Industrial Electronics (4331103) - Summer 2024 Solution

Question 1(a) [3 marks]

#

Explain two transistor analogies of SCR.

Answer: SCR can be represented as a two-transistor model with interconnected PNP and NPN transistors.

Diagram:

graph TD
    A[Anode] --- B1[PNP Base]
    B1 --- C1[PNP Collector]
    C1 --- E2[NPN Emitter]
    E2 --- B2[NPN Base]
    B2 --- C2[NPN Collector]
    C2 --- K[Cathode]
    G[Gate] --- B2
    E1[PNP Emitter] --- A
    E1 --- B2
    C2 --- B1

• Regenerative action: When gate current triggers NPN, it causes PNP to conduct, creating self-sustaining current
• Latching mechanism: Once both transistors are ON, gate loses control as feedback path maintains conduction

Mnemonic: “Push-Pull Network Triggers Sustained Conduction”

Question 1(b) [4 marks]

#

Explain working and characteristic of IGBT.

Answer: IGBT (Insulated Gate Bipolar Transistor) combines MOSFET input characteristics with BJT output capabilities.

Diagram:

graph TD
    G[Gate] --- MOS[MOSFET Section]
    MOS --- BJT[BJT Section]
    BJT --- C[Collector]
    E[Emitter] --- BJT

Characteristics Table:

• Input advantage: Voltage-controlled gate with high impedance requires minimal drive power
• Output advantage: Low on-state voltage drop even at high current densities

Mnemonic: “MOSFET Input, BJT Output, Makes Perfect Power Switch”

Question 1(c) [7 marks]

#

Explain construction, working and characteristic of DIAC.

Answer: DIAC (DIode for Alternating Current) is a bidirectional triggering device used in thyristor control circuits.

Diagram:

graph LR
    A[Terminal A] --- P1[P-region]
    P1 --- N1[N-region]
    N1 --- P2[P-region]
    P2 --- N2[N-region]
    N2 --- B[Terminal B]

Characteristics Curve:

Construction & Operation Table:

• Blocking state: Below breakover voltage, high resistance prevents current flow
• Conducting state: Above breakover voltage, negative resistance region enables sudden conduction
• Bidirectional: Functions identically for positive and negative voltages

Mnemonic: “Break Voltage Both Ways, Then Current Flows”

Question 1(c) OR [7 marks]

#

Explain construction and working of Opto-Isolator and Opto-SCR

Answer: Opto-devices use light to transfer signals while maintaining electrical isolation between circuits.

Opto-Isolator Diagram:

graph LR
    A[Input] --- L[LED]
    L --- G[Glass/Plastic]
    G --- D[Phototransistor]
    D --- O[Output]

Opto-SCR Diagram:

graph LR
    A[Input] --- L[LED]
    L --- G[Glass/Plastic]
    G --- S[Light-sensitive SCR]
    S --- O[Output]

Comparison Table:

• Electrical isolation: Complete electrical separation provides noise immunity and safety
• Signal transfer: Light coupling eliminates ground loops and voltage level issues
• Triggering: Light replaces gate current for SCR activation in Opto-SCR

Mnemonic: “Light Jumps Gaps While Electricity Stays Home”

Question 2(a) [3 marks]

#

Draw symbol and give application of 1) UJT 2) SCS 3) MCT.

Answer:

UJT (Unijunction Transistor):

SCS (Silicon Controlled Switch):

MCT (MOS-Controlled Thyristor):

Applications Table:

Mnemonic: “Unique timing, Controlled switching, Master power”

Question 2(b) [4 marks]

#

Explain importance of gate protection for SCR.

Answer: Gate protection circuits safeguard SCR against spurious triggering and voltage spikes.

Gate Protection Circuit:

Protection Table:

• Junction protection: Prevents reverse voltage damage to gate-cathode junction
• Noise immunity: Filters out electrical noise that could cause unwanted triggering

Mnemonic: “Guard the Gate to Prevent Problems”

Question 2(c) [7 marks]

#

List out various methods of triggering SCR and explain any three of them.

Answer: SCR triggering methods convert the device from blocking to conducting state through gate activation.

