Thursday, August 6, 2020

Synchronous Reluctance motor: a rare-earth free solution for electric vehicles

The Research Group at Department of Industrial & Information Engineering & Economics of the University of L’Aquila has been working for many years in the field of designing, prototyping and testing of innovative motors for automotive applications. It is recently involved in a H2020-GV European project focused on contributing to avoid the use of rare-earth magnets through the development of a next generation of electric drivetrains, ensuring the industrial feasibility for mass production while focusing on the low cost of the manufacturing technologies. 

The impact of internal combustion engine on the environment has led to efforts to replace it by alternative propulsion systems, among which the electric motor has become the primary candidate. The electric car market is booming and in the next twenty years a surge electric vehicles (EV) sales is expected which should exceed that of traditional ICE cars, as shown in Fig.1.
The new technologies for energy storage and powertrains play a critical role in the development of the electric vehicle market. At motor level, key components and innovative materials must be integrated in the current motor designs. Recent advances in materials, power electronics, control systems have contribuited to new energy efficient and performant powertrains adopting innovative motor technologies.

Trend of electric car sales (Bloomberg New Energy Finance)
1. Trend of electric car sales (Bloomberg New Energy Finance)

The basic characteristics of an electric motor

The requirements of the electric motors for EVs are different from the conventional ones used in the industrial applications. The most challenging specifications are a reduced size and high efficiency in an extended speed range. For good overloading and wide speed range capability, the machine is usually water cooled with a water jacket around the stator core.
The basic characteristics of an electric motor are the following:
1. high specific power and specific torque;
2. very wide speed range;
3. high efficiency over wide torque and speed ranges;
4. wide constant-power operating capability;
5. high reliability and robustness for vehicular environment;
6. reasonable cost.
Since the EV operates over a wide torque-speed range in various driving conditions, the motor design should be aimed to achieve overall energy saving over a driving-cycle of the vehicle (e.g. WLTP, NEDC, UDDS). There are many demands for developing propulsion systems with high power density, high efficiency and low cost.
The research in this field has been intense in the past few years and different types of electric machines have been studied and proposed. The vast majority of motor solutions rely on Permanent Magnet technology using rare-earth magnets. Table 1 summarizes the existing electric vehicles in the European & US markets, specifying the technological solution for the traction motors.

Traction Motors for Electric Vehicles
Table 1: Traction Motors for Electric Vehicles

From the Permanent Magnet Synchronous Motor

The Permanent Magnet (PM) Synchronous motors are more attractive and the main advantages are their inherently high efficiency, high power density, and high reliability.
The PM motors are relatively easy to control and exhibit excellent performance, in terms of maximum torque per ampere control and optimal extended speed operation. Different types of PM machines are proposed according to the position of PMs in the rotor and can be classified as surface or interior mounted magnets.
The high and volatile cost of raw materials for magnets makes uncertain their long-term availability, especially since the electric vehicle technology is going to be manufactured in mass production. Also, PM motors present several technical drawbacks that limit the performances of the motor, in particular the demagnetization effect if the temperature of the motor exceeds its operating conditions. Therefore, there is a growing attention in alternative solutions that include rare-earth (RE) free machines or reduced RE-PM machines.

The Synchronous Reluctance Motor

The Synchronous Reluctance Motor (SynRM) is becoming of great interest in the recent years and represents a valid alternative for electric and hybrid vehicles due to its simple and rugged construction. The main advantage of the SynRM relies on the absence of the rotor cage losses or PM losses, allowing a continuous torque higher than the torque of an Induction Motor (IM) of the same size. Other important features are:
1. the rotor is potentially less expensive than PM motors and IM ones;
2. the specific torque is acceptable and it is not affected by the rotor temperature;
3. the field-oriented control algorithm is simpler with respect to the one of IM drives.

Cross section of the 200 kW SynRM
2. Cross section of the 200 kW SynRM

The conventional SynRMs are known for their lower specific (peak) power and specific (peak) torque (compared to the PM motors), higher noise and lower power factor.
Despite these drawbacks, it is possible to obtain high torque density and high efficiency motors through an optimized rotor design.
Specific power in SynRM is enhanced by increasing the rotor operating speed and the flux-weakening region. Nevertheless, the optimal geometry for motor performances needs to be refined to guarantee the mechanical integrity of the rotor at high speed.

New solutions for the power traction systems of electrical vehicles

The Research Group at Department of Industrial & Information Engineering & Economics of the University of L’Aquila has been working for many years in the field of designing, prototyping and testing of innovative motors for automotive applications. The Group is recently involved in a H2020-GV European project labelled “RefreeDrive” – Rare Earth Free e-Drives for low cost manufacturing.

