High Speed Machines Drive Technology Forward Dr Sab Safi, C.Eng, Consultant/Specialist, SDT Drive Technology There is a continual demand for high speed advanced electrical machines and drives for wide-ranging applications in all market sectors. The motivation for their development varies considerably. However, common objectives are to improve efficiency, and, thereby, to conserve energy and reduce environmental pollution, increase power density, enhance functionality, and improve reliability and maintainability and is also being driven by legislation, consumer expectations regarding performance and more fierce competition in the market place. Many of the applications pose particularly severe challenges in terms of the rotational speed and/or space envelope constraints, the thermal operating environment, for example, whilst others are particularly challenging in terms of being highly cost-sensitive or safety critical. Past many industrial applications have utilized existing technology to achieve high rotational shaft speeds. Direct drive high-speed machines or a slow-speed electric motor coupled with a speed increasing gearbox has typically generated the necessary shaft speeds. But advances in high-speed motor technology, along with improvements in the cost and performance of power electronic drives, materials and non-conventional bearings along with more efficient cooling methods, permits an alternative approach using high-speed machines which can directly drive, or be driven by, a high-speed system such as turbine, compressor or other turbomachinery. This results in significant performance benefits such as reduction in motor generator size, as well as reduced motor generator cost and simplified integration. Integration of these small, highly efficient machines into the coupled equipment further reduces cost and complexity Recently, researchers are focusing on the design of high-speed, super high-speed, or even ultra-high-speed machines for applications such as turbo-chargers/ superchargers, compressors, spindles, blowers, pumps, flywheel energy storage systems, and machine tools that require higher speed drives. Such high-speed machinery would include gas compressors, pumps, centrifuges, distributed generation units (microturbines), spindles and flywheel energy storage, and high-speed motors and alternators as examples of electric machines.
The increasing interest in these types of machines is partially due to the very small size and weight achievable in comparison to machines using conventional design strategies. The higher the motor speed, the smaller the electric machine volume for the same power output. The volume (V) of the machine is proportional to the output power (Pout), and inversely proportional to phase current density (J), airgap flux density (rotor Bg), and the angular velocity (ωr). [ / ]. Increasing current density, flux density and rotor velocity can increase the power density. However, increasing current density and flux density is limited due to the magnetic flux saturation and copper loss. Although current density can be increased considerably by using super conducting materials, it is still very expensive and not suitable for low-power machines. Large volume of additional components required to provide cryogenic temperature for super conducting will reduce the power density greatly for low-power machines. Therefore, increasing rotor speed is desirable to increase power density of the machine. Well logging, aerospace, automotive, marine, nuclear power, industrial processing plants and space exploration are just a few examples of applications that are highly dependent on the existence of compact harsh environment energy conversion systems. For example, in industry machinery operates at greatly different speeds. A large cement mill or steel converter runs at fractions of a revolution per minute, while centrifugal compressors operate up to 20,000 rpm. The absorbed powers vary as widely, from fractions of a kilowatt to many megawatts for ship propulsion. In this respect, gearboxes are used to match the operating speed of the prime mover to the requirements. The combination of high-speed motors and gearing is well known for power tools. These drives, up to some kilowatts power at 20 to 30,000 rpm, do not pose problems for either gearing or bearings, and are well established technology. The advent of cost effective frequency converters has allowed the speed range of larger electric machines of 100 to 1,000 kw to be increased to 4,000-6,000 rpm. The prospect of a permanent magnet motor of 20 MW operating at higher speed offers the possibility of low mass, very compact geared or direct-drive, which have been used at these higher speeds for decades, i.e. in combination with both steam and gas turbines. Operating speeds cover a broad range, from 10 to 200 krpm. Increasing speed is one of the most powerful elements for improving if the application can stand it. The design of high speed machines is known to be very challenging because materials are operated closer to their mechanical limits. The high speed capability of machine is constrained by several parameters, such as rotor mechanical, thermal, and electromagnetic limits. Additionally, one can enlist the limits of the power electronic converters, especially the switching frequency. From an electromagnetic point of view, higher speeds means higher induced voltage with extra stress on the insulation. The skin effect due to high frequencies increases the AC resistance. Increasing speed presents mechanical integration complications. Among these complications are rotor-dynamic behaviours related to the phenomenon of critical speed. Part of the structure comes to close to the elasticity/plasticity limit, which makes material choice difficult, since most strong materials are non-magnetic and reverse. The centrifugal force wants to radially push out any component in the rotor. This may cause difficulties for rotors with windings or permanent magnets, close to the air gap. It is possible to use a thin non-magnetic bandage (e.g. fibre-glass) or high-strength metal sleeve or a proprietary advanced graphite-composite sleeve can be used that offer unique advantages to machine performance. The magnitude of the air gap also plays an important role. The bearing needs to be able to sustain, in a stable way, the envisaged speed. Transition to non-touching air bearings/ foil or magnetic bearings has created excellent opportunities for increasing speed and improving reliability.
