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Conventional rotating electrical machines are characterized by stator and rotor structures featuring a cylindrical geometry around the shaft rotational axis. Although advantageous for mechanical reasons, the cylindrical geometry results in overall machine shapes and dimensions that may be unsuitable for installation. This particularly occurs in shipboard applications, where electric motors and generators are subject to stringent room constraints and need to be fit in unusually shaped compartments. This paper presents the development and test of a dual-shaft rotating permanent-magnet electric machine prototype having a linear structure that facilitates its onboard use for such applications as electric propulsion and rudder actuation. In fact, the proposed machine topology has overall dimensions which can be adjusted to fit the space available for installation. The operating concept and the detailed electromechanical design of the machine are first described. Then the manufacturing and factory test of the prototype under inverter supply are illustrated. Finally, the validation of the prototype as a boat propulsion variable-speed inverter-fed motor is presented. It is proved that, despite of its highly non-conventional electromechanical design, the machine can be effectively fed from a general-purpose inverter for permanent magnet motors.

This paper reports on a research project commissioned by LEONARDO Aircraft S.p.A. to the DAEEE for improvement of diagnostic and protection systems of avionics generators. The arc faults arising in worn brushes and slip-rings supplying the propeller anti-ice rotating loads on board aircrafts are of concern here. Intermittent arc faults may arise, causing current and torque ripples potentially exciting torsional resonances leading to the breakage of the coupling joint (‘mechanical fuse’) between turbine and generator and, hence, to a permanent out of service of the latter. Common non-selective protections are not effective against these types of faults. The study of alternative solutions requires accurate simulations of the complex electromechanical system. This work moves from the construction of the system model, for time-stepping FEM-circuit co-simulations by Maxwell/Simplorer software. Then, fixed-frequency intermittent faults matching experimental tests are analyzed. The causes of failure are definitively determined and solutions to overcome the problem are proposed. A companion paper shows an original online and sensorless method for speed and torque estimation and detection of harmful torsional resonances resulting from intermittent arc faults.

This paper compaerethe analytical mathematical model of active magnetic bearings (AMBs) with a FEM based model, developed in the framework of the Ansys©software. In both analyses, attention will be given to the force generated by the bearings on the rotor of a synchronous machine coupled with a flywheel (the purpose of this system is the storage of energy using all the possible advantages offered by AMBs). Firstly, it is necessary to represent the basic mathematical model of an AMB and its application on both flat and cylindrical geometry. Secondly, FEM simulations are performed and the relevant results are compared. Then, Energy Storage System (ESS) shall meet the requirements set out in advance. Finally, all the results based on the mathematical model are used to studya real system.

The doubly-fed induction machine has long been reputed as a curiosity with few practical applications. Consequently, very often there was not enough space to treat this machine in undergraduate electrical machines curricula in the past. But times changed and today this machine is often an integral part of wind turbines as electric generator. This way, getting familiar with the relevant operating principles becomes a desirable task for undergraduate students. This paper proposes a circuit-based theoretical approach which may serve as a template for an undergraduate lecture. To this end, firstly the basic characteristics of the wind turbine are introduced. Afterward, the operating principles of the doubly-fed induction machine are explained with reference to the single-phase equivalent circuit. A method based on the superposition principle is proposed for circuit solving, exploiting the concept of ‘virtual’ synchronous speed. An example given in the paper further illustrates the principle of operation and shed light on the power flows.

A wound-rotor three-phase induction generator with eccentric rotor and parallel connections in both stator and rotor can be represented by five equivalent sequence-circuits of virtually centered-rotor machines, as mathematically proved in previous papers. The five circuits allow the calculation of stator and rotor fault-related currents, in form of 2(p±1)-pole space vectors for 2p-pole machines. The formal calculation of fault-related currents is very useful to define fault indicators and tools for diagnosis, and in case of parallel-connected stators (like in many wind generators) the split-phase current signature analysis becomes viable. In fact, by reversing the equations linking split-phase current amplitudes and eccentricity degrees, original fault indicators are carried out in this paper. The new indicators are validated by simulations of a 1800kW wind induction generator with static, dynamic, and mixed faults in full-load conditions. The simulations are carried out by using a full FEA model of the motor, implemented by time-stepping FEM-circuit co-simulation software (Maxwell-Simplorer). A good match is obtained between theoretical calculations and simulations.

