Condition-based maintenance (also called predictive maintenance) requires diagnostic tools to determine when significant insulation aging has occurred, and thus when maintenance is necessary to avoid an in-service failure. Over the years many off-line diagnostic tests and on-line monitoring systems have been proposed and implemented, especially on critical motors and generators. In the past decade, most of the research has concentrated on improving existing off-line diagnostic tests and on-line monitoring such as magnetic flux, partial discharge, temperature, endwinding vibration, etc. However, some newer tests such as polarization/depolarization current, dielectric spectroscopy and on-line leakage current monitoring have been introduced. These tests and monitoring systems are reviewed. Tests and monitoring systems for rotor winding insulation and stator winding insulation are addressed separately.

Condition monitoring and diagnostics of motor and stator windings – A review

This paper puts into perspective the state-of-the art knowledge on bearing currents and diagnosis tools, which rely on shaft voltages and bearing currents phenomena. Today, shaft signals measurements are progressively included in predictive health monitoring of large turbo and hydro generators. In addition, bearing fault diagnosis through shaft signals have recently gained some attention to improve reliability and estimation of the remaining useful life in electrical drives. The main objective of this paper is to provide diagnosticians with a broad overview of the current trends in fault diagnosis through shaft signals. Therefore, the study presents a critical review of the measurements methods, advantages, and disadvantages of fault diagnosis methods, and suggests future possible enhancements for bearing health monitoring through shaft signals.

A Comprehensive Study on Shaft Voltages and Bearing Currents in Rotating Machines

The limitations of the thermal, vibration, or electrical monitoring of electric machines such as false indications, low sensitivity, and difficulty of fault interpretation have recently been exposed. This has led to a shift in the direction in research toward applying new techniques for improving the reliability of condition monitoring. With the changing environment, the purpose of this article is to provide an overview of the recent trends in the industrial demand and research activity in condition monitoring technology. The new developments in insulation testing, electrical testing, flux analysis, transient analysis, and fault prognostics are summarized. The challenges and recommendations for future work for the new technologies are also explored to help support target research and development efforts according to industrial needs.

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Condition Monitoring of Industrial Electric Machines: State of the Art and Future Challenges

In modern electric equipment, especially electric vehicles, inverter control systems can lead to complex shaft voltages and bearing currents. Within an electric motor, many parts have electrical failure problems, and among which bearings are the most sensitive and vulnerable components. In recent years, electrical failures in bearing have been frequently reported in electric vehicles, and the electrical failure of bearings has become a key issue that restricts the lifetime of all-electric motor-based power systems in a broader sense. The purpose of this review is to provide a comprehensive overview of the bearing premature failure in the mechanical systems exposed in an electrical environment represented by electric vehicles. The electrical environments in which bearing works including the different components and the origins of the shaft voltages and bearing currents, as well as the typical modes of electrical bearing failure including various topographical damages and lubrication failures, have been discussed. The fundamental influence mechanisms of voltage/current on the friction/lubrication properties have been summarized and analyzed, and corresponding countermeasures have been proposed. Finally, a brief introduction to the key technical flaws in the current researches will be made and the future outlook of frontier directions will be discussed.

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Electrical bearing failures in electric vehicles

Rotor winding interturn short-circuit (RWISC) occurring in turbine generators has been plaguing power producers with respect to production safety and economy. RWISC is characterized by impaired fundamental component of the exciting magnetomotive force and resultantly reduced no-load electromotive force E 0 . In this paper, an inverse reasoning method is proposed to analyze RWISC. First, it is assumed that the generator's rotor winding is running in good condition and the generator's electromagnetic power (virtual power, VP) is inversely calculated from the obtained operational data on generators. Second, the difference between the virtual power and the actual electromagnetic power is calculated and then used to determine whether an RWISC occurs on the turbine generator. Three real cases of severe rotor winding shorts have demonstrated that this method proposed in this paper not only enables to diagnose RWISC, but also allows determining the RWISC level and its trending accurately. The method is appropriate for RWISC diagnosis, whether offline or online, and can be applied to make the generator sets in the power plant run safer, more economical, and more stable.

Diagnosis of Rotor Winding Interturn Short-Circuit in Turbine Generators Using Virtual Power

Online magnetic flux monitoring via permanently installed air-gap flux probes is a well-established technology to determine the presence of shorted turns in a turbine generator rotor winding. Flux measurements are normally performed using flux probes installed in the machine air gap (on a stator wedge) and connected to portable or permanently installed instruments. In this “flux probe” test, to achieve a reliable diagnostic, the signal from the flux probe has to be measured under different load conditions ranging from no load to full load. This requirement presents a serious obstacle when applying this method on base load units. Recently, a new design of magnetic flux probe installed on the stator tooth was implemented. In addition, new algorithms used to analyze measurements using different types of flux probes were developed to minimize the need for tests at different load conditions. A finite element model (FEM) has been created to verify these new algorithms in different loading conditions. Based on this model, and real-world measurements, it has been demonstrated that accurate detection of shorted turns can be obtained without the need to vary the load on the machine if suitable sensors and algorithms are applied. This paper describes the new method and its advantages, comparing results obtained from online measurements on working generators and the FEM created to simulate different rotor conditions.

