Mechanical Unbalance Research Articles

Current-Based Detection of Mechanical Unbalance in an Induction Machine Using Spectral Kurtosis With Reference

This paper explores the design, online, of an electrical machine's healthy reference by means of statistical tools. The definition of a healthy reference enables the computation of normalized fault indicators whose value is independent of the system's characteristics. This is a great advantage when diagnosing a broad range of systems with different power, coupling, inertia, load, etc. In this paper, an original method called spectral kurtosis with reference is presented to design a system's healthy reference. Its principle is first explained on a synthetic signal. This approach is then evaluated for mechanical unbalance detection in an induction machine using the stator current instantaneous frequency. The normalized behavior of the proposed indicator is then confirmed for different operating conditions, and its robustness with respect to load variations is demonstrated. Finally, the advantages of using a statistical indicator based on a healthy reference compared with a raw fault signature are discussed.

Fault Detection of the Electrical Motors Based on Vibration Analysis

Abstract Detection and diagnosis of faults while the system is running can help to reduce all kind of losses. Mechanical faults, usages, slackness's, cause different noises and vibrations with different amplitude and frequency against the normal sound and movement of the equipment. Induction motors are present in any processes and systems and for this it is important to known the types of faults which can occur during its working. This paper presents briefly some electrical and mechanical faults in induction motors and how these faults influence the motor vibration. Each type of fault, electrical faults or mechanical faults, produces a vibration whit a specific frequency. Through monitoring and analyzing the vibration spectrum these specific frequencies can be detect and through these we can detect usages, slackness and forces which act and produce the fault. It is an undestroyed method to find out the functioning problems. If the amplitude of these vibrations reach to a certain level the fault can be detect and identify. The paper presents a possible two degree of freedom model for electrical motors. Motion equations and transfer function are deduced and through simulation can be followed how the vibration spectrum is changing for some important faults, like electrical imbalance, mechanical unbalance, looseness, bearings, external effects, base structure or resonance, which can be reduce to mass or spring variation in the system model. To obtain the behaviour of the system, the equations of the motion and Matlab simulations was used.

Induction machine faults detection using stator current parametric spectral estimation

Current spectrum analysis is a proven technique for fault diagnosis in electrical machines. Current spectral estimation is usually performed using classical techniques such as periodogram (FFT) or its extensions. However, these techniques have several drawbacks since their frequency resolution is limited and additional post-processing algorithms are required to extract a relevant fault detection criterion. Therefore, this paper proposes a new parametric spectral estimator that fully exploits the faults sensitive frequencies. The proposed technique is based on the maximum likelihood estimator (MLE) and offers high-resolution capabilities. Based on this approach, a fault criterion is derived for detecting several fault types.The proposed technique is assessed using simulation signals, issued from a coupled electromagnetic circuits approach-based simulation tool for mechanical unbalance and electrical asymmetry faults detection. It is afterward validated using experiments on a 0.75-kW induction machine test bed for the particular case of bearing faults.

INVESTIGATION ON THE DIAGNOSIS OF SIMPLE AND COMBINES MECHANICAL FAULTS IN ASYNCHRONOUS MOTOR BASED ELECTRIC DRIVES

In this study, the problems of mechanical unbalance , parallel and angular misalignments and their combinations are analyzed experimentally. Such freq uent defects in the drives mainly in the major powe rs are also responsible for the bearings degradation. However, they have not raised the attention of rese archers, given the complexity of their modeling. The combination of the phasic current signal analysis and the neutral current by the FFT supplemented by visual i nterpretation of patterns models these defects resu lting from the 3D representation. The results obtained by using the proposed method show the efficiency to provide an accurate diagnosis of the state of the e lectric drive undergoing to isolated and combined mechanical defaults to a maintenance staff not nece ssarily expert of mechanical failure. The innovativ e approach validated experimentally on a 5.37 KW motor, offers an efficiency to provide an accurate diagnosis to a maintenance staff not necessarily co mposed of experts in this field.

