About this previous year paper
Subject: Electro-Mechanical Energy Conversion–I (EE401)
Exam Type: Theory
Semester: IV (4th Semester)
Session: 2016–17
Time: 3 Hours
Maximum Marks: 100
Section A – Short Answer Questions (10 × 2 = 20 Marks)
This section tests conceptual clarity in electromechanical principles and DC/AC machine basics.
Topics Covered:
Singly vs. Doubly Excited Systems: Magnetic field excitation comparison.
Co-energy: Definition and relationship to mechanical energy conversion.
DC Shunt Motor Direction Control: Reversing armature or field connections.
Energy Balance Equation: Based on conservation of energy in electric machines.
Applications of DC Shunt Motor: Lathes, fans, blowers, etc.
Transformer on DC Supply: Why it fails due to constant flux and heating.
DC Series Motor No-Load Operation: Explanation of overspeed risk.
Minimizing Hysteresis and Eddy Current Losses: Use of silicon steel and lamination.
Voltage Regulation in Transformer: Percentage voltage drop from no-load to full-load.
EMF Equation:
Given a 4-pole, wave-wound armature (720 conductors, 1000 rpm, flux = 20 mWb), find generated voltage.
Section B – Descriptive / Problem-Based Questions (5 × 10 = 50 Marks)
This section focuses on DC machines, transformers, and energy conversion analysis.
Key Questions:
Doubly Excited Magnetic System:
Show that torque = rate of change of magnetic energy with respect to displacement at constant current.
DC Generator EMF Derivation:
Derive expression for generated EMF in armature winding.
DC Series Motor Problem:
200V, 40A, 700 rpm, Ra=0.15Ω,Rf=0.1ΩR_a = 0.15Ω, R_f = 0.1ΩRa=0.15Ω,Rf=0.1Ω.
Field shunted with resistance equal to field resistance, load torque increased by 50%.
Find new speed and current (flux ∝ field current).
Hopkinson’s Test (Back-to-Back Test):
Given: V=230VV = 230VV=230V, Iline=30AI_{line} = 30AIline=30A, field currents 5A and 4A, armature resistance = 0.025Ω.
Calculate efficiency of both machines.
DC Motor Principle & Back EMF:
Working, significance of back EMF, and derive torque equation.
Transformer Problem (All-Day Efficiency):
200 kVA, 1000/250V, 50Hz transformer: O.C. test: 250V, 18A, 1300W
S.C. test: 80V, 200A, 2400W Load cycle:
Full load (0.8 pf) – 8 hrs Half load (unity pf) – 10 hrs
No load – remaining hrs Calculate all-day efficiency.
Sumpner’s Test & Polarity Test:
Explain procedure and purpose.
Parallel Operation of Transformers:
Necessity, conditions, and phase sequence matching for 3-phase systems.
Open-Delta (V–V) Connection:
Diagram and derivation showing
- Open-delta voltage ratio=13 of closed-delta.\text{Open-delta voltage ratio} = \frac{1}{\sqrt{3}} \text{ of closed-delta.}Open-delta voltage ratio=31 of closed-delta.
Section C – Analytical / Long Questions (2 × 15 = 30 Marks)
Questions Include:
DC Motor Starter:
Necessity, types (2-point, 3-point, 4-point).
Explain Three-Point Starter with neat diagram. Numerical: 230/230V, 3kVA transformer
O.C. test: 230V, 2A, 100W S.C. test: 15V, 13A, 120W
Find regulation and efficiency at full load, 0.8 pf lagging.
Armature Reaction:
Definition, effects (demagnetizing and cross-magnetizing).
Methods of reduction (compensating winding, interpoles).
Compare armature control vs. field control of DC motor speed.
Three-Winding Transformer:
Construction, working principle, and importance of tertiary winding (e.g., balancing unbalanced loads).
Explain Autotransformer construction, operation, and applications.
Major Topics Covered
Magnetic energy conversion (singly & doubly excited systems)
EMF and torque equations of DC machines Transformer testing (O.C., S.C., Hopkinson, Sumpner)
Efficiency and voltage regulation Armature reaction and speed control
Three-phase and auto-transformers Parallel and open-delta operation of transformers
Purpose of the Paper
This paper assesses both analytical and practical understanding of electrical energy conversion — how electrical and mechanical energies interact in DC machines and transformers.
It emphasizes derivations, efficiency calculations, and experimental methods, preparing students for practical lab work and industry applications.