This paper investigates a 3-D coupled-field finite-element method (FEM) used in simulation of temperature distributions in air-cooled asynchronous induction motors. The temperature rise in motors is due to Joule's losses in stator windings and squirrel cages, and heat dissipation by air convection and solid conduction. The Joule's losses calculated by 3-D eddy-current field analysis are used as the input for the thermal field analysis, which is deeply dependent on accurate air fluid field analysis. Moreover, a novel multi-component fluid model is proposed to deal with the influence of rotor rotation upon the air convection. A test prototype is designed and manufactured. The good agreement of the temperature distributions between the simulated and measured results validates the proposed methodology.

Calculation of Temperature Rise in Air-cooled Induction Motors Through 3-D Coupled Electromagnetic Fluid-Dynamical and Thermal Finite-Element Analysis

A significant method to characterize the electrical properties of materials and to evaluate the condition of the insulating systems is the dielectric response study which can be done in the time or frequency domain. The dielectric response measuring results can be affected by different parameters like measuring voltage, temperature, water content and aging condition of insulating system. In this work numerous measurements are presented, thus to understand, how the dielectric response measurement results at the insulating system of high voltage rotating machines can be affected by these parameters in the time and frequency domain. Because of the importance of the dielectric response application as a non-destructive diagnostic method the possibilities and limitations of this method to reflect the deterioration processes due to thermal and electrical stress were also investigated. In addition the possibility to transfer the measuring data from the time to frequency domain was investigated and the calculated and measured data were compared. For the test object actual stator bars were used that were manufactured with epoxy VPI technology.

Dielectric response studies on insulating system of high voltage rotating machines

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

Accelerated aging tests have been extensively used on motor and generator insulation systems to simulate in a short time period the deterioration mechanisms occurring during normal operation of many years, to identify insulation system designs with longer life time and to support the qualification of a new system. The contribution presents results from extensive investigations on the behavior of two VPI machine test insulation systems (reference and candidate) with thermal class F (155°C), which were subjected to three thermal overstresses in repeated cycles. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, insulation resistance and partial discharge quantities were measured after each thermal cycle. After that the end of life of the insulation systems were checked with ac voltage and impulse voltage. The results support the qualification by comparative evaluation and they indicate, how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.

Behavior of machine insulation systems subjected to accelerated thermal aging test

Epoxy nanocomposite samples with a good dispersion of alumina nanoparticles in epoxy matrix were prepared and experiments were performed to measure their partial discharge resistant characteristics. Epoxy alumina nanocomposites with 0.1, 1, 5, 10 and 15 wt% nanofillers were prepared in the laboratory and partial discharge (PD) experiments were conducted at a voltage of 10 kV for different durations using IEC (b) type electrodes. The degradation of the sample surfaces were analyzed using SEM techniques, surface profile studies, FTIR spectroscopy as well as PD studies. An attempt was made to understand the interaction dynamics between the nanoparticle and the epoxy chain by measuring the glass transition temperature of the nanocomposites. The partial discharge resistance obtained for the nanocomposites are compared with those of unfilled epoxy and epoxy microcomposites. It was observed that even with 0.1 wt% of nanofiller added to the epoxy matrix, the partial discharge resistance to degradation gets improved considerably. It was also observed that the inter particle distance has a significant effect on the discharge resistance to degradation. The improvement in the degradation resistance is attributed to the interactions between the nanoparticle and the epoxy chain. A possible mechanism for the surface degradation of nanocomposites has been proposed.

Partial discharge resistant characteristics of epoxy nanocomposites

This article describes the change in partial discharge (PD) pattern of high voltage rotating machines and the change in the tan /spl delta/ as a function of the applied test voltage during the aging processes as caused by the application of different stresses on stator bars. It also compares the PD patterns associated with internal, slot, and end-winding discharges, which were produced in well-controlled laboratory conditions. In addition, the influence of different temperature conditions on the partial discharge activities are shown. The investigations in this work were performed on model stator bars under laboratory conditions, and the results might be different from those obtained for complete machines, as rotating machines are complex PD test objects, and for example, the detected PD signals in a complete machine significantly depend on the transmission path from the PD source to the measurement device.

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

Behavior of model insulating systems for high voltage rotating machines under different stresses

In order to investigate the significance of the capacitance and dissipation factor measurements as a function of test voltage for insulating systems of high-voltage rotating machines a series of measurements have been done on simple epoxy resin specimens and actual stator bars which have been manufactured based on the epoxy VPI technology. The possibilities of proposition of a relation between the maximum variation of the dissipation factor and capacitance with applied test voltage and the relative volume of the air-filled voids were investigated. Also the stator bars have been aged by different accelerated aging procedures and the changes of the capacitance and dissipation factor as a function of voltage were studied during the aging procedures.

M. Farahani

Papers

Dielectric response studies on insulating system of high voltage rotating machines

Behavior of machine insulation systems subjected to accelerated thermal aging test

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

Behavior of model insulating systems for high voltage rotating machines under different stresses

Study of Capacitance and Dissipation Factor Tip-Up to Evaluate the Condition of Insulating Systems for High Voltage Rotating Machines