dynamics

High inertia:                                                    Slower acceleration/deceleration

Higher starting torque required                     Affects transient response

Q4(b) Motor power rating (short-time duty)

Calculate heating during operation               Ensure temperature rise within limits

Select motor with higher short-time rating

Q5(a) Dynamic braking in IM                      Motor disconnected from AC supply

DC injected into stator                                   External resistor dissipates energy

Components: Rectifier, braking resistor

Q5(b) Plugging in IM

Phase sequence reversed                              Produces strong braking torque

High current → heating                                Used for rapid stopping.

Q6(a) Firing angle & DC series motor

Increased firing angle → reduced armature voltage

Reduced voltage → reduced speed & torque
Enables controlled acceleration.

Q6(b) Dual converter operation

Two converters connected back-to-back      One operates as rectifier, other as inverter

Enables four-quadrant operation

Q7(a) VSI vs CSI in IM drive

Q7(b) Static rotor resistance control       External resistance inserted in rotor circuit

Increases slip → speed control                    Low efficiency due to power loss