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed, and the difference of failure pattern with respect to more common bar breakage mechanisms (overloads, heavy start-ups, etc.) is evidenced. Some theoretical models for modal frequency calculation are presented, which may help to identify the origin of vibrations and the fault root-cause in faulted drives. Dynamic simulations of the drive have been carried out for airgap torque calculation and bar length optimization.

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 study of electric machines subject to rotor eccentricity is usually performed using a winding and permeance function approach. In a companion paper, an alternative method is investigated based on conformal mapping. The geometry of a an eccentric round-rotor machine has been mapped into a uniform air-gap topology by means of Moebius conformal map. In this paper, the approach is used to derive explicit formulas to compute the unbalanced magnetic pull of the eccentric machine. It shown how, unlike conventional winding-function theory, the proposed method yields closed-form formulas which do not involve series expansions nor require numerical integrations and are thus particularly fit for implementation into fast simulation programs. Such expressions are assessed by comparison with Finite Element Analysis (FEA).

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed. The measurements are matched with dynamic simulations of the complete electromechanical system comprising a vibrating-cage motor and the static converter. Some theoretical models for modal frequency calculation are also presented, which may help identify the origin of vibrations and the fault root-cause in faulted drives.

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.

One of the key tasks in the modeling of electric machines is the analytical expression of self and mutual phase inductances as a function of the rotor position. Winding and permeance functions are a powerful tool for this purpose. This paper investigates the expression of the air-gap permeance function for a round rotor machine subject to pronounced eccentricity. An expression is sought in the form of a Fourier series expansion in order to make it suitable for modeling purposes. Although derived through non trivial algebraic passages, the final expression proposed is proved to be accurate and computationally efficient as the theoretically infinite sums involved in it exhibit a very rapid convergence to their asymptotical values.

The ships have long been an easy play-ground for hydraulic technologies. When high torques and low speeds are demanded on board ship, both technical and traditional reasons usually lead to hydraulic solutions. In fact, the conventional electrical machines are better suited for low-torque, high-speed applications, and coupling gears must be used to reduce the speed. However, oil-free direct-drive alternatives can be conceived, by new concepts of low-speed permanent-magnet actuators fed by low-frequency inverters. This paper shows the experimental results obtained with a linear permanent-magnet motor designed for direct coupling to rudder stocks or stabilizing fins. The prototype weights 700kg and features a thrust-to-weight ratio of 5. Even better results can be obtained by using composite materials for the machine frame. The machine features a full structural redundancy and inherent fault tolerance, thanks to the modular construction. The present research has been carried out in the framework of the Italian Defence's Research National Program.

An innovative design of a device embedding the functions of both power transformer and shunt reactor is presented. The device is thought for installation onboard substation platforms of long HV/EHV AC cable connected offshore wind plants. The compact design saves much space on the hosting offshore platform, which can be sized smaller and cheaper. Machine sizing as well as cost saving considerations are exposed in this paper.

This work shows a method to quantify rotor eccentricities in synchronous machines by exploiting the unbalance caused in the split-phase currents. The paper first develops a machine model comprehensive of eccentricities and parallel circuits in the stator, by using symmetrical components. Then, the model is used for formal calculation of the unbalanced currents. Finally, the equations are reversed to obtain eccentricity degrees from current measurements. Practical formulas are given for fault assessment, only requiring machine line voltage and synchronous reactance. The method can be applied on load. This paper provides full details of the theory underlying the method. The theory also clarifies some aspects about split-phase currents, not deepened before. It is proven that the air gap flux modulation due to eccentricities, acting through additional 2(p ±1) -pole flux waves in 2p-pole machines, stimulates additional currents, which circulate in the stator and turn into 2(p ±1)-pole rotating space vectors in the complex domain. Vector trajectories have shape and amplitude dictated by eccentricity type and degree, respectively. This study is limited to 2p-pole machines with p ≥ 2. The theory is corroborated by simulations of a practical 1950-kVA generator in this paper. Experimental proofs and simulations of a laboratory 17-kVA machine are provided in a sequel of this paper.

