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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.