Table 2: SynRM requirements for the target application
Table 2: SynRM requirements for the target application

This project is focused on contributing to avoid the use of rare-earth magnets through the development of a next generation of electric drivetrains, ensuring the industrial feasibility for mass production while focusing on the low cost of the manufacturing technologies.
The aim is to study and develop new solutions for the power traction systems of electrical vehicles, based on Brushless AC electrical machines rare-earth magnet free.
Through the development in the electric motor topologies within the project, advanced performance has been achieved in terms of specific power, power density and efficiency, compared to a current electric vehicle taken as a reference (Tesla Model S 60). The ReFreeDrive Consortium is composed of 13 partners in six European countries and the team of University of L’Aquila took in charge the design of high speed Synchronous Reluctance motor (200 kW peak power) for a full-electric premium vehicle. The goal was to design a liquid cooled SynRM than could satisfy the imposed requirements shown in the Table 2.

Performances while respecting the target components cost

The avenues indicated above have required a complex optimization process for matching the desired motor performances while respecting the target components cost. Specific sizing procedures and optimization algorithms have been used for the design refinement and the proposed solution presents an innovative rotor structure with “fluid shaped” barriers and radial ribs.
Fig.2 shows the cross section of the 6-pole SynRM.The rotor with asymmetric shape has multiple “ribs” that connect the segments to each other axially and transversally: these connections maintain enough mechanical integrity in the rotor structure when rotational forces are applied at high speed.

Table 3: Performance of the SynRM
Table 3: Performance of the SynRM

The optimal positioning and the optimal thickness of the rotor ribs have been refined by the “topology optimizer” coupled to a mechanical Finite Element software; this algorithm has allowed to optimize the quantity and the positioning of the mass needed by a mechanical part to sustain the stress.
The main motor performance at peak power and continuous power are listed in Table 3 while Fig.3 presents the efficiency map in motoring mode: the peak efficiency is about 96% and the maximum torque is 383 Nm with a peak power of about 250 kW at 6200 rpm. At maximum speed (18000 rpm), the maximum power is 56 kW: these performances fully satisfy the requirements given in the Table 2.

Efficiency map of the SynRM
3. Efficiency map of the SynRM

 

Rotor of the SynRM (1 pole): mechanical stress @ 18000 rpm
4. Rotor of the SynRM (1 pole): mechanical stress @ 18000 rpm

The mechanical equivalent stress map at max speed (18000 rpm) is reported in Fig.4: the results are satisfactory and confirm that the rotor structure is able to withstand mechanical stress at high speeds and it satisfies the mechanical limits of the chosen electrical steel.
A lower power machine (75 kW peak power) has been scaled from the 200 kW design by only changing the stator winding and stack length in order to contain the manufacturing costs.
Two protoypes have been realized and Fig. 5 shows the stator and rotor cores of the 75 and 200 kW sizes.

Stator and rotor cores of the SynRM prototypes
5. Stator and rotor cores of the SynRM prototypes

The power electronic has been designed by R13 Technology (a University of L’Aquila spin-off) for a direct integration with the SynRM housing sharing the motor cooling system.
The powertrains (75 and 200 kW) will be tested in real driving conditions on a test bench and vehicle demonstrators.

by Marco Villani, University of L’Aquila, Dept. of Industrial and Information Engineering and Economics

ABOUT THE AUTHOR
Marco Villani, University of L’Aquila, Dept. of Industrial and Information Engineering and Economics
Marco Villani, University of L’Aquila, Dept. of Industrial and Information Engineering and Economics

Marco Villani received the M.S. degree in electrical engineering from the University of L’Aquila, Italy, in 1985. He became an Assistant Professor of power converters, electrical machines, and drives in 1993. In 1990, he was Research Fellow at the University of Dresden, German, and in 1995 at the Nagasaki University, Nagasaki, Japan. In 1998 he cooperated in two SAVE projects concerning the “Energy efficiency improvements in threephase Induction Motors” and the “Barriers against energy efficient motor repair”. He has been involved in National Research Projects and took the responsibility of several research contracts between the University of L’Aquila and industrial partners. He is currently associate professor of Electrical Machines Design for the Master-level degree courses of Electrical Engineering at the University of L’Aquila. His research interests are focused on modeling and design of electrical machines, high efficiency induction motors, optimization techniques for the electrical machines design, Finite Element analysis of electric motors, design of PM synchronous motors and Reluctance motors for industrial, automotive and aerospace applications. He is author of more than 160 technical papers in scientific journals and conference proceedings.

Fault-tolerant electric drives for aeronautical applications

Fault-tolerant motors for flap application (FA) and tail rotor drive (ETRD)
3. Fault-tolerant motors for flap application (FA) and tail rotor drive (ETRD)

Fault-tolerant modular electric drives represent an already feasible solution to assure the reliability requisites of aeronautical applications. Prototypal studies and systems already operating in aircrafts confirm it. To understand better the characteristics and the specificities in this applicative ambit, we have talked about that with Professor Marco Tursini, ordinary professor of the Department of Industrial and Information Engineering and Economics at L’Aquila University.