Effect of mechanical unbalance induced by pregnancy on the muscle load of the erector spinae during a sit-to-stand motion

Effect of mechanical unbalance induced by pregnancy on the muscle load of the erector spinae during a sit-to-stand motion

Comparative investigation of diagnosis media for induction machine mechanical unbalance fault

For an induction machine, we suggest a theoretical development of the mechanical unbalance effect on the analytical expressions of radial vibration and stator current. Related spectra are described and characteristic defect frequencies are determined. Moreover, the stray flux expressions are developed for both axial and radial sensor coil positions and a substitute diagnosis technique is proposed. In addition, the load torque effect on the detection efficiency of these diagnosis media is discussed and a comparative investigation is performed. The decisive factor of comparison is the fault sensitivity. Experimental results show that spectral analysis of the axial stray flux can be an alternative solution to cover effectiveness limitation of the traditional stator current technique and to substitute the classical vibration practice.

Inertia effects as a possible missing link between micro and macro second-order work in granular media

This paper is concerned with a theoretical question as to the definition of instabilities in a granular assembly and its proper formulation at the microscopic level. Recently, this question has taken up much prominence with the emergence of intriguing failure modes such as diffuse failure associated to unstable plasticity of granular materials and microstructural instabilities. An analysis of the second-order work as a general and necessary criterion to detect instabilities is conducted both at the macroscopic and microscopic levels including large deformations. On the basis of a micromechanical analysis of a body consisting of arbitrary interacting particles in a representative element volume (REV), a general formula is derived to quantify the microscopic second-order work involving local variables on the grain scale. The latter emerges as a sum of a configurational term that involves contact forces between neighboring grains, plus a kinetic part consisting of the mechanical unbalance of intergranular forces under dynamics at incipient failure. The present analysis is thought to serve as a clarification of the question of failure in geomaterials typified by a transition from static to a dynamic regime with release of kinetic energy originating from microstructural interactions.

Elastic multiple-mass model for rotordynamic analysis of flexible electrical rotors

The paper presents an elastic multiple-mass model for rotordynamic analysis of flexible electrical rotors supported in sleeve bearings, considering mechanical unbalances and electromagnetic forces. This model has been especially developed for flexible electrical rotors, which operate near below or near above the first critical bending speed of the rotor. Using this simplified model, a static rotor active part eccentricity can be simulated and the orbital movement of the rotor can be calculated. Additionally, the influence of different balancing concepts—elastic balancing versus rigid balancing—on the shaft vibrations is analyzed. To verify the model, a finite element analysis was performed, which indicates a satisfactory match. On the one hand, the aim of the paper is to derive an elastic multiple-mass model for rotordynamic analysis of flexible electrical rotors for special boundary conditions. On the other hand, the aim is to show the mathematical coherences—based on a simplified model—between the rotordynamics, the oil film characteristics of the sleeve bearings, the elasticity of the rotor structure, the electromagnetics and the balancing concept.

Model-based control concepts for vibratory MEMS gyroscopes

In this contribution, a systematic method for the design of open- and closed-loop controllers for vibratory MEMS gyroscopes based on so-called envelope models will be presented. The methodology will be exemplarily carried out for a gyroscope with electrostatic actuation and read-out elements. The specifically designed capacitive actuators of the gyroscope are capable of compensating the system’s inherent mechanical unbalance (quadrature compensation) as well as the system’s response to an external angular rate (force feedback). The utilized envelope model solely captures the relevant system dynamics of the gyroscope while at the same time describing the actuation and read-out mechanisms simplified to a suitable level of detail thus providing the basis for an efficient and systematic control design.In order to demonstrate the proposed methodology, an optimized start-up strategy for the control of the primary oscillation is designed. Furthermore, the approach is utilized for the deviation of a basic quadrature controller for the secondary oscillation. In order to account for the typically weakly damped open-loop dynamics of the gyroscope and the transient coupling between the quadrature and the angular rate signal a more sophisticated combined concept of closed-loop quadrature and force feedback control is introduced. Both simulation and measurement results obtained for a prototype gyroscope validate the mathematical models and prove the feasibility of the proposed concepts.

Decoupled Quadrature and Force Feedback Control of Capacitive MEMS Gyroscopes

AbstractThis contribution presents the mathematical model of a capacitive MEMS gyroscope containing specific actuators which are capable of compensating mechanical unbalance (quadrature error) as well as the system response to an external angular rate (force feedback). Based on this model a quadrature controller is systematically designed. In order to eliminate the typically weakly damped open-loop dynamics of the gyroscope additionally a combined concept of closed-loop quadrature and force feedback control is introduced. The presented theory is validated by means of both numerical simulations and measurement results utilizing a prototype gyroscope.

