The news of the manufacturing stop for Yara Birkeland, the first autonomous-guide electric container ship, arouses the debate about the green future of intermodal logistics. The matter was the transition of the transport from road to sea, reducing noise, pollution and fine dusts, with more safety of local viability.
The means, focus of the project started by Vard and Yara International in collaboration with Kongsberg Gruppen, which designs and develops autonomous-guide systems for military and civil vehicles, has captured the attention of the entire world. The “green” container ship is aimed at delivering goods in Norway through fiords, transporting products from Porsgrunn manufacturing site to Brevik and Larvik cities.
The reduction of nitrogen dioxide and carbon dioxide emissions that would occur in the region are equal to those generated by about 40,000 travels per year of trucks.
The loading capacity of Yara Birkeland, which will cover a course of approximately 40 miles at a maximum speed of 10 knots, is equal to 150 TEU. Besides, also the loading and unloading phase will be sustainable: it will occur with electric equipment.
In the opinion of Svein Tore Holsether, president and managing director of Yara, “this new autonomous battery container ship accomplishes the transition of the transport from road to sea, reducing noise and fine dusts and improving the safety of local viability.”
The means was expected to be ready in the first months of 2020, becoming at fully autonomous guide in 2022, but Pandemic has postponed everything. Let us see …
Audi’s will of further strengthening its electric expansion has led to the development of the S versions of Audi e-tron and Audi e-tron Sportback, which adopt three electric motors, two at the rear axle, able to supply an overall maximum power of 503 HP. It is an absolute premiere for mass-produced models.
The electric four-wheel drive avails itself of the innovative electric torque vectoring function with active variable distribution of the torque on the rear axle. Reactivity, performance and feeling while driving reach a new dimension.
503 HP and 973 Nm of torque allow shifting from 0 to 100 km/h in 4.5 seconds.
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.
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.
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.
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.
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.
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.
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.
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 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 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.
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.
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.
«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.
«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.
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».
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.
«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.
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.
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.
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.
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.
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 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.
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 byBosch. 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.
The University of Southern Denmarkgives 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.
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”.
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.
Thanks to appropriate numerical optimisation techniques, it is possible to drastically reduce the losses that originate in permanent magnets due to eddy currents, with a small reduction in the torque that can be developed by the machine. In the same way, it is possible to design machines with concentrated windings with combinations of number of slots and poles traditionally considered incompatible or not feasible in symmetrical form. This is confirmed by the studies carried out by Professor Alberto Tessarolo, University of Trieste, and by examples of how this approach can be of great application interest.
by Gianandrea Mazzola in collaboration with Professor Alberto Tessarolo,
University of Trieste
In the construction technology of modern electrical machines, the use of so-called “concentrated” or “wound tooth” stator windings is becoming more and more frequent, replacing, where possible, the more traditional “distributed” windings. The difference between the two types of windings can be appreciated by the examples shown in figure 1. It will be observed that the distributed winding consists of “ample” coils which embrace a relatively large portion and connect leads arranged in “distant” slots (figure 1a). Conversely, concentrated windings consist of “tooth coils”, i.e. coils each wound around a tooth in the stator’s magnetic core (figure 1b and figure 1c).
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A drawback of concentrated windings is the fact that they, even when supplied with ideal currents, produce harmonic fields at the machine air gap which are capable of inducing losses due to eddy currents in permanent magnets and consequent overheating.
Moreover, it is not always possible to opt for concentrated windings. This is possible, in fact, at the state of the art, only for motors and permanent magnet generators in which the number of slots, indicated by Z, is similar (a little higher or a little lower) to the number of poles P.
In general, concentrated windings are usually considered feasible only if the number of slots Z and the number of poles P satisfy a precise algebraic relationship. More precisely, for the feasibility of winding, the quantity K, as shown in the following relation:
must be an integer number, having indicated by MCD (Z, P/2) the Maximum Common Divisor between Z and P/2. The above relation restricts the choice of the number of slots Z and poles P to a limited number of combinations (which we can define as “conventional combinations”). The limitation in question becomes particularly restrictive in the case of windings with more than three phases (m>3), as is often required to increase reliability. In the case of multi-phase windings, the scope of permissible poly-slot combinations is significantly reduced, thus significantly limiting the designer’s choice and precluding, in some cases, the adoption of wound tooth technology.
Large reduction of losses in the magnets, with small reduction in torque
In response to these critical issues, Professor Tessarolo has recently developed and proposed a methodology for the optimized design of concentrated windings, using multi-layer configurations. “Configurations in which – explains Professor Tessarolo – there can be several coils of different phases wound around the same tooth, as exemplified in figure 2, identifiable by different colours depending on the phase to which they belong. The methodology, which is based on a particular algorithm of quadratic optimization, nevertheless easily implemented in widespread computing environments (such as Matlab), permits reducing some of the drawbacks of concentrated winding machines. “In particular – observes Professor Tessarolo – this methodology makes it possible to reduce the risk of overheating of the magnets due to harmonic fields at the air gap and the problem of the limited number of combinations of project-acceptable poly-slots, especially in the case of a number of phases greater than 3. With regard to the reduction of ohmic losses in magnets, a multi-layer configuration optimized for wound tooth winding makes it possible to reduce the losses in magnets by up to 50%, at the price of a relatively limited reduction in the power developed by the machine. The potential for design optimization is exemplified in figure 3 for the combinations of 9 slots-8 poles and 12 slots-10 poles. The torque and losses of the magnets are normalized with respect to the value they assume for the traditional configuration (with a single coil for each tooth), represented by points A and C. Each point represents an optimized multi-layer design configuration.
