Question 1(a) [3 marks]

Draw the structure of IGBT and explain it.

Answer: IGBT combines MOSFET's input with BJT's output characteristics.

graph LR
    A[Gate] --> B[Oxide Layer]
    C[Emitter] --> D[N+]
    D --> E[P Body]
    E --> F[N- Drift Region]
    F --> G[P+ Substrate]
    G --> H[Collector]

Gate-Oxide Layer: Controls device switching
N+ Emitter: Source of electrons
P+ Collector: Forms BJT section

Mnemonic: "MOSFET Input, BJT Output, IGBT Throughout"

Question 1(b) [4 marks]

Draw and explain the construction of SCR. Also draw the characteristic curve of it.

Answer: SCR is a four-layer PNPN semiconductor device with three terminals.

graph LR
    A[Anode] --> B[P Layer]
    B --> C[N Layer]
    C --> D[P Layer]
    D --> E[N Layer]
    E --> F[Cathode]
    G[Gate] --> D

Characteristic Curve:

     I
     ^
     |                     Forward
     |                   Conduction
     |                      /
     |                     /
     |                    /
     |          Breakover|
     |               *   |
     |             /     |
     |Forward     |      |
     |Blocking    |      |
     |            |      |
     +------------+------+----> V
     |            |
     |            |
     |Reverse     |
     |Blocking    |
     |            |
     |            V

P-N-P-N Layers: Forms two transistors (PNP, NPN)
Gate Terminal: Triggers conduction
Holding Current: Minimum to maintain conduction

Mnemonic: "PNPN Layers Form Two BJT Pairs"

Question 1(c) [7 marks]

Explain the working of solid state relay using Opto TRIAC, Opto-SCR and Opto-transistor with the help of circuit diagram.

Answer: Solid state relays use optocouplers for electrical isolation between control and load circuits.

graph LR
    A[Control Circuit] --> B[LED]
    B --> C[Opto-isolator]
    C --> D[Power Switching Element]
    D --> E[Load Circuit]

    subgraph "Types"
    F[Opto-TRIAC]
    G[Opto-SCR]
    H[Opto-Transistor]
    end

SSR Type	Input Circuit	Isolation	Output Circuit	Applications
Opto-TRIAC	DC control signal	LED + TRIAC detector	TRIAC power switch	AC loads
Opto-SCR	DC control signal	LED + photo-SCR	SCR power switch	DC loads
Opto-Transistor	DC control signal	LED + phototransistor	Power transistor	Low power DC

Working Principle: Control signal activates LED → Light triggers photo-sensitive device → Switches power circuit
Zero-Crossing Detection: Reduces EMI by switching at zero voltage
No Mechanical Parts: Increases reliability and life

Mnemonic: "LED Illuminates, Photo-device Conducts, Power Flows"

Question 1(c OR) [7 marks]

Describe the working and constructional features of SCR, GTO and power MOSFET with the help of characteristic curve.

Answer:

Device	Construction	Characteristic Curve	Working Principle
SCR	PNPN 4-layer with gate	Latching - once ON stays ON	Gate pulse triggers, requires external commutation to turn OFF
GTO	Modified SCR with better gate control	Similar to SCR but can be turned OFF by gate	Negative gate pulse extracts carriers, turns OFF
Power MOSFET	Vertical structure with many cells	Non-latching - requires gate bias	Gate voltage creates channel, removed voltage turns OFF

graph TD
    subgraph "SCR"
    A1[Anode] --> P1[P Layer]
    P1 --> N1[N Layer]
    N1 --> P2[P Layer]
    P2 --> N2[N Layer]
    N2 --> K1[Cathode]
    G1[Gate] --> P2
    end

    subgraph "GTO"
    A2[Anode] --> P3[P Layer]
    P3 --> N3[N Layer]
    N3 --> P4[P Layer]
    P4 --> N4[N Layer]
    N4 --> K2[Cathode]
    G2[Gate] --> P4
    end
    
    subgraph "Power MOSFET"
    S[Source] --> N5[N+ Source]
    N5 --> P5[P Body]
    P5 --> N6[N- Drift]
    N6 --> N7[N+ Substrate]
    N7 --> D[Drain]
    G3[Gate] ---> P5
    end