Synchronous reluctance (SynRel) motors with rotor flux barriers are gaining increasing attractiveness in automotive applications thanks to their cheap, rugged and magnet-free rotor construction. When equipped with a split-phase stator winding and supplied from multiple inverters, these machines can exhibit further merits as traction motors in regard to enhanced fault tolerance compared to conventional three-phase solutions. Since SynRel motors are usually designed through iterative optimization techniques, it is highly desirable to have accurate and fast methods to predict their performance without the need for time-consuming finite element analysis (FEA) simulations. An analytical procedure is set forth in this paper to analytically model and simulate a SynRel motor with a split-phase stator winding through a magnetic equivalent circuit (MEC) technique. MEC parameters are computed from analytical formulas describing the air-gap magneto-motive force distribution and the magnetic field inside flux barriers. As an output, the air-gap flux density of the SynRel motor can be computed through the presented technique at any operating point. Results are positively assessed by comparison with FEA simulation on a sample SynRel motor including magnetic saturation effects.

A 1130kW squirrel-cage induction motor used for railway traction is analyzed in this paper in case of eccentric rotor. Since the motor has parallel-connected windings, the air-gap unbalance makes rise unbalanced currents in the stator branches, and the split-phase current signature analysis can be attempted for fault detection. The split-phase currents are better analyzed when transposed in the complex domain, by using symmetrical components. Moreover, the rotor cage currents must be carefully taken in account in the model, since they develop a powerful damping effect on the signature of static eccentricity. This paper shows that the faulted machine can be represented by the superimposition of five equivalent circuits corresponding to five centered-rotor virtual machines, useful for fault-related split-phase current calculation. The mathematical modeling shown in this paper is useful to carry out a diagnostic method, as shown in a companion paper. The latter also reports confirmatory simulations of the faulted 1130kW motor, based on winding function approach.

A squirrel-cage three-phase induction motor with eccentric rotor and parallel connections in the stator can be represented by five equivalent sequence-circuits of virtually centered-rotor machines, as mathematically proved in a companion paper. The five circuits allow the calculation of stator and rotor fault-related currents, in form of 2(p±1)-pole space vectors for 2p-pole machines. The formal calculation of fault-related currents is very useful for defining fault indicators and tools for diagnosis, and in case of parallel-connected stators (like in many traction motors) the split-phase current signature analysis becomes possible. This paper validates the sequence-circuit modeling by comparing the theoretical results with simulations of a 1130kW traction induction motor with static, dynamic, and mixed faults in both no-load and full-load conditions. The simulations are carried out by using a full mesh-model of the motor, implemented by winding functions. A good match is obtained between theoretical calculations and simulations. Moreover, the role of the cage damping on the eccentricity-related current signatures is evidenced. It is shown that the cage damps not only the signature of static fault but also that of dynamic fault, and that the damping itself is load-dependent in the latter case.

Surface Permanent Magnet (SPM) machines with slotless stator design are used is some applications where torque pulsations and/or the additional losses due to slotting effects need to be eliminated. These machines have a geometric structure allowing for an accurate analytical solution for the magnetic fields, which is desirable especially for optimization purposes. This paper derives a compact but exact analytical formula for the load torque computation in slotless SPM machines. The formula has the advantage of well highlighting the dependence of the load torque from machine construction details. It can be therefore useful in the design and dimensioning stage as a fast alternative to time-consuming Finite Element (FE) analysis.

