Sunday, July 21, 2019

Solar cars, a four-seats racing vehicle

Official presentation of the vehicle (© Ferrari Museum)

The Sun Energy, integrated by an appropriate level of technology for electric motors, can represent a valid alternative to conventional mobility. The four-seats solar cruiser, called “Emilia 4”, conceived by Bologna University in the ambit of Onda Solare project, is the first and unique solar racing vehicle for more passengers ever implemented in Italy. At its first exit, it won the American Solar Challenge, one of the most prestigious competitions in the world for solar cars.

Fiorenzo Sorcinelli

It dates back to just few weeks ago Onu scientists’ umpteenth alarm aimed at sensitizing worldwide governments about the imminence of a climatic change owing to the massive emissions of greenhouse gases into the atmosphere. The transport of freights and people has its own share of these emissions. We should just consider that in Europe more than 1/3 of energy is used in transports and it is obtained from petroleum products (petrol, diesel, LPG…). For the only Italy, and just for the road transport, this implies over 30 million tons of oil that every year literally go up “in smoke”. Then, we must add to this smoke the CO2 contribution amounting to 900,000 (equivalent) tons of natural gas and 1.1 million tons (equivalent) of biofuels … Nowadays, the urgency of an intervention is so evident and undeniable that the primary automotive industries are competing through the continuous release of hybrid or fully electric models.

Vehicle structure: we can notice the carbon structure and the titanium roll bar

The sustainable mobility is a crucial matter and they are working hard at it but the contribution of electric cars, even if in fast expansion, still represents a negligible market share (0.01%). Moreover, the perception that the electric does not solve the general problem of the pollution, but simply moves it elsewhere, is diffused.

This explains why, in this panorama, solar cars, powered by solar energy converted into electric by photovoltaic panels, succeed in catching increasing attention. These cars, in fact, besides featuring zero emissions, are conceived to reduce the energy consumption drastically by means of ultra-light cars, a perfect aerodynamics and the highest efficiency. In general, they are advanced prototypes, almost always single-seater, designed and implemented by forefront Universities and Research Centres, to take part in solar competitions that, like in the case of Formula 1 in automotive field, represent a test bench to test new technologies, innovations and solutions.

Construction of one of the two electric motors positioned inside the wheels

Concerning this, it is worth highlighting that the design of a solar vehicle is influenced by the quantity of energy input into the car, quite limited under standard conditions. It is so understandable how, even if some of the prototypes manufactured in these years have been ideated for public uses, commercial models of solar vehicles are not available, yet. On the contrary, at least observing them, these vehicles seem quite distant from a daily use.
Recently, however, the organizers of these competitions have adopted a strategy of competition regulation modification that leads to approach these vehicles to conventional transport means as much as possible, creating interesting synergies with the industrial world.

This direction is followed by the recent introduction of multi-passenger cruisers, a category devised in opposition to single-seater models. These new vehicles must comply with more severe requisites due to the presence of passengers, for instance concerning the safety and driveability. However, even the electric powertrain must evolve beyond a wider “battery pack”: inner spaces must be exploited differently, mechanics must be repositioned, the braking recovery becomes even more essential, and so on.
Concerning cruisers, then the multi-passenger concept must be adopted already in the design philosophy: in fact, it is no longer sufficient that you are implementing a solar car with which to run in the desert, but instead a functional and sustainable means of transport.

At Bologna University, in the ambit of Onda Solare project, with the implementation of the four-seats solar cruiser called “Emilia 4”, first and unique multi-passenger solar race vehicle ever made in Italy. With excellent results, considering that at its first exit it won the American Solar Challenge, one of the most prestigious competitions in the world for solar cars.

Design

The vehicle was born from a design and construction course based on the implementation of competing engineering instruments, rarely used in such integrated form, achieving a product unique of its kind. It features a high technological content, especially in terms of materials, structures, processes and related optimization logics that have implied the creation of overall Cad 3D models, the execution of manifold functional details for each component, the structural and fluid-dynamics controls with Fem codes, including simulated crash-tests, to proceed then to the prototyping of scale models, using more or less traditional techniques and materials, such as scale tests in wind tunnel, multi-objective optimization of geometries, also through the development of apposite codes.

The final data of the vehicle are the following: overall dimensions 4600x1800x1200mm; empty mass (batteries excluded) of 230kg; total weight of 620kg (4 passengers included); front surface of 1,60 m2; wheel base of 2.772 m; Cx of 0,20.

