C. Bruzzese

Trends in Fault Diagnosis for Electrical Machines: A Review of Diagnostic Techniques

The fault diagnosis of rotating electrical machines has received an intense amount of research interest during the last 30 years. Reducing maintenance costs and preventing unscheduled downtimes, which result in losses of production and financial incomes, are the priorities of electrical drives manufacturers and operators. In fact, both correct diagnosis and early detection of incipient faults lead to fast unscheduled maintenance and short downtime for the process under consideration. They also prevent the harmful and sometimes devastating consequences of faults and failures. This topic has become far more attractive and critical as the population of electric machines has greatly increased in recent years. The total number of operating electrical machines in the world was around 16.1 billion in 2011, with a growth rate of about 50% in the last five years [1].

Analysis and Application of Particular Current Signatures (Symptoms) for Cage Monitoring in Nonsinusoidally Fed Motors With High Rejection to Drive Load, Inertia, and Frequency Variations

In this paper, some original and effective fault indicators for broken-bar detection in power squirrel-cage induction motors are presented. A motor phase-current signature analysis can be performed by evaluating the typical ratios <i>I</i> ₍₇ _- _2s)_f /<i>I</i> _5f and <i>I</i> ₍₅ ₊ _2s)_f <i>I</i> _7f, <i>I</i> ₍₁₃ _- _2s)_f /<i>I</i> ₁₁ <i>f</i> and <i>I</i> ₍₁₁ ₊ _2s)_f /<i>I</i> _13f , etc., which appear in the phase-current spectrum of faulted motors fed by nonsinusoidal voltage sources. The main advantages of the new indicators are the following: (1) accentuate insensitivity to disturbs such as load torque, drive inertia, and frequency variations; (2) low dependence with respect to machine parameters (except the pole number); and (3) linear dependence on fault gravity. They can be directly applied on motors fed by open-loop low-switching frequency gate turn-off/thyristor converters. Railway traction drives are possible targets. Application to mains-fed motors can be tried too, if suitable harmonics are present in the plant supply. A detailed analytical formulation for fault indicators is furnished, based on the multiphase symmetrical component theory; theoretical results have been supported by experimental work, performed by using a motor with an appositely prepared cage, and successively, method validation was achieved on three other industrial motors.

Harmonic Signatures of Static Eccentricities in the Stator Voltages and in the Rotor Current of No-Load Salient-Pole Synchronous Generators

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.

Rotor bars breakage in railway traction squirrel cage induction motors and diagnosis by MCSA technique Part II : Theoretical arrangements for fault-related current sidebands

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.

Computationally Efficient Thermal Analysis of a Low-Speed High-Thrust Linear Electric Actuator With a Three-Dimensional Thermal Network Approach

Permanent-magnet synchronous linear motors (PMSLMs) are more and more frequently used as all-electric direct-drive actuators in those applications where a force needs to be developed along a fixed direction. In this paper, an accurate 3-D thermal model of a PMSLM is derived through a lumped-parameter network approach which exploits all the symmetries in the actuator structure to maximize the spatial density of nodes. Numerically efficient techniques are then proposed to solve the thermal network analytically. Some experimental validations are finally presented based on the thermal testing of a laboratory prototype.

Synchronous generator eccentricities modeling by improved MWFA and fault signature evaluation in no-load E.M.F.s and current spectra

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.

Magnetic optimization of a fault-tolerant linear permanent magnet modular actuator for shipboard applications

The reliability is a key requirement in electric actuators to be used for moving ship control surfaces. This paper addresses a fault-tolerant design for an innovative naval actuator based on an inverter-fed permanent magnet linear synchronous motor. Due to the highly non-conventional actuator concept, a detailed FEM modeling approach is presented in this paper for comparison of different design solutions. A fault tolerant modular stator structure is proposed. The modular winding can be designed based on an integer-slots or fractional-slots concept. The two solutions are compared in this paper, considering encumbrance and thrusting force performances. The linear motion can be reverted to rotary motion through a prismatic-rotoidal joint for coupling to on board ship steering gears used to drive control surfaces (rudders, stabilizing fins). A drive prototype has been built and is presently under testing to assess and refine the results of the FEM modeling presented in the paper.

Static and dynamic rotor eccentricity on-line detection and discrimination in synchronous generators By No-Load E.M.F. space vector loci analysis

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.

Model-based eccentricity diagnosis for a ship brushless-generator exploiting the Machine Voltage Signature Analysis (MVSA)

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.

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.