Luigi Alberti

A Coupled Thermal–Electromagnetic Analysis for a Rapid and Accurate Prediction of IM Performance

The design of electrical machines for extreme operating conditions must include a thermal analysis coupled with the magnetic analysis. However, the traditional coupling of the thermal and the magnetic models can result in an unacceptable increase in computation time, particularly if finite elements (FEs) are used for the machine analysis. This paper proposes a coupled thermal-magnetic analysis of an induction motor (IM) with the primary goal of achieving a rapid and accurate prediction of the IM performance. Only a minimum set of FE magnetic analyses is carried out so as to determine the parameters of the IM equivalent circuit. These parameters are nonlinear and are adjusted on the basis of the operating point. Then, this equivalent circuit is coupled with a lumped-parameter thermal network to predict the temperature in each part of the IM. Since both the equivalent circuit and thermal network solutions are analytic, the analysis converges very rapidly. At the same time, the FE analysis yields a precise estimation of the IM parameters used in the equivalent circuit. Some experimental results are reported, showing the accurate prediction of the proposed methodology.

Experimental Tests of Dual Three-Phase Induction Motor Under Faulty Operating Condition

This paper describes a set of experimental tests on a dual three-phase induction machine for fault-tolerant applications. Both three-phase and six-phase machine operations are considered. Different winding configurations are investigated and compared in case of both open-circuit and short-circuit faults. Experimental tests for each configuration are reported at no-load and under load operating conditions.

Theory and Design of Fractional-Slot Multilayer Windings

This paper deals with fractional-slot multilayer windings. They are becoming attractive since they allow to reduce the harmonic content of the windings magneto motive force. Some examples of such a type of windings are reported in literature, but they are limited to a few slot/pole combinations. On the contrary, in this paper the general theory is presented. General rules for the design of multilayer windings are illustrated. It is also investigated when the adoption of a multilayer winding makes possible to limit the torque ripple and the rotor losses of the machine. Several examples and experimental tests are reported to investigate the advantages and convenience of adopting multilayer windings.

Performance Comparison Between Switching-Flux and IPM Machines With Rare-Earth and Ferrite PMs

Switching-flux permanent magnet (PM) machines compared with interior PM machines are presented to be interesting-solutions in terms of torque per volume density, compactness, active material layout, and cooling capability. The flux linkage is bidirectional, and the voltage is almost sinusoidal. These machines are then interesting for ac drives and field-weakening operation. Despite the lack of torque density, the adoption of low-cost magnets is investigated. Demagnetization in PMs is a critical issue and influences the choice of the machine.

MMF Harmonics Effect on the Embedded FE-Analytical Computation of PM Motors

This paper aims to define a minimum set of finite-element (FE) solutions to be used in the design and analysis of saturated permanent-magnet motors. The choice of the FE solutions belonging to this set is strictly associated with the classical d-q axis theory, and it is described in terms of key points on the flux-magnetomotive-force diagram. When synchronous machines are considered, such a diagram has a regular shape, so that a huge reduction in FE field solutions is possible with no loss of accuracy. It is also shown that the torque computed by using the d-q axis theory is almost independent of the variation of the flux linkage with the rotor position. At last, this paper describes a technique in which few FE solutions allow the identification not only of the average torque but also of the main torque harmonics. As a result, the torque behavior versus rotor position can be rapidly predicted.

Design and Control of an Axial Flux Machine for a Wide Flux-Weakening Operation Region

This paper describes the strategy to design and control an axial-flux (AxF) surface-mounted permanent-magnet machine for achieving a wide flux-weakening (FW) operating region. By using a slotted stator with fractional-slot windings and additional cores enclosing end windings, the AxF machine satisfies the specification of a wide constant-power speed range. The design procedure is presented for increasing FW capability while obtaining low-harmonic back electromotive force and low cogging torque. This technique is applied to design an 8-Nldrm AxF prototype machine that exhibits about 3 : 1 FW range. To the aim of exploiting full capability of the machine, an FW controller is designed and implemented. This controller utilizes the voltage difference between the current regulator and the output voltage, limited by a voltage source inverter. With this method, the output torque in the FW region is higher than that achieved using the conventional FW method based on the voltage-magnitude feedback. The goodness of both design and control algorithm is proved by experimental tests on a prototype.

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.

Design of a Low-Torque-Ripple Fractional-Slot Interior Permanent-Magnet Motor

Despite a strong nonlinear behavior and a complex design, the interior permanent-magnet (IPM) machine is proposed as a good candidate among the PM machines owing to its interesting peculiarities, i.e., higher torque in flux-weakening operation, higher fault tolerance, and ability to adopt low-cost PMs. A second trend in designing PM machines concerns the adoption of fractional-slot (FS) nonoverlapped coil windings, which reduce the end winding length and consequently the Joule losses and the cost. Therefore, the adoption of an IPM machine with an FS winding aims to combine both advantages: high torque and efficiency in a wide operating region. However, the combination of an anisotropic rotor and an FS winding stator causes some problems. The interaction between the magnetomotive force harmonics due to the stator current and the rotor anisotropy causes a very high torque ripple. This paper illustrates a procedure in designing an IPM motor with the FS winding exhibiting a low torque ripple. The design strategy is based on two consecutive steps: at first, the winding is optimized by taking a multilayer structure, and then, the rotor geometry is optimized by adopting a nonsymmetric structure. As an example, a 12-slot 10-pole IPM machine is considered, achieving a torque ripple lower than 1.5% at full load.

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.

IPM Machine Drive Design and Tests for an Integrated Starter-Alternator Application

This paper deals with an integrated starter-alternator (ISA) drive which exhibits a high torque for the engine start, a wide constant-power speed range for the engine speedup, and a high-speed generator mode operation for electric energy generation. Peculiarities of this ISA drive are thus its flux-weakening capability and the possibility to large torque overload at low speed. The focus on the design, analysis, and test of an interior permanent-magnet motor and drive for a prototype of ISA is given in this paper. In details, this paper reports on the design of stator and rotor geometries, the results of finite-element computations, the description of control system, and the experimental results of prototype tests.