Low Inductance Effects on Electric Drives using Slotless Permanent Magnet Motors: A Framework for Performance Analysis

The use of slotless Permanent Magnet machines is becoming the first choice among many other conventional solutions when improved features, such as smooth control and very high speed, are needed. However, it is not often mentioned that in order to take advantage of the main peculiarities of this kind of machines, the whole electric drive system should be conveniently designed to avoid an unexpected decline in performance during the operation. Whenever the behavior of a motor needs to be studied by means of Finite Element Analysis (FEA) as a verification of a design process, the under-load condition is typically simulated by assuming ideal current waveforms as coil sources i.e. sinusoidal for Synchronous machines and quasi-squared for Brush-less DC (BLDC) machines. However the characteristic low inductance of slotless machine can lead to high current ripple and hence, torque ripple, if the supply system is not consequently designed. This work aims to define a framework capable of giving some useful information regarding the electric drive performance. Such a framework is based on a model representing the electric drive system which gives, indirectly, the supply current to a Finite Element (FE) motor model, in order to carry out a harmonic losses analysis based on a temporal discretization technique. A slotless motor prototype has been considered as an example and different solutions for the current ripple reduction are also discussed.