Thanks to appropriate numerical optimisation techniques, it is possible to drastically reduce the losses that originate in permanent magnets due to eddy currents, with a small reduction in the torque that can be developed by the machine. In the same way, it is possible to design machines with concentrated windings with combinations of number of slots and poles traditionally considered incompatible or not feasible in symmetrical form. This is confirmed by the studies carried out by Professor Alberto Tessarolo, University of Trieste, and by examples of how this approach can be of great application interest.

by Gianandrea Mazzola in collaboration with Professor Alberto Tessarolo,
University of Trieste

In the construction technology of modern electrical machines, the use of so-called “concentrated” or “wound tooth” stator windings is becoming more and more frequent, replacing, where possible, the more traditional “distributed” windings. The difference between the two types of windings can be appreciated by the examples shown in figure 1. It will be observed that the distributed winding consists of “ample” coils which embrace a relatively large portion and connect leads arranged in “distant” slots (figure 1a). Conversely, concentrated windings consist of “tooth coils”, i.e. coils each wound around a tooth in the stator’s magnetic core (figure 1b and figure 1c).

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A drawback of concentrated windings is the fact that they, even when supplied with ideal currents, produce harmonic fields at the machine air gap which are capable of inducing losses due to eddy currents in permanent magnets and consequent overheating.
Moreover, it is not always possible to opt for concentrated windings. This is possible, in fact, at the state of the art, only for motors and permanent magnet generators in which the number of slots, indicated by Z, is similar (a little higher or a little lower) to the number of poles P.

In general, concentrated windings are usually considered feasible only if the number of slots Z and the number of poles P satisfy a precise algebraic relationship. More precisely, for the feasibility of winding, the quantity K, as shown in the following relation:

must be an integer number, having indicated by MCD (Z, P/2) the Maximum Common Divisor between Z and P/2. The above relation restricts the choice of the number of slots Z and poles P to a limited number of combinations (which we can define as “conventional combinations”). The limitation in question becomes particularly restrictive in the case of windings with more than three phases (m>3), as is often required to increase reliability. In the case of multi-phase windings, the scope of permissible poly-slot combinations is significantly reduced, thus significantly limiting the designer’s choice and precluding, in some cases, the adoption of wound tooth technology.

Large reduction of losses in the magnets, with small reduction in torque

In response to these critical issues, Professor Tessarolo has recently developed and proposed a methodology for the optimized design of concentrated windings, using multi-layer configurations.
“Configurations in which – explains Professor Tessarolo – there can be several coils of different phases wound around the same tooth, as exemplified in figure 2, identifiable by different colours depending on the phase to which they belong.
The methodology, which is based on a particular algorithm of quadratic optimization, nevertheless easily implemented in widespread computing environments (such as Matlab), permits reducing some of the drawbacks of concentrated winding machines.
“In particular – observes Professor Tessarolo – this methodology makes it possible to reduce the risk of overheating of the magnets due to harmonic fields at the air gap and the problem of the limited number of combinations of project-acceptable poly-slots, especially in the case of a number of phases greater than 3.
With regard to the reduction of ohmic losses in magnets, a multi-layer configuration optimized for wound tooth winding makes it possible to reduce the losses in magnets by up to 50%, at the price of a relatively limited reduction in the power developed by the machine. The potential for design optimization is exemplified in figure 3 for the combinations of 9 slots-8 poles and 12 slots-10 poles. The torque and losses of the magnets are normalized with respect to the value they assume for the traditional configuration (with a single coil for each tooth), represented by points A and C. Each point represents an optimized multi-layer design configuration.

“For example, in configuration B for the 9/8 machine, losses are reduced by about 50% at the expense of a 6% reduction in the nominal torque,” says Professor Tessarolo. “In the D configuration for the 12/10 machine, the losses in the magnets can be reduced by about 70% at the price of a drop of only 4% in the nominal torque.”

The optimization also extends the field of acceptable poly-slot combinations

The proposed optimisation method also permits extending the range of possible poly-slot combinations.
“In other words – underlines Professor Tessarolo – the method provides a symmetrical multi-layer configuration for a concentrated winding with a generic number of Z slots and P poles, even if Z and P are not such as to give a whole K in the above-mentioned equation”.
For example, figure 4 shows the cross-section of an 8-slot, 6-pole (unconventional) machine compared to the conventional 9-slot, 6-pole machine; similarly, the cross-section of an 11-slot, 10-pole (unconventional) machine is compared to the conventional 12-slot, 10-pole machine.