Finite Element Analysis of Turbine Generator Rotor Winding Shorted Turns

Damper bars are often used in high output salient pole synchronous motor (SM) rotors for motor starting and stabilization of performance. Several cases of broken damper bars in the rotor of SMs that lead to motor starting failure and/or forced outage have recently been reported. However, detection of damper bar failures is difficult, since the damper bars are active only under the starting or load transients. Damper bar testing in the field currently relies on off-line visual inspection, and there is no means available for detecting failures on line. In this paper, a new method for detecting damper bar failures based on the analysis of airgap search coil measurements under the starting transient is proposed. Since airgap search coils are being increasingly installed in large SMs for detecting field winding short circuits, the proposed method can be implemented without additional hardware. The effectiveness of the proposed method is verified with two-dimensional finite element analysis and experimental testing on a custom-built 30 kW salient pole SM under controlled fault conditions. It is shown that the proposed method can provide reliable and sensitive detection of broken damper bars whenever the motor is started at low additional cost. The off-line single phase rotation test is also shown to provide reliable monitoring of damper bar condition.

Airgap Search Coil-Based Detection of Damper Bar Failures in Salient Pole Synchronous Motors

An interturn insulation failure in the field winding of synchronous motors is a common problem that can lead to degradation in performance and accelerated wear of motor components. Existing offline tests for detecting this fault are known to be unreliable and inconvenient to perform, as they require motor disassembly. Online monitoring based on airgap flux monitoring has been proven to be successful, but could lack sensitivity for cases where the number of turns per pole is large. In addition, field winding turn shorts can be intermittent depending on the centrifugal force and could be unobservable at standstill or under an steady state operation. Therefore, monitoring of the turn faults under standstill and starting conditions with high sensitivity and without motor disassembly is desirable. In this paper, the detection of field winding short circuits for salient pole synchronous motors based on an airgap search coil under the starting transient, and new methods for standstill testing without motor disassembly are proposed. Two-dimensional finite element analysis and experimental testing on a 30-kW motor are given to verify the claims made. It is shown that the proposed set of tests can provide sensitive and reliable detection of field winding short circuits under standstill and starting conditions.

Comprehensive Monitoring of Field Winding Short Circuits for Salient Pole Synchronous Motors

Failures in the field winding of synchronous generators cause degradation in performance and accelerated wear of components. A number of offline and online test methods has been developed over the years, and airgap flux monitoring has been proven to be the most effective solution for reliable detection of the fault. However, there are concerns on reduced sensitivity for cases where the number of turns per pole is large. In this letter, the fault indicators used in commercial applications for airgap flux based fault detection are analyzed for salient pole synchronous generators, and a new simplified method with improved sensitivity is proposed. Experimental testing on a 330-MW generator under controlled fault conditions is given to verify the claims. A comparative study shows that the new method can provide a simplified means of improving the sensitivity and reliability of the technology presently employed in the field.

Reliable Flux-Based Detection of Field Winding Failures for Salient Pole Synchronous Generators

The rotor windings of synchronous motors and generators are usually very reliable. However, the turn insulation in such machines will eventually degrade and puncture due to thermal aging, load cycling and/or contamination. Although turn shorts do not directly lead to machine failure, they can lead to high bearing vibration, may damage synchronizing systems within brushless motors and may limit output. Off-line tests are available to detect rotor winding shorted turns, however they may be unreliable since the rotor is not spinning for the test and if only a few shorts are present, the shorts may disappear once the rotor is spinning (or vice versa). With funding from the US Electric Power Research Institute, a new type of on-line magnetic flux test has been developed that is suitable for salient pole machines, and in particular the salient pole rotors in 4 (or more) pole hydrogenerators and synchronous compressor motors that are widely used for air separation. The test requires retrofitting a small magnetic flux probe to the stator core and instrumentation to interpret the small voltage signals that are measured from the probe.

Mladen Sasic

Papers

Finite Element Analysis of Turbine Generator Rotor Winding Shorted Turns

Airgap Search Coil-Based Detection of Damper Bar Failures in Salient Pole Synchronous Motors

Comprehensive Monitoring of Field Winding Short Circuits for Salient Pole Synchronous Motors

Reliable Flux-Based Detection of Field Winding Failures for Salient Pole Synchronous Generators

Using magnetic flux monitoring to detect synchronous machine rotor winding shorts