Search

Nowadays, the pollution caused by the massive use of fossil fuel is a well-known critical issue makes the design of the electrical machines a crucial task because the vast majority of the industrial or household applications are integrated with this kind of technology; hence improving the efficiency of electric motors is expected to result in an extraordinary benefit in terms of energy saving. On the other side, the increasing use of renewable energies (like wind energy, hydropower, tidal energy), combined with the distributed generation concept, call for new electric machine technology and for modern design approaches. This doctoral thesis has been mainly focused on finding analytical procedures to model and analyze some types of electrical machines which are of interest for renewable and distributed generation. The use of analytical approaches is, in some cases, fundamental because numerical methods, mostly based on Finite Element Analysis (FEA), are very inefficient in terms of required time and computation resources. The types of electrical machines considered in this work are various. The attention is first placed on the slotless surface permanent magnet (SPM) topology (with different types of rotor magnetization). Wound-field synchronous generators with both three phase or multiphase stator are then considered. For the various kinds of the machines taken into account, some modeling, design and analysis studies have been conducted in the attempt to fill some gaps in the existing technical literature. In the first part of the thesis the purely analytical modeling of slotless SPM machines is addressed. In the work, it has been considered how the stator slotless design can be combined with different surface permanent magnet rotor topologies. The main efforts of the study have been addressed to the purpose of finding an explicit analytical expression to compute slotless machine torque and no-load back-EMF, covering all the SPM rotor topologies of interest. The subsequent part of the thesis, is about the analytical computation of end-coil leakage inductance in concentric windings. For a good dynamic modeling of machines equipping this kind of winding, it is useful that their mathematical model is implemented and that model parameters are identified with good accuracy. The proposed technique shows a very good accordance compared with a 3D FEA simulation and is also validated though tests conducted on a dedicated experimental set-up. Multiphase machines are attractive for many energy-saving fault-tolerant applications thanks to their higher efficiency and intrinsic resilience to faults. A challenging task in the modeling of multiphase machines for design and simulation is identifying the self and mutual inductances due to leakage fluxes. In this thesis is also presented a novel approach for the leakage inductance determination in multiphase machines based on routine tests combined with very simple 2D magnetostatic FEA simulations. The subsequent part of the thesis is about the design of wound-field synchronous generators specifically required to operate in a distributed generation system where significant fluctuations are known to frequently occur in both voltage and frequency. Design provisions are investigated to improve the generator resilience to these grid disturbances. In the last part of this thesis the attention is focused on multiphase alternators interfaced to DC distribution systems through multiple rectifiers. As a new finding presented as a part of this investigation, it is shown that a short circuit fault occurring on AC/DC rectifier terminals generates a strongly oscillatory behavior with much larger current peaks than could be predicted with conventional models regarding short circuit transients in DC networks.

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.

One of the most common issues in permanent magnet motors for automotive applications is to improve their capability of operating over very wide speed ranges. For this purposes, various motor design techniques have been proposed in the literature. After a brief review of these techniques, the paper presents an idea for improving the flux weakening capability of permanent-magnet reluctance-assisted motors. The idea is based on using an auxiliary field circuit embedded in flux barriers and used to partly demagnetize the motor above the base speed. The field circuit is fed by a brushless permanent-magnet exciter and a diode rectifier. While at low speed the field is not energized, it automatically activates above the base speed by means of a centrifugal switch. As discussed in the paper, the auxiliary field current increases linearly with the speed so as to progressively reduce motor flux with a consequent beneficial reduction in the current to be fed by the inverter for flux regulation.

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).

An accurate prediction of medium-voltage induction motor (IM) power factor in rated conditions is important in the design stage to verify the machine compliance with specifications according to international testing standards. Laboratory and industrial experiences suggest that significant errors in full-load power factor calculation can result for two-pole IMs in particular, due to the eddy currents induced in the solid-steel shaft at rated slip. Such eddy currents are responsible for rejecting the main flux into rotor laminated yokes causing an increase in their saturation and, hence, in the required magnetizing current with respect to no-load conditions. This article proposes a method to study the phenomenon through a combination of analytical and simplified Finite Element (FE) calculations as a computationally-efficient alternative to conventional FE simulations. The results of the proposed approach are experimentally assessed by comparison with measurements on a set of built and tested medium-voltage two-pole IMs of different sizes, showing very good accuracy and computational performance.

The paper is aimed at an analytical prediction of the air gap flux density distribution of a spoke type interior permanent magnet machine, based on its magnetic equivalent circuit (MEC) incorporating the rotor position. The study is initiated by an elaboration of the machine MEC in two particular rotor positions corresponding to the maximum and null magnetic coupling between the stator and rotor which enable the prediction of the direct and quadrature components of the air gap flux density, respectively. The decomposition of the armature magnetic reaction into direct and quadrature components enables the generalization of the prediction of the air gap flux density waveform for any rotor position. The accuracy of the proposed approach is improved taking into account the stator and rotor slotting effects that are analytically incorporated considering complex permeance functions.

