Andreas Krings

Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis

This paper presents a practical approach to model thermal effects in directly cooled electric machines. The main focus is put on modeling the heat transfer in the stator winding and to the cooling system, which are the two critical parts of the studied machines from a thermal point of view. A multisegment structure is proposed that divides the stator, winding, and cooling system into a number of angular segments. Thereby, the circumferential temperature variation due to the nonuniform distribution of the coolant in the cooling channels can be predicted. Additionally, partial computational fluid dynamics (CFD) simulations are carried out to model the coolant flow in the cooling channels and also on the outer surface of the end winding bodies. The CFD simulation results are used as input to the analytical models describing the convective heat transfer to the coolant. The modeling approach is attractive due to its simplicity since CFD simulations of the complete machine are avoided. The proposed thermal model is evaluated experimentally on two directly cooled induction machines where the stator winding is impregnated using varnish and epoxy, respectively. A good correspondence between the predicted and measured temperatures under different cooling conditions and loss levels is obtained.

Temperature Influence of NiFe Steel Laminations on the Characteristics of Small Slotless Permanent Magnet Machines

High performance electrical machines can operate at temperatures of 100°C and beyond in rotor and stator cores. However, magnetic properties are generally measured at room temperatures around 23°C to 25°C according to the standards, even if it is known that the magnetization of some materials is substantially influenced by increasing temperatures. This paper investigates the thermal influence on the magnetic properties and iron losses in the stator cores of small slotless permanent magnet synchronous machines (PMSMs). The stator stack is made of thin nickel iron (NiFe) lamination sheets. Magnetic measurements of the stator core are conducted for different frequencies and flux densities at several temperatures between 25°C and 105°C. The obtained measurement data is afterwards used in finite element method (FEM) simulations to investigate the influence of the magnetic property change on the machine performance. For the PMSM in consideration, the FEM simulations show that an increased stator core temperature reduces the electromagnetic torque considerably; approximately 1/3 of the torque reduction due to increased rotor magnet and stator core temperatures (from 25°C to 100°C) can be attributed to the increased stator core temperature.

Influence of the Welding Process on the Performance of Slotless PM Motors With SiFe and NiFe Stator Laminations

The influence of the welding process during the manufacturing of small slotless permanent-magnet synchronous machines (PMSMs) is studied in this paper. The focus lies on the change of the magnetic properties in high-quality silicon-iron (SiFe) and nickel-iron (NiFe) electrical steel sheets with thicknesses of 0.1 and 0.2 mm. It is shown that the welding process changes the magnetic material properties significantly and increases the specific iron losses. Experimental results are provided for magnetic flux densities up to 1.5 T and frequencies from quasi-static to 200 Hz. The obtained measurement data is afterward used in finite-element method (FEM) simulations to investigate the influence of the magnetic property changes on the motor performance, particularly with regard to stator core losses.

Soft Magnetic Material Status and Trends in Electric Machines

This paper gives an overview on the history and trends of magnetic materials used in electrical machines and motors. The presented materials include silicon–iron, nickel–iron, and cobalt–iron lamination steels, as well as amorphous and nanocrystalline magnetic materials and soft magnetic composites. Development trends and current usage of these selected materials are presented, giving an outlook on the new magnetic material research with regard to electrical machine applications.

PWM Influence on the Iron Losses and Characteristics of a Slotless Permanent-Magnet Motor With SiFe and NiFe Stator Cores

This paper investigates the influence of switching frequency and modulation index combined with welding on the iron losses in thin silicon-iron and nickel-iron lamination sheets of a small slotless permanent-magnet synchronous machine (PMSM). First, measurements are conducted on welded and nonwelded stator ring cores for switching frequencies between 1 and 20 kHz and modulation indexes of 0.4, 0.7, and 0.9, keeping constant fundamental flux density peak values. This is possible by changing the dc-link voltage of the inverter. The obtained measurement data are afterward used in finite-element method simulations to investigate the inverter influence on the performance of the PMSM.

