About this previous year paper
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.