This paper describes some tests carried out on board an Italian Navy's ship for validation of a novel technique for on-line monitoring of shaft misalignments in working synchronous generators. The technique is based on the split-phase current signature analysis principle. Rotor eccentricities impact on the split-phase currents of stator windings with parallel connections, producing asymmetry in both amplitudes and phases. Symmetrical components can be applied to carry out a straightforward diagnosis starting from the measurement of those currents. This paper also describes the practical measurements, in presence of unbalanced and step-changing loads. Loads on board ships are usually unbalanced and may change suddenly. However, it is shown that the load-related unbalance can be separated from the eccentricity-related unbalance in the split-phase currents. The paper shows the basics of the split-phase current signature analysis, the practical installation of current transformers in the test generator, the instrumentation set-up, and the results of measurements with various loads, besides some check simulations carried out by using winding functions.

Synchronous generators (SGs) are vital ganglia of the ship. SGs supply survival/defense/attack systems, propulsion, etc., so deserving on-line monitoring. Rotor unbalances shorten bearing lifetime, with overheating and stator-rotor rubs. No technique is definitely established for on-line evaluation of rotor eccentricities (REs) in working SGs. Actual manual air gap checks are time-consuming, and require unit shut-down and dismounting. This paper shows a method based on measurement of split-phase currents in stator windings with parallel branches. A symmetrical component transformation reverts the split-phase currents in rotating space vectors. The (p±1)-order vectors provide an affordable estimation of REs in 2p-pole machines. Rogowski coils or current transformers can be installed in the terminal box, or on winding front-ends, for current measurement. A LabView virtual instrument (VI) was developed for method application. This paper also describes some tests carried out on board an Italian Navy's ship for on-line monitoring of shaft misalignments in a loaded 2MVA SG. Loads on board ships are usually unbalanced and may change suddenly. However, it is shown that the load-related unbalance can be separated from the eccentricity-related unbalance in the split-phase currents. Good assessment of air gap percentage reduction in the tested 2MVA machine was finally obtained.

2-Pole turbogenerators are usually very high power round-rotor synchronous machines used in generating plants. Turbogenerator monitoring is crucial, especially in nuclear plants. Very long turbogenerator rotors experience excessive run-out due to thermal, mechanical, and magnetic unbalances, resonances, etc. The rotor eccentricity causes vibrations and winding and frame loosening. Since the stators of turbogenerators are often parallel-connected for maximum power output rating, the rotor eccentricities also make rise unbalances in the split-phase currents. This paper resorts to the split-phase current monitoring for detection and estimation of rotor eccentricities. A method is proposed, based on a special theoretical analysis of 2-pole round-rotor machines, through symmetrical components. Time-stepping finite-elements are used for confirmatory simulations of a 500MVA machine.

The rotor eccentricity (RE) of synchronous generators (SGs) is assessed in this paper, through measurement of internal currents in stator windings with parallel loops. REs cause an airgap flux density amplitude modulation, resulting in four superimposed flux distributions with 2(p±1) poles in 2p-pole SGs, which correspond to four virtual rotors. The armature winding is decomposed through symmetrical components, which define two additional 2(p±1)-pole virtual armatures. Four fault-related 2(p±1) -pole virtual machines are so obtained, whose currents provide a good estimation of REs. Theory and simulations for a six-pole 1950kVA machine are provided in this paper. Simulations of the 1950kVA SG and tests on a four-pole 17kVA SG with regulated eccentric flange provided confirmation of the theoretical work.

This paper shows a method for rotor eccentricity (RE) evaluation in synchronous machines (SMs) by measurement of internal currents in stator windings with parallel connections. REs produce a space/time air gap flux amplitude modulation, resulting in additional 2(p±1)-pole flux distributions in 2p-pole SMs. The correspondent internal reaction currents are translated in the complex domain by a Fortescue's space-transformation. 2(p±1)-pole current space vectors trace epitrochoidal trajectories, and provide an affordable estimation of static and dynamic REs. Practical formulas were carried out for RE assessment, which require few machine parameters. Experiments were performed on a 17kVA SM with regulated eccentricity and reconfigurable stator windings. The results are compared with those obtained through transient time-stepping FEM simulations.