Electric characteristics

In terms of electric powertrain, the vehicle is designed for an average absorption of 21Wh/km at the cruise speed of 50km/h, consumption that allows an autonomy of about 750km.

The photovoltaic panel consists of 5m2 of monocrystalline silicon, with 326 high-efficiency SunPower cells (24% at 25°C) for a maximum panel power of 1,1kW. The converter for the photovoltaic panel is of Boost Converter typology, distributed generation, with a nominal power for each single stage of 200w and 98% overall efficiency.

The battery pack, which weighs 85kg of which 64kg of battery, is positioned in the central tunnel and features these characteristics:

  • 1344 Samsung lithium ion cells with nominal capacity of 3.4Ah
  • 48V nominal voltage
  • 331,2Ah current intensity
  • 16,1kWh total energy.
Housing of batteries and other electric devices

Motors consist of two surface permanent-magnet synchronous motors assembled on the rear wheels by means of external rotor directly coupled with the wheel and stator on the structure. Each motor, weighing 11kg, shows the following characteristics:

  • 1300W nominal power;
  • 3000W maximum power;
  • 35Nm nominal torque;
  • 125 Nm maximum torque and it allows a maximum speed of 110km/h.

The traction inverter is a three-phase inverter, with field orientation control able to deliver a maximum current (x1) of 200 ARMS.
This solution results in the 97% motion transformation efficiency.
The constant control of the entire electric powertrain is assured by 2700 sensors.

Structure

For the frame, they have chosen a carbon fibre monocoque body, vital for the implementation of a high-performance vehicle, light but also resistant and stiff. The structure has been made through processes in autoclave on carbon fibre tools, to grant dimensional stability during the polymerization cycle. These tools, on their turn, have been rolled on high-density aluminium or foam models, milled on big CNC machines. Most parts are composed by sandwich structures, to obtain the utmost resistance with an extremely low weight. We so obtain a vehicle that we judge the lightest in the world survey of “solar multi-occupant car”. The vehicle safety is based on the construction of a sturdy monocoque structure, made of reinforced composite, on which are mounted the other essential structural (and non-) elements. Only significant exception to the use of carbon fibre is in the safety cage, made with titanium alloy bars, mutually welded and bolted in the carbon fibre frame.

To know more about Emilia 4
Battery construction

The whole solar powertrain, including electric motors and all energy management and control systems, has been developed by the Inter-departmental Centre on Advanced Engineering and Materials of Bologna University, under the coordination of prof. Claudio Rossi.

Conclusion

This solar car has been a real challenge to network the manifold available competences on the national territory. However, it has also represented the emblem of their success. Able, last July, to cross the United States, from Nebraska to Oregon, travelling for 2,700 km and overcoming a pass of the Rocky Mountains at the altitude of 2,500 m, it transported 4 people on board, without ever turning to the grid recharge: only solar energy, or directly in nice days, or the one stored in the battery during the many thunderstorms. All this was awarded by the victory in the competition and by the technical prizes by “Mechanical Design Award” and “Battery Pack Design Award”.

They now work at the homologation and at the registration (category Le7) while a long road starts, the one that might lead to a real industrialization of solar vehicles. We believe, however, it is already clear that the Solar Energy, integrated by an adequate level of technology for Electric Motors, can represent a valid alternative to traditional mobility … if we want to devise something that can really protect our Planet.

Noise reduction by means of isotropic super finishing

Electric Motor

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.

Fig. 1 – Simplified representation of the smoothing carried out by a conventional tumbling barrel: the effect of media does not consist in the only removal of the protruding material but also in impressing of the base material. Therefore, even if prolonged for endless time, the tumbling process has a finishing limit due to the impressing of media themselves on the material surface

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).

Fig. 2 – In the case of ISF, the material removal is preceded by a chemical etching localized on the protrusions of the material itself: this allows using fundamentally “harmless” media for the base material not chemically etched: this permits the stock removal through successive flattening operations, which can be theoretically prolonged endlessly

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.

Fig. 3 – The characteristic that it is possible to operate only on protruding material allows the ISF to preserve a texture in depression, extremely favourable for the lubricant tribology

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.

Fig. 4 – Trend of residual stresses in an austenitic steel specimen (blue line), afterwards subjected to ISF treatment (red line)

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.

