On Medium Frequency Differential Mode Resonance Effects in Electric Machines

The contribution presents the new non-destructive diagnostic methods for the insulating material diagnostics of electrical rotary machine windings. The theory of the diagnostic methods is presented, and these methods are verified in laboratory conditions on Relanex insulating material. This material is used for insulation of electrical rotary machine windings. Attention is focused on determining the breakdown voltage correlation with diagnostic quantities. The relationship between breakdown voltage and the diagnostic quantities is also assessed by a non-traditional method, i.e., by a cluster analysis. More methods of the cluster analysis were used, and all results obtained were analysed and justified.

A density-based topology optimization approach is developed to investigate the distribution of conductive material in a discretized 2D-domain. The initial objective is to maximize the magnetic energy inside regions of interest (stator's teeth), then it is adapted to a multi-objective optimization to provide more control on the design. The developed framework is extended from one stator tooth made of iron, to multiple ones. This paper does not give a final optimized topology for winding in electrical machines but the first steps of a promising design method to achieve it.

The frequency response analysis method is more and more frequently used as a diagnostic tool for investigation of electrical device windings. The method is being used for identification of dislocations and deformations of transformer windings. This paper presents a novel application regarding possibilities of the frequency response analysis method in electrical machine windings. Of special importance is the early detection of winding failures in electrical machines, both during the manufacturing process and in operation. The results of investigations of winding faults in electrical machines of different construction are presented in this paper. The influence of turn-to-turn faults between adjacent winding wires on the admittance waveform was investigated.

This paper develops detailed models for predicting the voltage and current distributions in individual conductors of electric machine windings driven by pulse width modulation (PWM) inverter drives. This is the first step to better understand how PWM excitation voltage stresses the winding insulation of inverter-driven electric machines. A high-fidelity finite element (FE) model of electric machine windings was developed to account for frequency-dependent winding parasitic parameters. The model is capable of investigating the impacts that wire positions have on the winding parasitic parameters, hence the voltage and current distributions of individual conductors. A stator core with manually-wound windings was built to verify the fidelity of the FE model. Tests using both a form-wound winding and a random-wound winding with reduced number of conductors in slots were conducted to obtain the actual voltage and current distributions in individual conductors. Finally, the FE approach was applied to simulate a complicated winding structure with multiple serial turns and parallel branches to investigate the voltage and current distributions in individual conductors and the effects of wire positions.

It is shown that the reliability of electrical machines is mainly determined by the state of insulation of electrical windings. Most often, the stator windings break down. It has been established that the main causes of the loss of operability of the windings of electrical machines are their breakage, short circuit and decrease in insulation resistance. The equivalent circuits of the windings are considered, which make it possible to compose mathematical models and solve the main problems of technical diagnostics—assessment of the state of the winding and the search for a possible defect. The conducted experiments and the carried out analysis confirmed the correctness of the choice of equivalent circuits for the windings of electrical machines necessary for solving the problems of their technical diagnostics.

Research into electric machine windings with reduced magnetomotive force (MMF) space harmonic content has a long history. However, attention has often been confined to lower order harmonics with larger magnitudes. To more clearly and easily visualize the MMF space harmonic spectrum, this paper will explain the phenomenon of harmonic “families” intrinsic to a majority of electric machine windings. With a concise proof, it will be shown that within each family, the harmonic components have magnitudes that are inversely proportional to their orders. As a consequence, harmonics of the same family have the same winding factor. The number of families and, hence, different winding factors is found to be only a fraction of the number of slots of a unit machine. The family phenomenon greatly simplifies the determination of an MMF harmonic spectrum, envisions the behavior of higher order components, and predicts how a design change can improve the winding harmonic profile. When the family phenomenon disappears and the special cases with enlarged periods are also discussed. Two examples, both eliminating the 5th- and 7th-order harmonics but with very different harmonic spectra, are analyzed and compared to illustrate the power of the family phenomenon to aid the design of windings with improved electromagnetic performance.

Under the effects of alternating current (AC), the resistance and reactance of electric machine windings are different from their static state. As electric machine windings usually comprise many conductors, it makes the modeling and analysis very cumbersome using the finite element method. This paper presents an analytical method for computing AC resistance and reactance of electric machine windings placed in ferromagnetic slots, which is more convenient and efficient, and could be extended to the slots with different complex structures. First, the slot is divided into a number of layers based on the concept of discretization. Then, the magnetic field governing equation is established, and the distributions of magnetic field intensity and the current density of conductors under AC conditions are solved. By Poynting's theorem, the complex power flowing into the conductors is calculated, and then the effective resistance and reactance of the windings are obtained. Finally, the deduced analytical solutions are compared with the finite element method, and the results show that they have a good agreement for both single-layer winding and the double-layer windings with current phase difference. The presented analytical model is useful for understanding the AC effects and optimization of electric machine windings placed in ferromagnetic slots.

In this paper, the effects of a high temperature storage on phase inductance, ohmic and iron losses and coil quality of the stator windings are investigated. For better significance, 19 stators with different impregnation materials and methods are tested on Arrhenius basis for a useful life of 8000 hours. The results clearly show that, although the insulation resistance and phase-to-ground dissipation factor worsen as expected from literature, no impact of the insulation embrittlement on ohmic and AC winding resistance, inductance or dissipation factor can be observed. As a consequence, it is not necessary to include high-temperature-based aging into standard models for electrical machines. The only measurable high-temperature-induced aging effects seem to concern the winding insulation.