“From the comparison between conventional and non-conventional configurations – says Professor Tessarolo – it appears that the latter, in the face of a greater construction complication, in some cases show better performance. For example, the 9-slot and 6-pole machine in figure 4 has a high torque ripple which is about double that of the 8-slot and 6-pole machine. Or, to quote another example, the 11-slot and 10-pole machine has permanent-magnet losses around half those of the 12-slot and 10-pole machine”.

To give a more complete idea, the tables in figure 5 show a comparison between conventional (white cells) and non-conventional (grey cells) configurations in terms of winding factor and specific losses produced in the permanent magnets. It can be observed that some unconventional configurations have interesting and competitive values.

Operating benefits also for multi-phase machines

The possibility of using unconventional configurations can be particularly useful when designing multi-phase machines or machines consisting of several three-phase windings. This circumstance often occurs in applications that require continuity of service even in the event of a fault.
“For example – comments Professor Tessarolo – if you wanted to build a 12-phase machine, or with double three-phase winding, with eight poles, the conventional rules available in literature would force you to choose, to obtain a whole K from the above-mentioned report, a minimum number of 24 slots. It is clear that, for small machines, the use of Z equal to 24 could lead to unacceptable slot dimensions. The use of an optimized and unconventional multi-layer configuration can, in this case, be of help, making it possible to create a three-phase 8-pole, 9-slot double triad machine, as shown in figure 2”.
The prototype of this machine was also tested, recording the vacuum induced electromotive forces and then verifying the perfect electrical symmetry of the 9-phase winding, as shown in figure 6.

A further example of application is the 12-phase motor shown in figure 7, consisting of four three-phase windings offset by 90 degrees, each characterized by 7 slots and 6 poles. The choice of an unconventional winding in the case of what is shown in figure 7, was dictated by the need to have (for the maximum frequency allowed and the nominal speed) a total number of 24 poles, to be divided between the 4 independent units, of which the machine must consist for fault tolerance reasons. This resulted, for each unit, in a maximum number of 6 poles.

“The choice of 2 and 4 poles – underlines Professor Tessaroli – was not possible, as it led to excessive stator and rotor yoke thicknesses, such as to exceed the design dimensional constraints imposed on the radial dimensions. In this case, the project concerned the development of an electric outboard motor with integrated propeller, where space constraints were predominant. The number of poles for each unit was therefore fixed at 6, the choice of the number of slots such as to give an acceptable winding factor was between Z=9, Z=8 and Z=5, as shown in figure 5”.

The first (conventional) one was rejected because the torque ripple was too high. The only remaining options were therefore unconventional, i.e. 8 slots and 6 poles or 7 slots and 6 poles. The second was chosen because of its lower magnet losses and the almost zero torque ripple.
A prototype of the 7×4 slot and 6×4 pole three-phase quadruple winding machine was made (figure 8 a-b) and this was tested by connecting in parallel 2 of the 4 stator units and loading them respectively on a resistor star and on a diode rectifier bridge (figure 8 c-d). The results of the tests are shown in figure 9, where the waveforms recorded on the test bench are compared with those obtained by simulation of the machine with the finite element method in the time domain.

The results confirm the perfect symmetry of the machine and the excellent agreement between design forecasts and experimental behaviour. Similar waveforms, which do not show any unexpected phenomenon as a consequence of the choice of an unconventional winding, were also obtained by loading the other two machine units.
Professor Tessarolo concludes: “It can be said that the realization of concentrated electric windings, beyond traditional shapes and the classical limitations assumed for your project, have wide margins of optimization and extension. Provided that they are implemented on a multi-layer basis”.

It has been shown in these pages how, with appropriate numerical optimization techniques and the operating methodology proposed by Professor Tessarolo, it is possible to drastically reduce (even by more than 50%) the losses that originate in the permanent magnets due to eddy currents, with a small reduction in the torque which the machine is able to develop. It was also shown that, through similar optimization techniques, it is possible to design machines with concentrated windings with combinations of number of slots and poles traditionally considered incompatible or not feasible in symmetrical form. Finally, some application examples have been illustrated of how this can be of interest, especially (but not only) in the design of concentrated winding machines with more than three phases. It is therefore an operational approach and a methodology that, in fact, provides useful elements for greater freedom in design and execution.