SCR: High current capability, latching behavior
GTO: Self turn-off capability, higher switching speed
MOSFET: Voltage-controlled, fast switching, no secondary breakdown

Mnemonic: "SCR Latches, GTO Self-Extinguishes, MOSFET Channels"

Question 2(a) [3 marks]

Explain the methods to protect SCR against over current in details.

Answer: SCR over-current protection prevents device damage due to excessive current.

Protection Method	Working Principle	Implementation
Fast-acting Fuses	Melts quickly during fault	Series with SCR
Circuit Breakers	Trips when current exceeds threshold	Main circuit protection
Current-limiting Reactors	Limits di/dt and peak current	Series with SCR

Heat Sinks: Help dissipate excess heat
Snubber Circuits: Reduce current spikes during switching

Mnemonic: "Fuses Fast, Reactors Restrict, Breakers Break"

Question 2(b) [4 marks]

Explain any two methods to turn ON the SCR.

Answer: SCR can be turned ON through different triggering methods.

Triggering Method	Circuit Implementation	Characteristics
Gate Triggering	Pulse applied between gate-cathode	Most common, controlled
Voltage Triggering	Anode voltage exceeds breakover voltage	No gate control, emergency

graph TD
    subgraph "Gate Triggering"
    DC[DC Source] --> R1[Resistor]
    R1 --> SW[Switch]
    SW --> G[Gate]
    K[Cathode] --> GND[Ground]
    end

    subgraph "Voltage Triggering"
    VS[Voltage Source] --> SCR[SCR Anode]
    SCR --> RL[Load]
    RL --> GND2[Ground]
    end

Gate Triggering: Controls firing angle precisely
Voltage Triggering: Happens when forward voltage exceeds breakover voltage

Mnemonic: "Gate Gets Control, Voltage Ventures Automatically"

Question 2(c) [7 marks]

Enlist the various methods to turn OFF the SCR and explain each of it using circuit diagram in brief.

Answer: SCR commutation methods are techniques to turn OFF a conducting SCR.

Commutation Method	Circuit Principle	Applications
Natural Commutation	AC source crosses zero	AC circuits
Forced Commutation	External components force current to zero	DC circuits
Class A (Self)	Parallel LC oscillator	Simple circuits
Class B (Resonant)	LC circuit in series with SCR	Medium power
Class C (Complementary)	Second SCR to divert current	High power
Class D (Auxiliary)	Auxiliary SCR + LC	Controlled timing
Class E (External)	External voltage source	Reliable but complex

graph LR
    subgraph "Natural Commutation"
    direction LR
    AC[AC Source] --> SCR1[SCR]
    SCR1 --> L1[Load]
    L1 --> AC
    end

    subgraph "Class B Commutation"
    direction LR
    DC[DC Source] --> SCR2[SCR]
    SCR2 --> L2[Load]
    C[Capacitor] ---> SCR2
    L[Inductor] ---> C
    SW[Switch] ---> L
    end

Natural Commutation: Current naturally falls to zero in AC cycles
Forced Commutation: Artificially brings current to zero in DC circuits
Communication Classes: A through E progressively more complex and reliable

Mnemonic: "Natural Zeros, Forced Components, Classes Advance Reliability"

Question 2(a OR) [3 marks]

Explain the methods to protect SCR against over voltage in details.

Answer: Over-voltage protection prevents damage from voltage transients.