The highest-speed applications may reach as high as 250 krpm. The conventional metal or ceramic bearings, however, do not work at these speeds. Foil and magnetic bearing systems are most optimally utilized in these applications. A two-pole machine is selected for speeds above 50 krpm to have the lowest fundamental frequency at maximum speed for easy integration with the power electronics. Speeds within the range of 20 to 50 krpm are well within conventional bearing capabilities. However, for improved reliability, foil bearings are highly desirable, although the size of the foil bearings is inversely proportional to speed. Also, a higher number of machine poles can be selected for improving performance. The fundamental of the machine current is a multiple of mechanical frequency. However, machine current should be low enough for easy integration with the power electronics. Electrical fundamental frequencies above 1.5 khz should be avoided to prevent distorted stator currents, which result in increased machine losses and electromagnetic interference (EMI). In this speed range, 4-, 6- and 8-pole machines may be selected. For speeds below 20 krpm, foil bearings are not feasible due to increased size. Rotor-dynamic behavior usually does not create problems since the first critical speed is far above the operating speed range. An increased number of poles can be used for reduced machine sizes. The size of the magnetic bearings is not very sensitive to the operating speed ranges. Implementations at high speeds are more challenging since higher frequency bandwidth is required. This is due to the fact that the critical speeds are within the operating speed range. Acceleration and deceleration time may not be critical. Hence, the high torque-to-inertia ratio is not required in most applications. A ratio of L:D of two is selected as a rule. Electric machines with electronically controlled power conditioning units can experience metal bearing failures early in their projected life cycles. These failures can be caused by the interruption of the high-frequency current that flows through the bearings that can gradually damage bearing races. If the energy of the current pulses is sufficiently high, metal is transferred from the balls and the races to the lubricant. This phenomenon is known as electrical discharge machining. The common mode noise problem can be mitigated through different provisions for the design of electric machines and power conditioning units. The replacement of metal bearings with ceramic, magnetic or foil bearings may be the ultimate solution. High speed motor loss will become the key role to limit the motor drive performance. It is very important to keep the loss as low as possible for the motor to operate at high-speed. The friction loss, that includes bearing loss and windage loss, will also be very significant when the speed is very high. Therefore, proper rotor and bearing designs are needed. High Speed Machine Types & Characteristics - After a general discussion on the limitations of machines at high speeds, an overview of the main types with their high-speed variants is made.
All machines have relatively similar ratings for stator losses because stator construction is generally similar. Rotor losses are important and the permanent magnet machines (PMMs) are the preferred option rather than induction machines (IMs) and Switch reluctance machines (SRMs) where most of the losses are in the stator. Minimal eddy current losses are present in the magnet and in the rotor sleeve. Windage loss occurs in the mechanical air gap of the machine. This loss is a function of the size of the air gap, the tip speed of the rotor, the rotor and stator surface quality, and the medium in the air gap. If additional gas flow is introduced in the air gap for cooling or other purposes, the loss may be increased. PMMs machines are better due to their large air gaps and the smooth cylindrical shape of their rotors. SRMs do not score as high due to the complex shape of their rotors and the potential need to introduce gas flow into the air gaps for cooling purposes, which results in additional windage losses. The IMs are rated much lower compared to PMM, primarily due to the rough surfaces of their rotors, which are made of laminated steel and embedded copper or aluminium bus bars. Rotor thermal limitation is a primary characteristic that determines the ability of an electric machine to be integrated in a high-temperature environment. This characteristic primarily depends on material thermal properties. All PMMs are rated low due to the use of permanent-magnet materials, whose temperatures are limited to about 200 C. The SRM is better due to use steel lamination material operating up to 400 C. The IM is slightly worse than the SRM due to additional copper or aluminium bars. The temperature of a machine rotor is due to self-heating. Therefore, in some applications, the PMM with its inherently small rotor losses may perform better than other machines with higher temperature capabilities. The specifications for materials in terms of temperature and life, and the requirements of magnetic materials in term of saturation, loss and temperature stability have improved greatly. The availability of recently developed Sm-Co materials with operating temperatures up to 550 C will be of interest for