Triggering Methods Table:

1. Gate Current Triggering:

• Direct control: Small gate current initiates large anode current flow
• Current range: 10-100mA typically required depending on SCR rating

2. Light Triggering (LASCR):

• Optical control: Photons generate carriers at junction
• Isolation: Provides electrical isolation between control and power circuit

3. dV/dt Triggering:

• Rate sensitivity: Rapid voltage rise causes junction capacitance charging
• Prevention: Snubber circuits (RC networks) control voltage rise rate

Mnemonic: “Gates, Light, and Voltage Changes Turn SCRs On”

Question 2(a) OR [3 marks]

#

Explain working of solid state relay using opto-SCR.

Answer: Solid state relays (SSRs) use opto-SCR for contactless switching with electrical isolation.

SSR Block Diagram:

graph LR
    I[Control Input] --> LED[LED]
    LED --> OSCR[Opto-SCR]
    OSCR --> ZC[Zero Crossing Circuit]
    ZC --> TS[Thyristor Switch]
    TS --> O[Output Load]

Operation Table:

• Silent operation: No mechanical noise during switching
• Long life: No contact degradation as in electromechanical relays

Mnemonic: “Light Links Logic to Load”

Question 2(b) OR [4 marks]

#

Define snubber circuit and explain importance of snubber circuit.

Answer: A snubber circuit is a protective network that suppresses voltage and current transients in switching devices.

Basic RC Snubber:

Importance Table:

• Circuit protection: Extends thyristor life by limiting stress on the device
• Noise reduction: Minimizes electromagnetic interference in surrounding circuits

Mnemonic: “Suppress Noise Upsetting Balanced Behaviors Easily Restored”

Question 2(c) OR [7 marks]

#

List various commutation methods of SCR and explain any two of them

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

Commutation Methods Table:

1. Natural Commutation:

• Zero crossing: SCR turns off when AC crosses zero and anode current falls below holding
• Simplicity: No additional components required for commutation
• Limitation: Works only in AC circuits at fixed frequency

2. Forced Commutation (Class B):

• Auxiliary SCR: Second SCR (SCR2) discharges capacitor to reverse bias main SCR
• Timing control: Precise control over when SCR turns off
• Application: Used in DC circuits where natural commutation isn’t possible

Mnemonic: “Nature Follows Current, Forced Creates Current Collapse”

Question 3(a) [3 marks]

#

Explain advantages of polyphase rectifier over single phase rectifier.

Answer: Polyphase rectifiers offer significant improvements over single-phase designs in power applications.

Advantages Table:

• Output smoothness: Significantly less ripple requiring smaller filtering components
• Transformer utilization: Better utilization factor (0.955 vs 0.812) reduces transformer size

Mnemonic: “More Phases Mean Smoother Power”

Question 3(b) [4 marks]

#

Write short note on UPS.

Answer: UPS (Uninterruptible Power Supply) provides continuous power during main supply failure.

UPS Block Diagram:

graph LR
    A[AC Input] --> R[Rectifier]
    R --> C[DC Bus]
    C --> I[Inverter]
    I --> O[AC Output]
    C <--> B[Battery Bank]
    S[Static Switch] --> O
    A --> S

Types of UPS Table:

• Backup time: Typically 5-30 minutes depending on battery capacity
• Protection: Surge protection, voltage regulation, and frequency stabilization

Mnemonic: “Power Constantly Protected Under Switch”

Question 3(c) [7 marks]

#

Give function of Inverter and explain basic principle of Inverter also explain series inverter with neat diagram and waveform.

Answer: Inverters convert DC power to AC power by switching DC through a transformer or directly to create alternating waveforms.

Function Table:

Basic Principle Diagram:

graph LR
    D[DC Source] --> S[Switching Circuit]
    S --> T[Transformer/Filter]
    T --> A[AC Output]
    C[Control Circuit] --> S

Series Inverter Circuit:

Waveforms:

• Oscillation: Series LC circuit creates resonant oscillation when SCR triggers
• Commutation: SCR turns off naturally when current reverses through resonance
• Frequency: Determined by LC values: f = 1/(2π√LC)

Mnemonic: “Direct Current Switches To Alternating Current Through Resonant Circuit”

Question 3(a) OR [3 marks]

#

Explain basic principle of chopper.

Answer: A chopper is a DC-to-DC converter that switches DC input on/off to produce controllable average DC output.

Basic Chopper Circuit:

Principle Table:

• Switching action: Rapidly turns ON/OFF to chop DC input
• Pulse width modulation: Controls voltage by varying ON-time ratio

Mnemonic: “Chopping Creates Controllable DC”

Question 3(b) OR [4 marks]

#

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

Answer: SMPS (Switched Mode Power Supply) converts input power to regulated output using high-frequency switching.

