Mauro Bortolozzi

Study of faulty scenarios for a fault-tolerant multi-inverter-fed linear permanent magnet motor with coil short-circuit or inverter trip

A fault-tolerant machine used for safety-critical tasks must a) guarantee at least a reduced-performance operation in case of partial machine fault and b) guarantee avoidance of drive mechanical jam/stall in case of total machine fault, to allow for the intervention of the back-up systems. Classical hydrostatic transmissions used on board ships for critical tasks such as rudder and stabilizing fin steering gears fulfill both the requirements a), b) above, but recent proposals for substitution with full-electric drives (rotary motors coupled with multistage reduction gears) usually do not. Especially the requirement b) needs particular attention and increased complexity when dealing with geared drives. This paper proposes a linear permanent-magnet direct drive fulfilling both the requirements above, for (but not limited to) rudder/fin steering gears. The absence of gears grants the requirement b), whereas the full-modular structure satisfies a), with independently fed stator modules and multiple inverters. This paper addresses some fault scenarios including electrical failures, in the machine winding (short-circuited coils) and in the inverters (trip of one or more units). The performance degradation is studied and assessed for the cases considered by both simulations and measurements on a prototype.

Modeling of split-phase machines in Park's coordinates. Part I: Theoretical foundations

A split-phase machine is a special electric machine whose stator winding is split into multiple (N) three-phase sets. The possibility to supply it through N independent inverters makes it attractive especially when high power and reliability are required. So far, the dq0 refeence frame representation, originally introduced for three-phase machines, has been applied in detail to split-phase configurations in the N=2 case only. In this paper, the extension to an arbitrary number of stator sets is investigated from an analytical viewpoint. In particular, the paper shows that when N exceeds two, the d-axis and q-axis stator voltage equations are no more decoupled, in general, as it happens for N=1 and N=2. Such d-q cross-coupling is explained in terms of mutual leakage inductances, regardless of possible rotor saliencies and magnetic saturation. The implications of these results in terms of equivalent circuit representation will be presented in a companion paper (Part II).

Modeling of split-phase machines in Park's coordinates. Part II: Equivalent circuit representation

A split-phase machine is a special electric machine whose stator winding is split into multiple (N) three-phase sets. The possibility to supply it through N independent inverters makes it attractive especially when high power and reliability are required. So far, the dq0 refeence frame representation, originally introduced for three-phase machines, has been applied in detail to split-phase configurations in the N=2 case only. In this paper, the extension to an arbitrary number of stator sets is investigated from an equivalent circuit representation viewpoint. The equivalent circuit topologies proposed in this paper to represent a split-phase machine with N three-phase sets is directly derived from the theory presented in Part I. In particular, it will be stressed that as long as N is equal to 1 or 2, dq equivalent circuits remain decoupled as known from the literature, otherwise for N higher than or equal to 3, dq cross couplings must be taken into account in the equivalent circuit representation.

Analytical Computation of End-Coil Leakage Inductance of Round-Rotor Synchronous Machines Field Winding

The computation of end-coil leakage inductances of electric machines is a challenging task due to the complicated leakage flux 3-D distribution in the winding overhang region. In this paper, the problem of computing the field-circuit leakage inductance of round-rotor synchronous machines is addressed. The proposed method is fully analytical and descends from the symbolical solution of Neumann integrals applied to the computation of self-inductance and mutual inductance combined with the method of mirror images to account for core effects. With respect to existing analytical approaches, the methodology requires neither numerical integral solutions nor discretizing the end-coil geometry into small straight elements. The accuracy of the proposed technique for computing the mutual inductance between two single-end turns is assessed against measurements on a dedicated experimental setup. The extension of the method to the computation of the entire field-circuit end-coil leakage inductance is assessed by comparing with the 3-D finite-element analysis.

