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

Subject: Electrical Machines and Control (NEE409)

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 core theoretical understanding of electrical machines, control systems, and stability analysis.

Topics covered:

Transformer efficiency & voltage regulation: Definitions and significance.

Applications of DC motors: Industrial drives, lifts, cranes, electric vehicles, etc.

Torque–Slip characteristic: Graphical relation for a 3-phase induction motor.

Torque–Speed curve: Behavior of an AC servo motor.

Types of test signals: Step, ramp, parabolic, and sinusoidal inputs.

Force–Current Analogy: Analogous electrical elements for mechanical systems.

Asymptotes in Root Locus: Meaning and calculation of asymptotic angles.

PID Controller: Definition and role in feedback control.

Routh–Hurwitz Criterion: Used to determine stability of the system
s4+2s3+s2+3s+2=0s^4 + 2s^3 + s^2 + 3s + 2 = 0s4+2s3+s2+3s+2=0.

Applications of Autotransformer: Voltage regulation and starting of induction motors.

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

This section focuses on analytical derivations, control system plotting, and electrical machine problems.

Key Questions Include:

Open Circuit and Short Circuit Tests:

Procedure for single-phase transformer testing, equivalent circuit derivation, and efficiency/voltage regulation evaluation.

Root Locus Sketching:

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

Mechanical–Electrical Analogies:

Obtain force–voltage and force–current analogies, and write the system’s differential equations.

Polar Plots:

(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/gain relationships.

Damping Ratio Problem:

For G(s)=Ks(1+Ts)G(s) = \frac{K}{s(1+Ts)}G(s)=s(1+Ts)K​, find by what factor gain K must be multiplied to change damping ratio from 0.3 to 0.9.

DC Series Motor Speed Control:

Given: 200 V, 500 rpm, 25 A, Ra=0.5Ω,Rf=0.3ΩR_a = 0.5Ω, R_f = 0.3ΩRa​=0.5Ω,Rf​=0.3Ω.

Find resistance required to reduce speed to 250 rpm.

Transformer Losses:

Types: Core losses (hysteresis + eddy current) and copper losses; explain and methods to minimize them.

Three-Phase Transformer Construction:

Core and shell type diagrams and working principles.

Controllers:

Working and characteristics of P, PI, and PID controllers.

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

Questions Include:

DC Motor Speed Control:

Armature voltage control, flux control, and variable resistance methods with circuit diagrams and equations.

Scott Connection:

Conversion of 3-phase to 2-phase supply with phasor diagram and applications.

Draw the 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:

Define Bode plot and its importance in stability 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)​,
determine:

(i) Phase crossover frequency         (ii) Gain crossover frequency

(iii) Phase margin                            (iv) Gain margin

(v) Overall system stability

Major Topics Covered

Transformers: testing, efficiency, and losses

DC & AC machines: speed-torque characteristics and control methods

Root Locus, Polar, and Bode Plot analysis

System modeling & mechanical–electrical analogies

PID controller design and tuning

Routh–Hurwitz stability criterion

Purpose of the Paper

This exam evaluates understanding of both electrical machine performance and control system behavior. It tests how theoretical principles are applied to real-world systems such as DC motor drives, transformers, and automatic control loops.

Students are expected to combine mathematical modeling, graphical analysis (Root Locus, Bode Plot), and practical control logic to demonstrate their conceptual mastery.