SMPS Block Diagram:

graph LR
    A[AC Input] --> F[EMI Filter]
    F --> R[Rectifier & Filter]
    R --> S[Switching Circuit]
    S --> T[Transformer]
    T --> O[Output Rectifier]
    O --> OF[Output Filter]
    OF --> OUT[DC Output]
    FB[Feedback Control] --> S
    OUT --> FB

Blocks Function Table:

• High efficiency: 70-90% vs 30-60% for linear supplies
• Small size: High frequency allows smaller transformer and components

Mnemonic: “Filter, Rectify, Switch Through Transformer, Rectify, Filter”

Question 3(c) OR [7 marks]

#

Explain 1 phase half wave rectifier with waveform also explain 3 phase full wave rectifier with waveform.

Answer: Rectifiers convert AC to DC by allowing current flow in one direction while blocking reverse flow.

1-Phase Half Wave Rectifier:

1-Phase Half Wave Waveforms:

3-Phase Full Wave Rectifier:

3-Phase Full Wave Waveforms:

Comparison Table:

• 1-Phase Half Wave: Simplest design but with high ripple and poor efficiency
• 3-Phase Full Wave: Much smoother output with 6 pulses per cycle

Mnemonic: “Half Passes Only Peaks, Three Phases Fill Valleys”

Question 4(a) [3 marks]

#

Describe working of solar photovoltaic based power generation with block diagram.

Answer: Solar PV power generation converts sunlight directly into electricity through photovoltaic effect.

Solar PV System Block Diagram:

graph LR
    S[Solar Panel Array] --> C[Charge Controller]
    C --> B[Battery Bank]
    B --> I[Inverter]
    I --> L[AC Loads]
    C --> D[DC Loads]

Component Table:

• Energy conversion: Photons excite electrons in semiconductor material creating current
• Scalability: System size can be adjusted based on power requirements

Mnemonic: “Sunlight Produces Voltage, Batteries Invert Loads”

Question 4(b) [4 marks]

#

Explain use of SCR as static switch.

Answer: SCR functions as a solid-state switch with no moving parts for reliable and fast switching.

SCR Static Switch Circuit:

Applications Table:

• Latching action: Once triggered, continues to conduct until current falls below holding value
• High reliability: No mechanical wear due to absence of moving parts

Mnemonic: “Semiconductor Switching Controls Running Loads”

Question 4(c) [7 marks]

#

Describe the working principle of Induction heating and dielectric heating also give comparison of Induction heating and dielectric heating.

Answer: Both heating methods use electromagnetic principles to generate heat without direct contact.

Induction Heating Diagram:

graph TD
    A[AC Power] --> B[High Frequency Generator]
    B --> C[Work Coil]
    C --> D[Magnetic Field]
    D --> E[Eddy Currents in Workpiece]
    E --> F[Heat Generation]

Dielectric Heating Diagram:

graph TD
    A[RF Generator] --> B[Applicator Plates]
    B --> C[Electric Field]
    C --> D[Molecular Friction in Material]
    D --> E[Heat Generation]

Comparison Table:

• Induction heating: Works through electromagnetic induction creating eddy currents in conductive materials
• Dielectric heating: Causes rapid oscillation of polar molecules creating internal friction and heat

Mnemonic: “Induction Makes Metals Hot, Dielectrics Heat Non-Metals”

Question 4(a) OR [3 marks]

#

Draw and explain the circuit diagram of photo electric relay using photo diode.

Answer: Photo-electric relay uses light detection to control switching operations automatically.

Circuit Diagram:

Operation Table:

• Light detection: Photodiode conducts when illuminated, changing bias on transistor
• Switching: Transistor amplifies small photodiode current to drive relay coil

Mnemonic: “Light Drives Diode, Diode Drives Transistor, Transistor Drives Relay”

Question 4(b) OR [4 marks]

#

Draw the circuit diagram of AC power control using DIAC-TRIAC and explain it.

Answer: DIAC-TRIAC circuit enables smooth control of AC power through phase angle adjustment.

Circuit Diagram:

Operation Table:

• Phase control: RC network creates delay in triggering point within AC cycle
• Bidirectional operation: Works on both halves of AC cycle

Mnemonic: “Delay Initiates At Capacitor, Triggers Reliable Independent AC Control”

Question 4(c) OR [7 marks]

#

Explain IC555 three stage sequential timer working with waveform.