Stator inductance matrix diagonalization algorithms for different multi-phase winding schemes of round-rotor electric machines part II. Examples and validations

The stator winding of multi-phase machines can be designed according to different schemes, which differ by either the number or the spatial arrangement of phases. For control and analysis purposes, it is often useful that the stator inductance matrix is reduced to a diagonal form. This allows for the multiphase machine to be split into independently-controllable systems and directly leads to stator harmonic impedance determination. Simple and compact diagonalization algorithms have been established in a companion paper covering all possible multiphase schemes of practical importance with a unified approach, under the hypothesis of uniform air-gap and negligible magnetic saturation. In this paper, the theory and algorithms proposed for inductance matrix diagonalizations are illustrated and validated by applying them on a on several multi-phase winding schemes implemented in a prototype machine with a reconfigurable stator winding.

A new magnetic wedge design for enhancing the performance of open-slot electric machines

In electric permanent-magnet motors for traction applications, an open slot stator design is to be used when the winding is composed of flat-turn coils. The open slot design, combined with rotor permanent magnets, can produce remarkable cogging torque effects. This paper describes a magnetic wedge design which allows for a significant cogging torque reduction and, at the same time, allows for a fine adjustment of stator phase inductance values. The new wedge concept is applied in the paper to a 12-slot surface permanent-magnet motor. Finite-element analysis is used to study the wedge design effects on motor phase inductance and cogging torque amplitude.

Explicit Torque and Back EMF Expressions for Slotless Surface Permanent Magnet Machines With Different Magnetization Patterns

Slotless permanent magnet machines are attractive in some modern drive and power generation fields, where the cogging torque and additional losses need to be minimized or removed. The stator slotless design can be combined with different surface permanent magnet (SPM) rotor topologies. In this paper, explicit analytical expressions are derived to analytically compute the slotless machine torque and no-load back Electro-Motive Force in the case of segmented SPM rotor with parallel, radial, or Halbach-array magnetization patterns. The expressions are found by solving the magnetic field due to the slotless stator winding and to the permanent magnet blocks; the latter modeled through equivalent surface current densities. The accuracy of the method is successfully assessed by comparison with the finite-element analysis (FEA). The proposed formulas are an effective alternative to the FEA to quickly compare different design solutions as well as to optimize them. Application examples are provided in which the presented method is adopted to define the machine cross section that maximizes the torque density.

Investigation into induction motor equivalent circuit parameter dependency on current and frequency variations

Induction motors are presently the most widespread kind of electric machinery used for industrial and general purpose applications. The parameters of their equivalent circuit model can be determined with a combination of magneto-static and time-harmonic Finite Element Analysis (FEA) simulations. This paper investigates how equivalent circuit parameters depend on current and frequency variations. Calculation results obtained from FEA combined with analytical formulas are experimentally assessed by measurements on a small sample motor used for household appliances.

Cogging torque fast computation method for Surface Permanent Magnet machines based on winding function theory

Various analytical techniques have been proposed in the literature for cogging torque computation in Surface Permanent Magnet (SPM) machines. Among the various available methods, the ones found to be satisfactorily accurate are based on the Maxwells stress tensor, which involves both normal and tangential air-gap field and lead to quite involved calculation formulas. In this paper, a relatively simple alternative is proposed where permanent magnets are modeled as equivalent field circuits to which a well defined winding function can be associated. The approach leads to a plain cogging torque formulation where only the normal no-load air-gap flux density appears. The accuracy of the method is assessed by Finite Element (FE) analysis on some sample SPM machine geometries. The results are shown to match FE simulations with a satisfactory accuracy compared to Maxwell-stress-tensor-based analytical techniques available from the literature.

Improved analytical computation of rotor rectangular slot leakage inductance in squirrel-cage induction motors

Squirrel-cage induction motors are the most widely used type of electric machinery in industrial applications thanks to their rugged, cheap and robust construction. For the fast prediction of induction motor steady-state performance without the need for time-consuming finite-element analyses, the equivalent circuit parameters of the machine need to be calculated, including rotor slot leakage inductances. This paper, in particular, proposes a new improved analytical formula to determine the rotor slot leakage inductance in squirrel-cage induction motors with rectangular-shaped bars. The formula is obtained by solving Poissons equation in the slot domain. Results are assessed by comparison against Finite Element Analysis (FEA). The precision of the presented methodology is also compared to that of approximated analytical models available from the literature. The comparison shows that the accurate formula proposed is in excellent agreement with FEA with errors below 2%, while the simplified model generally leads to errors above 10% which increase as the height-to-width bar ratio decreases.