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.