To reduce the noise of electric motor transmissions, the technique that seems to offer the best performances for industrial productions is a variant of the so‑called isotropic super finishing (ISF), which can combine the typical finishing of the conventional ISF with the creation of compression stress states that are typical of the shot peening.

Francesco Chichi, Paolo Marconi

As you can easily guess, the shift from the reciprocating motor to the electric motor has unavoidably exerted a big impact on the relative transmission systems, both due to the different modalities of power delivery and to the different rotation speed. Less intuitive is how the passage to the electric motor has also implied the appearance of new problems, or better, has made some characteristics, which previously did not involve any criticality, emerge as problematic. Noise is one of them: first, due to the diminished typical noise of the propulsor; secondly, due to the increase of rotation speeds, which has implied a correspondent translation towards high frequencies also of the noise produced by gears, even more entering that range of frequencies that human ears hear as most “annoying”.

Therefore, for transmission systems intended for electric motors, the reduction of gears’ noise becomes fundamental, noise decrease that unavoidably is achieved through an optimization of the surface finishing of the same.
Unfortunately, precisely the different characteristics of power delivery of electric motors lead to the adoption of gears with increasingly small sizes compared to traditional ones, fact that undoubtedly does not support surface finishing processes. In this complete survey, the technique that seems to offer the best performances for industrial productions is a specific variant of the so-called isotropic super finishing (ISF), variant that also succeeds in matching the typical finishing of the standard ISF with the creation of the typical compression stress states of the shot peening.

Isotropic Super Finishing

With the term of “Isotropic Super Finishing” (often indicated with its acronym ISF) it is meant a vibration finishing treatment where the abrasive action of media on the surfaces of the components to be treated is enhanced by the presence of specific chemical agents.
Such chemical agents can selectively etch the asperities of the material, asperities that, after having been etched by the chemical action, are gradually removed by the mechanical action of media.

The primary target of such treatment is carrying out surface finishes with roughness in the order of 0.02 micros through a process at environmental temperature, granting an absolute uniformity of component profiles, with side benefits given by the annulment of eventual states of residual surface stresses and a slight increment of surface hardness.

In this of its formulations, ISF has been present on the market for about thirty years now: in this article, we are instead presenting a further implementation in which the chemical agent in liquid form is replaced by dedicated abrasive pastes, in order to achieve a process that not only allows an even better surface finish but can also perform surface stress states of compressive type that are partially analogue to those attainable through shot peening.
In its standard setup, ISF can be considered a derivation of the conventional tumbling barrel, those vibrating containers where components that need a cleaning of burrs, edges, witness bars or other macro-asperities are plunged into a “bed” of media having opportune hardness, shapes and sizes, and let vibrating for a long time (typically in the order of hours), until when the mechanical contact between media and pieces achieves the removal of this macro-asperities.

In the ideal case, the contact between media and element should occur tangentially to the surface of the element itself as much as possible, to obtain a removal of asperities but without creating new indentations on the material because of perpendicular clashes to the surface itself.
The limit of the traditional tumbling resides precisely in this: the removal of asperities occurs anyway for the mechanical effect of the collision between media and asperities of the material, shock that, even if controlled, will always and anyway have perpendicular components to the surface, and not only tangential to it.

This means that prosecuting “endlessly” a tumbling treatment, we cannot expect a corresponding “endless” improvement of smoothing because media, after removing asperities up to a certain level, become a source of damaging for the surface, too (fig. 1).
In the case of ISF, conventional media are combined with a chemical agent able to etch the mechanical resistance of the base material: in this way, media must act on a material whose macroscopic characteristics of mechanical resistance are diminished, and therefore the material removal is notably facilitated.

This allows using so less “aggressive” media from the mechanical point of view that they can no longer constitute a source of damaging for the base material, thus making finishing a progressive “flattening” by parallel levels (fig. 2).

This means that the process can be carried on “endlessly” in time, progressively incrementing the surface removal of the material and then finishing, reaching surface finishes with roughness in the order of 0.02 μm, moreover with surface “textures” characterized by a development essentially in depression, extremely favourable for the lubricant tribology (fig. 3).
Contextually with these, so to speak, geometric effects, a further benefit induced by ISF is an annulment of eventual residual traction stresses on the surface.