Answer: A three-stage sequential timer uses multiple 555 ICs to generate timed sequences for process control.

Circuit Diagram:

graph LR
    T[Trigger] --> IC1[555 Timer 1]
    IC1 --> O1[Output 1]
    IC1 --> D1[Delay]
    D1 --> IC2[555 Timer 2]
    IC2 --> O2[Output 2]
    IC2 --> D2[Delay]
    D2 --> IC3[555 Timer 3]
    IC3 --> O3[Output 3]
    IC3 --> R[Reset]
    R -.-> IC1

Waveform:

Sequential Operation Table:

• Cascading connection: Output of first timer triggers second, and so on
• Timing control: Each stage duration independently adjustable with RC values
• Applications: Industrial sequencing, process control, automated systems

Mnemonic: “First Stage Finishes, Second Starts, Third Succeeds”

Question 5(a) [3 marks]

#

Draw and explain solid state control of DC shunt motor.

Answer: Solid-state DC motor control uses SCRs to regulate voltage applied to the motor.

Circuit Diagram:

Control Method Table:

• Speed regulation: Controls armature voltage to vary motor speed
• Torque control: Maintains high starting torque with current limiting

Mnemonic: “SCR Controls Current Delivering Motor Power”

Question 5(b) [4 marks]

#

Explain working principle of stepper motor.

Answer: Stepper motors convert digital pulses into precise mechanical rotation through electromagnetic principles.

Stepper Motor Structure:

graph TD
    C[Controller] --> D[Driver]
    D --> P[Phase Windings]
    P --> R[Rotor Movement]

Operation Principle Table:

• Digital positioning: Each pulse rotates motor by precise angle
• Holding torque: Maintains position when energized without rotation

Mnemonic: “Pulses Produce Precise Positional Steps”

Question 5(c) [7 marks]

#

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

Answer: Programmable Logic Controller (PLC) is an industrial digital computer for automation control.

PLC Block Diagram:

graph TD
    P[Power Supply] --> CPU[Central Processing Unit]
    CPU --> M[Memory]
    CPU --> I[Input Module]
    CPU --> O[Output Module]
    I --> S[Input Sensors/Switches]
    O --> A[Actuators/Motors]
    CPU --> C[Communication Module]
    CPU --> P[Programming Device]

PLC Components Table:

• Scan cycle: Reads inputs, executes program, updates outputs continuously
• Programming languages: Ladder logic, function block, structured text, etc.
• Advantages: Reliability, flexibility, expandability, diagnostic capabilities

Mnemonic: “Power Centralizes Processing, Inputs/Outputs Make Automation”

Question 5(a) OR [3 marks]

#

Draw and explain construction of DC Servo motor.

Answer: DC servo motors provide precise position control with feedback for automation and robotics.

Construction Diagram:

Construction Table:

• Low inertia: Special design allows rapid acceleration/deceleration
• High torque-to-inertia ratio: Responds quickly to control signals

Mnemonic: “Precise Position Feedback Drives Exact Control”

Question 5(b) OR [4 marks]

#

Explain working of BLDC motor.

Answer: Brushless DC (BLDC) motors use electronic commutation instead of mechanical brushes and commutator.

BLDC Operation Diagram:

graph TD
    PS[Power Supply] --> C[Controller]
    C --> D[Driver Circuit]
    D --> W[Stator Windings]
    HS[Hall Sensors] --> C
    W --> R[Rotor Rotation]
    R --> HS

Working Principle Table:

• Commutation: Electronic switching sequences power to stator windings
• Efficiency: Higher efficiency due to elimination of brush losses
• Reliability: No brush wear or sparking, longer lifespan

Mnemonic: “Electronic Switching Creates Rotation Without Brushes”

Question 5(c) OR [7 marks]

#

Explain construction and working of VFD.

Answer: Variable Frequency Drive (VFD) controls AC motor speed by varying frequency and voltage.

VFD Construction Diagram:

graph LR
    A[AC Input] --> R[Rectifier]
    R --> D[DC Bus/Filter]
    D --> I[Inverter]
    I --> M[Motor]
    C[Control Circuit] --> I
    F[Feedback] --> C

Construction and Working Table:

• Speed control: V/f ratio maintained to provide constant torque
• Energy savings: Adjusts power to actual load requirements
• Soft start: Gradual acceleration prevents mechanical shock

Mnemonic: “Rectify, Filter, Invert Frequency For Motor Control”