Protection Method	Working Principle	Implementation
Snubber Circuits	RC network limits dv/dt	Parallel with SCR
Metal Oxide Varistors	Clamps voltage spikes	Parallel with SCR
Zener Diodes	Breaks down at set voltage	Anode-cathode protection

graph LR
    subgraph "Snubber Circuit"
    direction LR
    A1[Anode] --- R[Resistor]
    R --- C[Capacitor]
    C --- K1[Cathode]
    end

Snubber Circuit: Limits voltage rise rate (dv/dt)
MOV: Absorbs energy from voltage spikes
Thyristor Rating: Always use components with margin above circuit voltage

Mnemonic: "Snubbers Slow, Varistors Clamp, Zeners Zap"

Question 2(b OR) [4 marks]

Explain triggering of Thyristor in detail.

Answer: Thyristor triggering involves activating the device from blocking to conduction state.

Triggering Method	Working Mechanism	Advantages
Gate Triggering	Low power pulse at gate-cathode	Precise control
R-C Phase Shift	Varies phase angle for control	Simple circuit
UJT Triggering	Relaxation oscillator generates pulses	Stable timing
Light Triggering	Photons generate carriers (LASCR)	Electrical isolation

graph TD
    subgraph "UJT Triggering Circuit"
    direction LR
    DC[DC Source] --> R1[Resistor]
    R1 --> UJT[UJT Emitter]
    UJT --> C[Capacitor]
    C --> GND[Ground]
    UJT -- "Base 1" --> R2[Resistor]
    R2 --> GND
    UJT -- "Base 2" --> R3[Resistor]
    R3 --> DC
    UJT -- "Pulse Output" --> T[Transformer]
    T --> G[SCR Gate]
    end

Gate Current: Must exceed latching current
Gate Pulse: Width and amplitude critical for reliable triggering
Triggering Angle: Controls power delivered to load

Mnemonic: "Gate Gets Going, RC Rhythmically, UJT Uniformly, Light Liberates"

Question 2(c OR) [7 marks]

Design and explain snubber circuit for SCR. Also explain the importance of it.

Answer: Snubber circuits protect SCR from voltage transients and control switching behavior.

graph LR
    A[Anode] --- R[Resistor]
    R --- C[Capacitor]
    C --- K[Cathode]
    A --- SCR[SCR]
    SCR --- K
    A --- L[Inductor]
    L --- Load[Load]
    Load --- K

Component	Function	Selection Criteria
Resistor (R)	Limits discharge current	R > E/I₍max₎
Capacitor (C)	Absorbs voltage transients	C = I₍load₎/(dv/dt)
Optional Diode	Provides discharge path	Fast recovery type

Design Steps:

Calculate maximum dv/dt from SCR datasheet
Determine load current and circuit voltage
Select C to limit dv/dt below SCR rating
Select R to limit discharge current and provide damping

Importance:

dv/dt Protection: Prevents false triggering
Turn-off Support: Improves commutation
Switching Loss Reduction: Reduces power dissipation
EMI Reduction: Smooths voltage transitions

Mnemonic: "Resistor Restrains, Capacitor Catches, Diode Directs"

Question 3(a) [3 marks]

Explain the working of three phase Full Wave Rectifier using circuit diagram.

Answer: Three-phase full-wave rectifier converts three-phase AC to DC with six diodes.

graph LR
    subgraph "Three-Phase Source"
    A[Phase A]
    B[Phase B]
    C[Phase C]
    end

    subgraph "Bridge Rectifier"
    D1[D1]
    D2[D2]
    D3[D3]
    D4[D4]
    D5[D5]
    D6[D6]
    end
    
    A --> D1
    B --> D3
    C --> D5
    D1 --> P["\+"]
    D3 --> P
    D5 --> P
    N["\-"] --> D2
    N --> D4
    N --> D6
    D2 --> A
    D4 --> B
    D6 --> C
    
    P --> RL[Load]
    RL --> N

Six Diodes: Three for positive, three for negative half-cycles
Conduction: Each diode conducts for 120° per cycle
Output: Low ripple (4.2%) compared to single-phase

Mnemonic: "Six Diodes, Three Phases, Smooth DC"

Question 3(b) [4 marks]

Differentiate single phase and poly phase rectifier circuit.

