Cajetan Pinto

Stator circulating currents as media of fault detection in synchronous motors

Often found in critical, high power applications, synchronous machines require reliable condition monitoring systems. Large synchronous machines are typically designed with parallel connected windings in order to split the currents in parallel paths, delivering the total power at the terminals. Under ideal symmetrical conditions, no current will circulate between parallel branches of the same phase. However, when a motor fault breaks this symmetry, currents circulate between the branches. Thus, due to the fact that they are only non-zero under faulty conditions, circulating currents potentially represent a sensitive indicator of faulty condition. In this paper, the advantages of using the circulating current between parallel branches of the stator of a synchronous motor as an early indicator of motor faults are shown. Analysis is conducted both through simulation, via the use of finite element methods (FEM), and through experimentation using a specially-designed synchronous machine which allows various fault conditions to be investigated. Through comparison between experiment and simulation, the simulation tool is validated. Furthermore, it is shown that the circulating current is better suited for fault detection than either the branch or the stator current. It is concluded that an improved condition monitoring and protection system for a synchronous machine may be achieved if these currents are monitored.

Slip estimation of a large induction machine based on MCSA

This paper presents a newly developed algorithm for a large induction machine rotor slip estimation. The proposed algorithm is based on spectrum analysis of the stator current. The main idea is to find the best fit of spectrum components to the operating parameters. Motor slip is the result of the presented algorithm. Numerical calculations show that the method yields very accurate results and can be an important part of machine monitoring systems. Specially the presented method successfully deals with the slip estimation of a large machine with relatively small slip, for which very often standards methods may fail.

A signal processing approach to bearing fault detection with the use of a mobile phone

According to statistics, bearings are the most often failing elements of low voltage motors. At the same time diagnostics of rolling element bearings constitutes a well-established part of the rotating machinery condition monitoring domain. In many cases however the cost of installing a high-end accelerometer based bearing condition monitoring system, which is currently the most common approach in the industry, might be difficult to justify on non-critical machinery due to potentially long payback period on the investment. This text investigates the possibility of performing condition monitoring of rolling element bearings based on acoustic signals recorded by a standard, easily accessible mobile phone. The main difficulty in using mobile phone-embedded microphone for rotating machinery diagnostic purposes is the fact that the frequency response of the mobile phone microphone is very poor below 200Hz. The results presented in this text seem to indicate that with an appropriate signal processing approach, it is possible to indicate the presence of faults in the bearings.

Usage of acoustic camera for condition monitoring of electric motors

The paper presents a method of analysis of electric motors based on acoustic measurements. Acoustic signals were measured by microphones arranged in circular shape array called acoustic camera. Measurements by acoustic camera allow localization of sound source and by that separate sounds of interests from background noise can be obtained. Typically acoustic camera measurements are accompanied by a photography associated with acoustic measurement scene which is used for mapping of sound sources on the monitored area. As a reference, acoustic analysis is compared with vibration measurements. Vibration signals were measured by piezoelectric accelerometers and data collector dedicated for condition monitoring of electric motors. As presented in the result section, acoustic analysis appears as a valuable technique for condition monitoring of electric motors even in noisy industrial environment.

A novel non-invasive method for detecting missing wedges in an induction machine

This paper presents a newly developed algorithm for evaluating the health of an induction machine. The proposed algorithm is based on spectrum analysis of an impedance calculated using measured stator current and voltage signals. The main idea is to calculate the frequency spectrum of the impedance for each power phase and compare specific differences between phases. Experimental investigations show that the method yields very accurate results and can form an important part of a machine monitoring system. In particular the presented method is shown to be successful in detecting missing wedges in electric motors.

Bearing fault detection with the use of acoustic signals recorded by a hand-held mobile phone

It is well know that bearings are the most often failing elements of electric motors. Percentage of bearing failures vs. other failures is even more significant for the case of low voltage motors. At the same time diagnostics of rolling element bearings constitutes a well-established part of the rotating machinery condition monitoring domain. In many cases, however, the cost of installing a high-end accelerometer-based bearing condition monitoring system, which is currently the most common approach in the industry, might be difficult to justify on non-critical machinery due to potentially long payback period of the investment. Therefore, still the very first diagnostic is often performed by human ear and the assessment of the nature of the emitted sound. Considering an example of electric motors, it is typically the abnormal or excessive sound that is first recognized by the operators as the indicator of faulty motor operating condition. This text investigates the possibility of performing condition monitoring of rolling element bearings based on acoustic signals recorded by a standard and easily accessible mobile phone. The main difficulty in using mobile phone-embedded microphone for rotating machinery diagnostic purposes is the fact that the frequency response of the mobile phone microphone is very poor (below 200 Hz) where typically the bearing fault frequencies lie. The results seem to indicate that given an appropriate signal processing approach is taken, it is possible to indicate the presence of faults in the bearings.

Field current signature analysis for fault detection in synchronous motors

Often encountered driving critical applications, synchronous machines (SM) require reliable condition monitoring systems. In this paper, the advantages of using the field winding current of a synchronous motor as an early indicator of machine faults are demonstrated. Analysis is carried out, both through simulation, via the use of finite element methods (FEM), and through experimentation, using a specially-designed synchronous machine which allows various fault conditions to be investigated such as eccentricities and short-circuits. It is established from the comparison of the spectra, between healthy and faulty field current data, that the field winding current is well suited for fault detection as well as discrimination among the common faults in SMs. Further, it is shown that field winding current is quite sensitive to any problem of the stator, permitting the detection of stator short-circuits at a very early stage. Thus, it is concluded that an improved condition monitoring and protection system for SMs may be achieved if the field current is effectively monitored.

Low cost, hand held acoustic camera

The work described in this paper focuses on the design of a low cost, hand held acoustic camera. The novel process of creating the camera has been entirely engineered in ABB Ltd. Even though the creation of the acoustic camera with the use of 3D printing techniques may come at the cost of slightly decreased accuracy of the measurements, when compared with the commercially available solutions, however this is entirely acceptable in many applications. Additionally the costs saving related with the in-house manufacturing adds to the attractiveness of such an approach. The paper aims to present that with the advent of new technologies e.g. 3D printing it becomes possible for simple-design tools capable of delivering high quality results to be created by virtually anyone.

Condition monitoring of synchronous motors based on measurement of circulating stator currents

Condition monitoring based on currents circulating between parallel stator windings of a synchronous motor has been demonstrated. Circulating currents arise from any asymmetries in the motor that influence the air-gap magnetic field such as an eccentric rotor or an inter-turn short circuit. A measurement system based on six individual current sensors has been designed to acquire current data on an industrial motor with high fidelity in order to get a better understanding of the sensitivity of the circulating currents to the different motor states. The selection of appropriate sensors as well as their dielectric layout is discussed in detail. The frequency analysis of the acquired current waveforms yields a rich and complex spectrum, proving the very high sensitivity of circulating currents even to small motor asymmetries.

A novel non-invasive method for detection of static rotor eccentricity in an induction machine

This paper presents a newly developed algorithm for evaluating the health of an induction machine. The proposed algorithm allows for motor eccentricity detection on the basis of impedance spectrum analysis. The proposed method is based on a concept of performing the analysis in the frequency spectrum of the impedance for each power phase. The biggest advantage of the method lies in the fact that it exploits the information contained in both the current and the voltage signals. An experimental verification seems to be supporting the proposed method as an effective diagnostic tool for the detection of the static eccentricity of the rotor.