Fig. 5 – a) The stress induced by the media of the shot peening causes the compression plastic deformation of the surface material, orthogonally to such surface (red zones); b) due to the Poisson effect, to such compression plastic deformation corresponds a plastic deformation of opposite sign, in parallel to the surface; c) the underlying material, still in elastic field (blue zones), is stressed in traction, to which it reacts with an elastic response in compression, then generating the desired compression states

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.

Fig. 6 – Exemplificative representation of the stock removal process

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).

Fig. 7 – a) The occurred plastic stock removal on the surface of the component leaves a corresponding plastic deformation by traction on the material surface; b) The underlying material, still in elastic field, is stressed in traction, to which it reacts with an elastic response in compression, generating compression states for the material on the surface

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.

Fig. 8 – Trend of the surface residual stresses induced by ISF, carried out with abrasive pastes in relation to the starting material and to what obtainable through shoot peening
Fig. 9 – Comparison of the surface finishing for mechanically polished gear (on the top) and through ISF with abrasive pastes (on the bottom)

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.

Fig. 10 – SEM image concerning the state of the surfaces of a mechanically polished gear (on the top) and through ISF with abrasive pastes (on the bottom)

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.

Acoustic monitoring and background noise in industrial environment

acoustic analysis

Since any interruption in the manufacturing process can cause a serious financial loss for the company, it is very important to prevent unplanned shutdowns of electric machinery. Hence, monitoring and diagnosing the health of electric motors is crucial, and continues receiving more and more attention. One of the possibilities for performing diagnostics of electric machines is by analysing sound emitted by object of interests. The quality of acoustic monitoring is very much dependent on the background noise of the environment in which the machine is operated. Some attempts to create condition-monitoring methods based on acoustic analysis were made in the past (Refs. 1-4).
Recently, acoustic analysis has attracted more and more attention, and has been applied in many fields – speech recognition, for example. However, condition-monitoring methods based on acoustic analysis are still considered difficult to implement in an industrial environment due to the background noise.
The easy availability today of data collectors and sensors as accelerometers or current probes drives the use of many condition-monitoring systems based on those measurements. However, it is still very often the case that site engineers are asking for inspection of the machine when they notice abnormal sound.
Instead of isolation of the sound and its analysis, a typical “solution” is to perform measurements of vibration, current, temperature or voltages that are not always indicative of the problem. Even though what was reported was abnormal sound, existing solutions are trying to detect the fault by various types of measurements, as opposed to showing that sound is emitted by a specific part in the first place. This, in turn, might limit the amount of possible diagnostic decisions, thus limiting the amount of required effort.
For this reason, in many cases the first diagnostic attempt is made by highly experienced engineers who are able to initially detect and diagnose the problem by simply listening for the sound source. For many years, diagnostics in the industry were performed “by ear,” with subsequent assessment of the emitted sound. Still, the influence of background noise can strongly affect the quality of such a judgment.
Today’s trends in the job market lead to a situation where there is less and less people who are experienced enough to judge the condition of an object by listening to the sound it makes. It is the result of the fact that many people prefer to do office work rather than working in an industrial environment. As is shown, in the Global Employment Trend document (Ref. 5), or in the list of the top 10 jobs forecast for next decade (Ref. 6), this situation will be even more prevalent in the future. However, there remains a necessity of doing the initial investigation of objects to localize the abnormal sound to perform immediate action.
A solution of the described problems might lie in the usage of acoustic analysis for objects-of-interest diagnostics. Thus far, it has been relatively difficult to create a reliable, acoustic-based condition monitoring system due to the fact that sound measurements are always affected by background noise. However, recent technologies like acoustic cameras are able to successfully localize specific sound components and thus remove the influence of such noise (Refs. 7–8).
A variety of faults that can occur in induction machines have been extensively studied and many monitoring methods have been proposed to detect problems (Ref. 9). Most of those methods for condition monitoring of electric motors utilized vibration or motor current signature analysis (MCSA) (Refs.9–11). While vibration and current signature analysis-based monitoring techniques are well known and well-accepted, acoustic measurements are not so popular in industrial application.
This paper describes a diagnostic method for induction motors based on acoustic measurements, while vibration analysis is used as a reference for assessment of the value of acoustic measurements.