On-line mechanical unbalance estimation for permanent magnet synchronous machine drives

An on-line mechanical unbalance estimation for permanent magnet synchronous machine (PMSM) drives is presented. At high speeds, either rotor or mechanical load unbalance generates vibration that can cause mechanical failure such as a defect of a bearing or shaft. To prevent failure and minimise mechanical vibration, the amount of the rotating unbalance should be detected in real time. The information of the estimated unbalance can be utilised to manage an operation to achieve the durability of mechanical parts. The dynamic nature of a mechanical system with a PMSM with regard to unbalance is investigated. From this examination, the models of the stator current in time and frequency domains for PMSMs are presented to develop a robust real-time unbalance estimation scheme. Based on the modelling and analysis of a rotating mechanical system using a PMSM drive, a compact unbalance estimation algorithm is proposed. The algorithm is combined with an open-loop torque observer and an unbalance estimator using a filter. Experimental results show the validation of the developed model and the proposed on-line unbalance estimation scheme.

Air-gap force distribution and vibration pattern of Induction motors under dynamic eccentricity

A method for determining the signatures of dynamic eccentricity in the airgap force distribution and vibration pattern of induction machine is presented. The radial electromagnetic force distribution along the airgap, which is the main source of vibration, is calculated and developed into a double Fourier series in space and time. Finite element simulations of faulty and healthy machines are performed. They show that the electromagnetic force distribution is a sensible parameter to the changes in the machine condition. The computations show the existence of low frequency and low order force distributions, which can be used as identifiable signatures of the motor condition by measuring the corresponding low order vibration components. These findings are supported by vibration measurements and modal testing. The low frequency components offer an alternative way to the monitoring of slot passing frequencies, bringing new components that allow to discriminate between dynamic eccentricity and rotor mechanical unbalance. The method also revealed a non linear relationship between loading, stress waves and vibration during dynamic eccentricity.

Compensation of parasitic effects for a silicon tuning fork gyroscope

This paper refers to a silicon micromachined tuning fork gyroscope, which is driven via two piezoelectric thin film actuators. The device responds to an external angular rate by a torsional motion about its sensitive axis due to the Coriolis effect. The shear stress in the upper torsional stem, which is proportional to the angular rate, is detected via a piezoresistive readout structure. In addition to the wanted signal corresponding to the angular rate, there are unwanted contributions from the drive motion, e.g., from mechanical unbalances and from asymmetries of the piezoelectric excitation induced by fabrication tolerances. These effects, which disturb the sensor signal with varying contributions in amplitude and phase, have already been examined for capacitive surface micromachined sensors. In this paper, they are identified for a piezoelectrically driven, bulk-micromachined gyro and compared to results of FEM simulations. System-level simulations are performed and show possibilities to compensate the main parasitic effects. Results of eliminating the mechanical unbalance by femtosecond laser trimming are presented and compared with the simulations.

Unbalanced Hydraulic Forces Caused by Geometrical Manufacturing Deviations of Centrifugal Impellers

When an impeller has geometrical manufacturing deviations, synchronous vibration occurs from an unbalanced hydraulic force rotating with the impeller. This phenomenon is termed “Hydraulic unbalance.” In this study, experiments and analyses were carried out to study the hydraulic unbalance. We made several model impellers with intentional “manufacturing deviations”: in vane angle, vane pitch, and with eccentricity. Hydraulic forces were estimated from the rotor vibrations. The results of the experiments showed that the magnitude of the hydraulic force increased as the increase of the extent of manufacturing deviations. It was also shown that the amplitude and the phase of the vibration due to the unbalanced hydraulic force depend on the flow rate. This character of the unbalanced hydraulic force is quite different from that of the mass unbalance force, which is independent of the flow rate. A two-dimensional inviscid flow analysis was carried out to clarify the characteristic of the unbalanced hydraulic force. It was found that the hydraulic force can be divided into three components which are constant, linear and parabolic function of the flow rate. Applications of the present results show that the “hydraulic unbalance” of a high-speed pump impeller manufactured within a geometrical tolerance recommended by a typical standard can be larger than the ordinary “mechanical unbalance.” It was thus shown that the hydraulic unbalance is one of the important factors in the balancing of high-speed pump rotors.

