Chuck Yung & Cyndi Nyberg
Technical Support Specialists
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "Distribution Factor and Winding Conversion Issues" by Cyndi Nyberg and Chuck Yung, presented at the EASA Convention 2006, explores the complexities involved in modifying three-phase stator windings in electric motors. The authors begin by explaining the concept of the distribution factor (Kd), which is the ratio of the resultant voltage induced in a series-connected group of coils to the arithmetic sum of the magnitudes of the voltages induced in the coils. This factor is crucial in accounting for the asynchronous contribution of each coil to the overall torque of the motor.
Nyberg and Yung highlight the differences between concentrated and distributed windings. In a concentrated winding, such as a DC machine field coil, each coil is wound around a single pole, resulting in a distribution factor of 1.0. In contrast, a distributed winding, such as a lap winding in an AC motor, consists of multiple coils placed symmetrically in the slots of the stator core. The distribution factor for these windings is less than 1.0 due to the slight delay in the contribution of each coil to the torque.
The paper delves into the impact of different winding layouts on the distribution factor. For example, a standard salient pole winding with 12 groups of 3 coils has a different distribution factor compared to a consequent pole winding with 6 groups of 3 coils. The authors provide detailed calculations for determining the distribution factor based on the number of slots, poles, and the electrical angle between each slot.
Nyberg and Yung also address the challenges of converting windings from one type to another, such as from concentric to lap windings. They emphasize the importance of using the correct distribution factor in these conversions to avoid significant changes in motor performance. The paper includes several examples to illustrate the potential errors that can occur when the distribution factor is not correctly applied. For instance, converting a lap winding with 12 groups of 4 coils to a single-layer lap winding with 12 groups of 2 coils requires careful calculation to ensure the effective turns per coil remain consistent.
The authors explain that the selected winding pattern, whether sequential or skip-slot, affects the distribution factor and, consequently, the torque and flux densities of the motor. They provide formulas for converting concentric windings to lap windings and emphasize the need to consider the air gap density to maintain the same torque.
The paper concludes with practical examples of winding conversions, demonstrating the importance of accurate calculations to avoid unintended changes in motor performance. Nyberg and Yung stress that any winding change should be carefully considered, as errors in the new turns calculation can significantly impact the motor's torque and efficiency.
Key Points Covered:
- Definition and importance of the distribution factor (Kd)
- Differences between concentrated and distributed windings
- Impact of winding layouts on the distribution factor
- Challenges and calculations for converting windings
- Importance of using the correct distribution factor in conversions
- Practical examples of winding conversions
Key Takeaways:
- The distribution factor is crucial for accounting for the asynchronous contribution of each coil to the motor's torque.
- Concentrated windings have a distribution factor of 1.0, while distributed windings have a lower distribution factor.
- Different winding layouts affect the distribution factor and motor performance.
- Accurate calculations are essential when converting windings to avoid significant changes in torque and efficiency.
- Any winding change should be carefully considered to prevent errors in the new turns calculation.
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