Fault-tolerant electric drives arouse lively interest in aeronautical applications because they allow satisfying their strict reliability specifications. A modular design permits to obtain electric motors with high power density and high efficiency, as well as intrinsically fault-tolerant, without turning to the complete redundancy of the actuator.
Extending the approach to power electronics and to control, the use of electrical drives can be hypothesised not only in service motions but also in “safety critical” functions of aircrafts, such as primary flight controls.
To understand better the characteristics of multi-phase fault-tolerant drives in this operational ambit, the state-of- the-art and the new opportunities, we faced the issue with Professor Marco Tursini, ordinary professor of the Department of Industrial and Information Engineering and Economics at L’Aquila University, who reported as example some projects developed by his research team.

The added-value of modularity

In recent years we witness the growing use of electric drives in aircrafts, trend indicated by the English acronym MEA (More Electric Aircraft). The basic idea is replacing the conventional hydraulic and pneumatic actuation systems with more compact and lighter electric systems, to reduce consumptions and fuel costs, to allow longer flight routes and to decrease emissions.
The electric drives commonly used in industry cannot satisfy the reliability requisites demanded in aeronautics, especially in those functions on which depends the safety itself of the flight and of passengers.
These limits, connected with both the structure of motors and control electronics, have led researchers to explore new system architectures, able to guarantee the demanded reliability without turning to the complete redundancy, i.e. maintaining compactness and lightness features. «A possible solution – explains Professor Marco Tursini, Department of Industrial and Information Engineering and Economics at L’Aquila University – consists in considering the redundancy in terms of “phases” of the electric motor, directly in the design activity.
Such concept leads to the development of multiphase modular drives, able to satisfy the basic principles of the fault-tolerance of an electromechanical actuator, in other words the electric, magnetic and thermal insulation among phases».
We are hereunder illustrating some solutions developed on such principles by the research team of L’ Aquila University, coordinated by Professor Tursini, in the ambit of funded projects concerning the drive of flaps, of a cart lift system and of the tail rotor of a helicopter.

Examples of specific applications

The “flap actuator” (FA) is designed to move the flaps positioned on the trailing edge of aircrafts’ wings. Flaps are generally used just for some seconds during landing and take-offoperations and are “safety critical” systems of the aircraft because their failure affects the flight mission.

1. Electromechanical actuators for flap (FA) and cart lift system (CLS)
1. Electromechanical actuators for flap (FA) and cart lift system (CLS)

The CLS, Cart Lift System is instead a service lift used for the transport of foods and drinks between the hold and the passenger cabin in large aircrafts. Its failure affects passengers’ comfort and serenity and the same reliability specifications as for flaps are required. Both FA and CLS systems are based on a re-circulating ball screw device that translates the rotary motion to linear, whose mechanical view is illustrated in Figure 1.
The ETRD (Electrical Tail Rotor Drive) project, developed in the ambit of the European “Clean Sky” research programme, concerns instead helicopters with tail rotor drives of “Fenestron®” type, fully integrated into the terminal part of the tail beam, such as H 160 of Airbus Helicopters.
«In this case–Prof. Tursini underlines– the project target is the replacement of the current mechanical architecture, where the motion is taken by the main rotor and transmitted to the tail, with an electric motor directly coupled to the blades of the tail rotor, with the advantage, in addition to the system simplification, of guaranteeing a total decoupling between the speeds of the turbo-shaft engine and of the tail rotor, so notably widening the aircraft’s manoeuvre capability».
Fault-tolerance specifications impose that the loss of one phase owing to failure does not affect the capability of delivering the nominal power of the actuator, whereas with a further second loss it is possible to supply reduced power, decreasing torque or speed depending on applications, as we can deduce from Table 1.

Table 1 – fault-tolerance specifications
Table 1 – fault-tolerance specifications

The fault-tolerant motor

The modular approach of fault-tolerant drives finds its natural implementation in switched reluctance or permanent magnet motors of “brushless DC” type (PM-BLDC).
The structure of the latter, preferred due to the higher achievable power densities, is illustrated in Figure 3.

Fault-tolerant motors for flap application (FA) and tail rotor drive (ETRD)
3. Fault-tolerant motors for flap application (FA) and tail rotor drive (ETRD)

«The electric independence –Professor Tursini highlights– is first of all implemented by powering the phases separately. Phase windings are made with coils arranged on protruding poles and by alternating wound and unwound poles. This implementation favours the failure reduction, through both the physical separation among the coils of the different phases and through the minimal overlap among the respective wirings. The arrangement on alternate poles features a low reciprocal inductance among phases, guaranteeing a substantial magnetic independence and a minimal thermal interaction. These engineering solutions allow avoiding the failure in one phase affects the others».
For the flap application, they have adopted the solution with five-phase PMBLDC motor, internal rotor, 18 magnets on the rotor and 20 stator slots.
«Each phase –Professor Tursini explains– consists of a pair of coils wound on opposite stator teeth, arranged at 180 mechanical degrees one another and connected in series.
The electromagnetic structure is sized to limit the netshort circuit current to phase terminals, with the rotor in motion at the highest speed. With the short circuit that is powered by the voltage induced by rotor magnets and generates overheating and braking torque. In the case of CLS and ETRD motors, we have adopted a similar solution with six independent phases».

