Industrial Electronics (4331103) - Summer 2025 Solution

Question 1(a) [3 marks]

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Draw characteristics of Opto-Isolators, Opto-TRIAC and Opto-transistor.

Answer:

Characteristics of Opto-Electronic Devices:

Opto-Isolator	Opto-TRIAC	Opto-Transistor
Linear relationship between LED current and photodetector current	Non-linear triggering response with threshold	Linear current transfer characteristic
CTR (Current Transfer Ratio) is key parameter	Triggering occurs at specific current threshold	Collector current depends on base illumination

• CTR (Current Transfer Ratio): Ratio of output current to input current
• Trigger Current: Minimum current needed to activate the device
• Linearity: How proportional the output is to the input light

Mnemonic: “LTL - Light Transfers Like current flows – Linear for isolators/transistors, Triggered for TRIACs”

Question 1(b) [4 marks]

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Describe working & constructional features of IGBT.

Answer:

IGBT Structure and Operation:

graph TD
    A[Gate] --> B[Metal Oxide]
    B --> C[P+ Body]
    C --> D[N- Drift Region]
    D --> E[P+ Collector/Substrate]
    F[Emitter] --> C
    E --> G[Collector]
    style A fill:#91a6ff
    style B fill:#ffeead
    style C fill:#ff9e9e
    style D fill:#d9ffb3
    style E fill:#ff9e9e
    style F fill:#91a6ff
    style G fill:#91a6ff

• Voltage Controlled: Device is controlled by gate voltage like MOSFET
• Conductivity Modulation: P+ collector injects holes into drift region
• Low On-State Voltage: Conduction losses lower than MOSFET

Mnemonic: “IGBT MBC” - “Input from MOS, Body handles current, Collector acts like BJT”

Question 1(c) [7 marks]

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Explain working of SCR using two-transistor analogy.

Answer:

SCR as Two-Transistor Model:

graph TD
    A[Anode] --> B[P1]
    B --> C[N1]
    C --> D[P2]
    D --> E[N2]
    E --> F[Cathode]
    G[Gate] --> D
    
    subgraph PNP Transistor
    B
    C
    D
    end
    
    subgraph NPN Transistor
    C
    D
    E
    end
    
    style A fill:#91a6ff
    style B fill:#ff9e9e
    style C fill:#d9ffb3
    style D fill:#ff9e9e
    style E fill:#d9ffb3
    style F fill:#91a6ff
    style G fill:#91a6ff

Two-Transistor Explanation:

• Latching Mechanism: Once triggered, transistors keep each other ON
• Triggering: Small gate current → T2 turns ON → T1 gets base current → Both remain ON
• Holding Current: Minimum current needed to maintain regenerative action
• Turn-OFF: Anode current must fall below holding current

Mnemonic: “PPFF” - “Positive feedback Perpetuates Forward conduction”

Question 1(c) OR [7 marks]

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Explain the working of Solid state relay using Opto-SCR.

Answer:

Solid State Relay with Opto-SCR:

graph LR
    A[AC/DC Input] --> B[LED]
    B --> C[Photo-SCR/Detector]
    C --> D[Main SCR/TRIAC]
    D --> E[Output Load]
    F[Zero Crossing Circuit] --> D
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffee99
    style D fill:#ccffcc
    style E fill:#dddddd
    style F fill:#e6ccff

Working Principle and Components:

• Electrical Isolation: Complete separation between control and power circuits (>1000V)
• Zero-Crossing: Switching only at zero voltage reduces EMI/RFI noise
• Silent Operation: No mechanical clicks unlike traditional relays
• Long Life: No mechanical wear as in conventional relays

Mnemonic: “LIPO” - “Light In, Power Out - isolation guaranteed”

Question 2(a) [3 marks]

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Explain the working of snubber circuit for SCR.

Answer:

Snubber Circuit for SCR:

• dv/dt Protection: Prevents false triggering due to rapid voltage rise
• Turn-OFF Support: Helps in commutation by providing alternate path
• Energy Absorption: Absorbs energy from inductive loads during switching

Mnemonic: “CARD” - “Capacitor And Resistor Damp unwanted triggering”

Question 2(b) [4 marks]

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Write the differences between forced commutation and natural commutation.

