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

Subject: Electro-Mechanical Energy Conversion – I (EEE401)

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 checks conceptual understanding of basic principles in DC machines, transformers, and magnetic systems.

Topics covered:

Difference between singly excited and doubly excited systems

Definition of co-energy

Changing the direction of rotation in a DC shunt motor

Energy balance equation of a motor based on conservation of energy

Applications of DC shunt motor (e.g., fans, blowers, conveyors)

Why transformer does not operate on DC supply

Why a DC series motor cannot be started on no-load

Methods to reduce hysteresis and eddy current losses in transformers

Definition of voltage regulation of a transformer

Numerical: Calculate generated EMF for a 4-pole, wave-wound armature having 720 conductors at 1000 rpm with 20 mWb flux

Section B – Descriptive / Analytical Questions (5 × 10 = 50 Marks)

This section contains analytical and numerical questions focusing on energy conversion principles, DC machines, and transformer efficiency.

Key questions include:

Torque in Doubly Excited Magnetic Systems:

Derivation showing torque = rate of increase of magnetic energy w.r.t. displacement at constant current.

DC Generator EMF Equation:

Derive expression for generated EMF.

Numerical: DC series motor (200 V, 40 A, 700 rpm, field resistance 0.1Ω, armature 0.15Ω) — calculate new speed and current if field is shunted and load torque increases by 50%.

Hopkinson’s Test:

Two identical DC shunt machines tested back-to-back with given data (230 V, 30 A line current, field currents 5A & 4A, armature resistance 0.025Ω).

Calculate efficiency of both machines.

Working Principle of DC Motor:

Explain with significance of back EMF and derive armature torque equation.

Transformer Efficiency Calculation:

Given O.C. test and S.C. test data (200 kVA, 1000/250 V transformer).

Compute all-day efficiency with varying load conditions (full load, half load, no load).

Sumpner’s Test and Polarity Test:

Procedures, purpose, and precautions.

Parallel Operation of Transformers:

Necessity and conditions for successful operation of three-phase transformers.

Open-Delta (V–V) Connection:

Derive relation showing that

Section C – Long Analytical Questions (2 × 15 = 30 Marks)

These are detailed questions requiring explanations, derivations, and calculations.

Questions:

DC Motor Starter:

Importance of starter, types (2-point, 3-point, 4-point).

Explain three-point starter with diagram.

Numerical: Given transformer (230/230V, 3kVA) with O.C. and S.C. test data. Find regulation and efficiency at full load 0.8 pf lagging.

Armature Reaction:

Definition and effects in DC machines.

Methods to reduce effects (inter-poles, compensating windings).

Compare armature control and field control methods for DC motor speed regulation.

Three-Winding Transformer:

Construction, working, and importance of tertiary winding.

Explain Auto-Transformer: Principle, construction, advantages, and applications.

Major Topics Covered

Magnetic energy and torque equations

DC machine principles, testing, and control

Transformer testing (OC, SC, Hopkinson, Sumpner)

Voltage regulation and efficiency

Armature reaction and starters

Three-phase and auto-transformers

Parallel and open-delta operations

Purpose of the Paper

This paper evaluates both theoretical and analytical understanding of electro-mechanical systems — specifically DC machines and transformers.
It combines mathematical derivations, practical testing methods, and real-world applications, forming the foundation for advanced electrical machinery and power system studies.