Measurements Tools

Acoustic camera

The idea of the acoustic camera is to do sound source identification and quantification, and to create a picture of the acoustic environment through the processing of the multidimensional acoustic signals received via microphone array and to overlay that acoustic picture on the video picture (Ref. 7). Other possible acoustic camera applications include use as test equipment for non-destructive measurements for sound identification in vehicle interiors and exteriors (Refs. 7–8 and 12); trains and airplanes (Refs. 13–14); and for measurement in wind tunnels, etc. Additionally, some studies show the application of acoustic camera for unmanned underwater vehicles (Ref. 15), robots and robotized platforms, etc. It can also be used for passive acoustical sensing in battlefield environments (Ref.16). In this work, a 48-microphone acoustic camera was used for sound measurements; parameters for the microphones are presented in table 1.
acoustic analysisAcoustic holography technique was used for analysis of the sound source. Acoustic holography technique is a method that is used to estimate the sound field near a source by measuring acoustic parameters away from the source via microphone array. This is a well-known technique and its description can be found in (Refs. 16–17).

Vibration measurements

Vibration measurements are one of the most popular methods for condition monitoring of electric motors. Typically, piezoelectric accelerometers are used for measurements of the vibration. For the purpose of the present work, vibration measurements were taken as a reference for the sound measurements. Vibrations were collected with ABB’s MACHsense-P condition monitoring tool. MACHsense-P is a walk- around condition monitoring service tool provided by ABB that specifically focuses on electric motors. Vibration signals were measured using 4 simultaneous data capture channels and analysed for mechanical and electromagnetic defects. The frequency range used for analysis by MACHsense-P tool is from 0 Hz to 12,800 Hz. The vibration analysis presented in this paper is embedded functionality in the MACHsense-P tool.

Measurements analysis and comparison

All vibration and acoustic measurements where done in an industrial environment. Since induction motors are the most widely used machines in industry (Ref. 18), two of the same type three-phase induction motors were chosen. Nameplate details of the motors are presented in Table 2.
acoustic analysisBoth motors were located relatively close to each other, and both of them were driving centrifugal pumps of the same type through direct coupling. Both motors where operating at the same load level. Motor case 1 is considered healthy while motor case 2 is considered to have a combination of static eccentricity and soft foot. As soft foot typically results in static eccentricity, this combination of faults is very common.

Results based on vibration measurements

For both of the motor cases, vibration sensors were located horizon- tally on the center of the body of the motors. Figure 1A presents a vibration spectrum of the healthy motor case while Figure 1B presents a vibration spectrum of a combination of static eccentricity and soft foot motor case. Since static eccentricity can be typically visible in low-frequency range, both figures present frequencies from 0 Hz to 200 Hz. You may notice that Figure 1B contains a high peak – at around 100 Hz.
acoustic analysis
The value of this peak is above 0.12 gs, while in Figure 1A this peak is smaller than 0.02 gs. As presented in (Ref. 12), static eccentricity causes additional forces visible in vibration at frequency fecc – given by following equation:
fecc = 2 ∙ fline
Where fline is power supply frequency. In the above case, both motors were supplied by 50 Hz; therefore static eccentric-related frequency fecc is visible at 100 Hz. By taking the amplitude of fecc frequency as the static eccentricity indicator, it is clearly visible that the motor in case 2 reached a higher level of static eccentricity than the healthy motor from case 1. With the MACHsense-P the indicator of static eccentricity is calculated automatically.
acoustic analysis

Results based on acoustic measurements

For industrial applications, when performing measurements using a microphone, background noise can- not be avoided. The background noise can be filtered out by post-processing methods of the measured signals. This is possible due to the different nature of the measured sound. The background noise (including the  aerodynamic noise of the cooling device) is   usually  a broadband signal with a more or less constant spectrum (Ref. 1). On the contrary, the induction machine generates sound that is characterized by many pure tones – at least for the sound produced by electromagnetic origin. Reference 1presents a method where before operating the induction machine, a measurement of only the background noise is conducted. This spectrum of the measurements is later subtracted from the measured spectrum, with the induction machine in operation. However, this noise filtering approach is not accepted in industry because it affects the industrial process.
Reference 11 describes a method that isolates the frequencies related to the motor presented in electric current measurements. The same approach can be applied for vibration or acoustic signal. As presented in (Ref. 11), by knowing motor parameters and motor slip, all the frequencies related to motor condition can be identified. Likewise, all motor-related frequencies can be found and identified in the acoustic signal, even if the signal contains back- ground noise.
Figure 2 presents an acoustic spectrum of average signal via microphone array.
acoustic analysis
Figure 2A presents acoustic spectrum of a healthy motor case, while Figure 2B presents an acoustic spectrum of a combination of static eccentricity and soft foot motor case. Both figures are obtained for frequencies ranging from 0 Hz to 200 Hz. Similar to vibration cases, it is possible to notice that Figure 2B consists of a high peak at around 100 Hz, while Figure 2A does not. Value of this peak is above 600 mPa, while in Figure 2A this peak is smaller than 350 mPa. Those results are very similar to vibration-based results, and they are clearly indicating static eccentricity; however, in case of acoustic signal there is no assurance that this frequency emanates from the motor.
To solve this problem, the acoustic holography technique can be applied to find the sound source of the frequency of interest, in this case, 100 Hz.
acoustic analysis

