Akihito Nakahara

Eddy Current Analysis Considering Lamination for Stator Core Ends of Turbine Generators

In this paper, we introduce a calculation method for in-plane eddy currents produced by perpendicular flux at stator core-ends of turbine generators. In the method, the laminated core is subdivided into each electrical steel sheet by 3-D finite elements. The insulated layers of the sheets are modeled by the gap elements, which represent the magnetic resistance of the layers. The combination method of double nodes and anisotropic conductivity is proposed to consider the insulation between the steel sheets. It is especially required for the stator teeth with slits. The validity of the calculation method is verified by comparing the measured and calculated results in the case of a simple model that simulates the end part of the turbine generator. The method is also applied to the real machine to clarify the eddy current distribution.

High-Frequency Interbar Current Losses in Cage Induction Motors Under No-Load Condition

This paper describes a study of the interbar current losses in induction motors under no-load condition by comparing the measured and calculated results. The interbar resistances and losses are measured on the machine of the rotor with end rings removed. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The losses are calculated with three-dimensional (3-D) magnetic field analysis on finite-element model reflecting the measured interbar resistance. The calculated losses agree well with the dependence of the measured losses on the interbar resistances. Moreover, it is confirmed that the core heating treatment caused the loss reduction and the increase of the high-frequency impedances form measurement results.

Three Dimensional Harmonic Field and Eddy Current Analysis for Rotor End Region of Turbine Generator

In this paper, we report eddy current loss distribution in retaining rings of a turbine generator rotor end region. Main causes of the eddy current are harmonic fluxes in a steady state. Although the calculation required full model in three dimensional analysis, we used two different models. One is to analyze eddy current loss due to the magneto-motive force harmonic flux and another is to analyze the loss due to the slot harmonic flux. This enabled us to acquire results in reasonable times. By the analysis, distributions of eddy current loss caused by each harmonic flux are well studied. In addition, we propose a harmonic flux distribution analysis method based on a Fourier series expansion. We were able to clarify the higher harmonic flux and eddy current loss distributions.

Evaluation of increasing rates in eddy-current loss of the motor due to carrier frequency

This paper describes that a theoretical formula for evaluating loss ratio in eddy current of a permanent magnet (PM) motor fed by a PWM inverter is proposed. A formula for evaluating the increasing rates of eddy-current loss in iron core due to carrier frequency corresponding to inverter output parameters (i.e., DC-bus-voltage, modulation index and modulation methods) are derived. Calculated and measured increasing rates of eddy-current loss in iron core were compared. According to the results of this comparison, the difference between the calculated and measured results is within 10 % when the modulation index is less than 0.7. The increasing tendencies of the calculated and measured increasing rates of eddy current loss agree well. The proposed theoretical formula can be applied for improving the efficiency of motor-inverter drive systems.

In-plane eddy current analysis for end and interior stator core packets of turbine generators

This paper describes analyses of in-plane eddy currents, which are generated by the perpendicular flux at both of end and interior stator core packets of turbine generators. The 3-D finite element method considering lamination of the electrical steel sheets is applied to the calculation. The validity of the analysis is examined by a simple apparatus that simulates the stator core of the turbine generator. It is clarified that the in-plane eddy currents are generated not only at the end stator packets, but also at the interior packet due to the duct space. It is also clarified that the in-plane eddy current loss decreases as nearly half by the slits of the stator teeth.

Calculation method of circulating current in parallel armature windings in consideration of magnetic circuit

Recently, the capacity of turbo generators used in power plants is increasing in order to keep up with the growth of electric power consumption in the world. Turbo generators are consequently experiencing problems, including increasing electromagnetic force, temperature rise of armature coils, etc., as we try to increase the armature current to keep pace with the capacity increase. One way of avoiding these problems is to increase the number of parallel armature windings for decreasing the armature current per coil. However, the circulating current in the parallel windings is generated by the difference of the linkage flux of each winding, when the number of parallel windings is not a divisor of pole numbers. In this paper, we propose a simple method to calculate the circulating current by using a magnetic circuit in the design phase. We confirmed the proposed method has a similar accuracy and faster performance in comparison with the finite element method (FEM) analysis. And then we applied the proposed method to a calculation of the circulating current in 2- and 4-pole generators and considered the factors affecting the circulating current.

Circulating current in parallel connected stator windings due to rotor eccentricity in permanent magnet motors

Rotor eccentricity in permanent magnet (PM) motors produces a voltage imbalance between parallel circuits that then creates a circulating current. This circulating current causes conduction loss, which is one of the stray losses in motors. The conduction losses of circulating currents need to be reduced in order to make PM more highly efficient. This paper describes the effect of neutral point connections on circulating current caused by static rotor eccentricity in a PM motor, as the circulating current depends on the use of neutral point connections in stator windings. We developed a theoretical expression for a circulating current with a closed circuit and compared its validity with that of measured results. The results of how well the connections of neutral points reduced the circulating current loss are provided.

Interbar current losses in cage induction motors due to harmonic flux

The interbar current losses have been studied through measurements and calculations. However, the quantitative effect of the contact resistance between the rotor bar and the iron core inter-bar current losses is not fully understood because of difficulties both in measurement of contact resistance and its modeling in analysis. This paper describes a study of the inter-bar current losses in induction motors due to harmonic flux and compares the measured and calculated results. The interbar current losses due to harmonic flux are measured on the machine of the rotor with end rings removed. The slot insulation and core heating are applied to make differences in the interbar resistances. The losses decrease as the interbar resistances increase. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The inter-bar current losses are calculated with three-dimensional magnetic field analysis. The finite element model contains the iron core portion with corrected conductivity reflecting the measured inter-bar resistance. The calculated losses agree well with the dependency of the measured losses on the interbar resistances. Moreover, additional measurements of high-frequency impedances of rotors reveal the interbar loss reduction effects of core heating caused by increasing the high-frequency impedances.

Comparative electrical design of radial- and axial-flux permanent magnet synchronous machines under space limitation

Comparative electrical designs of radial- and axial-flux permanent magnet machines (AFPMs) under space limitation are described in this paper. The target machines are conventional radial-flux machines and axial-flux machines with a single stator between two rotors. An analytical equation for designing axial-flux machines is derived considering given dimensions. The torque densities are calculated under dimensional limitations. From calculated results, the torque density of axial-flux machines depends on axial lengths, though that of radial-flux machines is independent of axial lengths. The back yoke thickness and the stator slot height of axial-flux machines are included in the axial length. Axial-flux machines have the possibility of having higher torque density than radial-flux machines if the windings are filled in the slot at a rate proportional to the increase of the slot space. However, the optimal axial length of axial-flux machines depends on the electrical loading because of the magnetic saturation of the stator teeth.

In‐plane eddy current analysis for end and interior stator core packets of turbine generators

Purpose – The purpose of this paper is to investigate the distribution of in‐plane eddy currents in stator core packets of turbine generators, and to reveal the loss reduction effect by the slits in the stator teeth.Design/methodology/approach – The in‐plane eddy currents are calculated by a 3D finite element method that considers lamination of electrical steel sheets. First, this method is applied to a simple model that simulates the stator core of the turbine generators. The calculated losses are compared with the measured losses in order to confirm the validity. Next, the same method is applied to a 250 MVA class turbine generator.Findings – The validity of the calculation method is confirmed by the measurement of the simple model. By applying this method to the turbine generator, it is clarified that the considerable in‐plane eddy currents are generated not only at the end stator packets, but also at the top of the teeth of the interior packets due to the duct space. It is also clarified that the in‐p...