The fault-tolerant drive

The modularity principle suggests that each phase is powered and controlled by a dedicated fully independent subsystem. Each subsystem includes the functions that can be directly referred to a single phase: a power bus, a power stage (inverter), current and voltage sensors and a control device (microcontroller), and it can operate autonomously from the others.

BLDC control of 5-phase motor
5. BLDC control of 5-phase motor

«The functions shared among the phases –Professor Tursini further points out– like the mechanical position sensor and primary power buses must be opportunely redundant to reach system reliability requisites». The current is controlled with a proportional-integral regulator (PI).
An orientation logic, enslaved to the position sensor, generates the current control. In PM-BLDC motors, the current is controlled in phase with the trapezoidal voltage induced by magnets (back-EMF), to generate constant torque, as shown in Figure 5.
Each phase shares in the overall torque with a proportional contribution to its own current. «An outer ring –Professor Tursini adds– adjusts the motor speed, drawn by the mechanical sensor, still with PI regulator.
A redundant communication bus allows phase microcontrollers to share the speed control, the position measurement and other useful information to equalize the control among phases».
The power stage of each module is composed by a single-phase inverter, as per Figure 6.

Fault-tolerant multi-phase inverter
6. Fault-tolerant multi-phase inverter

In case of fault on a motor phase, the latter is disconnected by cutting off the power switches of the correspondent single-phase inverter.
«The short circuits on inverter branches –Professor Tursini adds – are primarily managed in hardware, detected by the drivers of power switches and reported to the microcontroller that provides for controlling the concerned branch in cut-off.
The short circuits persisting on the inverter and the short circuit on the levelling condenser are protected by the fuse cut-off at the stage input, which equally cuts out the phase but avoids the short circuit of the power DC bus».

Fault tolerance

The architecture of the fault-tolerant drive allows the control adaptation in automatic in case of loss or cut-out of one or more phases caused for failure, as we can assess in Figure 7.

Control dynamics and the cutoff of 2 phases (5-phase motor for flap (FA) @300 rpm, 12.7 Nm)
7. Control dynamics and the cutoff of 2 phases (5-phase motor for flap (FA) @300 rpm, 12.7 Nm)

«In such eventuality in fact –Professor Tursini highlights – we have a transient reduction of the produced torque and a consequent decrement of speed. The latter, surveyed by speed regulators, results in a rise of the current control for sound phases, which restores the value of the average produced torque to the pre-fault level and the speed to the controlled level».
Owing to the loss of phases caused by failure, the torque ripple around the average value increases, hand in hand with the number of missing phases and with the adjacency of their angular arrangement in the motor, but this aspect is secondary in a “safety critical” system. «The fault-tolerant drive–Professor Tursini confirms – must then be sized fixing in the specifications the number of phases that can be lost and considering the overload of sound compensating phases». If we assume the torque/current relation as linear, the overload factor of a fault-tolerant motor is given by:
Where m is the number of phases of the motor and x is the number of inactive phases (cut off) owing to faults or corrective fault actions. For a 5-phase motor, it is valid F1 = 1.25 and F2 = 1.66 respectively for one or two missing phases. «Actually –Professor Tursini adds–aeronautical actuators are sized to exploit magnetic circuits at best. Consequently, the active phases operate in conditions of high saturation in case of fault and the current overload is higher. The Finite Element electromagnetic analysis allows obtaining more accurate indications concerning this since the design phase».
Table 2 reports, as example, the results of the Finite Element analysis for the FA actuator, including the torque/current ratio (TCR) in the operation with sound phases and with fault.

Table 2 – motor performances for FA at nominal torque (12.5 Nm)
Table 2 – motor performances for FA at nominal torque (12.5 Nm)

Guaranteed reliability requisites

What reported validates that modular fault-tolerant drives already represent an immediately feasible solution to guarantee the reliability requisites of aeronautical applications. Prototypal studies and systems already in use in aircrafts confirm it. «Compared to hydraulic and pneumatic systems –Professor Tursini ends– electric drives offer more user-friendliness in installation and maintenance, reduced overall dimensions and weights, hypothetically allowing the reduction of fuel consumption and of consequent emissions. This is a more and more central aspect in a globalized society taking care of environmental issues. Since 2008, the European Commission, in partnership with the major European aerospace industries, has promoted and funded “Clean Sky” Consortium, technological initiative for an “ecologically favourable” evolution of the air transport».


FOCUS ON RESEARCH
Marco Tursini was born in 1960 in L’Aquila province and graduated in Electrical Technology Engineering at L’Aquila University in 1987. In the same year, he started his scientific collaboration with L’Aquila University where he is currently Ordinary Professor of Power Converters, Electrical Machines and Drives and President of the Master Didactic Area Council in Electrical Engineering. In his career, he carried out research activity by the Swiss Federal Institute of Technology in Lausanne and by Nagasaki University.

