(SEM IV) THEORY EXAMINATION 2024-25 ELECTRICAL MACHINES-I

This document contains the complete B.Tech (Semester IV) Theory Examination 2024–25 question paper for Electrical Machines–I (BEE402). The paper spans two printed pages, carries 70 marks, and evaluates a student’s understanding of DC machines, transformers, electromechanical energy conversion, armature reaction, commutation, torque equations, machine characteristics, and transformer testing methods.
 

Section A – Short Conceptual Questions (14 Marks)

Section A includes seven 2-mark questions, covering essential principles of electrical machines (Page 1):

Definition, significance & application of Co-energy

Relation between electrical & mechanical degrees using induced EMF waveform

Meaning and purpose of interpoles in DC machines

Why a series motor must not run without load (speed–torque curve)

Polarity test of a single-phase transformer

Star/Delta connections in 3-phase transformers

Conditions for parallel operation of three-phase transformers

These questions test core conceptual clarity of DC and transformer fundamentals.

Section B – Analytical & Application-Based Questions (21 Marks)

Students attempt any three of the five questions. These involve numerical calculations, derivations, and descriptive explanations (Page 1).

Topics include:

Energy Conversion

Deriving force & torque equations from energy and co-energy

Commutation in DC Machines

Process of commutation, its types, and methods to improve commutation

Field Resistance Change Problem

A numerical problem involving:

DC shunt motor (250 V, Rₐ=0.5 Ω, Rf=250 Ω)

Change in speed when 250 Ω resistance is added into field circuit

Transformer Efficiency

Transformer (50 kVA, 1100/440 V)

Maximum efficiency at 85% load, 0.8 pf lagging

Compute efficiency at half load, 0.6 pf lagging

Speed Control of DC Motors

Methods of speed control

Detailed explanation of Ward–Leonard method

This section strengthens understanding of practical numerical applications.

Section C – Long Descriptive Questions (35 Marks)

Each question contains two alternatives, and students must attempt one part.

Q3 – Torque Derivation / Energy Conversion Principle (Page 1)

Derivation of torque expression for cylindrical rotor machines

OR

Principles of electromechanical energy conversion using block diagrams, energy balance & energy flow

Q4 – Armature Reaction / Generator Characteristics (Page 1)

Armature reaction using MMF waveforms & its effects

OR

Characteristics of different DC generators + numerical problem on compounding of shunt generator
(240 V, 36 kW, 500 turns)

Q5 – Starters / DC Series Motor Problem (Page 2)

Need of starter in DC motor + construction & working of 4-point starter

OR

DC series motor (230 V, 20 A, 1500 rpm) → find series resistance to maintain rated torque at:

Starting

1000 rpm

Q6 – Transformer Tests / V–V Connection (Page 2)

OC & SC test data given for 1-kVA transformer → find:

Equivalent circuit

Parameters

Efficiency & voltage regulation (full load, 0.8 pf lagging)

OR

Meaning of V-V (open delta) connection; proof that rating = 57.7% of delta-delta rating

Q7 – Sumpner’s Test / Parallel Operation (Page 2)

Explain Sumpner’s (Back-to-Back) Test with diagram

OR

Conditions of parallel operation, plus numerical load-sharing problem:

Two 150 kVA single-phase transformers

Leakage impedances:

A: 1 + j3 Ω

B: 1.2 + j3.2 Ω

Share load of 240 kW at 0.8 pf lagging

These questions test deeper understanding, graphical interpretation, and thorough analysis of DC machines and transformers.