Industrial Electronics (4331103) - Winter 2022 Solution

Question 1(a) [3 marks]

Draw the construction of SCR and explain it.

Answer: SCR (Silicon Controlled Rectifier) is a four-layer PNPN semiconductor device with three terminals: Anode, Cathode, and Gate.

Diagram:

• P-N-P-N Layers: Four alternating semiconductor layers
• Gate Terminal: Controls turn-on of the device
• Current Flow: Anode to cathode when triggered

Mnemonic: "Silicon Controls Rectification" - SCR controls current flow in one direction only when triggered.

Question 1(b) [4 marks]

Draw construction of TRIAC and explain it.

Answer: TRIAC (Triode for Alternating Current) is a bidirectional three-terminal semiconductor device that conducts in both directions when triggered.

Diagram:

• Bidirectional Operation: Conducts in both directions when triggered
• Gate Control: Single gate controls conduction in both directions
• Equivalent Circuit: Acts like two SCRs connected in anti-parallel
• AC Applications: Widely used for AC power control applications

Mnemonic: "TRI-direction AC controller" - Controls current in both directions in AC circuits.

Question 1(c) [7 marks]

Describe construction & working of Opto-Isolators, Opto-TRIAC, Opto-SCR, and Opto-transistor. And list their applications.

Answer: Opto-isolators use light to transfer electrical signals between isolated circuits.

Diagram:

• Electrical Isolation: Complete separation between input and output
• Noise Immunity: High resistance to electrical noise
• Speed: Response times in microseconds range

Mnemonic: "LOST" - Light Operates Semiconductor Terminals in all opto-devices.

Question 1(c) OR [7 marks]

Describe Explain working of SCR using two transistor analogies. List the various industrial applications of SCR.

Answer: SCR can be modeled as two interconnected transistors: PNP (T1) and NPN (T2).

Diagram:

Working Principle:

Industrial Applications of SCR:

• Power Control: AC/DC motor speed control
• Switching: Static switches, solid-state relays
• Inverters: DC to AC conversion
• Protection: Overvoltage protection circuits
• Lighting: Light dimmers, illumination control

Mnemonic: "POWER" - Power control, Overvoltage protection, Welding machines, Electronic converters, Regulated supplies.

Question 2(a) [3 marks]

Define Triggering in SCR and explain any two triggering techniques.

Answer: Triggering is the process of turning ON an SCR by applying appropriate signal to its gate terminal.

Two Triggering Techniques:

• Gate Triggering: Most common method using electrical pulse
• Light Triggering: Uses photosensitive semiconductor properties

Mnemonic: "GET" - Gate Electrical Triggering is the most common method.

Question 2(b) [4 marks]

Write the differences between forced commutation and natural commutation.

Answer:

• Forced Commutation: Actively turns OFF SCR using external circuit
• Natural Commutation: SCR turns OFF when AC current crosses zero

Mnemonic: "FACE" - Forced Active Commutation requires External components.

Question 2(c) [7 marks]

Design the snubber circuit for SCR.

Answer: Snubber circuit protects SCR from high dV/dt and limits rate of voltage rise.

Diagram:

Design Steps:

• Resistance R: Limits discharge current of capacitor
• Capacitance C: Absorbs transient energy and limits dV/dt
• Protection: Prevents false triggering and damage
• Power Rating: R must have sufficient power rating

Mnemonic: "RCSS" - Resistance-Capacitance Saves Silicon from Stress.

Question 2(a) OR [3 marks]

Define commutation and Explain class-E commutation for SCR.

Answer: Commutation is the process of turning OFF an SCR by reducing its anode current below the holding current level.

Class-E Commutation:

Diagram:

• Auxiliary SCR: Controls the commutation process
• Resonant Circuit: Forms LC resonant circuit
• Operation: Auxiliary SCR triggers capacitor discharge to reverse-bias main SCR
• Application: Used in inverters and choppers

Mnemonic: "ACE" - Auxiliary Capacitor Extinguishes conduction.

Question 2(b) OR [4 marks]

Explain Triggering of Thyristor.

Answer:

• Gate Triggering: Most common and controllable method
• Parameter Control: Pulse width, amplitude, and rise time
• Gate Sensitivity: Varies with temperature
• Protection: Required against unwanted triggering

Mnemonic: "VITAL" - Voltage, Illumination, Temperature And Level are all triggering methods.

Question 2(c) OR [7 marks]

Explain methods to protect SCR against over voltage and current in details.

Answer:

Overvoltage Protection:

Diagram:

Overcurrent Protection:

Diagram:

• Coordination: Protection devices must work in coordination
• Response Time: Critical for effective protection
• Multiple Layers: For critical applications, several methods are combined

Mnemonic: "SCOPE" - Snubbers, Clamps, Overload sensors, Protectors, and Electronic limiters.

Question 3(a) [3 marks]

List the differences between single phase rectifier and poly phase rectifier.

Answer:

• Ripple Factor: Single phase has higher ripple compared to poly phase
• Form Factor: Better in poly phase systems
• Size/Weight: Poly phase systems have better power/weight ratio

Mnemonic: "PERCH" - Poly phase has Efficiency, Ripple improvement, Capacity, and Higher ratings.

