(SEM IV) THEORY EXAMINATION 2024-25 ELECTRICAL MACHINES AND CONTROLS

ELECTRICAL MACHINES & CONTROLS – B.Tech (Semester IV), REE-409

This is a 3-hour, 70-mark theory examination designed to assess a student's understanding of transformers, alternators, DC/AC machines, control systems, stability analysis, torque characteristics, and feedback systems.

The paper is divided into three sections, testing conceptual clarity (A), analytical/descriptive understanding (B), and advanced problem-solving & control system applications (C).
 

SECTION A — Short Answer Questions (2 × 7 = 14 Marks)

This section contains 7 brief questions to test essential concepts of machines & control systems.

Topics include:

Frequency of induced EMF in an alternator             Why alternators are rated in kVA

Armature reaction in alternators                              Synchronous impedance

Methods of determining voltage regulation of alternators          Types of control systems

Writing characteristic equation of given system

These check definitions, formulas, and basic understanding.
 

SECTION B — Descriptive / Analytical Questions (7 × 3 = 21 Marks)

Attempt any 3 out of 5.

Topics covered:

(a) Transformer Performance

Open-circuit & short-circuit test data                       Draw equivalent circuit (HV side)

Calculate efficiency at 90% load, 0.8 PF

(b) Autotransformers

Definition, merits & demerits                                     Convert 1100/2200 V, 1000 kVA two-winding transformer to autotransformer

Determine voltage & power rating

(c) DC Motor Starting & Speed Control

Need for starter                                     Working of 3-point starter

Numerical: speed change when flux is reduced by 1%

(d) 3-Phase Induction Motor

Derive torque equation                          Draw torque–slip characteristic

Condition for maximum torque

(e) Voltage Regulation of Alternator

Using:

OCC & SCC test data                  Calculate regulation at

0.8 lagging PF                             0.707 leading PF

This section evaluates machine analysis, numerical problem solving, and conceptual engineering understanding.
 

SECTION C — Long / Advanced Control System Questions (7 marks each)

Each question offers choice (a or b).

Topics include:

Q3 – Servomotors / Routh Stability

Two-phase servomotor with torque–speed characteristics
OR

Routh stability → range of K for which system has 0,1,2 RH-plane poles

Q4 – Time-Domain Specifications / Open & Closed Loop

Find gain K & velocity feedback Kh for given overshoot & peak time; calculate rise & settling times
OR

Open-loop vs closed-loop systems + advantages of feedback

Q5 – Mechanical Modeling / Root Locus

Derive differential equation & draw mechanical equivalent
OR

Complete root locus for given G(s):

Centroid, asymptotes                      Breakaway point

Crossing of jω-axis                          Angle of departure

Q6 – Controllers / Nyquist Stability

PI, PD & rate-feedback controller effects on 2nd-order system
OR

Nyquist stability criterion for small and large values of K

Q7 – Bode Plot / Resonant Frequency

Draw Bode plot for given transfer function

Gain crossover                                 Phase crossover

Gain margin & phase margin          System stability

OR

Find resonant frequency, resonant peak, and bandwidth for given second-order system

This section evaluates control theory, system stability, frequency response, and dynamic system analysis.
 

 OVERALL PURPOSE OF THE EXAM

This question paper examines whether the student can:

Analyze transformer, alternator, DC motor, and induction motor behavior

Understand tests like OC, SC, OCC, SCC, and voltage regulation

Perform machine numericals (efficiency, speed, torque, voltage regulation)

Understand fundamentals of servomotors and control systems

Apply Routh, Nyquist, and Root Locus methods

Evaluate time-domain specifications

Understand Bode plots and frequency response

Build differential equations for mechanical systems

It tests combined knowledge of electric machines + classical control system theory, essential for electrical engineering.