Conclusions

In this paper, an acoustic-based technique for the condition monitoring of electric motors was presented. Vibration analysis was used as a reference for assessment of the value of acoustic measurements. Acoustic measurements were performed via 48-microphone acoustic camera. Two induction motor cases were examined a healthy motor case and a combination of static eccentricity with soft foot case. For fault case, respective frequencies were identified in both vibration and acoustic signal. Based on acoustic holography technique, the fault-related acoustic frequency source was localized in the center of the body of the faulty machine. As presented in the results section, one can say that acoustic signals can be successfully used for condition monitoring of electric motors in noisy industrial applications. Obviously, single acoustic signal is disturbed and noisy compared to vibration signals; therefore sound localization technique via acoustic camera was needed to solve this problem. An additional benefit of sound analysis is the fact that the acoustic sensors need not be attached directly to the motors, which is often difficult in industrial applications.
(Maciej Orman and Cajetan T. Pinto)

References
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Impact on efficiency of core materials

Materials and efficiency

No matter whether intended for a small or big household appliance, or used to drive hybrid and/or electrical cars or, moreover, applied in industrial ambits, the electric motor goes on confirming an unprecedented global growth trend.

Challenge that involves materials

Component manufacturers, producers and end users focus on this faster and faster diffusion, orienting their commitment not only to the reduction of costs but also to the optimization of performances and to the pursuit of the highest efficiency. A challenge that directly involves also the materials that constitute the core of electric cars, from the magnetic lamination to the stator pack.

Stefano Fortunati
Stefano Fortunati, Senior Technical Manager Stainless Steel & Electrical Steel Metallurgy and Manager of the Research Unit of CSM Terni.

We talked about that with Mr. Stefano Fortunati, Senior Technical Manager, Stainless Steel & Electrical Steel Metallurgy and manager of the Research Unit of CSM Terni, with specific focus on the “white good” and automotive sectors. «For several years now – explains Mr. Fortunati – efficiency has represented a key development factor, also to comply with the regulations provided on national and international scale. For this reason, the household appliance and automotive fields, as well as others, have paid ever-rising attention to the high efficiency of electric motors for a long time».
Approach, the latter, that has motivated and determined a production shift towards higher and higher quality classes.
«Including – underlines Mr. Fortunati – the producers of magnetic laminations used for the implementation of electric motor cores».

The decreasing demand for raw materials

A context that highlights, compared to the past, a decrease of the demand for less valuable raw materials (classes with very low percentage values of Silicon or similar) and of “high” thicknesses (even 0.65 mm).
The current trend is preferring, also in the “white goods” world, lamination thicknesses included between 0.5 and 0.35 mm, and steels characterized by a higher Silicon content.  «Important aspect – underlines Mr. Fortunati – is also the higher care and the control of the crystalline microstructure, besides the choice and the selection of increasingly clean/pure steels (i.e. free from second non-metal phases)».
Actually, in fact, a low level of internal cleaning and purity, combined with a distribution of defects and second phases, increases the likelihood of constraints to the movement of magnetic domains during the magnetization. This implies a rise of losses and, therefore, an efficiency decrease.

