About this previous year paper
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
| VSI | CSI |
|---|---|
| Voltage source | Current source |
| Output voltage controlled | Output current controlled |
| Requires DC capacitor | Requires DC inductor |
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
| Type | Example |
|---|---|
| Constant torque | Conveyors |
| Variable torque | Fans, pumps |
| Constant power | Machine tools |
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
| Feature | VSI | CSI |
|---|---|---|
| Input | DC voltage | DC current |
| Control | PWM | Current control |
| Size | Compact | Bulky |
Q7(b) Static rotor resistance control External resistance inserted in rotor circuit
Increases slip → speed control Low efficiency due to power loss