Answer:

Comparison of Commutation Methods:

• Timing Control: Forced commutation offers better timing control
• Circuit Size: Natural commutation results in smaller circuit size
• Reliability: Natural commutation has fewer components to fail

Mnemonic: “DANCE” - “DC needs Active commutation, Natural for AC, Costs Extra for forced”

Question 2(c) [7 marks]

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

Answer:

UPS Block Diagram and Operation:

graph LR
    A[AC Input] --> B[Rectifier/Charger]
    B --> C[Battery Bank]
    C --> D[Inverter]
    B --> D
    D --> E[Output Filter]
    E --> F[AC Output]
    G[Control Circuit] --> B
    G --> D
    H[Bypass Switch] --> F
    A --> H
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffffb3
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#b3e0ff
    style G fill:#ffee99
    style H fill:#ffddbb

UPS Operation Modes:

• Online UPS: Power always flows through rectifier/inverter (double conversion)
• Offline UPS: Power flows directly to load, switches to battery when power fails
• Line-Interactive: Similar to offline but with voltage regulation
• Backup Time: Depends on battery capacity and load requirements

Mnemonic: “BRIC” - “Battery Ready when Input Cuts off”

Question 2(a) OR [3 marks]

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Explain pulse gate triggering method of SCR.

Answer:

Pulse Gate Triggering Method:

• Precise Control: Exact timing of SCR turn-on
• Noise Immunity: Less susceptible to false triggering
• Power Efficiency: Low average gate power consumption
• Isolation: Can be coupled through pulse transformer or opto-isolator

Mnemonic: “TRAP” - “Timed, Reliable, Amplitude-controlled Pulses”

Question 2(b) OR [4 marks]

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List the commutation methods of SCR and explain any one in detail.

Answer:

Commutation Methods of SCR:

Detailed Explanation of Class E (Capacitor Commutation):

graph TD
    A[DC Source] --> B[SCR1]
    B --> C[Load]
    C --> D[Ground]
    A --> E[Commutating Capacitor]
    E --> F[Auxiliary SCR2]
    F --> D
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffffb3
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#ffcccc

• Working Principle: When SCR1 is ON and carrying load current, firing SCR2 connects pre-charged capacitor across SCR1, reverse biasing it
• Turn-OFF Time: Determined by capacitor value and circuit resistance
• Applications: DC choppers, power control circuits, inverters
• Advantages: Simple circuit, reliable operation, cost-effective

Mnemonic: “CARE” - “Capacitor Applies Reverse voltage for Extinction”

Question 2(c) OR [7 marks]

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

Answer:

SMPS Block Diagram and Operation:

graph LR
    A[AC Input] --> B[EMI Filter]
    B --> C[Rectifier/PFC]
    C --> D[High Frequency Inverter]
    D --> E[HF Transformer]
    E --> F[Rectifier/Filter]
    F --> G[Output DC]
    H[Feedback Control] --> D
    F --> H
    style A fill:#b3e0ff
    style B fill:#ffddbb
    style C fill:#ffcccc
    style D fill:#ccffcc
    style E fill:#ffffb3
    style F fill:#e6ccff
    style G fill:#b3e0ff
    style H fill:#ffee99

SMPS Working Principles:

• High Efficiency: 70-95% efficient compared to 50-60% for linear power supplies
• Size Reduction: High-frequency operation allows smaller transformers
• Regulation: Feedback loop maintains stable output despite input/load changes
• Protection: Built-in overcurrent, overvoltage, and thermal protection

Mnemonic: “RELIEF” - “Rectify, Energize at high frequency, Isolate, Extract DC, Feedback”

Question 3(a) [3 marks]

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State the method to protect SCR against over voltage.

Answer:

SCR Overvoltage Protection Methods:

• Voltage Rating: Always use SCR with voltage rating 2-3 times normal operating voltage
• Rate-of-Rise: Protect against fast transients with snubber circuits (dv/dt protection)
• Breakdown Voltage: Never exceed reverse breakdown voltage of SCR junction
• Coordinated Protection: Use multiple methods for critical applications

Mnemonic: “SCRAM” - “Snubber Circuits Reduce Abnormal Maximum voltages”

Question 3(b) [4 marks]

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State any four advantages of polyphase rectifiers over single-phase rectifiers.

