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Irregularities in the magnetic circuits of motors may result in spurious voltages that lead to shaft currents through the shaft, bearings, beating supports and closing through the machine framework. The IEEE Standard Test Procedure for Polyphase Induction Motors and Generators discusses the shaft current and presents a measurement method for recording either the voltage across the ends of the shaft or the current. This paper discusses an alternative measurement approach and its application to the identification of shaft current in a large induction machine. Procedures were developed for measuring the shaft current. The procedures include the shunt current method and the measurement using the Rogowski coil. Only the Rogowski coil measurement yields accurate measurements of shaft currents whereas the other method either yields inaccurate measurement or may result in other problems for the machine's integrity. The theory and justification for the superiority of the Rogowski coil method is presented along with supporting test data.

The presence of a steel core in the commonly used aluminum conductor, steel reinforced (ACSR) on overhead transmission lines causes an increase in the ac resistance of the conductor, as a result of the magnetic induction in the core. This induction, which is higher with an odd number of aluminum layers, causes hysteresis and eddy current power losses in the steel, and a redistribution of current in the layers of aluminum wires. The effects of the total current in the conductor and the temperature of the steel core on the current distribution, the ac/dc resistance ratio and the power loss are determined for a Grackle conductor with three layers of aluminum wires. It is shown that the resistance ratio and the power loss can be significantly reduced at higher currents by careful design of the lay length (pitch) of each layer of aluminum wires.

To calculate the radial current distribution and core loss within a concentric-lay steel-cored conductor, the magnetic properties of the core must be known for any service condition. The paper describes comprehensive measurements on a length of the steel core taken from a 54/3.77 mm + 19/2.24 mm (Grackle) ACSR conductor. Curves are given to illustrate the effects of magnetic field strength, tensile stress and temperature on the modulus, real and imaginary parts of the complex relative permeability, the hysteretic angle, the loss tangent, the total core loss and the hysteresis loss.

Almost any metal can be used in the design of the squirrel cage of an induction machine. Obviously lead is not used because of its low melting point and magnesium because of its low flash point. Others are rejected because of the cost, lack of strength, resistance, thermal conductivity, or manufacturability including ease of joining. Basically, the choice falls to two materials, aluminum and copper and their alloys. For lowest cost of manufacture, especially for mass production, aluminum surpasses copper. In order to determine which of the two materials is the best takes the combined skills of an electrical, a mechanical and a metallurgical engineer. This paper discusses some of the considerations in the choice of rotor bar material. Keywords Rotor bar desgn, rotor bar failure, induction motors

This paper discusses rotational losses and how they are produced in the core materials of induction motors. These losses are largely caused by flux that rotates in the plane of the machine laminations. This suggests that steel specification for applications to rotating machines should be given in terms of rotational loss data as a material characteristic, in much the same fashion as Epstein test results are provided for alternating losses. If a standardized test for rotational losses were to be used, steel producers could rationally investigate the effects of composition and processing variables. This is necessary in order to produce low loss steels for motor applications. Reduction of rotational losses in motor cores could significantly lower ac machine operating costs and contribute to the growing interest and design of high-efficiency induction motors. The paper describes a test procedure for determining rotational losses in a sample. It then compares the results with standardized tests from an Epstein test procedure. It is seen that there are significant differences in loss results obtained for the rotational test versus the alternating current test. The development of analysis tools for determining the amount of rotational flux and degree of polarization in a machine core is also assuming greater importance as a tool for machine designers. The authors have investigated a rime harmonic finite element formulation utilizing Magnet 2D, a commercially available package. The paper includes a brief analysis of a typical problem using this tool. Keywords: Rotational Losses, Measurement of Rotational Losses, Rotational Flux, Induction Motors, MLQ Steels.

Analysis of the radial flow of heat in a multi-layer stranded conductor carrying current indicates that most of the heat is conducted through the very thin air gaps at the contacts between strands in adjacent layers and through the triangular or rectangular voids between layers. The theory predicts that, with constant current, the radial temperature difference increases as the axial tension decreases, and as the air pressure decreases. To confirm these predictions, a length of 91/4.04 mm AAC conductor was tensioned within a vacuum chamber, and the temperature of each layer of wires was measured for various total currents. It was found that the radial temperature difference increased with increasing resistive power loss per unit length, with decreasing axial tension, and with decreasing air pressure. The effective radial thermal conductivity is independent of the power loss, and increases with increasing axial tension and increasing air pressure. The calculated effective gap at the contacts is 0.5 to 0.9 micrometer.

This paper considers the problem of magnetic flux variation in the rotors of vent-cooled, two pole induction machines. The solution to this problem is important, not only to reduce stray losses in such machines, but also to identify the location of hot spots to estimate their effects on the design. A second objective behind the research was to reduce the magnitudes and, as a result, the variations in shaft magnetic fluxes. Reducing shaft fluxes will result in reduced shaft failures due to fatigue that is exacerbated by repeated heating and cooling. A procedure is suggested for examining the effects of the placement of the vent holes on flux variations, especially in the shaft. Once the effects of flux variations are known, the heating effect can be deduced.

An accurate estimate of the skin effect factor in the rotor bars of squirrel cage induction machines is important for the calculation of starting torque since the developed torque is directly proportional to the bar resistance. Past studies have given good estimates of this factor for the portion of the bars embedded in the rotor iron, but little is known about the effect in the rotor bar extension beyond the rotor core before entering the end ring. Depending on the length of the bar extension, the effect on the starting torque calculations has the potential to become significant. In this study, the end region of the machine was modeled using a commercial three-dimensional finite-element package in order to gain some insight into this phenomenon. Index Terms--Bar extension, large induction motors, skin effect factor, 3-D finite element.