Jawaharlal Nehru Technological University Hyderabad B.Tech III Year ELECTRICAL MACHINE DESIGN Previous Question Paper (March - 2021 - R18)

The design of an electrical machine is a complex process requiring a balance between performance, cost, and reliability. A good design must satisfy the following major considerations:

1. Electrical and Magnetic Loading: The output of an electrical machine is directly proportional to the product of specific magnetic loading (average flux density over the armature surface) and specific electrical loading (ampere-conductors per meter of armature periphery). Selecting optimal values for these loadings is critical to determine the size and efficiency of the machine.

2. Thermal Considerations: Heat generated due to copper losses, iron losses, and mechanical losses must be dissipated effectively to prevent insulation failure. The design must ensure that the temperature rise remains within the limits defined by the insulation class (e.g., Class A, B, F, or H). Proper ventilation and cooling methods are essential for long-term reliability.

3. Insulation Requirements: Insulation must withstand the operating voltage, thermal stress, and mechanical vibrations. The choice of insulation material depends on the voltage level, environmental conditions, and the expected life of the machine.

4. Mechanical Constraints: The machine must be mechanically robust to withstand electromagnetic forces, centrifugal forces during high-speed operation, and vibrations. The shaft, bearings, and frame must be designed to maintain alignment and structural integrity.

5. Efficiency and Power Factor: A good design aims to minimize losses (I²R, hysteresis, eddy current, and friction/windage) to maximize efficiency. For AC machines, maintaining a high power factor is crucial to reduce the rating of the supply system and improve overall performance.

6. Cost and Material Availability: The design should be economically viable by minimizing the use of expensive materials like copper and high-grade silicon steel. Standardized parts and manufacturing processes should be used to reduce production costs.

7. Regulatory Standards: The design must comply with national and international standards (such as IS, IEC, or IEEE) regarding performance, safety, and testing procedures.

The design of a three-phase transformer involves determining the main dimensions of the core and windings to satisfy the specified kVA rating, voltage, frequency, and efficiency requirements. The process is primarily governed by the output equation and the relationship between the core area and the window area.

The output equation for a three-phase transformer is given by:

Where f is frequency, Bm is maximum flux density, Ai is net iron area of the core, Aw is window area, and Kw is the window space factor.

The design steps for main dimensions are as follows:

1. Core Area (Ai): The net iron area is determined by selecting a suitable flux density (Bm) based on the grade of silicon steel used. For a stepped core, the area is calculated using a stacking factor to account for insulation between laminations.
2. Window Area (Aw): The window area is chosen based on the required space for high-voltage and low-voltage windings, including insulation clearances. It is related to the core area through the ratio of window height to width.
3. Core Diameter (d): For a circular core, the diameter is derived from the net iron area. The core is typically designed as a stepped core to maximize the utilization of the circumscribing circle.
4. Yoke Design: The yoke area is usually designed to be 10% to 15% larger than the limb area to reduce flux density and iron losses, as the yoke carries the same flux as the limb.
5. Window Dimensions: The height of the window is determined by the winding height plus necessary clearances for top and bottom yokes, while the width is determined by the space required for the LV and HV windings and the core-to-winding insulation.

The final dimensions are optimized to balance the cost of copper (winding weight) and iron (core weight), typically aiming for a design where the cost of iron equals the cost of copper for minimum total cost.