The complex relative permeance function is a suitable tool to predict the air gap magnetic field in slotted stator electrical machines. In the literature, the complex permeance function is usually identified by means of numerical techniques based on complex techniques such as conformal mapping or subdomain method. In this paper, an alternative approach is proposed. This method is based on solving the magnetostatic Laplace equation over a portion of the slotted air gap by imposing appropriate boundary conditions in the slot opening region. Such boundary condition tries to represent the theoretical trend of the magnetic field divergence near the corner-shaped ferromagnetic regions. Furthermore, the Carter theory for the slot opening consideration is used. A fully analytical formulation for the complex relative permeance function is obtained, and its accuracy is assessed using finite element analysis.

Direct-drive generators to be used in off-shore wind farms are very large low-speed electric machines which pose remarkable design and manufacturing challenges. Demand forecasts for this kind of machines are urging manufacturers to work out design and technological solutions capable of facilitating series production at competitive costs. This paper presents the development of an interior-permanent magnet generator design and technology aimed at reducing series manufacturing costs while preserving good performance levels. The focus is placed on two of the most critical issues in the machine design and analysis, namely the prediction and reduction of eddy-current losses in stator conductors and in permanent magnets. The proposed design solutions are validated through the construction and testing of a 780 kVA generator prototype conceived for easy scalability to higher power ratings (up to around 2.5 MVA) by core length increase.

In permanent-magnet motors used for vehicle traction application a well-known problem is the need to weaken the flux at high speeds. This is generally done by injecting a suitable demagnetizing current along the d axis. An alternative method has been recently proposed by the authors based on an Interior-Permanent-Magnet (IPM) synchronous motor equipped with a mechanical device which activates at high speeds by centrifugal force causing a significant reduction in the permanent magnet flux with no need for injecting d-axis stator currents. The performance of a road vehicle traction drive employing this machine under a maximum-torque-per-ampere control is investigated in this paper assuming a typical driving cycle. It is shown that remarkable benefits can be achieved in terms of energy saving, overload capability enhancement and speed range extension potential.

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 analys is (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.

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.

Round conductive bars embedded in semi-closed slots are frequently used in the construction of electric machinery, for instance in the squirrel cage of induction motors. Frequently, such as in the study of induction motor steady-state performance at rated slip, it is useful to estimate the slot leakage inductance of these conductors under the hypothesis of no eddy currents and no magnetic saturation. This is usually done through simple approximated analytical formulas available in the literature. In this paper, an improved explicit ready-to-use leakage inductance expression for circular bars embedded in semi-closed slots is derived by solving Poisson's equation in the slot domain. The precision of the proposed formulation is assessed against Finite Element Analysis (FEA) for various slot geometries and is shown to always give very accurate results, with errors below 2%. Conversely, approximated simplified formulas available from the literature are demonstrated to possibly give large errors, which exceed 20% for some explored slot geometries.

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 Poisson's 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.

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 are established in this paper covering all possible multi-phase schemes of practical importance with a unified approach, under the hypothesis of uniform air-gap and negligible magnetic saturation. In a companion paper, All the proposed equations are experimentally assessed though dedicated tests on several multiphase winding schemes.

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.

The accurate determination of stator leakage inductances is presently an open issue in the analysis and testing of multi- phase electric machines. Calculation methods are available, which involve complicated and often poorly precise three-dimensional (3-D) analyses. Experimental determination techniques, using measurements on the wound stator with the rotor removed, are also possible, but quite impractical, as they need to be performed during machine manufacturing or require rotor withdrawal. In this paper, a new approach is proposed to determine all the stator self- and mutual leakage inductances of a nine-phase synchronous machine based on a minimal set (a couple) of magnetostatic finite- element (FE) simulations, and on the measurements taken during no-load and short-circuit routine tests. The procedure is applied to a wound-field salient pole nine-phase synchronous generator for validation, showing good accordance with the results obtained from measurements on the machine with the rotor removed. A discussion is also proposed on the possibility to extend the presented procedure to other multiphase topologies

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.

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.

DC power generation for MVDC grids is in the easiest way realized by a cascade connection of a synchronous generator and a diode rectifier. Due to large peak fault currents, including oscillating transient components, it is important to know in depth the short circuit behaviour of diode rectifiers, mainly during the first moments after the fault. This is helpful to be able to perform a suitable sizing of the system and its protections. In order to obtain an accurate estimation of the transient short circuit current, models for software simulation of 6-pulse and 12-pulse DC generator systems have been implemented. This paper shows the validation of such models by the comparison of simulation results with experimental tests.

The increasing complexity of electric grids including renewable sources causes significant impacts in terms of power quality. This implies that new generators are to be designed so that they can withstand larger and larger disturbances in the amplitude and frequency of the grid to which they are connected. This purpose can be attained by appropriate provisions in the generator electromagnetic design. In this paper, some case studies will be presented comparing the transient stability performance of possible alternative electromagnetic designs of a 4.7-MVA 6-kV for power station based on an Organic Rankine Cycle. The fault-tolerance capabilities of the alternative design solutions are assessed through lumped-parameter simulations in the Matlab-Simulink environment used to predict machine compliance with typical transient functional requirements.

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 Maxwell's 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.

Permanent magnet synchronous machines, especially if designed with open stator slots, may suffer from important cogging torque issues. A special magnetic wedge design has been presented in a companion paper to cope with this problem. The design of such wedge is herein processed through a genetic algorithm in order to find the optimal configuration, which leads to cogging torque minimization. An optimal design configuration is found and compared to non-optimal ones to quantify the benefits that can be achieved by genetic optimization.

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.