This paper shows that a static eccentricity makes rise a double fundamental frequency ripple in the rotor current of salient-pole synchronous machines. This ripple leads, under conditions ruled by the stator windings, to precise signatures in the no-load voltage spectrum. Both rotor current ripple and voltage harmonics can be used for diagnosis. The fault-related voltage harmonics are theoretically previewed in this paper through analysis of the windings. Simulations performed by using the winding function approach confirm the theoretical predictions. A four-pole 15-kVA generator was used for experiments, featuring an innovative flange with exact and easy regulation of mixed-type eccentricities. Rotor current monitoring has advantages of single-sensor measure and accentuate fault sensitivity. Experiments also showed an additional rotor-rotation frequency ripple in the rotor current, in case of mixed-type fault.

This paper studies the reaction currents of a synchronous machine (SM) with general rotor eccentricity (RE) and parallel paths in the stator. REs make rise loop currents which can be translated in the complex domain through a Fortescue's space-transformation. Extensive simulations show that mainly 2(p±1)-pole current space vectors are stimulated by REs in 2p-pole SMs, with typical cardioid-shaped patterns. The simulation results were justified by using multi-phase symmetrical components, and theoretical formulas were obtained for fault severity assessment starting from loci amplitudes. Experimental verification was shown in a companion paper, where a Labview virtual instrument detects natural and artificial REs in a 17kVA four-pole generator.

This paper shows a laboratory implementation of a new on line monitoring method for detection of synchronous generator (SG) rotor eccentricities (REs). A 17kVA 4-pole wound-rotor salient-pole SG with reconfigurable stator poles and a regulated mechanical flange has been instrumented with Hall sensors, data acquisition board (DAB), and a LabView virtual instrument (VI) for RE detection through internal current space vector analysis (ICSVA). The ICSVA has been theoretically described in a companion paper; test-bed and experimental performances are described here. REs make rise loop currents in the SG's stator with parallel connections. Pole currents are transformed through a Fortescue's matrix, and 2(p±1) pole current space vectors are used for RE detection in 2p-pole SGs. The analytical signals were elaborated by direct detection of maximum and minimum amplitude of cardioid trajectories, and in alternative, by FFT. Both the methods were tested and discussed. A commissioning procedure for practical method set-up is also presented.

Static eccentricity detection can be performed by measurement of a double fundamental frequency ripple in the rotor current of synchronous machines. The ac ripple leads to clear signatures in the no-load terminal voltage spectra, pushing up some voltage harmonics normally smaller in the healthy machine. Rotor current ac ripple and related voltage harmonics can be used for diagnosis, since they are sensitive fault indicators. In Part I of this work, general criteria to preview the fault-related voltage harmonics for no-load machines affected by static eccentricity are carried out, based on the analysis of the winding structure. Simulations performed by using a winding function-based model, which takes in account the field shape in the inter-pole regions, confirm the theoretical predictions. In Part II, a 15kVA generator with regulated flange has been used for experimental validation with various eccentricities.

In this paper a study about the different effects of static and dynamic rotor eccentricities on the external electric variables of a brushless salient-pole synchronous generator (SG) is presented. Air-gap irreularities and related monitoring techniques for synchronous machines have been studied in the past mainly about large vertical hydro-generators [1], but similar issues have been recently recognized as worthy of attention for on-board ship generators. Static and dynamic rotor eccentricities were simulated in this work, for a medium power ship brushless generator, by using a dynamic model including inductances computed by parametric 3D FEM and MWFA-based analyses of the faulty machine, and taking in account the damping effect of the parallel paths inside the machine. Currently voltage, and no-load e.m.f. sleady-state waveforms were analyzed by FFT. No-load vollage harmonics are sensitive fault indicators, whose harmonic content directly reflects level and type of eccentricity, and which can be used ror diagnostic purposes.

