THEORY EXAMINATION (SEM–IV) 2016-17 ELECTROMECHANICAL ENERGY CONCERSION-II

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.