About this previous year paper
INDUSTRIAL ELECTRONICS (NEC021)
SECTION – A
(Short Answer Questions | 10 × 2 = 20 Marks)
(a) Types of Power Diodes
Power diodes are classified as: General purpose diodes
Fast recovery diodes Schottky diodes
(b) Power Electronic Circuits
Common power electronic circuits include: Rectifiers
Inverters Choppers
AC voltage controllers Cycloconverters
(c) Comparison between MOSFET and BJT
MOSFET is a voltage-controlled device with high input impedance and fast switching, while BJT is a current-controlled device with lower input impedance and slower switching.
(d) Latching Current and Holding Current
Latching current: Minimum anode current required to keep SCR ON immediately after triggering.
Holding current: Minimum anode current below which SCR turns OFF.
(e) Input Power Factor
Input power factor is the ratio of real input power to apparent input power drawn from the AC supply.
(f) Effect of Freewheeling Diode in 3-Pulse Midpoint Converter
Freewheeling diode provides a path for load current during negative half cycle, reduces output voltage ripple, and improves power factor.
(g) Applications of Current Source Inverter
Induction motor drives Traction systems
Industrial motor speed control
(h) Principle of Current Limit Control Strategy
This strategy limits output current to a safe value by adjusting firing angle or duty cycle, protecting power devices and load.
(i) Advantages of Current Commutated Chopper
Simple commutation circuit
High reliability
Suitable for high power applications
(j) Advantages of Induction Motor over DC Motor
Induction motors are rugged, low-cost, low-maintenance, spark-free, and suitable for harsh industrial environments.
SECTION – B
(Attempt any five | 5 × 10 = 50 Marks)
(a) Static V–I Characteristics of SCR
The SCR has three regions: Forward blocking region
Forward conduction region Reverse blocking region
When gate pulse is applied in forward blocking region, SCR turns ON and remains ON until current falls below holding current.
(b) Triggering Methods of Thyristor Thyristor triggering methods include:
Gate triggering Forward voltage triggering
dv/dt triggering Temperature triggering
Light triggering
Gate triggering is the most commonly used method.
(c) Single Phase Half Wave Controlled Rectifier with R-L Load and Freewheeling Diode
In this circuit, SCR conducts during positive half cycle when triggered. During negative half cycle, freewheeling diode allows current to circulate through load, maintaining continuity and reducing output ripple.
(d) Performance Factors of Line Commutated Converters
Performance factors include: Input power factor
Efficiency Ripple factor
Voltage regulation Harmonic distortion
(e) Parallel Inverters
Parallel inverters operate by sharing load current. They improve reliability, allow higher power handling, and ensure continuity of supply in case one inverter fails.
(f) Control Strategies for Varying Duty Cycle (α)
Duty cycle can be controlled by: Time ratio control
Current limit control PWM (Pulse Width Modulation)
These strategies control output voltage and speed.
(g) Step-Up / Step-Down Chopper
A step-down chopper reduces average output voltage below input voltage, while a step-up chopper increases output voltage above input voltage using energy storage elements.
(h) Single Phase Full Converter Drive
In continuous conduction mode, current never falls to zero, giving smooth torque.
In discontinuous mode, current becomes zero during part of cycle, causing torque pulsations.
SECTION – C
(Attempt any two | 2 × 15 = 30 Marks)
3) Three Phase Full Wave Bridge Inverter
A three-phase full wave bridge inverter uses six thyristors or switches. It converts DC input into three-phase AC output by firing devices in a proper sequence.
It is widely used in motor drives and industrial power systems.
4)
(i) Variable Frequency Control of Induction Motor
Speed control methods include:
V/f control PWM control
Slip power recovery Rotor resistance control
Maintaining constant V/f ratio ensures constant torque.
(ii) Numerical: DC Motor Speed Control
Given data is substituted into converter equations to calculate:
Firing angle of converter
RMS thyristor current
Input power factor
(Full step-wise solution written in exam format.)
5)
(i) Slip Power Recovery System – Kramer Drive
Kramer drive recovers slip power from rotor circuit and feeds it back to supply, improving efficiency of induction motor during speed control.
(ii) Two Transistor Analogy of Thyristor
SCR is represented as two interconnected transistors (PNP and NPN). Regenerative action between the transistors explains latching and switching behavior.