Answer:

Parameter	Single Phase Rectifier	Poly Phase Rectifier
Input	Single AC source	Multiple AC sources (3 or more)
Diodes Required	2 (half-wave), 4 (full-wave)	3 (half-wave), 6 (full-wave)
Ripple Factor	0.482 (full-wave)	0.042 (3-phase full-wave)
Transformer Utilization	Lower (0.812)	Higher (0.955)
Output Waveform	Pulsating	Much smoother
Efficiency	Lower	Higher
Applications	Low power applications	Industrial power supplies

Form Factor: Lower in poly-phase (better quality DC)
Power Handling: Polyphase handles higher power more efficiently
Circuit Complexity: Polyphase more complex but better performance

Mnemonic: "Single Pulses Heavily, Poly Provides Smoothly"

Question 3(c) [7 marks]

Describe the application of series, parallel and bridge type Inverter.

Answer:

Inverter Type	Circuit Topology	Applications	Characteristics
Series Inverter	Resonant LC with load in series	Induction heating, Ultrasonic generators	• High frequency • Voltage source • Self-commutating
Parallel Inverter	Resonant LC with load in parallel	Uninterruptible power supplies, Solar inverters	• Current source • Better efficiency • Wider load range
Bridge Inverter	H-bridge with 4 switches	Motor drives, Grid-tied systems, General purpose	• Voltage/current source • Most versatile • Various control methods

graph TD
    subgraph "Series Inverter"
    DC1[DC Source] --> S1[SCR]
    S1 --> L1[Inductor]
    L1 --> C1[Capacitor]
    C1 --> RL1[Load]
    RL1 --> DC1
    end

    subgraph "Parallel Inverter"
    DC2[DC Source] --> L2[Inductor]
    L2 --> S2[SCR]
    S2 --> RL2[Load]
    C2[Capacitor] --> RL2
    RL2 --> DC2
    end
    
    subgraph "Bridge Inverter"
    DC3[DC Source] --> Q1[Q1]
    DC3 --> Q3[Q3]
    Q1 --> Q2[Q2]
    Q3 --> Q4[Q4]
    Q2 --> DC3
    Q4 --> DC3
    Q1 -- "Load" --> Q4
    Q3 -- "Load" --> Q2
    end

Series Inverter: Best for fixed-frequency, fixed-load applications
Parallel Inverter: Handles load variations better
Bridge Inverter: Most widely used for general applications

Mnemonic: "Series Sings at High Frequency, Parallel Performs with Variety, Bridge Brings Versatility"

Question 3(a OR) [3 marks]

Explain the working of three phase Half Wave Rectifier using circuit diagram.

Answer: Three-phase half-wave rectifier uses three diodes to convert three-phase AC to DC.

graph LR
    subgraph "Three-Phase Source"
    A[Phase A]
    B[Phase B]
    C[Phase C]
    N[Neutral]
    end

    subgraph "Half-Wave Rectifier"
    D1[D1]
    D2[D2]
    D3[D3]
    end
    
    A --> D1
    B --> D2
    C --> D3
    D1 --> P["\+"]
    D2 --> P
    D3 --> P
    P --> RL[Load]
    RL --> N

Three Diodes: Each conducts during positive half-cycle of its phase
Conduction: Each diode conducts for 120° per cycle
Output: 13.4% ripple (higher than full-wave)

Mnemonic: "Three Diodes, Three Phases, One Direction"

Question 3(b OR) [4 marks]

Enlist the different types of charging technology and compare it.