“For example, in configuration B for the 9/8 machine, losses are reduced by about 50% at the expense of a 6% reduction in the nominal torque,” says Professor Tessarolo. “In the D configuration for the 12/10 machine, the losses in the magnets can be reduced by about 70% at the price of a drop of only 4% in the nominal torque.”
The optimization also extends the field of acceptable poly-slot combinations
The proposed optimisation method also permits extending the range of possible poly-slot combinations. “In other words – underlines Professor Tessarolo – the method provides a symmetrical multi-layer configuration for a concentrated winding with a generic number of Z slots and P poles, even if Z and P are not such as to give a whole K in the above-mentioned equation”. For example, figure 4 shows the cross-section of an 8-slot, 6-pole (unconventional) machine compared to the conventional 9-slot, 6-pole machine; similarly, the cross-section of an 11-slot, 10-pole (unconventional) machine is compared to the conventional 12-slot, 10-pole machine.
“From the comparison between conventional and non-conventional configurations – says Professor Tessarolo – it appears that the latter, in the face of a greater construction complication, in some cases show better performance. For example, the 9-slot and 6-pole machine in figure 4 has a high torque ripple which is about double that of the 8-slot and 6-pole machine. Or, to quote another example, the 11-slot and 10-pole machine has permanent-magnet losses around half those of the 12-slot and 10-pole machine”.
To give a more complete idea, the tables in figure 5 show a comparison between conventional (white cells) and non-conventional (grey cells) configurations in terms of winding factor and specific losses produced in the permanent magnets. It can be observed that some unconventional configurations have interesting and competitive values.
Operating benefits also for multi-phase machines
The possibility of using unconventional configurations can be particularly useful when designing multi-phase machines or machines consisting of several three-phase windings. This circumstance often occurs in applications that require continuity of service even in the event of a fault. “For example – comments Professor Tessarolo – if you wanted to build a 12-phase machine, or with double three-phase winding, with eight poles, the conventional rules available in literature would force you to choose, to obtain a whole K from the above-mentioned report, a minimum number of 24 slots. It is clear that, for small machines, the use of Z equal to 24 could lead to unacceptable slot dimensions. The use of an optimized and unconventional multi-layer configuration can, in this case, be of help, making it possible to create a three-phase 8-pole, 9-slot double triad machine, as shown in figure 2”. The prototype of this machine was also tested, recording the vacuum induced electromotive forces and then verifying the perfect electrical symmetry of the 9-phase winding, as shown in figure 6.
A further example of application is the 12-phase motor shown in figure 7, consisting of four three-phase windings offset by 90 degrees, each characterized by 7 slots and 6 poles. The choice of an unconventional winding in the case of what is shown in figure 7, was dictated by the need to have (for the maximum frequency allowed and the nominal speed) a total number of 24 poles, to be divided between the 4 independent units, of which the machine must consist for fault tolerance reasons. This resulted, for each unit, in a maximum number of 6 poles.
“The choice of 2 and 4 poles – underlines Professor Tessaroli – was not possible, as it led to excessive stator and rotor yoke thicknesses, such as to exceed the design dimensional constraints imposed on the radial dimensions. In this case, the project concerned the development of an electric outboard motor with integrated propeller, where space constraints were predominant. The number of poles for each unit was therefore fixed at 6, the choice of the number of slots such as to give an acceptable winding factor was between Z=9, Z=8 and Z=5, as shown in figure 5”.
The first (conventional) one was rejected because the torque ripple was too high. The only remaining options were therefore unconventional, i.e. 8 slots and 6 poles or 7 slots and 6 poles. The second was chosen because of its lower magnet losses and the almost zero torque ripple.
A prototype of the 7×4 slot and 6×4 pole three-phase quadruple winding machine was made (figure 8 a-b) and this was tested by connecting in parallel 2 of the 4 stator units and loading them respectively on a resistor star and on a diode rectifier bridge (figure 8 c-d). The results of the tests are shown in figure 9, where the waveforms recorded on the test bench are compared with those obtained by simulation of the machine with the finite element method in the time domain.
The results confirm the perfect symmetry of the machine and the excellent agreement between design forecasts and experimental behaviour. Similar waveforms, which do not show any unexpected phenomenon as a consequence of the choice of an unconventional winding, were also obtained by loading the other two machine units.
Professor Tessarolo concludes: “It can be said that the realization of concentrated electric windings, beyond traditional shapes and the classical limitations assumed for your project, have wide margins of optimization and extension. Provided that they are implemented on a multi-layer basis”.
It has been shown in these pages how, with appropriate numerical optimization techniques and the operating methodology proposed by Professor Tessarolo, it is possible to drastically reduce (even by more than 50%) the losses that originate in the permanent magnets due to eddy currents, with a small reduction in the torque which the machine is able to develop. It was also shown that, through similar optimization techniques, it is possible to design machines with concentrated windings with combinations of number of slots and poles traditionally considered incompatible or not feasible in symmetrical form. Finally, some application examples have been illustrated of how this can be of interest, especially (but not only) in the design of concentrated winding machines with more than three phases. It is therefore an operational approach and a methodology that, in fact, provides useful elements for greater freedom in design and execution.