Practical Investigations on Cobalt–Iron Laminations for Electrical Machines

The adoption of high-performance soft magnetic material can be considered a possible way to respect the challenging design specifications for aerospace and automotive electrical machines. This paper presents an accurate noninvasive investigation of magnetic and energetic properties of cobalt-iron stator cores. In particular, the focus lies on the annealing processes aimed to maximize the magnetic saturation and the mechanical strength. Three cobalt-iron stator core samples coming from different annealing processes have been analyzed, both from the BH characteristics and losses viewpoints. Stator slot effects on the magnetic measurements have been investigated by finite-element method. A fringing effect correction factor to be applied on the measured BH curves is as well reported. In order to complete the experimental campaign analysis, micrographic analysis of the material structure is presented, as well as the iron losses separation coefficients.

Experimental characterization of magnetic materials for electrical machine applications

Most of the volume in classical electrical machines is occupied by magnetic materials for creating an as good as possible magnetic path. In machines with planar flux paths (2D geometries) the material is laminated to reduce eddy current losses in the core. Manufacturers typically supply reference values from Epstein frame measurements as a material performance specification and an easy way for comparison. However, the magnetic flux path in electrical machines is more complex due to rotating magnetic fields passing through the stator (with or without teeth), the rotor, and the airgap. Therefore, other material characterization methods, such as ring core measurements, are more suitable to characterize the magnetic materials for electrical machines. This paper gives a short introduction on different methods to characterize magnetic materials and a detailed description of building up a measurement system for characterizing ring core samples or electrical machine stator cores. The system is developed with regard to the IEC standard for measurements on magnetic ring core samples and can easily be build up in any lab with a power amplifier and a standard industrial control system equipped with analog input/output interfaces. Finally, reference measurements demonstrate the performance of the system.

New Magnetic Materials for Electrical Machines and Power Converters

T HE REQUIREMENTS FOR HIGH EFFICIENCY AND HIGH POWER DENSITY ELECTRICAL MACHINES AND POWER CONVERTERS are steadily increasing, especially in the field of aviation and transportation. One way for their satisfaction is to use soft and hard magnetic materials with performances. New high performance magnetic materials have been introduced in the market during recent few years, showing high saturation flux density, low specific losses, excellent behaviors at very high frequencies. On the other hand, high cost and high sensitivity to magnetic property degradation due to manufacturing process require by the users a good knowledge of their characteristics, otherwise the performance they are able to give cannot be fully exploited. The main objective of this Special Section is to collect the latest advances and developments in the field of the innovative magnetic materials with particular focus in electrical machines and power converter applications. Since the Special Section has to be considered application oriented, papers focused on chemical and physics aspects will be considered out of scope. In addition, experimental and validation results are mandatory, hence, papers including simulation results only will be rejected. Editors invite original manuscripts presenting recent advances in these fields with special reference to the following topics:

Characteristics comparison and selection guide for magnetic materials used in electrical machines

This paper presents an up-to-date magnetic material investigation and overview on magnetic materials used in rotating electrical machines. The focus is on small to medium-sized high-performance and high-efficiency permanent-magnet and induction motors for different application scenarios. The investigated materials include silicon-iron, nickel-iron, and cobalt-iron lamination steels, as well as soft magnetic composites and amorphous magnetic materials. The technical focus is on the magnetic properties and iron losses as well as the manufacturing influence and required thermal treatments during the manufacturing process. A new loss to flux density factor is introduced to compare the B H magnetization curve and the iron losses of different materials in the same diagram. The aim of the paper is to give the machine designer an efficient material overview and selection guide during the early machine design process.

Manufacturing influence on the magnetic properties and iron losses in cobalt-iron stator cores for electrical machines

The manufacturing influence on the magnetic properties and iron losses in stator cores made of cobalt-iron is investigated in this study, with a focus on the annealing and stacking process. Hysteresis and iron loss curves from measurements as well as a microscopic structure analysis are presented. FEM simulations are used to investigate the fringing effect in the stator cores due to the stator slots and others geometrical effects. A correction factor is derived and applied to the measurements. Furthermore, it is shown, that for cobalt-iron lamination sheets, the annealing process with an exact temperature and duration is absolutely essential to achieve expected magnetic and mechanical properties in the material.