THEORY EXAMINATION (SEM–VI) 2016-17 INDUSTRIAL ELECTRONICS

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.