Subject: Electromechanical Energy Conversion – II (NEE401)
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 basic understanding of synchronous machines, induction motors, and stepper motors.
Topics Covered:
Rotor vs. Stator Field Winding: Advantages of placing field winding on rotor and armature on stator.
Synchronous Reactance: Definition and role in voltage regulation.
Cogging in Induction Motors: Phenomenon of magnetic locking between stator and rotor teeth.
Why Synchronous Motor is Not Self-Starting: Need for auxiliary means to start.
Squirrel-Cage vs. Wound Rotor Motors: Construction and operational differences.
Distribution and Pitch Factors: Influence on generated EMF in alternators.
Power Flow Diagram of 3-Phase Induction Motor: From input to mechanical output.
Necessity of Starters: For limiting inrush current during starting.
Why Induction Motor Runs Below Synchronous Speed: Slip and torque relationship.
Stepper Motor Calculation: Stepping angle for a 3-phase, 24-pole permanent magnet stepper motor.
Section B – Descriptive / Analytical Questions (5 × 10 = 50 Marks)
This section focuses on analytical and numerical problems involving synchronous generators and induction motors.
Key Questions Include:
Construction and EMF Derivation of Alternator:
Constructional features, working principle, and derivation of EMF equation.
Synchronizing Power Calculation:
For a 1500 kVA, 3-phase, star-connected 6.6 kV, 8-pole, 50 Hz generator with 0.6 p.u. reactance, find synchronizing power at 0.8 pf lagging.
Active Power in Salient Pole Machines:
Derive the power equation and compare salient vs non-salient pole synchronous machines.
Three-Phase Induction Motor:
Working principle and torque-speed characteristics explaining stable and unstable regions.
Numerical on Induction Motor Performance:
746 kW, 3-phase, 50 Hz, 16-pole motor with rotor impedance (0.02 + j0.15) Ω.
(i) Speed at maximum torque
(ii) Ratio of full-load to maximum torque
(iii) Rotor resistance for max torque at starting.
Starters and Speed Control:
(i) DOL (Direct-On-Line) starter working
(ii) Pole-changing method for speed control.
Single-Phase Induction Motor:
Using double revolving field theory, explain why it is not self-starting.
Methods to make it self-starting (split-phase, capacitor start, shaded pole).
Numerical on Single-Phase Motor Equivalent Circuit:
Given data from Blocked Rotor Test and No-Load Test, determine circuit parameters.
Section C – Long Analytical Questions (2 × 15 = 30 Marks)
These require detailed explanations and numerical calculations.
Questions Include:
Effect of Excitation in Synchronous Generator:
Discuss how varying excitation affects current, power factor, and stability when connected to an infinite bus.
Explain Deep-Bar and Double-Cage Rotor induction motors.
Numerical on Induction Motor:
A 400V, 6-pole, 50Hz motor with torque of 120 Nm and rotor frequency 1.5 Hz.
Find:
(i) Shaft power and developed power (considering 8 Nm friction loss)
(ii) Rotor ohmic loss
(iii) Total power input
(iv) Efficiency if total stator loss = 500 W.
Discuss effects of space harmonics on 3-phase induction motor performance.
Parallel Operation of Alternators:
Process and conditions for synchronism.
Define hunting in synchronous machines — causes, effects, and remedies.
Major Topics Covered
Synchronous machines (construction, EMF, excitation control, hunting)
Alternator synchronization and power equations
Induction motors (principle, torque-speed, efficiency, losses)
Starting and speed control methods
Stepper and single-phase induction motors
Numerical applications in EMF, torque, and efficiency
Purpose of the Paper
This exam assesses understanding of AC machines (synchronous & induction) and their electromechanical interactions.
It combines theoretical concepts, mathematical derivations, and practical problem-solving, ensuring students can analyze and control machine performance under various operating conditions.
