Medium–Large Induction Machines

In this article, the accuracy levels of different scaling procedures for starting current measurements are addressed through the statistical analysis of locked-rotor tests involving hundreds of medium-large induction machines. This is performed to ascertain the confidence level of the predicted full-voltage starting currents from the reduced-voltage factory measurements, a necessity for motors with a strict tolerance to the starting current level. It is shown that, for traditional scaling methods employing only measurements, this accuracy level is 88-90% for scaling step from 0.6 to 1.0 p.u. voltage. This figure could be raised to 95-97% through a novel method that also uses finite element method (FEM) simulations for each individual test voltage. The differences between FEM and voltage measurements are documented and used for correcting the full-voltage FEM simulation. Further, the use of FEM is shown to help in the understanding of the causes and locations of measured voltage-dependent saturation uncertainties. The region (end-core, overhang, or main core) that contributes the most to saturation uncertainties is shown to be identifiable through origin/parametric dependencies, leading to a better product understanding and more reliable scaling methods in future.