Answer:

Advantages of Polyphase Rectifiers:

• Form Factor: Lower form factor means better DC quality
• Ripple Frequency: Higher ripple frequency is easier to filter
• Balanced Load: Polyphase draws balanced current from supply
• Size Reduction: Smaller filter components needed

Mnemonic: “HERBS” - “Higher efficiency, Even load, Reduced ripple, Better PF, Smaller filters”

Question 3(c) [7 marks]

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

Answer:

Solar PV Power Generation System:

graph LR
    A[Solar PV Array] --> B[Charge Controller]
    B --> C[Battery Bank]
    C --> D[Inverter]
    D --> E[AC Loads]
    D --> F[Grid Connection]
    B --> G[DC Loads]
    H[Maximum Power Point Tracker] --> B
    A --> H
    style A fill:#ffffb3
    style B fill:#ffcccc
    style C fill:#b3e0ff
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#ffddbb
    style G fill:#e6ccff
    style H fill:#ffee99

System Components and Functions:

• Grid-Tied Systems: Connected to utility grid, can sell excess power
• Off-Grid Systems: Standalone systems with battery storage
• Hybrid Systems: Can operate in both modes with battery backup
• Efficiency: Typical system efficiency 15-20% from sunlight to usable electricity

Mnemonic: “SIMPLE” - “Sun In, Maximum Power, Local Energy”

Question 3(a) OR [3 marks]

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State the method to protect SCR against over current.

Answer:

SCR Overcurrent Protection Methods:

• Current Rating: Always use SCR with current rating above maximum operating current
• di/dt Protection: Limit rate of current rise to prevent junction damage
• Thermal Management: Proper heatsinking to prevent thermal runaway
• Coordination: Protection device must act before SCR is damaged

Mnemonic: “FIRE” - “Fuses Immediately Restrict Excessive current”

Question 3(b) OR [4 marks]

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Explain basic principle of DC chopper.

Answer:

DC Chopper Basic Principle:

graph LR
    A[DC Input] --> B[Switching Device]
    B --> C[Filter]
    C --> D[DC Output]
    E[Control Circuit] --> B
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffffb3
    style D fill:#ccffcc
    style E fill:#ffee99

• Basic Equation: Vout = Vin × Duty Cycle (for step-down chopper)
• Operating Principle: Rapid switching controls average voltage
• Advantages: High efficiency, precise control, compact size
• Applications: DC motor drives, battery charging, DC voltage regulation

Mnemonic: “DISC” - “Duty cycle Influences Switching to Control output”

Question 3(c) OR [7 marks]

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Draw the circuit diagram of 3-Φ Full Wave rectifier using diode and explain it’s working.

Answer:

3-Phase Full Wave Diode Rectifier (Bridge Configuration):

Working Principles:

• Ripple Frequency: 6 times the input frequency (300/360Hz for 50/60Hz input)
• Ripple Factor: Approximately 4.2% (much lower than single-phase)
• Average Output Voltage: Vdc = 1.35 × Vrms (line voltage)
• Conduction Angle: Each diode conducts for 120° of cycle

Mnemonic: “PRESTO” - “Pairs of diodes Rectify Efficiently, Six Times per cycle Output”

Question 4(a) [3 marks]

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Write the applications of Induction heating.

Answer:

Applications of Induction Heating:

• Industrial Advantages: Fast heating, energy efficiency, clean process
• Control Benefits: Precise temperature control, repeatable results
• Environmental Impact: Reduced emissions compared to fossil fuel heating
• Metallurgical Quality: Improved material properties in many applications

Mnemonic: “HAMMER” - “Hardening, Annealing, Melting, Medical, Eddy-current cooking, Reshaping metals”

Question 4(b) [4 marks]

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Draw and explain the circuit of controlling AC load using TRIAC and DIAC.

