Medium-Large Induction Machines Starting Currents: Scaling Accuracy & Saturation Uncertainties

Accuracy levels of different scaling procedures for starting current measurements are addressed through statistical analysis of locked-rotor tests involving hundreds of medium- large induction machines. This is done so as to ascertain the confidence level of predicted full-voltage starting currents from reduced-voltage factory measurements, a necessity for motors with strict tolerance on proven starting current level. It is shown that for traditional scaling methods employing measurements only, this accuracy level is 88-90% for scaling step from 0.6 to 1.0 per unit voltage. It is shown that this figure could be raised to 95-97% through a novel method that also utilizes FEM simulations for each individual test voltage. The differences between FEM and measurements are documented and utilized 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 better product understanding and more reliable scaling methods in future.