Design Features of High Speed PM Machines Fengxiang WANG, Ming ZONG, Wenpeng ZHENG, Enlu GUAN School of Electrical Engineering, Shenyang University of Technology Shenyang 10023, China 1432 E-mail: wangfx@shenyang.cngb.com Phone & Fax: 86-24-2569 Abswact- The high-speed permanent magnet (PM) machine has been widely investigated since it has small size, high efficiency and power density. However, due to the high rotor speed and high frequency ofthe stator flux variation, the design of a high-speed machine is quite different from designing a conventional machine with low speed and low frequency. As a example, the design features of a high- speed PM generator driven by micro turbines are introduced in this paper. The choice of rotor structure, the design of stator core and winding, the bearing structure and the control system are discussed in some details. A Choice of PM Material The PM material properties have the significant meaning for a high-speed machine. Usually the following factors are considered for the choice of PM material: I) For the high power density and high efficiency of the machine, the PM material should have good magnetic properties normally including remanence B,, coercive force H, and maximum energy product (Bwma,. Kq Words-- high speed, permanent magnet machine, design features 1. INTRODUCTION Based on the operation principle of electric machinery, the output power is proportional to the machine speed for a constant torque. In other words, the higher speed, the smaller machine size for a given output power. The permanent magnet (PM) machine is favorite with high-speed operation due to its simple structure and high power density. High speed PM motors can find application in various fields, such as energy storage flywheel, grinding spindle and milling spindle for machine tools, centrifugal compressor and blower for natural gas pipeline, vacuum pump for vacuum and analytical equipment and so forth. Recently more attention is paid to the development of high speed PM generators driven by micro-turbines. For small gas turbines, it is advantageous to omit the gearbox that normally reduces the shaft speed for driving conventional low speed generator. The result is more efficient and reliable. For high-speed motors and generators, the rotor speed is normally above 30000 rimin and may exceed I00000 r/min, and the frequency of flux variation in the stator teeth and core can be more than I kHz. It is quite different to design such a high-speed machine from designing a common machine with low speed and low frequency. This paper will pay attention to the discussion of the design feature of high speed PM machines. 2) The magnetic properties of the PM should be stable, and the demagnetization curve is expected to be linear within the operation temperature range. Therefore, the PM material should withstand high operation temperature. 3) The price ofthe PM materinl should not be too high. Combining the above considerations. the sintered Nd-Fe-B material would be the best candidate. B Deterniinotion of Rotor Diameter and Length From the point of view to reduce the centrifugal force, the rotor diameter should be chosen as mail as possible since the centrifugal force is proportional to the square of rotor speed.. However, The rotor diameter cannot be too small due to the following reasons: I) In order to produce the required electromagnetic torque and octput power, the rotor should have enough space for housing the permanent magnets, and the stator should also have enough size for housing the windings. 2) The rotor should have enough not only strength but also rigidity. Therefore, the rotor cannot be too long, and should have a proper ratio of the length to the diameter. The rotor diameter must be determined carefully based on the electromagnetic and mechanical calculations. C Rotor Struchre 1) Choice of Pole Numbers: There are normally two choices ofthe pole number: 2 or 4 poles, which have both advantages and disadvantages as described as follows. For a 2-pole machine, the rotor has the simple structure and good strength and rigidity. The permanent magnet can adopt the integral structure and be magnetized in whole, which insure the mechanical and electromagnetic symmetry of PM rotor. Another advantage of the 2-pole machine is that the frequency of the stator flux sand current is only half the 11. PM ROTOR DESIGN It is apparent that the PM rotor is a key part and more attention should be paid to the mechanical consideration of the rotor design for a high-speed motor or generator. - 66 frequency of the 4-pole machine, which is beneficial to the reduction of the losses of stator core and copper. Compared with the 4-pole machine, the main drawbacks of the 2-pole rotor are that the endings of the stator winding will be longer and the sectional area of the stator yoke will be larger. to withstand large centrifugal forces and must be encapsulated in some high-strength material. One method for the rotor encapstilation is to use a cylinder enclosure made by nonmagnetic stainless steel. The magnet is intended to be shrink-fitted into the steel enclosure as shown