Electrical machine windings designed for low-voltage applications typically comprise conductors coated with an appropriate grade of electrical insulation. Multi-material Additive Manufacturing (AM) enables production of windings with complex geometries, where the conductors’ electrical insulation is directly incorporated. Copper and ceramics have been recognised in the literature as promising candidates for multi-material AM coils. Ceramics typically exhibit sufficient dielectric strength for such low-voltage applications and are expected to enhance the heat dissipation from the windings due to superior thermal conductivity compared to conventional insulation materials. This paper presents the design of electrical machine windings for multi-material AM and demonstrates the design flexibility and potential performance gains available. A Permanent Magnet (PM) generator has been selected for illustration. The advantages of windings incorporating shaped profile copper conductors with ceramic insulation is evaluated in terms of thermal behaviour and power losses. The theoretical analyses have been supplemented with experimentally derived data.

This article presents a novel use of three-dimensional printed direct winding heat exchangers (3-D-DWHX) to improve the thermal management of the stator in high power density electric machines. The 3-D-DWHX is in direct contact with the stator windings, enabling high continuous current densities. The use of dielectric polymer for the heat exchanger allows direct contact with the windings without an additional insulation material. By occupying the otherwise unused space between double layer concentrated windings, there is no significant impact on the electromagnetic design, and a smaller machine can be realized. The result is an increased continuous power rating and power density of electric machines. The polycarbonate-aluminum flake 3-D-DWHX was validated in a nonencapsulated motorette test, achieving a continuous current density of 20.4 ARMS/mm2 for a hotspot temperature of 148 °C. Finite element analysis shows that the same design, when encapsulated, can achieve 33.3 ARMS/mm2 current density with a hotspot temperature of 195 °C, corresponding to a continuous specific power of 11.8 kW/kg.

This paper proposes a systematic approach for the development of electric machine windings with reduced magnetomotive force (MMF) space harmonic content. By identifying the levels and slot position angles in the winding function of an electric machine winding, the winding factor, the magnitudes of individual harmonic components and thus the MMF spectrum of a winding can be represented mathematically as functions of the levels and angles. With numerical optimization, these functions can act as the constraints and the objective function. A typical formulation is to limit the magnitudes of undesired harmonics and to maximize the winding factor of the working component. The result of the optimization is often a new winding layout with potentially unequally pitched slots and different turns for the coils within a phase, and reduced or even eliminated harmonic orders of design interest, while maintaining a reasonable winding factor. The mathematical representation and design approach is applicable to both distributed windings and fractional-slot concentrated windings (FSCWs), with no special requirements on the slot/pole combination and the number of layers. Two design examples, a mixed-layer distributed winding and a 24-slot, 22-pole double-layer FSCW, are shown to have improved electromagnetic performance with finite element simulations.

Alternating current (ac) windings in electrical machines basically have a twofold purpose. In electrical generators ac windings are place where electromotive force (EMF) should be induced. In electrical motors, primarily ac windings goal is to produce rotating magneto-motive force (MMF) wave. In any case, ac windings should be designed in such a manner that induced EMF or generated rotating MMF wave consists predominantly of the fundamental sinusoidal component. Therefore, the starting point of an induction machine study is the analysis of MMF waves in the air gap of such machines. This assumes knowledge of winding distribution along the air gap from stator as well as from rotor side. The magnetic flux is analogous to the electric current. The MMF which sets up the magnetic flux is analogous to the EMF. The MMF is equivalent to a number of turns of wire carrying an electric current. If either the current through a coil (as in an electromagnet) or the number of turns of wire in the coil is increased, the MMF will be higher; and if the rest of the magnetic circuit remains the same, the magnetic flux increases proportionally [1]. At least two different approaches exist for describing winding distribution.

International audience

This paper studies the design of electric machine windings with reduced magnetomotive force (MMF) space harmonic content by adjusting the slot distribution and coil turns. Closed-form expressions for the winding factor and harmonic magnitudes established from a mathematical representation of winding functions are used to formulate the MMF space harmonic suppression problem. The framework is applicable to both distributed windings and fractional-slot concentrated windings (FSCWs) and can be solved with different techniques. No special requirements on the slot/pole combination and the number of layers are necessary. One example uses a linear algebraic technique to solve simultaneous equations and the result is a mixed-layer distributed winding. The process also reveals the criterion when multiple requirements on the winding factor and harmonic magnitudes can be satisfied concurrently. Contour plots are included to provide additional graphical insight. Another example designs a 24-slot 22-pole FSCW subset with numerical optimization. The resultant winding is a four-layer FSCW with two subsets and 3/4 of the harmonic orders are eliminated. The performance improvement of this winding is verified with finite-element simulations.

The continuous development in the field of industrial automation and electric mobility has led to the need for more efficient electrical machines with a high power density. The improvement of electrical machines’ slot filling factors is one of the measures to satisfy these requirements. In recent years, this topic has aroused greater interest in the industrial sector, since the evolution of the winding technological manufacturing processes allows an economically sustainable realization of ordered winding arrangements, rather than random ones. Moreover, the manufacture of electrical machines’ windings must be preceded by an accurate design phase in which it is possible to evaluate the maximum slot filling factor obtainable for a given wire shape and for its dimensions. For this purpose, this paper presents an algorithmic approach for the evaluation of maximum slot filling factors in electrical machines under an ideal geometric premise. In particular, this algorithm has a greater degree of flexibility with respect to the algorithm approaches found in the literature, since the study has been extended to round, rectangular and hexagonal wire sections. Furthermore, the slot filling factor calculation was carried out both for standard and non-standard slots. The algorithmic approach proposed can be considered as an additional useful tool for the fast design of electrical machine windings.