1981 Power Engineering Society Prize Paper Transient, Three-Dimensional, Finite-Element Analysis of Heat Flow in Turbine-Generator Rotors

The problem of three-dimensional, transient heat flow in the rotors of large turbine-generators is solved using a finite element formulation and employing new arch-shaped elements. Applications to negative sequence current heating and to rotor mechanical unbalance caused by thermal asymmetries are discussed. The method is shown to agree well with analytical calculations for heat flow in cylinders. Isothermal plots on a rotor isometric illustrate the pattern of heat flow.

Transient, Three-Dimensional, Finite-Element Analysis of Heat Flow in Turbine-Generator Rotors

The problem of three-dimensional, transient heat flow in the rotors of large turbine-generators is solved using a finite element formulation and employing new arch-shaped elements. Applications to negative sequence current heating and to rotor mechanical unbalance caused by thermal asymmetries are discussed. The method is shown to agree well with analytical calculations for heat flow in cylinders. Isothermal plots on a rotor isometric illustrate the pattern of heat flow.

Z band abnormality characterized by interwoven structure in human cardiac muscle.

The fine structure of a Z band abnormality which closely resembled that of nemaline body was studied using myocardial biopsies. The abnormality was found in 10 out of 103 examined patients with various cardiac muscle diseases. Ultrastructurally, they were characterized by interwoven textures of fine filaments in a form similar to the nemaline body. However, they differed in two points from the nemaline body: first, most of them were interposed between the sarcomeric arrangement as an intramyofibrillar architecture, and second, two parallel filaments with periodic bridges were demonstrated in this abnormality. In the present study, the interwoven Z band abnormalities occurred regardless of the disease's sort, the patient's age, and the hypertrophic grade in the muscle cell. Meanwhile, some degenerative changes, myofibrillar lysis and mitochondrial degeneration, were frequently detected in the cardiac muscles with the abnormalities. Pathologically, the interwoven form of the Z band abnormality seemed to indicate an unbalance of the compensative mechanism at the cell level.

The Drift of Two Degree of Freedom Gyro

Two-degree of freedom gyro is used as the position gyro of the high-speed air craft. The author measured the drift of air-driven two degree of freedom gyro to which the forced vibration about the vertical axis was given. He pointed out the following four items as the causes of the drift. (1) The effect of air to drive the rotor; When the angle between the axes of outer gimbal and of rotor is not rectangular, the air generates the torque about the axis of inner gimbal. (2) The effect of forced vibration; Forced vibration is given to the gyro through the bearings, having the characteristic of negative frictional resistance. (3) Unbalance of mechanism; It is obvious that this causes the drift. (4) The friction of rotor bearings; He compared the results of experiments with the theory, taking into account the causes mentioned above, and obtained the good agreement.

The Gulf Absolute Magnetometer

In response to the need for a more accurate method of measuring the vertical magnetic intensity in the field, voiced to me in 1938 by Captain N. H. Heck, of the United States Coast and Geodetic Survey, the Gulf Research and Development Company built an absolute magnetometer of new design for the determination of declination (D), vertical intensity (Z), and horizontal intensity (H). The declination is measured in the customary manner by means of a fiber‐suspended magnet. The intensity‐measurements are made by comparing the field of a Helmholtz coil with the component of the Earth's field that is to be measured. H is obtained by the sine‐galvanometer method. The zero‐field detecting device for Z is a special vertical field‐balance in which the polarity of the magnet can be reversed by discharging a condenser through magnetizing coils fastened to the case. The effect of residual mechanical unbalance is thus made to disappear from the average of two sets of observations taken with the magnet oppositely polarized, The Helmholtz coil‐constant for the Z‐magnet is determined by suspending the Z‐magnet in the sine‐galvanometer housing, and comparing the value of H so obtained with an observatory base‐line.From preliminary tests at Cheltenham in April, 1940, the declination‐error was 1.′6. The precision of the intensity‐measurements is limited entirely by the stability of electrical standards in the field. This is estimated to be between 0.01 and 0.02 per cent. Six standard cells, two standard resistors, and one Rubicon potentiometer comprise the electrical measuring‐equipment. The determination of Z on the basis of the Cheltenham base‐line for H and the electrical equipment of the CIW sine‐galvanometer No. 1 was 17γ greater than the Cheltenham base‐line for Z. This discrepancy can be accounted for either by pier‐differences or by a magnetic impurity in the construction of the Z‐balance. The tests were interrupted by the war. Since little remains to be done to the instrument, it is hoped that it will be made ready for the postwar observational program at an early date.