J.L. Kohler

An on-line method to detect incipient failure of turn insulation in random-wound motors

Induction motors used in various mining industry applications fail frequently because they are subjected to severe operating conditions. Periodic test procedures are inappropriate for many installations and the cramped and hazardous mining environment makes unscheduled motor replacements difficult and time-consuming to perform. An online detector of insulation degradation is proposed and demonstrated analytically and experimentally. This detector, termed the effective negative-sequence impedance, is computed from the voltage and current phasors at the motor terminals. It can be used frequently, even continuously, and does not interfere with normal motor operation. With this detector, motor replacements can be scheduled, thereby reducing production downtime associated with emergency maintenance actions. >

Detection of A-C machine winding deterioration using electrically excited vibrations

Presented in this paper is a theoretical review of the relationships that should exist between electrical winding parameters and the mechanical vibration of AC machine elements under normal and faulted operating conditions. Also included is data from an experimental study that relates stator vibration and bearing vibration to selected winding faults in a synchronous machine. Consideration of these results indicates a significant relationship between electrical deterioration and mechanical vibration and thus provides the motivation for additional study and a basis for future applications.

Condition monitoring of induction motor stator windings using electrically excited vibrations

A study designed to investigate the effects of stator winding deterioration on the frequency spectra of mechanical vibration in induction machines is presented. Theoretical motivation for this condition monitoring approach is provided and experimental observations of bearing vibration are presented to provide verification. Results of the study show that specific vibration frequencies may be monitored in order to provide an assessment of stator winding integrity. This information may be used by maintenance personnel as an aid in distinguishing between electrical and mechanical problems or as supporting data for identifying electrical winding breakdown before it progresses to a catastrophic failure. Advantages of this approach are that it has the potential to provide information related to the air-gap flux created by winding deterioration that may not be readily available from voltage and current monitoring and that it is an easily-implemented procedure that may be used to supplement other monitoring techniques.

Prediction of electrical behavior in deteriorating induction motors

Methods for predicting electrical behavior in deteriorating induction machines are reviewed. The procedures require the assumption of a specific load speed and the electrical operating parameters are calculated using models developed for this purpose. The approaches are general in that an arbitrary number of winding deterioration processes may be simulated on the stator and/or rotor. Forms of deterioration can include high or low impedance leakage between phases, between coils in a single phase, or between a phase and ground. Verification includes comparison with experimental data taken on motors that have been modeled as well as with data on motors that have not been specifically simulated. One application of this analysis is the generation of databases required to design and develop performance monitoring, deterioration detection, or deterioration classification systems for critical motor installations.<<ETX>>

Techniques for improved predictive maintenance testing of industrial power systems

The authors review the procedures available for the periodic testing of industrial power system components, with particular reference to the mining industry. The inherent limitations of these tests are discussed, and appropriate applications for these tests are suggested. The use of negative-sequence monitoring as an inexpensive but powerful detector of incipient failure in industrial power systems is proposed. It is emphasized that negative-sequence monitoring does not interfere with normal operations and is continuous. In laboratory experiments, it has been proven to be a sufficient indicator for detecting electrical deterioration within a squirrel-cage induction motor. It is concluded that, despite the effects of an unbalanced voltage supply on the measurement and the present uncertainty over how to compensate for these effects, negative-sequence monitoring can be a significant and powerful addition to a plants motor testing program. >

Condition monitoring of slip-ring induction motors

Abstract A technique for real-time condition monitoring of slip-ring induction motors is presented. The proposed method utilizes on-line sensing and analysis of the rotor current to assess the integrity of the motor. The theoretical basis for this approach is presented; the results of an experimental investigation are given. The paper is concluded with a discussion of the efficacy of the proposed method in industrial power systems.

Safety issues and the use of software-controlled equipment in the mining industry

Equipment control functions that were once hardwired are being implemented with software and very large scale integrated (VLSI) devices. Often this transition has resulted in increased flexibility, improved quality and decreased costs. At the same time, it has created new concerns and challenges concerning worker safety. The visible and well-defined ladder diagram for relay-logic has been replaced by programs in which the exact outcome for varied inputs can be more obscure. In the coal mining industry, efforts to automate longwall mining systems have resulted in semiautonomous machines operating within the same space as workers. This paper describes an effort initiated by the National Institute for Occupational Safety and Health (NIOSH) to identify the safety issues related to the use of processor-controlled equipment in mining. Specific findings in the areas of human factors, hardware and software safety are presented in this paper, and a brief description of a plan to address identified weaknesses is given.

Experimental investigation of on-line methods for incipient fault detection [in induction motors]

Condition-based maintenance (CBM) of industrial equipment is generally recognized as being the most cost-effective means for improving equipment availability. However, prerequisite to successful implementation of CBM is a reliable detector of failing components. One such detector, termed the effective negative-sequence impedance, had previously been identified as an indicator of induction motor stator winding degradation. However, a limitation of this detector is that it does not change in a predictable manner under certain motor operating conditions. This paper presents an experimental investigation of an improved technique for online detection of induction motor stator winding degradation. The paper begins with a brief description of the detectors, followed by a detailed description of the experimental setup, the experiments conducted and results.

DC trolley fires-a new solution to an old problem

Coal mine trolley fires remain a serious threat to miner safety. Over the past twenty years, nearly 70 such fires have been reported to MSHA, and of these, most were due to the inability of the circuit breaker to discriminate between a legitimate load and an illegitimate fault current. Despite the extensive research into this area in the 1970s and early 1980s, it is still a problem. The problem also persists in rail transit systems used for mass transportation. This paper reviews the problem, summarizes past work, and presents two viable techniques, one passive and one active, for discriminating between load and fault currents. >

Decision functions for electric power system signals

Abstract An investigation of electric power system component deterioration has led to the development of a mathematical procedure which can be used to predict incipient failure or assess the integrity of individual components while they are operating in the power system.