Answer:

Charging Technology	Working Principle	Advantages	Disadvantages
Constant Current (CC)	Fixed current until voltage threshold	Simple, low cost	Longer charging time
Constant Voltage (CV)	Fixed voltage with declining current	Fast initial charge	Current not limited at start
CC-CV	Starts with CC, switches to CV	Optimal charging profile	Requires controller circuit
Pulse Charging	Current pulses with rest periods	Reduces heat, extends battery life	Complex control circuit
Trickle Charging	Very low constant current	Maintains charge	Not suitable for main charging
Fast Charging	High current with intelligent control	Significantly reduced charging time	Heat generation, battery stress
Wireless Charging	Inductive coupling	Convenient, no cables	Lower efficiency, alignment issues

Battery Types: Different technologies suit different battery chemistries
Charging Profiles: Must match battery specifications to avoid damage
Temperature Management: Critical factor in charging efficiency and safety

Mnemonic: "Current Consistently, Voltage Varies, Pulse Pauses, Trickle Tops, Fast Finishes"

Question 3(c OR) [7 marks]

Explain the working of Solar Photovoltaic (PV) based power generation with the help of block diagram.

Answer: Solar PV systems convert sunlight directly into electricity through the photovoltaic effect.

graph LR
    S[Sunlight] --> PV[Solar PV Panels]
    PV --> C[Charge Controller]
    C --> B[Battery Bank]
    C --> I[Inverter]
    B --> I
    I --> L[AC Loads]
    C --> DC[DC Loads]

Component	Function	Types
Solar Panels	Convert light to DC electricity	Monocrystalline, Polycrystalline, Thin-film
Charge Controller	Regulates battery charging	PWM, MPPT
Battery Bank	Stores energy	Lead-acid, Lithium-ion, Flow
Inverter	Converts DC to AC	Pure sine wave, Modified sine wave
Distribution System	Delivers power to loads	Off-grid, Grid-tied, Hybrid

Photovoltaic Effect: Light energy creates electron flow in semiconductor material
Maximum Power Point Tracking: Optimizes power extraction under varying conditions
Grid Integration: Can operate standalone or connected to utility grid

Mnemonic: "Sunlight Strikes Semiconductors, Controllers Charge, Batteries Bank, Inverters Interface"

Question 4(a) [3 marks]

State the merits and demerits of Induction heating.

Answer:

Merits of Induction Heating	Demerits of Induction Heating
Rapid heating without direct contact	High initial installation cost
Precise temperature control	Requires electrical power source
Energy efficient (80-90%)	Limited to electrically conductive materials
Clean and pollution-free	Requires proper cooling systems
Localized heating possible	EMI generation may affect nearby electronics
Uniform heating throughout material	May require specialized coil designs

Working Principle: Eddy currents induced in workpiece generate heat
Applications: Melting, hardening, annealing, welding

Mnemonic: "Fast, Focused, Efficient but Costly, Conductive, Complex"

Question 4(b) [4 marks]

Draw the circuit of sequential timer using IC-555 and explain its working.

Answer: Sequential timer provides multiple timed outputs in sequence.

graph TD
    VCC[+VCC] --> R1[R1]
    R1 --> RST1[Reset IC1]
    VCC --> R2[R2]
    R2 --> TR1[Trigger IC1]
    VCC --> R3[R3]
    R3 --> THR1[Threshold IC1]

    IC1[555 Timer 1] -- "Output" --> C1[C1]
    C1 --> TR2[Trigger IC2]
    
    IC2[555 Timer 2] -- "Output" --> C2[C2]
    C2 --> TR3[Trigger IC3]
    
    IC3[555 Timer 3] -- "Output" --> LOAD[Load]

Working:

First 555 timer operates in monostable mode
Output triggers second timer when first timing cycle completes
Second timer triggers third timer
Each timer's period determined by its RC time constant

RC Values: T = 1.1 × R × C determines each stage's timing
Cascading: Multiple stages provide sequential timing events
Applications: Process control, industrial sequencing

Mnemonic: "One Timer Triggers Another Sequentially"

Question 4(c) [7 marks]

Draw the schematic circuit for single phase AC power control using TRIAC and explain it in detail.

