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This paper describes an implementation of the time-stepped multislice finite-element method including a rotor inter-bar model in simulations of large skewed synchronous machines. The general form of the system equations is developed with an improved definition of inter-bar loop currents in order to enhance the numerical stability at large inter-bar resistances. In order to reduce simulation time, finite-element analysis (FEA) algorithms are implemented to exploit multiple processing cores. The simulations predict the flux density distributions and induced voltage harmonics at various inter-bar resistances for a synchronous generator under open-circuit operation. The simulated voltage harmonics are compared to test results and results obtained using conventional two-dimensional FEA and traditional skew factors. Index Terms--Finite-element analysis, synchronous machines, inter-bar model.

Derating a motor for operation on an unbalanced voltage system is still much of a black art although National Electrical Manufacturers Association and International Electrotechnical Commission have generic derating charts for this application. Numerous papers have been written in an attempt to provide a theory for this operating mode and the losses that may be generated. This paper will discuss the instrumentation and testing of a four-pole 1120-kW induction motor that was tested at three different levels of unbalanced voltage. Results will be presented on the stator and rotor heating, the speed-torque and locked rotor performance, as well as noise, vibration and shaft voltage. The problem has also been analyzed using time-stepping finite-element analysis. Index Terms--Induction motor, rotor, stator, temperature, unbalanced voltage.

Surge tests with short rise times are used to ensure turn insulation integrity in form-wound vacuum-pressure impregnated (VPI) coils prior to resin treatment. Use of this test migrated from the evaluation of fully processed generator bars and random wound stators. The dielectric properties of the insulation in these systems are fully developed at the time of testing. This is not true of 'green' (unimpregnated) VPI coils. Although surge testing is a valuable manufacturing quality test, an excessive transient electric field in uncured coils may cause irreversible changes to the insulation. These changes may affect the life of the insulation system after complete impregnation and cure. This paper describes laboratory tests and finite element simulations that examine the effect of applied surge test voltage and number of applied pulses on voltage endurance (VE) life. The sympathetic voltage response in a single coil as a function of its location within a winding is described for a stator undergoing green surge testing as part of a quality assurance program. Results of VE testing and sample dissection data are presented. Test voltage was identified a more significant factor on VE life than the number of pulses applied. Index Terms--Coils, dielectric, failures, finite-element analysis (FEA), green, hipot, surge test, unimpregnated, winding.

Drives and electric motors are sometimes paired in order to improve efficiency and process control or to eliminate gears and reduce maintenance costs. Sometimes the addition of the drive to an existing installation has not been the success that was originally envisioned and substantial derating has occurred or the original motor or drive replaced. One industry standard for product certification requires that there be a 30[degrees]C margin in the temperature rise if a motor intended for use on a drive is acceptance tested on sinusoidal power. The intent of this temperature margin is to allow for the additional heating losses due to inverter harmonics. Machine design, especially the ventilation, has a significant impact on the difference in temperature rise due to the increased harmonic losses. This paper will discuss a number of different ventilation methods present in motors. The results of some tests done on machines using the same load on both sinusoidal power and inverters are also presented. Index Terms--Harmonic losses, inverter, temperature, winding.

It is not a simple task to measure temperatures on the rotors or rotor components of large induction and synchronous machines during steady-state operation. The single greatest obstacle to obtaining these data is related to getting the data from the rotating element. Other problems are associated with instrumentation noise, sensor type, and attachment. This paper describes a number of techniques that have been used by the authors and their associates over the last 30 years. Test and calculated temperatures are presented for one machine. Also, test data from a number of machines are presented that show, for this particular population of enclosures and ventilation schemes, the steady-state temperatures at the design load can range from the value of the stator rise to greater than 70 [degrees]C higher. Index Terms--Induction, rotor, synchronous, temperature rise, temperature sensors.

Stator winding failures are generally attributed to groundwall or turn insulation failures. The failure mechanism involves the gradual development of a Weakness in the insulation. This weakness is usually caused by a trapped void that is continuously enlarging due to ever-increasing corona or partial discharge activity. The failure mode is from the inside out. Recent studies of failures in actual machines, and of coils on voltage endurance testing, indicate that failures can occur from the outside inward. Surface contamination leads to intense surface discharge and tracking. This surface discharge can lead to rapid winding failure following the inward motion of the corona activity. This paper looks at tracking as one failure mechanism and describes a series of tests on standard insulation materials aimed at comparing the antitracking capability of the materials. The tests show that combinations of insulation materials can reduce the antitracking capacity of a rather robust insulation system and predispose it to failure. Index Terms--Insulation, partial discharge, surface discharge, tracking, winding failure.

A large portion of the stator core of a rotating electric machine is subjected to magnetic flux that rotates in the plane of the laminations. Iron losses under rotating flux conditions can differ considerably from those due to alternating fields. It has long been suspected that this difference may be responsible for some of the discrepancy between predictions of no-load core loss at the design stage and values obtained by factory test. At present, no international standard exists for determining iron losses due to rotational flux. This paper describes an apparatus capable of making such measurements and briefly reviews the state of the art in this area of research. Measured loss curves are given for seven grades of nonoriented (NO) electrical sheet. Index Terms--Iron losses, rotational hysteresis.

Iron losses in electrical machines are difficult to predict. Discrepancies between tested and calculated no-load core losses are sometimes large. In induction motors, a large portion of the stator core is subjected to flux that rotates in the plane of the laminations. Losses due to rotating flux differ from those observed under alternating flux conditions. This paper describes direct approaches for coupling rotational iron loss measurements with finite element analysis (FEA) results that yield the distribution of rotational flux in induction motor cores. Using these methods, the prediction of no-load core loss can be made to include the effects of rotational iron losses. Index Terms--Induction motors, iron loss, no-load core loss.

This paper describes an implementation of the time-stepped multislice finite-element method including a rotor inter-bar model in simulations of large skewed synchronous machines. The general form of the system equations is developed with an improved definition of inter-bar loop currents in order to enhance the numerical stability at large inter-bar resistances. In order to reduce simulation time, finite-element analysis (FEA) algorithms are implemented to exploit multiple processing cores. The simulations predict the flux density distributions and induced voltage harmonics at various inter-bar resistances for a synchronous generator under open-circuit operation. The simulated voltage harmonics are compared to test results and results obtained using conventional two-dimensional FEA and traditional skew factors. [ABSTRACT FROM AUTHOR]