THEORY EXAMINATION (SEM–IV) 2016-17 ELECTRICAL MACHINES AND CONTROL

Subject: Electrical Machines and Control (EEE409)

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 your basic understanding of transformers, DC machines, control systems, and stability analysis.

Topics covered:

Efficiency & Voltage Regulation of a transformer — definition and calculation principles.

Applications of DC Motors (e.g., electric traction, cranes, elevators).

Torque–Slip Characteristics of a 3-phase induction motor — shape and behavior explanation.

Torque–Speed Curve of an AC servo motor.

Test Signals: Step, ramp, parabolic, and sinusoidal inputs.

Force–Current Analogy: Relation between electrical and mechanical systems.

Asymptotes in Root Locus: Determining their angles.

PID Controller: Definition and control law.

Routh Criterion: Using it to determine system stability for given characteristic equations.

Applications of Autotransformer: Voltage regulation, starting of induction motors, etc.

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

This section focuses on transformer testing, control theory, and machine performance

Key Questions Include:

Open Circuit and Short Circuit Tests on a Single-Phase Transformer:

Procedure, purpose, equivalent circuit parameters, and efficiency calculation.

Root Locus Sketching:

For system G(s)=Ks(s2+4s+8)G(s) = \frac{K}{s(s^2 + 4s + 8)}G(s)=s(s2+4s+8)K​.

Analyze stability and response using graphical techniques.

Force–Voltage and Force–Current Analogies:

Write differential equations for a given mechanical system.

Polar Plot Construction:

(i) G(s)=1s(1+s)G(s) = \frac{1}{s(1 + s)}G(s)=s(1+s)1​

(ii) G(s)=10s(s+1)G(s) = \frac{10}{s(s + 1)}G(s)=s(s+1)10​

Sketch and interpret phase margins and gain margins.

Amplifier Gain and Damping Ratio Relation:

For G(s)=Ks(1+Ts)G(s) = \frac{K}{s(1 + Ts)}G(s)=s(1+Ts)K​, find gain factor for damping change (0.3 → 0.9).

DC Series Motor Speed Control Problem:

Given data: 200 V, 500 rpm, 25 A, armature resistance 0.5 Ω, field 0.3 Ω — find external resistance to reduce speed to 250 rpm.

Transformer Losses:

Core and copper losses — explanation and minimization methods.

Three-Phase Transformer Construction:

Core-type and shell-type designs with labeled diagrams.

Controllers:

Working principles and comparative behavior of P, PI, and PID controllers.

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

This section combines machine theory with advanced control system analysis

Questions Include:

DC Motor Speed Control:    Explain armature voltage control, field flux control, and variable resistance methods in detail.

Scott Connection:   Conversion of 3-phase to 2-phase supply with diagrams and applications.

Sketch Root Locus for G(s)=Ks(s+4)(s+5)G(s) = \frac{K}{s(s + 4)(s + 5)}G(s)=s(s+4)(s+5)K​.

Bode Plot Analysis:

For G(s)=16(1+0.5s)s2(1+0.125s)(1+0.1s)G(s) = \frac{16(1 + 0.5s)}{s^2(1 + 0.125s)(1 + 0.1s)}G(s)=s2(1+0.125s)(1+0.1s)16(1+0.5s)​:

Draw Bode plot and determine:        Phase Crossover Frequency

Gain Crossover Frequency                Phase Margin

Gain Margin                                      Overall System Stability

Major Topics Covered

Transformer testing and losses

DC motor speed control and torque-speed relationships

3-phase induction motor and servo motor characteristics

Analogies in mechanical–electrical systems

Control system analysis (Root Locus, Polar Plot, Bode Plot)

PID control theory and tuning methods

Stability testing using Routh–Hurwitz criterion

Purpose of the Paper

This paper tests the student’s ability to:

Understand and analyze electrical machine operations and characteristics.

Perform control system stability analysis using classical techniques.

Apply mathematical modeling to mechanical–electrical analogies.

Correlate transformer and motor theory with modern automation and control.

It bridges electrical machine design with control systems engineering, preparing students for both practical and analytical aspects of electrical system control.