Professor Marco Tursini, ordinary professor of the Department of Industrial and Information Engineering and Economics at L’Aquila University
Professor Marco Tursini, ordinary professor of the Department of Industrial and Information Engineering and Economics at L’Aquila University

Since the early Nineties, he has been manager of numerous research projects, in both industrial ambit (Texas Instruments, Indesit, Gefran, Denso, Umbra Cuscinetti) and in national funded projects (MIUR, PRIN, PON, Industria 2015) and European (Clean Sky). His interests concern the sector of electric machines and drives, with specific focus on modelling, simulation and control, “sensorless” drives, failure analysis and diagnostics, “realtime” simulation and rapid prototyping techniques. In this ambit, he is author of over 140 scientific publications. IEEE member and reviewer of numerous international journals and conferences, since 2011 he has been member of the Editorial Board of the international review “Electric Power Components & Systems”. In 2014 he promoted the birth of “R13 Technology” spinoff of L’Aquila University in the sector of industrial electronics, electric motors and drives, automation and measuring systems.


 

From nanomaterials to the new generation of electric motors

Conventional asynchronous motor (credits: Prof. Chiricozzi)
Conventional asynchronous motor (credits: Prof .Chiricozzi)

Conventional materials and technologies are by now reaching their performance limit. It is then clear the need of turning to a “revolutionary” synergy that succeeds in tracing a new frontier of development able to change the entire system of the electrical machine industry.

Electrical machines can still boast a notable improvement potential and, nevertheless, they are considered a quite mature technology. The constant improvement has been certainly achieved by using magnetic cores made of special steels, or with low magnetic loss, and high power density permanent magnets that provide low-loss excitation for machines, without however determining a revolutionary development. A real step forward, for a gradual change in the improvement of electrical machines’ performances, can occur only with the new rising materials, like for instance nanomaterials, also when combined with advanced technologies. A winning synergy can be established between new materials and forefront process technologies and it will undoubtedly be able to innovate the industry of electrical machines.

More efficiency and sustainability through nanoscale materials

The radical progresses that might substantially influence the efficiency and the performances of electrical machines require the introduction of new materials needed to replace the ones traditionally used.
Multifunctional electromagnetic materials, such as carbon nanotubes, better known as CNT, and magnetic-core superparamagnetic nanoparticles are the new materials that can be used to replace respectively copper and iron in the conventional motor, and to design a light high-performance electric motor that can be a state-of-the-art example of future energy conversion devices.
The need of increasing the overall efficiency of the motor system, in fact, has never been so high. Owing to the increment of energy costs and the serious concerns for global CO2 emissions, reaching the highest possible efficiency of the motor system has become a fundamental priority. Motorized systems used by manufacturing industries play an important role in national energy profiles. The variety and the differentiation of future systems will be oriented towards a higher power density, with increasingly efficient and compact motors.
However, conventional materials and technologies are by now reaching their limit. Therefore, it is evident the need of a “revolutionary” technology that by will enabled just by turning to new materials: “nanomaterials” might change the entire system of the electric machine industry.
The materials used to make motors have not changed a lot since 1910 to today. Copper and iron are still used and rare earth magnets are exploited to obtain the nest performances. Today there is instead the possibility of using nanoscale materials to develop next-generation electric motors that are sustainable and more efficient. The future forefront electrical motor will require the integration of the fundamental research concerning the development of nanoscale materials with the re-engineering of the electromagnetic design of the motor itself, in order to overcome the limits of existing materials.
Carbon nanotubes, CNT, represent an extraordinary family of new materials characterized by as extraordinary electrical, thermal and mechanical properties.
Carbon nanotubes feature a density of 1.5 g/cm3 against 8.96 of copper; resistivity reaches 1 x 10-8 Ωcm, against 1.7 x 10-6 of copper; ≈ 105 A/cm2 is instead the current density, against 103 of copper. Thermal conductivity and breaking strength appear quite different, too: 3,000 W/mK and 1,000 N/mm2 for carbon nanotubes, against respectively 400 W/mK and N/mm2 of copper, as synthesized in the  table (1).

Exploiting advantageously these intrinsic properties is based on the capability of systematically synthesizing, characterizing and integrating standardized materials into real devices. Therefore, almost the entire electric motor can be eventually manufactured with nanomaterials.
The main nanomaterials under development phase are the following:
1. Strong, light, electrically and thermally conductive CNT wire.
2. Superparamagnetic nanoparticles.
3. Hard magnetic nanoparticles.
4. Composites reinforced with carbon nanotubes.