Answer: TRIAC-based AC power control regulates power to loads through phase angle control.

graph LR
    AC[AC Supply] --> F[Fuse]
    F --> T[TRIAC]
    T --> L[Load]
    L --> AC

    AC -- "Phase Detection" --> ZC[Zero-Crossing Detector]
    ZC --> TC[Timing Circuit]
    TC --> G[Gate Drive]
    G --> T

Component	Function	Selection Criteria
TRIAC	Bidirectional power switch	Current rating > load current
DIAC	Triggers TRIAC symmetrically	Breakover voltage < trigger voltage
RC Network	Phase shifting for firing angle	R determines firing angle range
Snubber Circuit	dv/dt protection	Based on TRIAC specifications

Operation Principle:

RC network creates phase shift from AC input
DIAC breaks over when capacitor voltage reaches threshold
DIAC triggers TRIAC at specific phase angle
Varying R changes phase angle, controlling power

Firing Angle: 0° (full power) to 180° (zero power)
Applications: Light dimmers, heater control, motor speed control
Advantages: Smooth control, no moving parts, high reliability

Mnemonic: "Resistance Changes Phase, DIAC Delivers Pulse, TRIAC Transmits Power"

Question 4(a OR) [3 marks]

Enlist the merits and demerits of Dielectric heating.

Answer:

Merits of Dielectric Heating	Demerits of Dielectric Heating
Uniform heating throughout material	High initial equipment cost
Rapid heating (even for insulators)	High frequency power source required
Selective heating possible	Not effective for conductive materials
Energy efficient for certain materials	RF radiation safety concerns
Clean and pollution-free	Complex impedance matching requirements
Works with non-conductive materials	Power loss in transmission lines

Working Principle: Dipole rotation in high-frequency electric field generates heat
Applications: Plastic welding, wood drying, food processing

Mnemonic: "Uniform, Rapid, Insulator-friendly but Expensive, Complex, RF-intensive"

Question 4(b OR) [4 marks]

Draw the circuit diagram of photo-electric relay using LDR and explain its working.

Answer: Photo-electric relay uses light-dependent resistor to detect light and control a relay.

graph LR
    VCC[+VCC] --> R1[Load Resistor]
    R1 --> C[Collector]
    VCC --> RL[Relay Coil]
    RL --> C
    C --> Q[Transistor]
    Q --> GND[Ground]
    B[Base] --> Q
    R2[Base Resistor] --> B
    VCC --> LDR[LDR]
    LDR --> R2
    RL -- "Diode" --> VCC

Working:

LDR resistance decreases when light falls on it
Voltage divider (LDR + R2) provides base current to transistor
Transistor turns ON when sufficient base current flows
Relay activates when transistor conducts

Light Threshold: Adjustable via potentiometer
Applications: Automatic lighting, counting systems, alarm systems
LDR Characteristics: Resistance inversely proportional to light intensity

Mnemonic: "Light Lowers Resistance, Transistor Turns, Relay Responds"

Question 4(c OR) [7 marks]

Draw the circuit of DC power control using SCR with UJT in triggering circuit and explain in detail.

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

graph LR
    DC[DC Source] --> F[Fuse]
    F --> SCR[SCR]
    SCR --> L[Load]
    L --> DC

    DC --> R1[R1]
    R1 --> P[Potentiometer]
    P --> C1[Timing Capacitor]
    C1 --> E[UJT Emitter]
    E --> UJT[UJT]
    UJT -- "Base 1" --> R2[R2]
    R2 --> GND[Ground]
    UJT -- "Base 2" --> R3[R3]
    R3 --> DC
    UJT -- "Pulse Output" --> T[Transformer]
    T --> G[SCR Gate]
    G --> K[SCR Cathode]

Component	Function	Selection Criteria
UJT	Generates trigger pulses	η (intrinsic standoff ratio) = 0.5-0.8
R₁+P	Timing resistor	Controls charging rate of C₁
C₁	Timing capacitor	Determines pulse frequency
Transformer	Isolates UJT circuit from SCR	Pulse transmission capability
SCR	Main power control	Current rating > load current

