John Raymond Krahn

On-line capacitance and dissipation factor monitoring of AC stator insulation

A new on-line technique for monitoring the insulation condition of ac motor and generator stator windings is proposed. The approach uses a newly developed High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g., the insulation current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low-cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology was carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor discussed in this paper was aged at high temperature (255 °C) as the load cycled between 0 % and 200 % every 5 minutes. Although this highly accelerated life test does not represent how a motor ages in service under real operating conditions precisely, the principal goal was to prove the capability of the new HSCT to accurately detect the insulation current and quantitatively monitor motor insulation gradual aging and health. On-line data from the HSCT correlated well with off-line data from a commercial capacitance and DF bridge. It is hoped that the benefits of the on-line motor health monitoring are fully realized and the method extended to other electrical assets as well.

Computational study of dielectric composites with core-shell filler particles

Phase field modeling and computer simulation is employed to study dielectric composites with core-shell filler particles for high-energy-density applications. The model solves electrostatic equations in terms of polarization vector field in reciprocal space using fast Fourier transform technique and parallel computing algorithm. Composites composed of linear constituent phases (matrix, core, and shell materials) of different dielectric constants are considered. Inter-phase boundary conditions are automatically taken into account without explicitly tracking inter-phase interfaces in the composites. The core-shell structures of filler particles are systematically investigated in terms of shell thickness and dielectric constant with respect to core size and matrix dielectric constant, respectively. The effects of filler particle size, shape, and orientation are considered. It is found that core-shell structures of filler particles provide effective means to mitigate local electric field concentration in diel...

Motor current signature analysis during accelerated life testing of form wound induction motors

The work presented in this paper describes motor accelerated thermo-mechanical aging and helps set the stage for on-line monitoring/data analysis as an effective means to detect a fault as electrical machine develops it and identify its type in real time. As a first step towards this goal, Motor Current Signature Analysis (MCSA) is applied to the data collected during accelerated thermo-mechanical aging of a formed wound induction motor tested to failure. The characteristic frequencies corresponding to detecting broken bars, stator inter-turn shorts, static/dynamic eccentricities and gradual degradation of the machine insulation are identified and sought from the collected data. To accomplish this accelerated aging, the motor was started/stopped frequently and subjected to overload in a steady state manner to subject it to severe thermo-mechanical stresses. Detailed analysis of experimental data is presented. It is hoped that this effort would pave the way for future work where on-line monitoring and diagnostics become a standard tool for life extension, lower maintenance cost, condition based loading, etc. for electrical assets.

Online capacitance and dissipation factor monitoring of AC motor stator insulation

A new on-line technique for monitoring the insulation condition of AC motor stator winding is proposed. The approach uses a newly developed, High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g., the insulation leakage current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation leakage current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low-cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology is carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor used in this work was aged at high temperature (255 °C) as the load cycled between 0 % and 200 % every 5 minutes.

Corrosion Testing of a Subsea Motor and Stator Winding Insulation System

In subsea applications, turbomachinery is subjected to severe gas and liquid mixtures. This is due to the fact that equipment installed on the ocean floor is severely limited. Typical gas/liquid separators and gas processing equipment are not used to clean the gas before it enters the turbomachinery. This means that the turbomachines can have multiphase mixtures and also corrosive gases such as hydrogen sulfide or carbonic acid (carbon dioxide and water) present in their flow streams among other contaminants. In many of these subsea applications, the electric motor driving the turbomachine is cooled using the process gas. Therefore the internals of the motor must withstand the attack of the corrosive mixtures and erosion with multiphase flow. A series of environmental static tests were conducted on a full scale subsea prototype motor to evaluate the performance of various materials when subjected to corrosive gas mixtures. These tests were conducted at various time intervals with different levels of pressure and temperature to simulate the conditions of a subsea environment gas flow. This paper reviews the test matrix and the test set-up used to conduct these tests. Also, it highlights the safety measures used to manage the inherent risks of testing with large quantities of corrosive and toxic gases mixtures and the different techniques that were implemented to follow the trends of the health of the insulation system.Copyright