in Fig. 1. Besides the centrifugal forces, the enclosure inust be rigid and hence have a high Young's modulus. From the point of view of electromagnetic design. the smaller thickness of enclosure is desirable since the non-magnetic steel enclosure is equivalent to increasing the air-gap and it reduces the thickness of PM and produces eddy-current losses. Enclosure 111. STATOR DESIGN As mentioned above, the key points of the PM rotor design are the strength and stiffness consideration for the high-speed operation. For the stator design, the main problem is the high frequency of the stator current and flux. A. Choice of Stator Core 2) Roror Encapsirlation: The permanent magnet is not able It is well known that the stator core losses per kilogram can be exDressed as where B andfare the density and frequency of the magnetic flus in the stator core, ko is the loss coefficient per kilogram of the stator core inaterial tested under the condition of the frequencyfo and flux density Bo. Based on (I), if the stator core inaterials and flus densities are the same for a 3000r/inin inachine and a 60000 rimin machine, the ratio of core losses per kilogram for the two machines will be Fig. I PM rotor with steel cnclosure The other method for the rotor encapsulation is to use a carbon fiber bandage covering the permanent magnet as shown in Fig. 2, which is provided in [3]. In compare with the steel enclosure, the thin carbon-fiber bandage can make the air-gap smaller and the PM thickness larger, which are beneficial to the power density. However, the carbon-fiber acts like a thermal insulator and is an adverse factor for the watt dissipating of the rotor. An important issue is to ensure that the rotor cannot reach the temperature limitation, which would demagnetize the magnets. Therefore, the rotor- cooling problem should be considered in earnest. From the above comparison it can be seen that the stator core losses will be a serious problem for a high-speed machine due to the high frequency of the stator flux variation. The only method to cope this problem is to choose available materials with low core losses, such as special soft magnetic alloys, amorphous steel and powder ferrite cores. Special soft magnetic alloy such as NiFe steel is very expensive. The powder cores are cheap but inadequate today. Amorphous steel has very low hysteretic losses. Unfortunately it is quite difficult to manufacture and to achieve a high fill factor due to the thin sheet and rough surface. The losses per kilogram of amorphous steel is only 0.2-0.25 times of the losses of cold rolled steel plate with thickness of 0.27\". B. Structure ofStator Core There are normally 3 different types of stator core structure: multi-slot, minimal slot and slotless, as shown in Fig. 3. I) Multi-dol Core: Multi-slot core has 12 even more slots. The advantage of the multi-slot core is that the stator winding ._ .. .. . Fig. 2 PM rotor covered by a carbon-fiber bandage enclosure a) multi-slot b) minimal slot C) slotless Fig. 3 'Three different structures of stator core -67- can have distributed and short-pitch coils, which is helpful for eliminate some harmonic components of MMF. However, the fundamental component of will also be weakened due to the distribution and short-pitch of the winding. The pulsation frequency of magnetic field on the rotor surface produced by the stator slot opening due to the slot number increasing will increase, which will increase the losses ofthe rotor surface. 2) Miiiiinal Slor Core: The minimum number of slots for shows that the average flux densities in the air-gap for the 6- slot and slotless core with the same PM rotor are 0.8 and 0.6 respectively. The axial length of the slotless core has to be larger than the slotted core to compensate the flux density reduction. C. Smor Winding three-phase winding will be 6. Compared with the multi-slot structure, the 6-slot core is more effective for the winding availability. The pulsation frequency of inagnetic field on the rotor surface is reduced due to the reduction of the slot number. The harmonic components of MMF cannot be eliminated is the drawback ofthe 6-slot core. 3) Slodes.~ Core: The slotless core, as shown in Fig. 3c, is a ring core with smooth surfaces. The large air-yap between the rotor and stator core is used for housing the stator winding. Compared with the cores having slots, the magnetic field distribution in the air-gap is inore uniform, which can be seen from the coinparison of Fig. 4a and 4b. With the result, the rotor losses produced by the magnetic field pulsation will be reduced. However, the large air-gap will weaken the strength ofthe magnetic field. The magnetic field analysis I Two points for the stator winding design of the high-speed machine should be specially considered. The first is how to reduce the additional losses due to the high frequency of the stator current. And the second is how to reduce the ending length of the stator winding to make the rotor shorter in axial direction. The stator winding is exposed to high frequency current and high frequency magnetic field. The skin effect should be