PRESENTED BY:
S.Brindha & V.Bhuvaneshwari

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Abstract
New emerging applications in the areas of portable power generation, small turbo compressors and spindles require the development of ultrahigh-speed, low power electrical drives. A 500000 r/min, 100W electrical drive system is presented. Because of the ultrahigh-speed requirements, standard machine design and power electronic topology choices no longer apply and the complete drive system has to be considered. A permanent magnet machine with a slotless litz-wire winding is used, which results in a low motor inductance and a high fundamental machine frequency. Three different combinations of power electronic topologies and commutation strategies have been experimentally investigated. A voltage source inverter with block commutation and an additional dc–dc converter is selected as the most optimal choice for the power electronics interface as it results in the lowest volume of the entire drive system due to lower switching losses, no heat sink cooling required, a small number of semiconductor devices, and relatively simple control implementation in a low cost digital signal
processor.
I. INTRODUCTION
NEW APPLICATIONS are appearing for super high-speed drive systems in air-compressors, spindle drives and drills and miniature portable power generation systems . By increasing the speed, the volume of the electrical machine decreases for the same power rating, which has significant advantages
especially for portable power generation. The definition of super high-speed electrical machines is dependent on the required power level and the rated speed of the machine. The drives presently used in spindles and drills lie in the definition range of high-speed although there are applications emerging for super high-speeds.
This paper presents a new ultrahigh-speed drive system (see Fig. 1) that is capable of operating at 500 000 r/min with a power output of 100 W for a portable power gas turbine unit and for an air-compressor application. Such a drive system presents significant challenges in terms of the design of the electrical machine power electronics interface that includes a bidirectional power capability and the drive controller.
Future automotive fuel cells will require low power compressors which are small and lightweight and directly driven by high-speed electrical drives. The present spindle drives mainly use induction motors , but a significant number are being replaced by PM machines due to higher efficiency and power density, especially since the speed range extends up to 250 000 r/min with a constant power of approximately 100 W . The trend to smaller, higher precision work pieces, together with higher productivity requirements and smaller drill holes, will facilitate the development of ultrahigh-speed electrical drive systems.
Emerging application areas exist for air compressors, ultrasmall drills and spindles, and portable power generation produced by mesoscale gas-turbines. All these applications require a low power, ultrahigh-speed electrical drive system. In this paper, a 500 000 r/min, 100-W motor/generator drive system that can be coupled directly to a gas turbine is presented.In Section II, the complete drive system is discussed because the machine, power electronics interface and control cannot be treated in isolation. Section III describes the PM machine construction and the resulting electrical parameters. Since the designed machine has a high fundamental frequency and low inductance, the standard voltage source inverter (VSI) cannot be assumed to be the optimal choice.
II. DRIVE SYSTEM OVERVIEW
The main application is for a portable power unit where the output is used to power portable electronic devices of up to 100 W. The output voltage is specified as dc with a voltage range from 28 to 42 V as this level enables it to be utilized in both aircraft and automotive electrical systems. For most applications, the requirements are a small size, high efficiency, and simple control structure since high dynamical control of the torque or speed is not required.
For the emerging applications that require ultrahigh-speeds at low output powers, new electrical drive systems need to be developed where both the machine and power electronics designs are considered together. The application, either as the driving source and/or load, must be considered because mechanical couplings between the systems are not feasible. The machine selection and construction influence the power electronics and
the use of standard rotor angle position sensors are no longer feasible.
To achieve a small size for the overall system, all the
components in the system must be considered. The electrical machine scales with torque and the machine type, where for smaller machines the torque to volume ratio becomes smaller. At the rated speed, only a brushless machine can be used due to low frictional losses. For the power electronics converter, the size is dependent on the power, power losses, topology and passive components. The control electronics scale with control power and finally is projected to research projects with power outputs of 10 to 100 W.
For the industrial gas turbines, the grid-connected generator is coupled to the turbine through a gearbox and operates at a lower speed. Microgas turbines
that have direct connected high-speed permanent magnet (PM) generators/starters delivering 10 to 100 kW are becoming more prevalent For example, the Capstone microgas turbine operates at 90 000 r/min with a power output of 30 kW . Several international research groups are investigating ultramicrogas turbines with power outputs of up to 100 W for use in
portable power applications .Only a few of these
projects consider the electrical system, although the electrical drive system is an integral part of the total system to start and generate electrical power from the turbines.
For compressor systems, the power and speed trends are similar to the turbines. One application is a fuel cell air compressor that requires 120 000 r/min at 12 kw and another complexity and the level of functionality for integration of this system. Fig.1. Block diagram of the ultrahigh-speed drive system. Necessary systems are drawn as solid lines, while topology dependent parts are shown with dashed lines.
At the 100 W power level, the power electronics and electrical machine will be rather small, but control electronics can become large depending on the topology, commutation strategy and position
detection system. Therefore, reducing control complexity is also important.
Fig. 1 shows the overall block diagram of the drive system. The electrical machine is selected as a PM machine (Section III). The main function of power electronics is to form ac currents for the machine and to allow for bidirectional power flow. Depending on the inverter topology and commutation strategy implemented, an additional dc–dc converter or fast
analog current control may be required. This is also valid for the external passive components, where external inductors are required for a high-frequency VSI or external capacitors are required for a current source inverter (CSI). The drive control is implemented in a digital signal processor (DSP) and this allows the drive to communicate with other control systems.