In this paper a study about the different effects of shaft eccentricities on the external electric variables of a brushless salient-pole synchronous generator for ship onboard service is presented. Static and dynamic rotor eccentricities were simulated by using a dynamic mesh-model including inductances computed by an improved MWFA-based analyses and by 3D FEM, and taking in account the damping effect of the parallel paths inside the machine. Current and voltage steady-state waveforms were analyzed by FFT; no-load voltage harmonics appeared as good candidates as fault indicators, given their dependence on level and type of eccentricity. Some voltage measurements on the real machine are also reported and discussed.

The continuous enhancement of microprocessor (muP) performances, operating systems (OSs) and communication networks are pushed forward by the rising request of graphical, multimedia and control applications and of high-speed industrial and general-purpose communication tools. OSs have been embedded in a ever-growing number of applications, and their use has been recently extended to industrial drives. In this work an experimental PWM motor drive has been realized by using a common x86-based platform supported by a Linux-compatible operating system. Thanks to a multi-tasking embedded OS, the drivepsilas CPU can simultaneously perform parallel processes such as drive control, drive and machine diagnostics, and high-speed communication toward field and upper plant levels, each process being assigned with a different priority level. The drive is mainly dedicated to future research about these enhanced functionalities. Hardware, devices, and Linux-OS resources used for drive realization have been described, with experimental results.

In this paper a study about the different effects of static and dynamic rotor eccentricities on the external electric variables of a salient-pole synchronous generator is presented. Air-gap irregularities and related monitoring techniques have been studied in the past mainly about large hydro-generators (with practical applications), but similar problems have been recognized for on-board ship synchronous generators. These latter require a different approach, i.e. non-invasive monitoring. Static and dynamic rotor eccentricities were simulated in this work, for a ship-application sized generator, by using a dynamic model including inductances computed by parametric 3D FEM analysis of the faulty machine. Current, voltage, and no-load e.m.f. steady-state waveforms were analyzed by FFT and space-vector approach. No-load e.m.f. space vector is a sensitive fault indicator since its amplitude largely increases with the level of absolute eccentricity; furthermore, it is possible to discriminate the static eccentricity from the dynamic eccentricity utilizing the space vector loci ovality.

In this paper an accurate modeling of air-gap length function in polar and interpolar regions of non- isotropic synchronous generators is presented, for use in the MWFA (modified winding function approach), to obtain accurate machine simulations with eccentricities. Air-gap in interpolar regions is usually poorly modeled in current literature. Inductances obtained by the improved MWFA have been compared with 3D-FEM model obtained ones. External no-load e.m.f. and current spectra have been evaluated for a medium-power alternator with non-aligned rotor, and with various static and dynamic eccentricity degrees. Fault signatures appear all over the spectra, with variable entities, whose comprehensive evaluation can be used for fault severity assessment.

Inverter-fed cage induction motors can run at variable frequency and speed, and the diagnostic techniques to be applied for these motors must take in account frequency variations. Especially in variable frequency railway traction drives, bar breakage problems have been reported as a consequence of large sixth harmonic torques produced by the non-sinusoidal feeding. MCSA can be applied for motor diagnostics in these cases, but suitable indicators must be used for reliable fault severity assessment. Harmonic current side-band (HCSB) based Indicators (HCSBIs) for broken bars detection present a good rejection to frequency variations, as well as to motor load level, drive inertia, and motor parameters. In this paper, three industrial motors with broken bars have been tested for HCSBIs performance evaluation with different feeding frequencies, and the results have been reported and discussed.

