(SEM IV) THEORY EXAMINATION 2022-23 ELECTRICAL MACHINES- I

This question paper belongs to B.Tech Semester IV – Electrical Machines-I (KEE-402).
The exam duration is 3 hours and the total marks are 100.

The paper evaluates a student's understanding of:       Electromechanical energy conversion principles

DC machines: generators & motors                               Windings, starters, speed control

Transformers: equivalent circuit, tests, regulation, efficiency

Special transformer connections & 3-phase configurations

Torque equations and energy balance in electrical machines

The paper is divided into three major sections – A, B and C.

2. Section A – Short Conceptual Questions (20 Marks)

This section contains 10 questions, each worth 2 marks, requiring brief explanations.

Topics include:

● Magnetic Circuit in Energy Conversion

Reason for using magnetic medium in electromechanical systems.

● Block Diagram of Energy Conversion

General structure showing electrical system → magnetic system → mechanical output.

● Commutator as Mechanical Rectifier

How commutation converts AC to DC in DC machines.

● Lap vs Wave Winding

Differences in conductor distribution, number of parallel paths, and current handling capability.

● Speed Control of DC Machines

Field control, armature control, voltage control, etc.

● Necessity of Starters

High starting current problem in DC motors.

● Condition for Maximum Efficiency of Transformer

ηmax occurs when copper losses = iron losses.

● Voltage Regulation

Definition, significance, and calculation method through no-load and full-load conditions.

● Conditions for Parallel Operation of Transformers

Same voltage ratio, polarity, impedance magnitude and angle, and phase sequence.

● 3-Phase to 2-Phase Conversion

Connection diagram (Scott connection) and naming.

This section tests the fundamentals and theoretical base of electrical machines.

3. Section B – Descriptive & Analytical Questions (30 Marks)

Students must attempt any three of the five questions.
Each question carries 10 marks, requiring detailed derivations, diagrams, and explanations.

The topics include:

● Singly-excited vs Doubly-excited Systems

Differences + derivation of electromagnetic torque for singly excited linear system.

● Characteristics of DC Shunt Generator

Magnetization, internal, and external characteristics.

● 3-Point Starter

Construction, working, no-volt coil, overload protection, modifications in 4-point starter.

● Equivalent Circuit of Transformer

Primary & secondary referred circuits with core loss and leakage reactances.

● Open-Delta (V-V) Connection

Construction, VA rating reduction, phase diagram, and applications.

This section evaluates understanding of machine models, tests, and characteristics.

4. Section C – Advanced, Application-Oriented & Problem-Solving Questions (50 Marks)

This section contains 5 parts (Q3–Q7).
From each part, the student must attempt one option, each worth 10 marks.

Part 3 – Energy Conversion & Torque

Choice between:

● Torques in Doubly-Excited System

Components: electrical torque, reluctance torque, field interaction torque with derivation.

OR

● Energy Balance Equation of Motor

Derivation using conservation of energy, explaining all mechanical and electrical energy terms.

Part 4 – Commutation & Generator Characteristics

Choice between:

● Commutation in DC Machines

Process, causes of poor commutation, and improvement methods (interpoles, capacitor, brush shifting).

OR

● Compound Generator Characteristics

External characteristics, applications + numerical problem for generated voltage (long-shunt & short-shunt).

Part 5 – DC Motor Control & Testing

Choice between:

● Speed Control Zones

Different speed regions + detailed field-flux control method and speed ranges.

OR

● Swinburne’s Test

No-load loss test, procedure, limitations vs Hopkinson’s test.

Part 6 – Transformer Tests & Autotransformer

Choice between:

● Transformer Equivalent Circuit + Efficiency + Regulation

Using open-circuit and short-circuit test data to compute efficiency & regulation.

OR

● Autotransformer Copper Saving

Derivation + numerical problem (power transformed vs conducted).

Part 7 – Transformer Vector Groups & 3-Phase System

Choice between:

● Vector Groups

Yy0, Dy11, Yd1 etc., connection diagrams, phasor diagrams, and applications.

OR

● 3-Phase Transformer Bank Problem

Star-Delta (Y-Δ) connection calculations: secondary voltage, current, and kVA output.

This section tests problem-solving ability, numerical accuracy, and practical understanding of electrical machines.

Final Summary (Purpose of WHAT DESCRIPTION)

This WHAT DESCRIPTION provides a complete, long, clear, structured explanation of:

The aim of the question paper

What each section contains

What type of questions are asked

Expected depth of answers

Key concepts covered in ELECTRICAL MACHINES-I

It helps students understand the entire exam structure and conceptual scope.