An additional auxiliary power supply is required for the gate drivers, DSP and other control electronics. For control of the drive, either a position sensor or sensorless control has to be implemented in hardware and/or software on the control platform. The choice and design of the electrical machine is now considered, followed by the consideration of the various power electronic topologies and the control system options.
III. ELECTRICAL MACHINE DESIGN
The main challenges in the design of the electrical machine are the losses due to the high frequency in the stator core and windings, the rotor dynamics and a rotor design that minimizes the mechanical stresses and eccentricity. A PM machine is chosen with the aim of a low system volume, since PM flux
density remains constant for decreasing machine volume. In contrast, the flux density in an electrically excited motor decreases with decreasing size and is therefore not suitable for this application.
High-speed operation requires a simple and robust rotor geometry and construction, and excessive mechanical stresses can be limited with a small rotor diameter. Therefore, a radial-flux machine is selected with a cylindrical PM encased in a titanium retaining sleeve. A length-to-diameter ratio of 1 : 1 is defined as this leads to a short shaft which increases the critical speeds. For the highest torque density, high-energy rare Earth magnets such as sintered NdFeB or SmCo are the only choices. A Sm2Co17 based magnet is chosen because of its outstanding thermal characteristics with operating temperatures up to 350 ◦C. A new self-supporting, slotless litz-wire winding is used to keep the rotor losses low and the stator core manufacturing simple . The large air gap due to the ironless rotor and slotless winding results in a very low winding inductance of 6.2 μH. The peak value of the back electromotive force (EMF) is set to 16 V to allow the use of low on-resistance power MOSFETs in the power electronic converter and fulfill the specifications of operating from a low voltage dc bus.
The diametrically magnetized, cylindrical PM leads to a sinusoidal back EMF. The PM has only two poles to keep the fundamental frequency low. Nevertheless, the frequency of the currents and the magnetic field in stator winding and core reaches 8.3 kHz at rated speed. This leads to high eddy-current and hysteresis losses.
The copper losses are minimized with a litz-wire winding. Compared to a standard winding the copper loss is reduced by 70%. The stator is manufactured from high frequency silicon steel; however, the iron losses outweigh the copper losses and therefore new magnetic materials such as amorphous or nanocrystalline-iron-based materials are being
investigated.
The machine has an active length of 15 mm and a stator diameter of 16 mm. A cross section representation of the machine and the nominal values and parameters of the machine are given in Table I. A detailed electromagnetic and mechanical design description is presented in , the test bench setup and measurement results are shown in and the machine is optimized for efficiency . For machinery with speeds in the range of 500000 r/min, the selection of a suitable bearing is the main issue. The choice is
influenced by the operating conditions and design limitations such as required stiffness, operating temperatures, atmosphere, allowed volume, and lifetime. These factors are very much dependent on the application, and usually the bearing is considered
as part of the application, therefore the possible choices are only compiled and briefly compared and no recommendation is given.
High-speed ball bearings are commonly used in the dental industry, and bearings are available for speeds exceeding 500 000 r/min. The main advantages of ball bearings are the robustness and small size. The main disadvantages are the limited operating temperature and a lifetime dependent on lubrication,
load and speed.
Static air bearings, dynamic air bearings, and foil bearings levitate the rotor with air pressure, either generated with an external supply (static) or by spinning the rotor (dynamic and foil). These bearings all show low friction losses and a long lifetime.
Foil bearings are reported for speeds up to 700 000 r/min and temperatures up to 650 ◦C, but are not commercially available and require a complex design procedure.
Magnetic bearings levitate the rotor using magnetic forces and have similar advantages to air bearings. However, active magnetic bearings require sensors, actuators and control, which results in high complexity and an increased bearing volume.
In summary, all bearings apart from ball bearings have no wear and just air friction and therefore a long lifetime and low losses. Because of their simplicity, robustness, small size and avoidance of auxiliary equipment, ball bearings are selected for the initial motor–generator test bench. Since a mechanical
coupling at these speeds is not feasible, an overlong rotor containing two magnets, one for a motor and one for a generator, is supported by two high-speed ball bearings.
IV. POWER ELECTRONICS SELECTION AND OPERATION
The very high fundamental frequency and low motor inductance, coupled with the requirement for a small and lightweight design, present significant design challenges for the power electronics interface. The requirements depend on the application; however, for most of them, a compact size and low weight is needed, and also because at these speeds the electronics tend to be bigger than the machine itself. There are several possible converter topologies for connecting a dc voltage bus to a three-phase machine. To select a suitable topology and commutation strategy, both the converter and the machine have to be considered in terms of losses, number of dditional passive components and method of rotor position and speed sensing. In all the applications, there is no need for high dynamics in speed control or even position control, and therefore sensorless position estimation is desirable because in most applications there are space restrictions, especially in the axial direction where a position sensor is usually placed. This section first compares three different combinations of converter topology and commutation strategies and then compares different methods of position
sensing