THEORY EXAMINATION (SEM–VI) 2016-17 CONVENTIONAL & CAD OF ELECTRICAL MACHINES

CONVENTIONAL & CAD OF ELECTRICAL MACHINES – EEE023

B.Tech (SEM VI) | Section-wise Solved Answers

SECTION – A

(Explain the following – 2 marks each)

(a) Limitations in designing of machines

The design of electrical machines is limited by thermal constraints, magnetic saturation, mechanical strength, insulation limits, cost, and available materials. Designers must balance performance, efficiency, and safety.

(b) Output design equation of core and yoke of a transformer

The output of a transformer depends on core area, flux density, frequency, and window space. The design equation links kVA rating with magnetic and electric loading of the core and yoke.

(c) Specific electrical loading

Specific electrical loading is defined as the total current carried by all conductors on the armature per meter of armature periphery. It is expressed in ampere-conductors per meter.

(d) Classification of insulating materials

Insulating materials are classified based on temperature limits into Class A, E, B, F, and H. Higher classes withstand higher operating temperatures and increase machine life.

(e) Factors affecting size of rotating machines

Machine size depends on output power, speed, magnetic loading, electrical loading, cooling method, and permissible temperature rise.

(f) Difference between single-layer and two-layer winding

In single-layer winding, each slot contains only one coil side, whereas in two-layer winding, each slot contains two coil sides belonging to different coils.

(g) Standardization

Standardization refers to adopting uniform dimensions, ratings, and specifications to reduce cost, simplify manufacturing, and ensure interchangeability.

(h) Advantages and limitations of CAD

CAD allows faster design, optimization, and accuracy. However, it requires skilled operators, high initial cost, and depends on accurate input data.

(i) Electrical specific loading

Electrical specific loading is the current loading per unit circumference of the armature and directly influences machine output.

(j) Dielectric losses

Dielectric losses occur in insulating materials when subjected to alternating electric fields, causing energy dissipation in the form of heat.

SECTION – B

(Attempt any five – explained clearly)

(a) Determination of full-load MMF of salient pole synchronous generator

The full-load MMF is calculated considering armature reaction and field excitation. It involves resolving direct-axis and quadrature-axis components of armature current.

(b) Arrangement of LV and HV windings in transformers

In 3-phase core-type transformers, LV winding is placed nearer to the core to reduce insulation cost, while HV winding is placed outside.
In shell-type transformers, windings are sandwiched to reduce leakage flux.

(c) Electrical specific loading constancy

Electrical specific loading remains constant if current density, conductor-to-slot area ratio, slot width-to-pitch ratio, and slot depth are constant, ensuring uniform machine design.

(d) Machine ratings

Continuous rating allows operation indefinitely at rated load.
Short-time rating permits operation for a limited duration.
Intermittent rating involves periods of load and rest.

(e) Output equation of 3-phase shell-type transformer

The output equation relates kVA rating to flux density, current density, window space factor, and frequency. Stepped cores are used to approximate circular cross-section and reduce losses.

(f) Dielectric loss, loss angle, and power factor

Dielectric loss occurs due to imperfect insulation. Loss angle represents phase difference between voltage and current. Power factor indicates efficiency of insulation under AC conditions.

(g)

(i) Desired properties of magnetic materials
High permeability, low hysteresis loss, high saturation flux density, and low eddy current loss.

(ii) Effect of insulation class
Higher insulation class allows higher temperature rise, increasing machine output and lifespan.

(h) Simpson’s rule

Simpson’s rule is a numerical integration method used to calculate ampere-turns per meter in machine teeth by approximating magnetic field distribution.

SECTION – C

(Attempt any two – long answers)

(3) CAD of three-phase alternator

A flowchart is developed to calculate main dimensions, stator slots, conductors per slot, and winding details.
Input data includes rating, voltage, speed, frequency, magnetic loading, and electric loading.

(4) Design of stator frame of salient pole alternator

Given data such as kVA rating, voltage, frequency, poles, and specific loadings are used to calculate stator dimensions, number of conductors, and slots.

(5)

(i) Gap expansion factor

It accounts for slotting effect in air gap and increases the required MMF of induction motors.

(ii) Increasing starting torque of squirrel cage motor

This can be achieved by increasing rotor resistance, using deep-bar or double-cage rotors, and optimizing rotor slot design.