Carbon nanotubes can replace the copper wire

The best carbon nanotubes have proven a conductivity widely exceeding best metals’. Therefore, the future windings implemented with such materials might have a double conductivity (100 MS/m) compared to current copper ones.
If we keep electrical machine design parameters unchanged and we replace only copper with future carbon nanotube yarns, we can reduce losses in windings by Joule effect to half of the machine’s current losses. Carbon nanotube yarns are much lighter than copper and more ecologic, too.
Therefore, the copper replacement with nanotube yarns should significantly decrease CO2 emissions connected with the production and the operation of electrical machines. Moreover, the sizes and the masses of the machine might be reduced and motors might also operate at significantly higher temperature than current ones.

To facilitate the handling of CNT, carbon nanotubes are woven to form multifibre yarns like the one in the photo: twenty layers with diameter resembling human hair’s

In fact, a further highlight of CNT wire concerns the resistivity that decreases when the temperature rises (at environmental temperature, the resistivity temperature coefficient is negative, -0.2 x 10-3 /K), which is an advantage versus copper. Reducing the electric resistance of the nanotube yarn will be a future target to increase the power and to improve the efficiency of carbon motors.
Another advantage of CNT fibres consists in the fact they are constituted by very thin sub-conductors that should considerably limit the possibility of skin effects at the frequency of the electrical machine. Moreover, supposing future commercial CNT wires will be manufactured starting from multi-fibres that can be easily transposed, neither circulating currents are likely to appear. Both the skin effect and circulating currents can be very noxious in traditional high-current windings and result in higher resistance in AC.
Therefore, it should be possible to implement electrical machines with carbon windings that can work at higher operational temperatures than those today usually applied for their design. This is due to the fact there will be no increase of losses by Joule effect at the temperature rise.
Although it is not foreseen carbon nanotube conductors will soon replace copper ones, it is plausible to expect that in the short term CNT materials allow the creation and the development of new applications and new devices, wherever the same CNT wires can take advantage, compared to copper, in terms of lightness, flexibility, flexural strength, resistance to corrosion, high resistance and high modulus of elasticity, which is 103 kN/mm2.

Superparamagnetic nanoparticles, hard magnetic and CNT reinforced composites

As previously mentioned, among the main nanomaterials under development phase stand out also superparamagnetic nanoparticles and hard magnetic nanoparticles. The first, used in a polymeric matrix, produce an assembly of particles with low hysteresis and high saturation, then it is possible to manufacture a light-material magnetic core, which transports the same flux as the ferromagnetic one, or a higher one, and minimizes the loss of parasitic currents.
Hard magnetic nanoparticles are instead based on not critical, easily accessible, stable and easily recyclable 3D metals, like iron (Fe), Cobalt (Co) and Nickel (Ni), with a magnetization at high saturation and magnetic anisotropy.
Such nanoparticles can be used as constitutive elements for the manufacturing of sintered and alloyed permanent magnets, as replacement of permanent magnets currently used in electric motors containing critical rare earth elements at high cost. This new generation of permanent magnets can share in solving the sector dependence on rare earth metals, at the same time improving magnetic performances. This might be important for the development of electric cars based on rare earth elements for their powerful electric motors. Since nanocomposites feature better magnetic force/weigh ratios, the new devices, like electric motors, can be equipped with lighter magnets, so reducing their sizes and the weight as well. In that way, it would be possible to enhance the device performances while decreasing implementation costs, making the mass production much simpler.

Planning for the development of nanoscale materials and electric motor. CNT wire prototypes, magnetic material and CNT composite have been already produced in the UC Nanoworld laboratory

The last, but not least, family of nanomaterials under development phase includes the composites reinforced with carbon nanotubes. These materials will be increasingly available on trade with the rise of the production of CNT that, due to their intrinsic properties, will provide a much higher resistance than carbon nanotube fibres, in addition to a higher thermal conductivity.
Prerogatives, the latter, that will allow implementing the structure of the electric motor, the housing and the shaft, increasingly light and thermally conductive.
Hoping that progresses will be constantly confirmed in all four nanoparticle structures mentioned, there will be soon the conditions for the development of a “new generation of electrical motors”, which will be manufactured almost in their entirety with nanoscale and nanostructured materials.

New frontiers, trends and future developments

In today’s global more and more competitive market, the demands for higher power density, compact motors, lower costs and more reliability are driving the need of introducing an innovative approach to the use of new materials and advanced technologies, to allow the development of a new generation of electric machines and systems. The recent technological progresses in nanomaterials have opened new horizons and marked out the new frontiers for macroscopic applications.
The carbon nanotube wire, CNT, the magnetic core made with superparamagnetic nanoparticles and permanent magnets made with hard magnetic nanoparticles are new materials that can be used to replace respectively copper, iron and rare earth permanent magnets in conventional motors; this will permit to design a high-performance light electric motor that can be an example of a new generation of energy conversion devices. Other advantages of carbon motors undoubtedly concern also the fast acceleration, the high torque and the capability of operating at high temperatures and at higher speeds. The state-of-the-art motor must be then designed according to the synergy of the integration of different materials that exceed the limits of existing materials and exploit the properties of new materials.
The collaboration must reach an integration level where universities, designers and producers will work together to improve the understanding of the advantages and the challenges regarding the application of new materials and of advanced technologies, to allow the development of a new generation of electric motors that will play a fundamental role in industry and will envisage a new future for the society. I hope this challenge will be increasingly sustained and supported by potential stakeholders, so that Italy can release on the market the first electric motor made with nanomaterials and respective advanced technologies.