The added-value of the magnetic permeability

«The demand coming from the market – adds Mr. Fortunati – consists then in the availability of raw materials with improved characteristics, precisely to enhance efficiency. It is undoubted that, from the point of view of the material research, this aspect represents a “driving force”. However, it is not sufficient for the attainment of the targets». It is relevant also the aspect of design and engineering change the electric motor is living. From a more general point of view, in the past the only (or anyway the main) key design parameter was represented by magnetic losses, by power losses.
«The progress and the design evolution – explains Mr. Fortunati – have led in time to recognize a higher value to the parameter of the material magnetic permeability. Material that, for applications on rotary machines, must assure an easy magnetization in isotropic manner in all directions of the plan of laminations constituting the ferromagnetic core». Concerning this, from the point of view of the material crystallography, it is necessary to grant that the (crystallographic) directions of easy magnetization are homogeneously distributed in the plan.
Relying on improved crystalline texture means enhancing the magnetic permeability to achieve higher applicative efficiency.
Given that and knowing that magnetic losses, heating and power losses occur in the core magnetization, it is important also to evaluate the necessary current intensity. Succeeding in magnetizing the conductive material optimally allows using a lower quantity of it.

Materials and efficency
Manufacturing cycles of steels for electric motors.

«Moreover, also less – adds Mr. Fortunati – is the current that passes inside windings. This means reducing losses. An approach that considers not only the magnetic lamination, i.e. the ferromagnetic material inside the core and its behaviour. But also how much conductive material and the necessary current intensity to achieve the expected result».
A sign that shows and highlights the great liveliness that is permeating the design of electric motors. Of whatever typology they are (induction, synchronous, asynchronous, reluctance motors and so on).
A further guideline in the general development of motors, for both the household appliance sector, the automotive and industry in general is oriented towards the research of thinner and thinner materials.
Worth considering, then, the material resistivity element, faced choosing materials prevailingly beneficiated with Silicon but also with Aluminium or Manganese. With a limit, because increasing the widely used Silicon with values exceeding 3.5%, the material becomes too fragile, making their industrial manufacturing and their use very critical. Then, a further strategy consisted in orienting the research towards the improvement of the material surface quality and of its cleaning.
«Differentiating elements – underlines Mr. Fortunati – that however lead to a sort of selection among the steel mills that today can grant the production in conformity with these requisites».

The best compromise between thickness and purity

Thickness and electric resistivity (due to the content of Silicon, Aluminium and Manganese) represent determinant factors in automotive ambit, because the law that rules magnetic losses includes the frequency multiplier factor. In other words, in alternate field, their contribution becomes very important in industrial and/or domestic ambit.
«Even more determinant – underlines Mr. Fortunati – in the automotive industry, where they usually deal with variable frequencies and high frequencies, with a reference factor for the lamination characterization that reaches 400 Hz».
We should consider that the best attainable materials by a conventional chain feature magnetic losses (measured at an operation induction of 1.5 Tesla – 50 Hz, with reference thickness of 0.35 mm) with values around 2 W/kg.
Referring instead to the characterization of similar materials, optimized for automotive applications (measured at 400 Hz), such value notably increases, up to 14-15 W/kg.
Therefore, the general trend is demanding and finding better and better solutions in terms of losses and crystalline textures, to improve their polarizability, i.e. the magnetic permeability. Dominant factor, the latter, to obtain a higher power density.
As already underlined, all seem oriented towards increasingly thin materials but marked out by worse mechanical properties, then with a potential more difficult applicability in some ambits.

The electric resistivity

The other aspect and conflicting factor is represented by the percentage of elements that increase the electric resistivity (first of all Silicon) contained in the material. «If on one hand – explains Mr. Fortunati – they prefer materials with high alloy content to improve losses, on the other hand this intrinsically hinders the opportunity of increasing the magnetic permeability».

Materials and efficency
Pilot line for magnetic lamination blank at CSM.

Today, then, it seems that research has chosen orienting the development of products with improved texture, possibly limiting the alloy content to grant a better magnetic permeability. And, meanwhile, acting on other awarding aspects to improve losses, such as steel cleaning, thickness and surface quality. «An improvement model, the latter, – notices Mr. Fortunati – more difficult to seize but leading in the future to products with better usability, for those working now at new motor projects and for the more and more challenging market requirements. Without forgetting that the trend towards the “ultra-thin” is not always positively judged by the manufacturers of cores and stator packs, owing to the sometimes critical necessary machining and more complex processes to be managed».