Such phenomenon is widely documented in literature and, in general, we can assume that such effect is connected to the fact that the material removal anyway constitutes a relaxation element of stress states (several measuring techniques of residual stresses are just based on this presupposition), and that such removal is particularly accentuated precisely where residual stresses are more tractive (just think of the stress corrosion phenomenon).
Figure 5 reports a comparison of the stress states present on a sample of austenitic steel purposely hardened to induce traction surface stresses: the determination of residual stresses was carried out by means of X ray diffraction (XRD technique) by 2Effe Engineering laboratory at Soiano del Lago (BS), with the in-depth analysis carried out by progressively removing the surface material through electrochemical etching.

From liquid to paste

As highlighted by realities of industrial productions of fast transmission gears, it is by now almost impossible assuring their demanded performances, especially in terms of duration and fatigue strength, without turning to the shot peening treatment, a treatment that, in extreme synthesis, provides for hitting the surface of the component under machining with a flow of particles projected by a nozzle under the thrust of compressed air, so that the combination of:

• Kinetic energy owned by particles;
• Mass of particles;
• Material of particles (or, more precisely, their breaking point);
• Hardness of particles in relation with the hardness of the material to be treated
• succeed in inducing on the material surface a plastic deformation in orthogonal sense to the surface itself.

In its turn, due to the well-known Poisson effect, to such plastic deformation that is orthogonal to the surface, corresponds the onset of compression stress states in parallel to the surface itself, according to the mechanism illustrated overall in figure 5.

Therefore, it is a treatment of essentially mechanical nature, because the ameliorative effects it introduces are mechanical and the mechanisms through which such effects are generated are mechanical, too.
From the quantitative point of view, the induced stress states take a maximum absolute value up to 70% – 80% of the yield point of the material, and their effect propagates up to about 0.1 mm of depth.
Unfortunately, just their geometrical conformation, especially the adoption of a generally very small module, can highly complicate the efficacious application of the shot peening treatment to the gears of electric transmissions, precisely because of the geometrical difficult of “wetting” perpendicularly, with the media flow, the contact surfaces of the tooth.

Luckily, the collision with the media perpendicularly projected to the surface is not the only modality to achieve a controlled plastic deformation of gear surfaces (or of any other component): it is well-known that also stock removal machining can achieve a similar effect, as the stock removal passes through a “tear” in which the material is first brought to plastic deformation and then to exceed its elongation point (fig. 6).
Concerning the surface left free from the cut, the plastic deformation by traction that has preceded the chip separation leaves, due to an elastic spring-back mechanism resembling the one just seen for the shot peening, an elastic compression state in parallel to the surface itself (fig. 7).

The inspiring principle of ISF with abrasive pastes precisely consists in using vibration-finishing media to carry not so much a chemical agent but instead an abrasive paste, taking back the surface stock removal process from a chemical ambit to a mechanical ambit and actually reproducing a grinding process on infinitesimal scale, as the grain of the abrasive paste is the tool and the media is the tool holder.

The experimentations personally carried out on steel samples, previously seen, have led to determine, for this treatment, surface stress states that are qualitatively analogue to those induced by the shot peening, even if with quantitative values reduced from 80% – 70% to 30% – 25% of the yield point and penetration depth decreased from 100 μm to about 30 μm (fig. 8).

Conclusions

The so-called “vibration finishing treatments”, in other words all those treatments that are based on the repeated relative contact between the component under treatment and specific media acting also as vector of a third substance, are included by full right in the field of surface engineering, inside what I define “modification treatments”, i.e. aimed at enhancing the characteristics of the material surface through endogenous modifications, and not through the coating and deposition of exogenous material.

Isotropic Super Finishing, ISF in the international technical language, is a specific vibration finishing treatment that sets the target of the progressive removal of surface material through the combination of a chemical action that diminishes the resistance of the protruding material and of a mechanical action of media removing the so weakened material, in a finishing process that can be theoretically prolonged endlessly but that actually, with execution times of 6-12 hours, grants a finishing in the order of 0.02 μm on any engineering material.

A new approach to ISF provides for the replacement of the chemical agent with an abrasive paste that, conveyed by media, obtains the removal of the protruding material with a purely mechanical process mirroring what happens in a grinding operation. In this case, the material removal mechanism by exceeding the breaking point in elongation, grants the creation, by reaction, of elastic compression states on the surface, with a mechanism partially recalling the shot peening; in comparison with the latter, stress states are inferior in both absolute terms and in terms of reached depth, but with the advantage of a much better surface finish.