by Enzo Chiricozzi, Professor Emeritus of Electrical Machines University of L’Aquila

ABOUT THE AUTHOR
Enzo Chiricozzi, Professor Emeritus of Electrical Machines University of L’Aquila
Enzo Chiricozzi, Professor Emeritus of Electrical Machines University of L’Aquila

Enzo Chiricozzi graduated in Electrotechnical Engineering at “La Sapienza” University in Rome. Full Professor of Electric Machines at the Faculty of Engineering of University of L’Aquila, he was appointed to various roles, like: President of the Graduation Course in Electric Engineering, Director of the Department of Electric Engineering, Dean of the Faculty of Engineering, Deputy Rector for the University Placement and the relationships with the labour world. In 2012, with decree by MIUR, Ministry of Education, University and Research, he received the conferment of the title of Professor Emeritus of University of L’Aquila. His research activity, developed in the scientific area of Electric Machines and Electric Drives, with in-depth study of various themes (torsional oscillations of big generating units; analysis of the dynamics of a heterogeneous group of asynchronous motors; the saturation problem in electric machines; high energy efficiency asynchronous motors; analysis of the dynamic operation of electric generators of synchronous and asynchronous type operated by wind motors; drives in DC and in AC controlled by microprocessors), has been acknowledged by the scientific community on a national and international scale.

“Baby electric motors”, strollers with e-stroller system

Nine parents out of ten pay attention to strollers’ comfort and safety and, concerning this, Bosch has ideated a new system that marks its entry in a new market.
In the wind tunnel, with 7-degree intensity according to Beaufort scale, the air hits the stroller at a speed of 60 km/h and strongly shakes the canopy, but the stroller does not move thanks to the new e-stroller system by Bosch. It is much more than an electric traction, it is a stroller assistant with a complete range of comfort and safety functions. Besides the thrust support and the automated braking function, this system is in fact provided with an alarm function, with a series of highly technological sensors and the possibility of connecting it to the smartphone through an app.
The traction system includes two silent electric motors on the rear axle, a Bluetooth module and a system of smart sensors. These sensors, used also in smartphones, measure also the stroller’s speed and acceleration, detecting also the type of surface travelled.

Test benches for engineering students to asses industrial electronic equipment

Test benches
Test benches

The University of Southern Denmark gives young engineering students an opportunity to be part of developing the green technology of the future in a creative study environment with modern teaching facilities and dedicated teachers.
The engineering students are taught electronics and challenged with real-life industrial equipment driven by electric motors and drives manufactured by Nidec.
As part of the teaching projects, the institute and the students have access to several advanced electronic test benches, which gives them a good insight into the handling, testing and research of different combinations of industrial electronic equipment and teaches them to test torque, speed, load, etc. under realistic conditions.
The test benches use Nidec Leroy-Somer motor units (servo motors, LSMV and IMfinity induction ranges) and Control Techniques Unidrive M600/M700 drives and combine different set-ups of advanced variable speed drives and high-tech electric motors, which have the flexibility to test a wide variety of electronic equipment.
Each semester, the electronic engineering degree programme includes project work on a current topic, for example how to design the electronic controller of an electric go-kart motor.
In connection with the project, the students have access to a genuine go-kart with an electric motor as well as a corresponding test bench with programmable drives and electric motors.

Energy storage for the electric car. Dry electrode coating technology

Energy storage

Researchers at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden have developed a new production process with the aim of efficient and environmentally friendly future battery production. They coat the electrodes of the energy storage cells with a dry film instead of liquid chemicals. This simplified process saves energy and eliminates toxic solvents. A Finnish company is currently successfully testing the new IWS technology in practice.

Better and more cost-efficient production methods for energy storage are increasingly in demand, especially in Germany: all major automobile manufacturers have launched ambitious electric vehicle programs that will ensure a sharp rise in demand for batteries. So far, German companies have been purchasing the cells for this purpose in Asia.
There are two main reasons driving this trend: Asian technology groups have many years of experience in the mass production of battery cells and a lot of energy is consumed in these processes. Production at locations with high electricity prices, such as Germany, is, therefore, very high-cost.

No more toxic solvents – lower electricity costs
It is exactly this fact the Saxon Fraunhofer engineers want to change: “Our dry transfer coating process aims to noticeably reduce the process costs in electrode coating,” emphasizes IWS project manager Dr. Benjamin Schumm. “Manufacturers can eliminate toxic and expensive solvents and save energy costs during drying. In addition, our technology also facilitates the use of electrode materials that are difficult or even impossible to process wet-chemically.”
But exactly these materials are needed for future batteries with higher energy density. “For all these reasons, we think that our technology can help to achieve internationally competitive battery cell production in Germany and Europe.”

