Emanuele Fornasiero

Impact of MMF Space Harmonic on Rotor Losses in Fractional-Slot Permanent-Magnet Machines

The fractional-slot permanent-magnet (PM) machines are used in many applications due to their high torque density, low torque ripple, and high efficiency. However, the fractional-slot machines are characterized by high contents of space harmonics in the air-gap MMF distribution. Such harmonics cause flux variation in the air gap, and the main consequence is the induced losses in the rotor. Depending on the combination of slots and poles of the machine, there are different harmonic contents and then rotor losses. By means of a simple model of the rotor losses, this paper investigates the link between the rotor losses and the combination of the slots and the poles of the fractional-slot PM machines.

Slot Harmonic Impact on Rotor Losses in Fractional-Slot Permanent-Magnet Machines

Fractional-slot permanent-magnet (PM) machines have several advantages, but they suffer from the high content of space harmonics in the air-gap magnetomotive force (MMF) distribution. The MMF space harmonic amplitude and frequency depend on the particular combination of numbers of slots and poles. Such harmonics induce losses in the rotor since they are not synchronous with the rotor speed. This paper compares several kinds of machines with the same stator lamination but with different numbers of phases; three-, five-, and seven-phase PM machines are considered. Several winding configurations are compared, changing the number of poles but maintaining the same number of slots. Although in each case there is a reduction in terms of harmonic content, it is shown that rotor loss reduction is lower than expected. Some considerations about the harmonic impact are given. Although no experimental results are given, this paper refers to previous works in which rotor losses have been investigated and measured experimentally.

An Overview of Rotor Losses Determination in Three-Phase Fractional-Slot PM Machines

Three-phase fractional-slot PM machines are more and more used in many applications. In spite of the several advantages, these machines exhibit high contents of space harmonics in the air-gap MMF distribution, whose amplitude depends on the particular combination of number of slots and poles. The main consequence of such harmonic contents is the induced losses in the rotor. The aim of this paper is to link the rotor losses to the combination of slots and poles of the PM machine. The effort is to determine a general rule to single out easily if a machine topology is suitable or not, as far as the rotor losses are concerned. Both overlapped and non-overlapped coil fractional-slot windings are considered.

A General Approach to Determine the Rotor Losses in Three-Phase Fractional-Slot PM Machines

Three-phase fractional-slot PM machines are more and more used in many applications. In spite of the several advantages, these machines exhibit high contents of space harmonics in the air-gap MMF distribution, whose amplitude depends on the particular combination of number of slots and poles of the machine. The main cause of such harmonic contents is the induced losses in the rotor. The aim of this paper is to link the rotor losses to the combination of slots and poles of the PM machine. The effort is to determine a general rule to single out easily if a machine topology is suitable or not, as far as the rotor losses are concerned.

Rotor Losses Measurements in an Axial Flux Permanent Magnet Machine

MMF space harmonics and slot openings produce considerable rotor losses in permanent magnet (PM) machines, especially if fractional-slot windings are adopted. This paper aims to measure the rotor losses of a 12-slot 10-pole axial flux phase modulation machine. Both MMF space harmonics and slot openings are considered. The prototype is an axial flux machine with open slots, equipped with three different rotor disks: a disk without PMs, a disk with energized PMs, and a disk with de-energized PMs. A three-layer analytical model is used to interpret the experimental results, e.g., to split the losses in the different parts of the rotor.

Considerations on selecting fractional—slot windings

The paper focuses on the optimal selection of a fractional-slot winding for PM machines. The choice of the proper combination of slots and poles, and the corresponding winding layout, has a major impact on the PM machine performance, such as torque ripple, torque density, harmonic contents and then induced rotor losses, as well as capability to limit the short-circuit current and other fault-tolerance features. Among the very large possibilities to choose a winding configuration, the paper gives useful indications for a proper selection considering several aspects. The winding choice criteria are given using analytical equations, so that their implementation results to be easy. In this way, the collection of such criteria becomes an helpful tool in the design process.

Sensitivity Analysis of Torque Ripple Reduction of Synchronous Reluctance and Interior PM Motors

The main drawback of reluctance machines is a high torque ripple, due to the interaction between the stator magneto-motive force and the rotor structure. Adopting a rotor configuration characterized by several flux barriers per pole, there is a high influence of the rotor geometry on the machine performance, in terms of both average torque and ripple. An optimization is often required to determine the optimal rotor geometry so as to achieve a high and smooth torque. Then, the geometry determined above should guarantee good performance for various operating points (i.e., changing the current amplitude and phase), as well as for small variations of the geometry. This paper investigates this aspect, showing the results of optimizations carried out on various machines. The impact of the geometry parameters is taken into account and the sensitivity of the optimal solution to the geometry variation is pointed out. The paper highlights the difficulty to get a robust geometry as far as the torque ripple reduction is concerned. Finally, a few experimental results on a Synchronous Reluctance motor prototype will be presented, compared with Finite Element Analysis simulations for validation.

Effect of Stator and Rotor Saturation on Sensorless Rotor Position Detection

This paper shows how the saturation either of the stator or the rotor of a permanent magnet (PM) machine affects the machine performance, in terms of sensorless rotor position detection capability. An interior permanent magnet (IPM) motor with three rotor flux barriers is considered. The motor geometry is modified so as to achieve a higher iron saturation in the stator and in the rotor. Finite element (FE) simulations are carried out to highlight the effect of such saturation.

Design of synchronous reluctance motor for hybrid electric vehicles

The paper deals with the design of a synchronous reluctance motor suited for hybrid electric vehicles. At first, the focus is on rotor design. In particular an useful approach is proposed to choose the proper angles of the flux-barrier ends with the aim of reducing the torque ripple due to the slot harmonics. In the second part a comparison among different rotor arrangements (one, two and three flux-barriers per pole) is presented. Furthermore an improvement in terms of low ripple and high average torque is given thanks to the ”Machaon” configuration. The third part of the paper deals with the analysis of the improved layout. The current vector control method used to maximize the torque according to the voltage and current constraints is shown. Saturation effects are taken into account. The impact of electrical design on the mechanical characteristics of the rotor (and natural frequencies) is discussed as well. At last the predicted mechanical characteristics of the reluctance motor are presented: torque versus speed behavior, power losses and power factor. The efficiency map of the machine based on Finite Elements analysis is also reported.

Effect of stator and rotor saturation on sensorless rotor position detection

This paper shows how the saturation either of the stator or the rotor of a permanent magnet (PM) machine affects the machine performance, in terms of sensorless rotor position detection capability. An interior permanent magnet (IPM) motor with three rotor flux barriers is considered. The motor geometry is modified so as to achieve a higher iron saturation in the stator and in the rotor. Finite element (FE) simulations are carried out to highlight the effect of such saturation.