Working Principle:

UJT relaxation oscillator generates pulses
Potentiometer varies charging rate, changing pulse frequency
Pulses are coupled through transformer to SCR gate
SCR conducts for portion of cycle based on trigger timing

Control Range: From minimum to maximum power
Advantages: Precise control, high efficiency
Applications: DC motor control, heating elements, battery chargers

Mnemonic: "Resistor Regulates Rate, UJT Unleashes Pulses, SCR Switches Current"

Question 5(a) [3 marks]

Explain the hall effect sensor in BLDC driver circuit.

Answer: Hall effect sensors detect rotor position in BLDC motors for precise commutation timing.

graph LR
    subgraph "BLDC Motor"
    R[Rotor with Magnets]
    S[Stator Windings]
    H1[Hall Sensor 1]
    H2[Hall Sensor 2]
    H3[Hall Sensor 3]
    end

    H1 -- "Position Signal" --> C[Controller]
    H2 -- "Position Signal" --> C
    H3 -- "Position Signal" --> C
    C -- "Commutation Signal" --> D[Driver Circuit]
    D -- "Phase Current" --> S

Hall Sensor	Function	Output
Position Detection	Senses magnetic field of rotor	Digital (ON/OFF)
Placement	120° apart for 3-phase motors	Provides 6 unique states
Signal Processing	Inputs to microcontroller	Determines switching sequence

Working Principle: Voltage generated perpendicular to current and magnetic field
Commutation Sequence: Each sensor pattern corresponds to specific switching combination

Mnemonic: "Magnet Moves, Hall Senses, Controller Commutates"

Question 5(b) [4 marks]

Draw and explain solid state circuit to control speed of single phase Induction motor using TRIAC.

Answer: TRIAC-based speed control for induction motors uses phase control principles.

graph LR
    AC[AC Supply] --> F[Fuse]
    F --> T[TRIAC]
    T --> M[Induction Motor]
    M --> AC

    AC -- "Zero Crossing" --> ZC[Zero-Crossing Detector]
    ZC --> MC[Microcontroller]
    MC --> OI[Opto-Isolator]
    OI --> T
    S[Speed Control] --> MC

Working Principle:

Zero-crossing detector identifies voltage zero-crossings
Microcontroller calculates delay based on speed setting
After delay, gate pulse sent through opto-isolator to TRIAC
TRIAC conducts for remainder of half-cycle
Varying firing angle controls voltage to motor, adjusting speed

TRIAC Rating: Must handle starting current (5-7× running current)
Speed Range: Limited at low end due to motor characteristics
Applications: Fans, pumps, small machine tools

Mnemonic: "Zero Detected, Delay Determined, TRIAC Triggered"

Question 5(c) [7 marks]

Explain the construction and working of BLDC motor using diagram. Also enlist its applications.

Answer: Brushless DC motors use electronic commutation instead of mechanical brushes.

graph LR
    subgraph "BLDC Motor Construction"
    S[Stator with Windings]
    R[Rotor with Permanent Magnets]
    H[Hall Effect Sensors]
    end

    subgraph "Control System"
    HS[Hall Sensor Signals] --> C[Controller]
    C --> D[Driver Circuit]
    D --> S
    end

Component	Function	Types/Variations
Stator	Contains copper windings	Slotted/slotless designs
Rotor	Permanent magnets	Surface/interior mounted
Hall Sensors	Position detection	60°/120° configurations
Controller	Commutation logic	Microcontroller-based
Driver	Power switching	MOSFET/IGBT-based

Working Principle:

Hall sensors detect rotor position
Controller determines correct energizing sequence
Driver powers appropriate stator windings
Magnetic interaction produces rotation
Process repeats continuously

Applications:

Computer cooling fans and hard drives
Electric vehicles and hybrid cars
Industrial automation and robotics
Medical equipment (pumps, ventilators)
Drones and RC models
Home appliances (washers, refrigerators)
Precision instruments

Mnemonic: "Magnets Move, Sensors See, Electronics Energize"

Question 5(a OR) [3 marks]

Explain the working of variable frequency drive (VFD).

