JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III ELECTRICAL MACHINE DESIGN Previous Question Paper (August - 2022 - R18)

Insulating materials are classified based on their maximum permissible operating temperatures to ensure longevity and prevent thermal degradation. The classification is defined by international standards (IEC 60085):

• Class Y (90°C): Materials like cotton, silk, paper, and cellulose without impregnation.
• Class A (105°C): Cotton, silk, and paper impregnated with oil or varnish; organic materials.
• Class E (120°C): Synthetic resin enamels, cotton, and paper laminates with high-grade bonding resins.
• Class B (130°C): Inorganic materials like mica, glass fiber, and asbestos bonded with synthetic resins.
• Class F (155°C): Similar to Class B but with more thermally stable bonding resins like epoxy or silicone-alkyd.
• Class H (180°C): Inorganic materials bonded with silicone elastomers or resins.
• Class C (>180°C): Inorganic materials such as mica, ceramics, glass, and quartz used without binders or with specialized high-temperature binders.

The design of electrical machines is governed by fundamental electromagnetic, thermal, and mechanical principles to achieve optimal performance, efficiency, and cost-effectiveness.

• Electromagnetic Principles: Design relies on Faraday’s law of induction and Ampere’s circuital law. The core dimensions are determined by the output equation, which relates the machine's output to the magnetic loading (flux density) and electric loading (ampere-conductors per meter).
• Thermal Principles: Heat generated by copper losses (I²R) and iron losses (hysteresis and eddy currents) must be dissipated to keep temperatures within the limits of the insulation class. Design involves calculating the cooling surface area and heat transfer coefficients.
• Mechanical Principles: The machine must withstand mechanical stresses, including centrifugal forces on rotors, electromagnetic forces on windings during short circuits, and vibration. Shaft diameter and frame rigidity are determined based on these mechanical loads.
• Economic Principles: The design process aims to minimize the cost of active materials (copper and iron) while meeting performance specifications. This often involves iterative optimization to balance efficiency, weight, and manufacturing complexity.