Pilot plant successfully started in Finland
This potential is also seen by Fraunhofer’s Nordic partners: The Finnish battery company “BroadBit Batteries”, together with IWS, has commissioned a pilot plant in its Espoo factory, which coats electrodes with dry electrode material instead of wet pastes, as has been common in industry up to now. BroadBit uses it to produce new types of sodium ion batteries. “The demand for our technology is high, even in Germany,” reports Benjamin Schumm. On a laboratory scale, the IWS can already coat electrode foil with a remarkable production speed of several meters per minute. In this respect the Dresden engineers can show the potential for transferring the technology to the production scale.
Limits of classic wet chemistry until now, cell producers have mostly coated their battery electrodes in a complex wet-chemical process. First, they mix the active materials, intended later to release the stored energy, with additives to create a paste. In this process they add organic solvents, which are expensive and usually toxic. In order to protect operators and the environment, elaborate precautions for occupational safety and reprocessing are necessary. Once the paste has been applied to thin metal foils, a further expensive process step begins: Dozens of meter long heating sections dry the coated films before they can be further processed. This drying procedure usually causes high electricity costs”.

Binding molecules form a cobweb
The new film transfer technology for dry electrode coating, on the other hand, operates without these ecologically damaging and expensive process steps: the IWS engineers mix their active material with binding polymers. They process this dry mixture in a rolling mill known as “calender”.

This is what the electrodes coated with the new dry transfer coating technology look like. Fraunhofer IWS process enables battery electrodes to be produced on a pilot scale without using toxic solvents (Fraunhofer)

The shear forces in this system tear entire molecular chains out of the binder polymers. These “fibrils” join with the electrode particles as in a spider web. This provides the electrode material with stability. The result is a flexible dry electrode material layer. In the next step, the calender laminates the 100 micrometer thick film directly onto an aluminum foil, thus creating the battery electrode.

On the way to the solid state fireproof battery
“In this way, we are also able to process materials for new battery generations where classical processes fail,” says Benjamin Schumm. These include, for example, energy storage systems that use sulfur as active material or solid-state batteries which employ ion-conducting solids instead of flammable liquid electrolytes. “These batteries will be able to store more energy in the same volume than today’s lithium-ion batteries,” says the IWS scientist with a view to the future.
“However, these solid electrolytes can lose their functional properties in contact with solvents. A solvent-free coating process is significantly better qualified to produce these storage media.” On the way of processing electrodes for all solid state batteries the researchers have reached one important milestone by applying their dry film technology using extremely low binder contents.

Process could replace classic paste processes
The Dresden engineers now aim at enhancing their technology in cooperation with industrial partners in order achieve its breakthrough. In the BMBF-funded “DryProTex” project, for example, they are further developing the dry transfer coating process together with the companies Saueressig, INDEV, Netzsch Trockenmahltechnik and Broad-Bit Batteries. The partners expect a fundamental change in battery production: “The technology offers great potential to replace conventional processes for paste-based electrode production on the long run,” concludes Benjamin Schumm. In the DryProTex project material, process and equipment developments are conducted with the aim of realizing process design for industrial scale dry cathode production.

To know more about this Research visit Fraunhofer web site>>>

Non-conventional design of concentrated windings

Fig. 1b

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

Professor Alberto Tessarolo, Trieste University

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).

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.

Fig. 2 Example of multi-layer concentrated winding. The coils wound around the teeth are distinguished by different colours depending on the phase to which they belong
Fig. 3 Magnetic losses and developable torque of machines with (a) 9 slots and 8 poles; (b) 12 slots and 10 poles, designed in optimized multi-layer configuration

“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”.

Fig. 4 Cross section of an 8-slot and 6-pole (unconventional) machine compared with the conventional 9-slot and 6-pole machine; similarly, the cross section of an 11-slot and 10-pole (unconventional) machine is compared with the conventional 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.

Fig. 5 Winding factors and specific losses in permanent magnets for machines with different combinations of slots and poles. Grey background cells represent unconventional configurations

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.

Fig. 6 Vacuum electromotive forces in a simulation machine (continuous stroke) and in a measuring machine (dotted traces) for 12-phase machines with unconventional concentrated winding with 9 slots and 8 poles
Fig. 7 Cross section of 12-phase motor consisting of 4 non-conventional concentrated windings with 7 slots and 6 poles

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.

Fig. 8 Prototype made (a) and its installation on a test bench. Test configurations with two winding units placed in parallel and loaded (a) on star resistors and (b) on diode rectifier
Fig. 9 Voltage and current waveforms, from measurement and simulation to finite elements, of the machine working from a generator loaded on (a) resistor star and (b) rectifier bridge

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.

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)

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