Question 3(b) [4 marks]

Draw the circuit diagram of three phases Half Wave Rectifier and explain its Working.

Answer: Three-phase half-wave rectifier converts three-phase AC into pulsating DC using three diodes.

Diagram:

Working:

• Each diode conducts when its phase voltage is most positive
• Conduction angle of each diode is 120°
• Ripple frequency is 3 times the input frequency
• Average output voltage = 3Vm/2π (where Vm is peak phase voltage)
• Ripple factor = 0.17 (much lower than single-phase half-wave)

Mnemonic: "THREE-D" - THREE Diodes conducting sequentially.

Question 3(c) [7 marks]

Describe the working of UPS & SMPS with the help of block diagram.

Answer:

UPS (Uninterruptible Power Supply):

Diagram:

SMPS (Switched Mode Power Supply):

Diagram:

• UPS Efficiency: 80-90%, provides backup power
• SMPS Efficiency: 70-90%, much smaller than linear supplies
• Regulation: Both provide regulated output voltage

Mnemonic: "BRIEF" - Battery backup, Rectification, Inversion, Efficient switching, Feedback control.

Question 3(a) OR [3 marks]

Explain the Principle & working of Chopper circuits.

Answer: Chopper is a DC-to-DC converter that converts fixed DC input voltage to variable DC output voltage.

Diagram:

Principle:

• Switch (typically SCR, MOSFET, or IGBT) rapidly connects and disconnects source to load

• Output voltage controlled by duty cycle (ON time / total time)

• Average output voltage = Input voltage × Duty cycle

• Time Ratio Control: Varies duty cycle, keeping frequency constant

• Frequency Modulation: Varies frequency, keeping ON time constant

• Applications: DC motor control, battery-powered vehicles

Mnemonic: "CHOP" - Control High-speed Operation with Pulses.

Question 3(b) OR [4 marks]

Compare single-phase and Poly-phase rectifier circuits.

Answer:

• Harmonic Content: Lower in poly-phase systems
• TUF (Transformer Utilization Factor): Higher in poly-phase systems
• Cost-Effectiveness: Poly-phase more economical for high power

Mnemonic: "PERIPHERY" - Poly-phase Efficiency Ripple Improvement Power Handling Economy Rating Yield.

Question 3(c) OR [7 marks]

Describe the working of solar Photovoltaic (PV) based power generation with the help of block diagram.

Answer: Solar PV power generation converts sunlight directly into electricity using semiconductor materials.

Diagram:

Working Principle:

• Photovoltaic Effect: Photons from sunlight knock electrons free in semiconductor

• Cell Structure: P-N junction creates electric field

• Voltage Generation: Typical cell produces 0.5-0.6V DC

• Array Configuration: Series-parallel connections for desired voltage/current

• Efficiency: Typically 15-22% for commercial panels

• Applications: Residential, commercial, industrial power generation

Mnemonic: "SOLAR" - Semiconductors Oriented Light-to-electricity Array Regulation.

Question 4(a) [3 marks]

List the advantages of static switch.

Answer:

• Reliability: No mechanical wear and tear
• Speed: Much faster than mechanical switches
• Isolation: Can provide electrical isolation

Mnemonic: "SAFE" - Speed, Arc-free, Fast response, Endurance.

Question 4(b) [4 marks]

Draw the circuit diagram of A.C. Power control using DIAC-TRIAC and Explain it.

Answer: DIAC-TRIAC circuit provides smooth AC power control for resistive and inductive loads.

Diagram:

Working:

• Variable resistor R2 controls charging rate of capacitor C

• When capacitor voltage reaches DIAC breakover voltage, DIAC conducts

• DIAC delivers trigger pulse to TRIAC gate

• TRIAC conducts for remainder of half-cycle

• Process repeats for both half-cycles

• Phase Control: Controls power by varying firing angle

• Applications: Light dimmers, heater controls, motor speed control

• Power Range: Can control from near-zero to full power

Mnemonic: "DIRECT" - DIAC Initiates Regulated Energy Control in TRIAC.

Question 4(c) [7 marks]

Describe function of DC power control circuit using SCR with UJT in triggering circuit.

Answer: UJT-triggered SCR circuit provides precise control of DC power to the load.

Diagram:

Working Principle:

• Precise Control: UJT provides stable, predictable triggering
• Applications: Battery chargers, DC motor speed control, temperature control
• Advantages: Low cost, high reliability, good temperature stability
• Control Range: Wide range from near-zero to full power

Mnemonic: "SCRUP" - SCR Using Pulse from UJT for Power control.

Question 4(a) OR [3 marks]

Enlist applications of dielectric heating.

Answer:

• Material Requirements: Works best with poor conductors that have polar molecules
• Frequency Range: Typically 10-100 MHz
• Advantages: Uniform heating, faster processing, energy efficiency

Mnemonic: "POWER" - Plastics, Organics, Wood, Edibles, and Rubber processing.