Answer:

AC Load Control with TRIAC and DIAC:

Circuit Operation:

• Phase Control: Adjusting R1 changes phase angle at which DIAC triggers
• Power Control: Varying firing angle controls average power to load
• Bi-directional Control: Works on both half-cycles of AC input
• Applications: Light dimmers, fan speed control, heater control

Mnemonic: “CRAFT” - “Capacitor and Resistor Adjust Firing Time”

Question 4(c) [7 marks]

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Explain Spot Welding with Working and Applications.

Answer:

Spot Welding Process and Applications:

graph TD
    A[Step 1: Material Positioning] --> B[Step 2: Electrode Contact]
    B --> C[Step 3: Current Flow]
    C --> D[Step 4: Heat Generation]
    D --> E[Step 5: Weld Formation]
    E --> F[Step 6: Cooling]
    style A fill:#ffffb3
    style B fill:#ffcccc
    style C fill:#b3e0ff
    style D fill:#e6ccff
    style E fill:#ccffcc
    style F fill:#ffddbb

Spot Welding Working Principle:

Applications of Spot Welding:

• Automotive: Car body assembly, sheet metal joining
• Electronics: Battery tabs, small component assembly
• Appliances: Refrigerators, washing machines, dishwashers
• Aerospace: Aircraft panel assembly, lightweight structures
• Medical: Surgical instruments, implantable devices
• Consumer Products: Metal furniture, containers, toys

Mnemonic: “PCAFRI” - “Position, Compress, Apply current, Form nugget, Release after cooling, Inspect”

Question 4(a) OR [3 marks]

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Write the applications of Dielectric heating.

Answer:

Applications of Dielectric Heating:

• Process Benefits: Volumetric heating (heats throughout, not just surface)
• Speed Advantage: Significantly faster than conventional heating
• Quality Improvement: More uniform heating, better product quality
• Energy Efficiency: Direct energy transfer to material

Mnemonic: “FITPP” - “Food, Insulation drying, Textiles, Plastics, Pharmaceutical products”

Question 4(b) OR [4 marks]

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Write short note on SCR Delay timer.

Answer:

SCR Delay Timer:

graph LR
    A[Trigger Input] --> B[RC Timing Circuit]
    B --> C[SCR]
    C --> D[Relay/Output Device]
    E[Power Supply] --> B
    E --> C
    E --> D
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffffb3
    style D fill:#ccffcc
    style E fill:#e6ccff

• Timing Principle: RC charging time determines delay period
• Accuracy: Typically ±5-10% of set time
• Applications: Industrial process control, sequence control, protection circuits
• Advantages: Simple design, reliable operation, cost-effective

Mnemonic: “TIME” - “Timing Is Managed by Electronics”

Question 4(c) OR [7 marks]

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Explain the working of SCR as static switch. Write the advantages of static switch.

Answer:

SCR as Static Switch:

Working Principles:

• DC Operation: Requires commutation circuit for turn-off
• AC Operation: Natural turn-off at zero crossing
• Control Methods: Direct gate drive, pulse triggering, opto-isolation
• Protection: Requires snubber circuits, current limiting

Advantages of Static Switches:

• Reliability: Higher MTBF (Mean Time Between Failures)
• Compatibility: Works with electronic control systems
• Voltage Isolation: Can incorporate opto-isolation
• Surge Handling: Better transient protection with proper design

Mnemonic: “FANS” - “Fast switching, Arc-free operation, No moving parts, Silent operation”

Question 5(a) [3 marks]

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What is DC Drive? Give Classification of DC Drives.

Answer:

DC Drive Definition and Classification:

Classification of DC Drives:

• Power Range: From fractional HP to several thousand HP
• Control Precision: From basic to high-precision (0.01%)
• Response Time: From milliseconds to microseconds
• Protection: Various built-in protection features

Mnemonic: “PQCAS” - “Power rating, Quadrants, Control type, AC input phases, Switching method”

Question 5(b) [4 marks]

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Draw and explain the construction of variable reluctance type Stepper motor.