A glance at future trends

The current trend seems then to pursue the research of increasingly thin materials but we should consider right now the possible complications. It is then necessary to operate in parallel, for instance from the process point of view.
«Displacing for instance blanking – adds Mr. Fortunati – from mechanical systems towards solutions that adopt laser technologies, less affected by the thickness effect and that grant a minor degradation of the magnetic behaviour around the blanking/cutting areas. Meanwhile, we should imagine the manufacturing of super-thin laminations with a multilayer technology that can decrease the magnetic loss while facilitating their use».
Another element, anyway known for a long time, but really driving towards the smart use of these materials for applications such as household appliances but, especially in automotive ambit (where sizes and weights play a determinant role) is the stress control.

The role of stress control

When they design a motor, they tend in fact to obtain the best performances through the choice of the number of poles, number of teeth and so on, thus manufacturing a blank to implement the stator pack, even complex (with several cuts, slots and so on), compact and with very small sizes.

M
The attainment of the best efficiency of an electric motor involves several aspects. Among them, also the choice of the core materials.

«Material – adds Mr. Fortunati – that must be produced at the best possible state, without internal residual mechanical stresses that negatively influence the magnetic behaviour. It is clear that the blanking operation and, more in general, assembling processes, generate mechanical stresses and residual deformations and, actually, they deteriorate the material, with effects that become serious proportionally to the high magnetic quality of the starting material».

The use of “semi-finished” materials

The manufacturers of electric machine cores can face this aspect exploiting a stress-relieving annealing, to be carried out after the implementation of the stator pack, compacted and provided with windings.
«This process phase – explains Mr. Fortunati – is prerogative only of some companies that, instead of treating non-oriented grain material called “fully-finished”, use “semi-finished” materials. In other words, materials manufactured at lower costs and typically with relatively low percentages of Silicon, supplied not in a finished state but annealed, not decarburized and further cold-rolled, with a very low deformation level. The successive static annealing of the assembled core carried out by the user is calibrated to complete the material purification and to obtain the desired crystalline microstructure».
The analysis of this type of structure, as delivered by the steel producer, shows initial very bad magnetic specifications. Nevertheless, the stator pack producer improves the intrinsic properties through annealing (performed according to particular treatments and parameters provided for by law), which transforms the inner structure and solves the problem of residual stresses, induced by cuts and assembling operations. A process, the latter, that facilitates a calculated metallurgic process and recovers optimal magnetic characteristics. These are the peculiarities that identify “semi-finished” materials, spread especially by geographical areas.
Concerning this, Japan and United States have registered their broad diffusion for some time now, unlike what is happening in Europe. We are referring to materials that, even if considered of not excellent qualitative level, combine the advantage of a lower Silicon percentage (then better magnetic permeability), with the plus of lower cost and the solution of the criticality dictated by blanking.
«I believe that this represents an interesting field of research and development – ends Mr. Fortunati – still according to the logic of finding the best and optimal compromise able to satisfy the entire manufacturing chain, from the producer of raw materials to the motor manufacturer».

materials and efficiency
Centro Sviluppo Materiali, Terni.

Research and development activity where CSM (Centro Sviluppo Materiali) represents a qualified partner on a domestic and international scale, supporting constant innovation, process and product improvement and the attainment of higher and higher qualitative targets.

Experience and innovation at the service of materials

CSM (Centro Sviluppo Materiali) was established in 1963 as research centre of IRI in the steel industry. Today it represents the most important Italian private reality of RINA Group in the ambit of the innovation and improvement of product and process. A structure whose mission consists in developing and transferring, in industry and market at world level, the technological innovation in the strategic sector of materials and relative design, production and application technologies.
The innovation skills of CSM are based on the know-how gained in fifty years of industrial projects in metal and ceramic materials and in the integration of enabling technologies. Internet of Things, Additive Manufacturing, Big Data and Data Analytics, Augmented Reality, Collaborative Robotics and Digital Twins are some of the key words of the new context. The Centre can claim a long experience in these issues, achieved through innovation projects in the sector of materials, developed in collaboration with manufacturing companies of primary importance on a world scale.
Today CSM operates for the steel manufacturing industry and for diversified fields, such as oil and gas, aerospace, special materials and products, engineering and plant engineering, environment and energy. A strict collaboration with big, medium and small businesses, both engineering, manufacturing and service providers. All project phases, including simulations, preparations and analysis of samples and mock-ups, as well as all testing steps, take place in its own structures. Acknowledged as important pole of the European research and innovation network, CSM has always maintained a strong relation with steel partners at international level, too.
(Marcella Trapp)