Heterogeneous Air Gap and Its Consequences Modelling Residual Magnetic Attraction Force

Magnetic Pull (UMP), is common in rotating electrical machines. When within acceptable levels, RMAF does not compromise the equipment integrity. However, when excessive, it can lead to structural failures, as observed in several hydroelectric generators.

A mathematical model is developed, starting from the canonical form of the circumferential distribution of the magnetic flux density; an expedited model is proposed. The comparison between the simulation results and the measured data shows excellent agreement, indicating that RMAF analysis, based on the expedited model equation of magnetic induction, is sufficient to obtain accurate results, thereby avoiding the need for advanced functions, unconventional waveforms, or Fourier series components.

Two Brazilian case studies illustrate the practical consequences of excessive RMAF. In Case I, an 80 MVA generator with an interference-fitted rotor rim exhibited geometric deformations and high pulsating forces, resulting in fatigue in the structural welds. After corrective actions, including geometry correction and increased rim interference, all parameters returned to acceptable limits, with a significant reduction in RMAF.

In Case II, a 140 MVA generator with a floating rotor rim exhibited even more severe air gap variations (39.4% compared to the 10% limit), leading to structural failures. Similar repairs to those in Case I were implemented, restoring operational integrity.

The article concludes by emphasising the importance of continuous monitoring and the use of advanced tools for evaluating geometric and magnetic forces. The proposed methodology, based on over 30 years of research, enables early failure detection, reduces the risk of damage, and optimises preventive maintenance in vertical generators.