Answer: Variable Frequency Drives control motor speed by varying the frequency and voltage.

graph LR
    AC[AC Supply] --> R[Rectifier]
    R --> DC[DC Bus]
    DC --> I[Inverter]
    I --> M[Motor]

    C[Controller] --> I
    S[Speed Reference] --> C
    F[Feedback] --> C

VFD Section	Function	Components
Rectifier	Converts AC to DC	Diodes or SCRs
DC Bus	Filters and stores energy	Capacitors, inductors
Inverter	Converts DC to variable AC	IGBTs or MOSFETs
Controller	Manages frequency/voltage	Microprocessor

V/f Control: Maintains constant V/f ratio for stable torque
Operating Range: Typically 10-200% of rated speed
Efficiency: High efficiency across wide speed range

Mnemonic: "Rectify to DC, Invert to AC, Vary Frequency"

Question 5(b OR) [4 marks]

Draw and explain the circuit to control speed of Universal motor.

Answer: Universal motors can run on AC or DC and allow simple speed control methods.

graph LR
    AC[AC Supply] --> F[Fuse]
    F --> T[TRIAC]
    T --> M[Universal Motor]
    M --> AC

    AC --> R1[R1]
    R1 --> DIAC[DIAC]
    DIAC --> G[TRIAC Gate]
    R1 --> C1[C1]
    C1 --> P[Potentiometer]
    P --> F

Working Principle:

RC network creates phase shift from input voltage
Potentiometer adjusts phase shift amount
DIAC triggers when voltage reaches breakover
TRIAC conducts for remainder of half-cycle
Adjusting potentiometer varies firing angle and motor speed

Speed Range: Wide control range (10-100%)
Torque Characteristics: Decreases somewhat at lower speeds
Applications: Power tools, household appliances, sewing machines

Mnemonic: "Resistance Changes Phase, DIAC Delivers, TRIAC Conducts"

Question 5(c OR) [7 marks]

Draw the block diagram of PLC and explain the function of each block in brief. And enlist the advantages and applications of it.

Answer: Programmable Logic Controllers (PLCs) are industrial computers for automation control.

graph LR
    subgraph "PLC System"
    PS[Power Supply]
    CPU[Central Processing Unit]
    IM[Input Modules]
    OM[Output Modules]
    MEM[Memory]
    COM[Communication Interface]
    end

    PS --> CPU
    PS --> IM
    PS --> OM
    PS --> COM
    
    IM --> CPU
    CPU --> OM
    CPU <--> MEM
    CPU <--> COM
    
    FS[Field Sensors] --> IM
    OM --> ACT[Actuators]
    COM <--> HMI[HMI/SCADA]
    COM <--> NET[Network]

PLC Block	Function	Types/Characteristics
Power Supply	Provides regulated power	Typically 24VDC or 110/220VAC
CPU	Executes program, processes I/O	Scan-based operation
Input Modules	Interface with field sensors	Digital, analog, special
Output Modules	Control field devices	Relay, transistor, triac
Memory	Stores program and data	RAM, EEPROM, Flash
Communication	Network connectivity	Ethernet, Profibus, Modbus

Advantages:

Reliability in harsh industrial environments
Flexibility for reprogramming
Compact size compared to relay-based systems
Built-in diagnostics and troubleshooting
Modular expandability
High-speed operation
Cost-effective for complex control systems

Applications:

Manufacturing production lines
Process control in plants
Material handling systems
Building automation
Power generation and distribution
Water/wastewater treatment
Packaging machinery
Food processing

Mnemonic: "Power Provides, CPU Computes, Inputs Inform, Outputs Operate, Memory Maintains"