Question 4(b) OR [4 marks]

Draw and explain three stage IC555 timer circuit.

Answer: Three-stage IC555 timer circuit provides sequential timing operations.

Diagram:

Working:

• First timer activated by external trigger

• Output of first timer triggers second timer

• Output of second timer triggers third timer

• Each timer can be independently adjusted

• Applications: Industrial sequencing, process control, animation effects

• Timing Range: Microseconds to hours with proper component selection

• Features: Stable timing, immune to supply variations

• Advantages: Simple design, reliable operation, low cost

Mnemonic: "THREE-SET" - THREE Stage Electronic Timers in sequence.

Question 4(c) OR [7 marks]

Describe the working principle of Induction heating. And List merits-demerits of Induction heating.

Answer: Induction heating uses electromagnetic induction to heat electrically conductive materials.

Diagram:

Working Principle:

• High frequency AC in work coil creates alternating magnetic field
• Magnetic field induces eddy currents in workpiece
• Eddy currents generate heat due to material resistance
• Heating occurs within the workpiece, not from external source

• Frequency Range: 1 kHz to 1 MHz depending on application
• Applications: Heat treatment, melting, brazing, soldering

Mnemonic: "EDDY" - Electromagnetic Device Develops Yield of heat.

Question 5(a) [3 marks]

Draw & explain solid state circuit to control dc shunt motor speed.

Answer: Solid-state circuit for DC shunt motor speed control uses SCR to control armature voltage.

Diagram:

• Armature Voltage Control: SCR controls voltage to armature
• Field Winding: Connected directly to DC supply
• Speed Control: By varying SCR firing angle
• Advantages: Smooth control, high efficiency, compact size

Mnemonic: "SAFE" - SCR Armature Firing for Efficient control.

Question 5(b) [4 marks]

Explain working principle of stepper motor.

Answer: Stepper motor converts electrical pulses into discrete mechanical movements.

Diagram:

Working Principle:

• Energizing stator windings in sequence creates rotating magnetic field
• Permanent magnet rotor aligns with magnetic field
• Each pulse creates rotation by exact "step" angle
• Step angle determined by motor construction (typically 1.8° or 0.9°)

• Precise Positioning: Movement in exact increment steps
• Open-Loop Control: No feedback needed for position control
• Holding Torque: Maintains position when energized

Mnemonic: "STEP" - Sequential Triggering Enables Precise positioning.

Question 5(c) [7 marks]

Draw the block diagram of PLC and explain the function of each block.

Answer: Programmable Logic Controller (PLC) is a digital computer used for automation of industrial processes.

Diagram:

• Scan Cycle: Input scanning → Program execution → Output updating
• Advantages: Reliability, flexibility, modular design, easy troubleshooting
• Applications: Manufacturing automation, process control, material handling
• Programming: Ladder logic, function block diagram, structured text

Mnemonic: "PILOT" - Processing Inputs and Logic for Outputs with Timing control.

Question 5(a) OR [3 marks]

Draw and explain the construction of DC servo motor.

Answer: DC servo motor is designed for precise position and speed control.

Diagram:

Components:

• Armature: Low inertia for quick response

• Field System: Provides magnetic field (permanent magnets in modern motors)

• Commutator & Brushes: Electrical connection to rotating armature

• Feedback Device: Position sensor (encoder/resolver/tachometer)

• Housing: Contains bearings and mounting provisions

• High Torque-to-Inertia Ratio: Allows quick starts and stops

• Linear Torque-Speed Characteristics: Enables precise control

• Low Electrical Time Constant: Fast response to control signals

Mnemonic: "SAFE" - Sensitive Armature with Feedback for Exactness.

Question 5(b) OR [4 marks]

Draw and explain the circuit to control speed of a DC series motor.

Answer: DC series motor speed control circuit using SCR.

Diagram:

Working:

• Bridge rectifier converts AC to DC

• SCR controls average voltage to motor

• Firing angle controlled by potentiometer

• Series field and armature current is the same

• Speed varies inversely with voltage at low loads

• Armature Voltage Control: Primary method for speed control

• Torque Characteristics: High starting torque maintained

• Speed Range: Typically 3:1 for stable operation

Mnemonic: "SCRAM" - SCR Controls Rectified Armature and Motor speed.

Question 5(c) OR [7 marks]

Explain construction, working of Stepper motor Give and its applications

Answer: Stepper motor is an electromechanical device that converts electrical pulses into discrete mechanical movements.

Construction:

Diagram:

Working Principle:

• Digital pulses energize stator windings in sequence
• Magnetic field rotates in steps around stator
• Rotor follows magnetic field in precise angular steps
• Direction controlled by sequence of energization
• Speed controlled by pulse frequency

Types of Stepper Motors:

Applications:

• CNC machines and 3D printers
• Robotics and automation
• Camera lens focusing mechanisms
• Precision positioning systems
• Medical equipment
• Office equipment (printers, scanners)
• Automotive applications (headlight positioning)
• Small consumer devices

Mnemonic: "REACT" - Rotation Exactly At Controlled Timing.