In this work the results of a measuring and simulation campaign performed on an experimental induction motor with artificial broken bars are presented, aiming to extract spectral current components useful for fault detection. With non- sinusoidal feeding a lot of frequencies other than (1-2s)f (lower- sideband, LSB) and (1+2s)f (upper-sideband, USB) arise, in proximity of low-order current harmonics, that can be good fault indicators. Current signature analysis-based techniques (CSABT) can take advantage by these harmonic current sidebands (HCSBs) since they are less dependent on load, frequency, drive inertia and motor parameters, so producing a better fault estimation. The measures were matched with simulations carried out by using a topologically complete mathematical complex model. Machine parameters were obtained by 2D and 3D FEA. A comparison between LSB/USB-based indicators (LUSBBIs) and HCSB-based indicators (HCSBBIs) suggests to use the latter whenever a broken bar diagnosis must be attempted for inverter-fed motors.

Experimental work is presented in this paper, that clearly indicates the good behavior of some novel rotor bar fault indicators for converter-fed squirrel cage induction motors. The ratio of the fifth harmonic upper sideband on the seventh harmonic and the ratio of the seventh harmonic lower sideband on the fifth harmonic seem to be good broken bar indicators, largely independent on load conditions and drive inertia, and with sufficient rejection to frequency variations. We remember that classical indicators used in MCSA (such us the ratio of the first harmonic lower sideband on the first harmonic) depend heavily on those conditions. The relationship between the new fault indicators and the number of broken bars is quite linear, too. Harmonic current side-band based indicators (HCSBIs) have been previously derived by the authors from the multi-phase symmetrical component theory (MPSCT). Results of broken bar tests conducted on an experimental cage motor with variable fault gravity and load level have been reported and commented in this paper, for HCSBIs performance evaluation about fault severity assessment in case of broken bars.

Some new fault indicators for rotor bar breakages detection in squirrel cage induction motors have been theoretically previewed and experimentally proved. They are based on the sidebands of phase current upper harmonics, and they are well suited for converter-fed induction motors. The ratios I(7-2delta)f/I5f and I(5+2s)f/I7f are examples of such new indicators, and they are not dependent on load torque and drive inertia, as classical indicators (based on lower and upper sideband of first harmonic) do. So, the MCSA technique effectiveness is greatly improved, when applied on motors fed by low switching frequency converters (with natural harmonics) or by high switching frequency converters (with harmonic injection). Applications with grid-connected motors can be studied, too. Motor mathematical modeling was based on the multiphase symmetrical components theory; experimental work was performed by using a prototype with an appositely prepared cage, and successively method validation was achieved on other three industrial motors

By exploiting an appositely-made cage induction motor, same measurements of stator and bar currents were done with progressive rotor damage (increasing number of broken bars), for a complete characterization of motor current spectra under fault conditions. We used a three-phase wound-rotor induction machine that has been converted in a squirrel-cage machine with current-measuring capability on two bars. A multiphase symmetrical components-based model was carried out, for a better theoretical understanding of sidebands genesis, and for experimental results matching. Lower (LSB) and upper (USB) sidebands of fundamental current were detected, that characterize bar breakages; moreover, bar current spectra confirm the presence of direct and reverse slip-frequency multiphase symmetrical components (which produce rotor current unbalance), that are forecasted by mathematical theory. Some tests were performed by applying MCSA technique for fault-severity assessment; LSB and USB-based indicators have been tested and discussed

In a companion paper (Part I) the bar breakages for a 1.13 MW induction motor used in high speed railway traction drives were studied, by simulating the whole system (converter + motor) and by analyzing frequencies and amplitudes of current sidebands, for various loads (both active and inertial) and with increasing fault severity. It was recognized that faults can be characterized by (1-2s)f sideband, but results vary with load torque and drive inertia. Other sidebands appear less dependent from these disturbs, like (5+2s)f. Unfortunately, no works presented simple and general formulas about sidebands. In this paper, theoretical formulations for computation and prediction of rotor fault-related sideband frequencies and amplitudes are given, aiming to obtain relations useful for diagnostic purposes (fault severity assessment by MCSA), and by using Part I as a case-study. A decomposition by multiphase symmetrical components is performed, taking in account space harmonics. Formulas for (1-2s)f sideband amplitude are carried out, extendable to many other sidebands.