THEORY EXAMINATION (SEM–IV) 2016-17 ELECTRO-MECHANICAL ENERGY CONVERSION I

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=3​1​ 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.