Answer:

Variable Reluctance Stepper Motor Construction:

• Tooth Configuration: Typically rotor teeth fewer than stator teeth
• Step Angle: Determined by: Step angle = 360° ÷ (Number of rotor teeth × Number of phases)
• Construction Simplicity: No permanent magnets or windings on rotor
• Operating Principle: Magnetic reluctance path seeks to minimize when phases energized

Mnemonic: “STAR” - “Stator energizes, Teeth Align with minimum Reluctance”

Question 5(c) [7 marks]

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Explain the working of VFD (Variable Frequency Drive).

Answer:

Variable Frequency Drive (VFD) Working:

graph LR
    A[AC Input] --> B[Rectifier]
    B --> C[DC Bus/Filter]
    C --> D[Inverter]
    D --> E[AC Motor]
    F[Control System] --> B
    F --> D
    G[Operator Interface] --> F
    H[Feedback Sensors] --> F
    style A fill:#b3e0ff
    style B fill:#ffcccc
    style C fill:#ffffb3
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#ffee99
    style G fill:#ffddbb
    style H fill:#d9ffb3

VFD Components and Functions:

Working Principles:

• Speed Control Equation: Motor Speed (RPM) = (Frequency × 120) ÷ Number of poles
• Torque Control: Maintaining V/F ratio controls torque output
• Soft Start: Gradual frequency/voltage ramp-up reduces inrush current
• Braking Methods: Regenerative, dynamic, or DC injection braking
• Energy Savings: Significant energy savings at reduced speeds
• Advanced Features: PID control, network communication, programmable functions

Mnemonic: “DRIVE” - “DC conversion, Regulation, Inverter creates, Variable frequency, Efficient motor control”

Question 5(a) OR [3 marks]

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What are Hall effect sensors and what is their role in DC motors?

Answer:

Hall Effect Sensors in DC Motors:

Role in DC Motors:

• BLDC Motors: Critical for electronic commutation (replacing mechanical commutator)
• Precision: Higher accuracy than mechanical sensors
• Reliability: No mechanical wear, longer service life
• Integration: Can be integrated with drive electronics

Mnemonic: “MAPS” - “Measures position, Aids commutation, Provides speed data, Senses magnetic fields”

Question 5(b) OR [4 marks]

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Explain working principle of stepper motor.

Answer:

Stepper Motor Working Principle:

graph TD
    A[Step 1: Energize Phase A] --> B[Rotor aligns with Phase A]
    B --> C[Step 2: Energize Phase B]
    C --> D[Rotor aligns with Phase B]
    D --> E[Step 3: Energize Phase C]
    E --> F[Rotor aligns with Phase C]
    F --> G[Step 4: Energize Phase D]
    G --> H[Rotor aligns with Phase D]
    H --> A
    style A fill:#ffffb3
    style B fill:#ffcccc
    style C fill:#b3e0ff
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#ffddbb
    style G fill:#d9ffb3
    style H fill:#ffee99

• Position Control: Precise angular positioning without feedback
• Step Angle: Common step angles are 1.8° (200 steps/rev) or 0.9° (400 steps/rev)
• Holding Torque: Maintains position when phases energized at standstill
• Open-Loop Control: No position feedback normally required
• Speed-Torque: Torque decreases as speed increases

Mnemonic: “STEPS” - “Sequential Triggering of Electromagnetic Phases causes Stepping”

Question 5(c) OR [7 marks]

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

Answer:

PLC Block Diagram and Functions:

graph TD
    A[Power Supply] --> B[CPU/Processor]
    C[Input Interface] --> B
    B --> D[Output Interface]
    B --> E[Memory]
    F[Programming Device] --> B
    G[Communication Interface] --> B
    style A fill:#ffffb3
    style B fill:#ffcccc
    style C fill:#b3e0ff
    style D fill:#ccffcc
    style E fill:#e6ccff
    style F fill:#ffddbb
    style G fill:#d9ffb3

Functions of Each Block:

• Scan Cycle: Sequential process of reading inputs, executing program, updating outputs
• Programming Languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), Sequential Function Chart (SFC)
• Modularity: Expandable with additional I/O modules
• Robustness: Designed for harsh industrial environments
• Reliability: Typically MTBF >100,000 hours

Mnemonic: “PICO MPC” - “Power, Inputs, CPU, Outputs, Memory, Programming interface, Communication”