considered. The wire of stator winding is divided into several individually insulated strands to reduce the skin effect. The radius of the individual round conductor should be less than the depth of penetration expressed as follows -<.Iwhere u=2 nfis the angular frequency of the magnetic field, U. and a are respectively the magnetic permeability and conductivity ofthe wire material. For a 2-pole machine, the ending of the stator winding is rather long and needs a large axial space as shown in Fig. 5. The long rotor is undesirable from the point of view to increase the rigid of the rotor. * 1 (3) I a) slotted core Fig. 5 Schematic diagram ofstator winding endings In order to save the axial space, the connection mode ofthe coils can be changed from the axial direction into the radial direction, in which the coils are directly nested on the annular core, as shown in Fig. 6. Compared with Fig. 5, the ending length of the stator winding in axial direction is reduced remarkably, as the result the axial length of the whole rotor can be shorted. IV. BEARING DESIGN b) slotless core Fip. 4 Comparison of magnetic field distribution for slotted core and SlDlleSs core The common mechanical bearings cannot be used for s a high-speed machine because the friction will shotten the running life of the bearings. With the development of new technology, two kinds of contactless bearings, the air usually needs 2 radial and 1 axial magnetic bearings, as shown in Fig. 8b. Amolifier Electric magnel Rotor 4 Sensor a) Operation principle Fig. 6 Schematic diagram ofring-type winding endings bearing and magnetic bearing, are available for the high- speed machines. Basically, air bearings use a thin film of pressurized air to support a load. This type of bearing is called a \"fluid film\" bearing. The fluid is able to transfer forces because as the fluid is pushed through the bearing gap it generates a pressure profile across the bearing area. Fluid film bearings offer a number of advantages over mechanical bearings, such as no suffer from wear or heat generation due to friction, zero static and running friction, high stiffness and self centering etc. The rotary run out of the air bearings is about 25 nm. Therefore, the accurate design and manufacture with extremely tight tolerances of the air bearings are required. A hybrid structure of air bearing with radial and axial air bearings is as shown in Fig. 7. Fluid Radial Axial Controller b) Radial and axial magnetic bearings Fig. 8 Schematic diagram of magnetic bearings and conlrol system V. CONTROL SYSTEM DESIGN The control system for different application of high-speed machine is different. The distributed power system of the high-speed PM generator driven by micro-turbine is a typical application of high-speed machine. The control system block diagram of such a power generation system is as shown in Fig. 9. The high-speed generator is actually a generator/motor. During starting, the machine works as a motor to make the micro-turbine starting since the turbine cannot be self-started. The electric power of the motor comes from the battery through the DC/DC and DC/AC convener. The machine speed increases gradually by adjusting the frequency and amplitude of the voltage applied to the motor. When the speed reaches a preset value, the turbine is tired and starts to work. When the turbine can output rated power, the machine will finish motoring operation and work as a generator. The high frequency output AC power is converted into constant frequency (50Hz) and constant voltage (38OV line to line) through the AC/DC and DC/AC converter. The specially designed Line and EMC (electromagnetic compatibility) filter are used to filter the harmonic components to ensure the output AC power quality. Fig. 7 Structure afhybrid air bearings In the magnetic bearings, the force for rotor suspension is produced by the electromagnetic field. The magnetic bearings can be classified as different types based on their structure and application, such as passive, active and hybrid magnetic bearings etc. The active magnetic bearings are widely used in the high-speed machines. The operation principle of a typical active magnetic bearing can be illustrated using Fig. Sa. The controller receives the rotor-offset information from the sensor and then controls the rotor in the proper position through dynamically adjusting the winding current of the electric magnet. For a rotary machine, the rotor suspension -69- generator Micro Turbine power density. The key points of the PM rotor design are the strength and stiffness consideration for the high-speed operation. For the stator design, the main problem is the high frequency of the stator current and flux. Two kinds of contactless bearings, the air bearing and magnetic bearing, are available for the high-speed machines. ACIDC convener DCIAC ln\"ener Bi-directional DCIDC Convener (ChargcrIStaner) Battery Electric Fig. 9 Control bock diagram ofa high- speed generator driven by micra-turbne 131 141 VI. CONCLUSION The permanent magnet (PM) machine is favorite with high-speed operation due to its simple structure and high 151 -70-