aerospace applications Cooling options are an important feature that is determined by the location of losses. Machines with minimal or no losses in the rotor are easier to cool and create fewer complications for the cooling system. The PMM is preferring option because of minimal losses in the rotor. The induction machine ( IM) and SRM are equally rated much lower compared with the PMM due to their substantial rotor losses. Rotor mechanical limitations are directly related to the high-speed capabilities of the electric machine and the ability to integrate the rotor with a high-speed load. The stiffness of the rotor is paramount to high-speed integration. Compatibility with different bearing systems is important for achieving a high-speed. Transition from conventional bearings to magnetic bearings is one of the most powerful provisions. Rotor stiffness and a large air gap are the two most important parameters for accommodating this integration. PMM machines are the preferred choice because of lower sensitivity to air-gap size as well as their high power density and efficiency. IM and SRM performances are very sensitive to air-gap size. Therefore, these machines present challenges for successful integration with foil or magnetic bearings. Therefore, high-speed capability is a composite characteristic resulting from several other parameters, such as rotor mechanical limitations, rotor losses, windage losses, rotor thermal limitations, and machine complexity. The PMM is better solution primarily due to high rotor stiffness and low sensitivity to the air gap compared with induction and switch reluctance motors. An important advantage of such motors/generators actuators is that high speed operation can be introduced, avoiding risks of demagnetisation of the permanent magnets. Moreover, the peripheral magnet structure in the rotor reduces heat dissipation and provides higher overall efficiency.
Considering all the above issues & characteristics, multiparametric scans of the salient geometric parameters, pole numbers, bearing, materials have been initiated in this first part of the concept development study using advanced computational tools. This is supported in turn by empirical data from detailed knowledge about nonlinear mechanics, electromagnetics, thermodynamics, etc. that has been acquired by years of electric machine drive experience. SDT has developed a high-speed permanent magnet motor that can deliver between 2 kw-5kw and integrated control system, which is designed to run at up to 50,000 rpm and have integrated controllers. To further increase the capabilities of proprietary high speed motor systems, SDT use a technique phase advance control which allowing the delivery of high output torque at high speeds. This method allows phase current to build up in a motor winding before back EMF reaches any significant level. In this development the following materials have been used which have properties that greatly influence motor performance: the permanent magnet, core ferromagnetic materials, magnet wires, winding insulation. However, since SmCo magnets have a much higher temperature stability it was decided to use them for the magnet poles. The two important characteristics of the ferromagnetic materials that have influence on the motor performance are the maximum saturation flux density and the specific core loss. The high saturation cobalt iron alloys have high maximum saturation flux density and a relatively low specific core loss. This high speed machine solution is highly scalable and provides a solution for where consideration is being given to a variable speed drive to better match the drive to varying load requirements to reduce energy consumption. Suitable applications are currently being found in automotive, commercial vehicles, spindle, aerospace, industrial pumping and many other industries where compact, high performance machines and flexible transmission technology are demanded. Given the challenging requirements of high-speed, high power motors in a small package, reliability is best ensured using a comprehensive engineering approach such as thermal, structural, dynamic as well drive electronics, and high performance bearings. An electric motor is a complex piece of equipment, covering many engineering disciplines. Careful consideration must be given to all aspects of motor design when evaluating the impact of high rotational speeds and increased frequencies. Ultimately, the final design will be a tradeoff between multiple aspects of machine design, including rotor tip speed, rotordynamics, and cooling. However, whilst well established machine types, such as induction machines and wound-field machines, continue to be improved, and significant progress is being made in new technologies such as high temperature superconducting machines, at present, permanent magnet brushless and switched reluctance are arguably the most important classes of machine. The high speed and temperature PM and SR motors are no longer expected to limit the life of machine operation; with accompanying high performance materials, advanced power electronics and bearing development, operations are limited only by available power. Scalable high temperature motor and bearing developments allows their widespread usage in demanding and new emerging applications. www.sdtdrivetechnology.co.uk