Tadashi Fukao

An analysis of bearingless AC motors

Several types of AC bearingless motors are proposed. These bearingless motors have conventional four-pole stator windings and additional two-pole windings, whose currents produce radial forces acting on the rotor. General expressions of the machine inductances and radial forces are derived for the cylindrical rotor and salient-pole motors. No-load characteristics of laboratory squirrel-cage induction and reluctance-type synchronous bearingless motors are provided. The test motors were successfully driven by the control circuits. >

Super high speed electrical machines - summary

There is increasing interests in super-high-speed motors and generators in industry applications such as microgas turbines, compressors, blowers, pumps, hybrid electric vehicles, turbo-molecular pumps, machine tool spindle drives, information storage disk drives and etc. In this paper, the state of the art survey of super-high speed electrical machines presented. Recent development of high speed and power ranges are summarized. Various structures of electrical machines, including induction, permanent magnet, claw pole, homopolar, axial gap synchronous and switched reluctance machines, are reviewed from the point of view of high rotational speed characteristics. Emerging technologies such as bearingless operation and intelligent controls are introduced in other papers of the panel session

Design and analysis of permanent magnet-type bearingless motors

Magnetic bearings have been applied to high speed and high power electric machines for machine tools, turbomolecular pumps, etc. Bearingless motors can be expected to realize high speed and high power ratings because magnetic bearing functions are integrated into high-speed motors, which results in a simplified structure with short shaft length. In this paper, permanent magnet type bearingless motors, having built-in capability to achieve high power factor and high efficiency, are proposed. The shaft is suspended and centered by electromagnetic forces produced by currents in the additional radial force windings of the stator slots. At first the relationships of these radial forces, currents and voltages are derived analytically. Moreover, the relationships between radial forces and permanent magnet thickness are found. The optimal permanent magnet thickness is determined. The ratio of radial force over current as well as the peak air-gap flux density are discussed. These relationships are confirmed by prototype machines.

Basic characteristics of a consequent-pole-type bearingless motor

We describe the basic characteristics of a consequent-pole bearingless motor. The consequent-pole-type rotor has buried permanent magnets (PMs) polarized in the same radial direction. We carried out an analysis to find the optimum pole number for the machine to produce stable magnetic suspension. The results indicate that there is decoupling of the radial suspension forces from the drive torque when eight or more poles are used. Here, we compare the torque and suspension force generation with those of a conventional surface-mount PM rotor. The suspension force is several times higher for the consequent-pole rotor; however, the torque decreases by 12%. We built a test machine and confirmed the torque and suspension characteristics. We also compared the test machine with other conventional bearingless motors.

Characteristics of a permanent magnet type bearingless motor

Super high speed and high power electric machines are required for turbomolecular pumps and spindle drives. High rotational speed and high power drives can be achieved with bearingless motors. In this paper, a bearingless motor with principles of permanent magnet type synchronous motors is proposed. High power factor and high efficiency can be expected in permanent magnet type bearingless motors. The proposed bearingless motor is a 4-pole permanent magnet synchronous motor, in which additional 2-pole windings are wound together with 4-pole motor windings in stator slots. With currents of 2-pole windings, radial magnetic forces are produced to support a rotor shaft. Principles of radial force production of surface-mounted permanent magnet bearingless motors are analyzed mathematically. It was found that radial forces are efficiently produced by employing thin permanent magnets on the surface of rotor iron. A test machine was built to measure inductance functions as well as relationships between voltages and currents.<<ETX>>

Performances of bearingless and sensorless induction motor drive based on mutual inductances and rotor displacements estimation

A self-sensing bearingless motor is considered as an effective solution to reduce cost and shorten a shaft length. In this paper, a novel estimation method of a rotor displacement is proposed. The method is based on the detection of currents induced by mutual inductances, which vary as a function of the rotor displacements. A high-frequency carrier voltage is superimposed on a motor main terminal voltage. The induced carrier-frequency current component is distinguished from the suspension-winding current. The carrier signal is selected high enough to suspension-current components. However, the carrier current is disturbed in transient conditions. The disturbed current results in a vibration of the estimated rotor displacements. A suspension-current estimator is proposed to reduce this vibration and to obtain the difference between the detected current and the estimated current. As a result, the disturbance vibration is significantly reduced. It is shown that a successful magnetic suspension is realized with the proposed method.

A stable operation of induction type bearingless motor under loaded conditions

A control method of induction type bearingless motors, which realizes stable operations under loaded conditions, is proposed. Without a proposed control method, it is shown that both the amplitude and the direction of radial force vary in accordance with loads. These variations result in interference of radial force commands in two perpendicular axes. The interference becomes a serious problem as a load increases. A shaft finally touches down without proper compensation. It is found that the decoupling of the radial force commands can be realized with compensation of both amplitude and phase angle of 4-pole motor current. A decoupling controller is proposed with a principle of field oriented controller of conventional induction motors. The results of experiments show the validity of compensations. It is shown that induction type bearingless motors can be operated stably under loaded conditions, as well as transient conditions.

A method of determining the advanced angle of square-wave currents in a bearingless switched reluctance motor

Bearingless switched reluctance motors, which can control rotor radial positions with magnetic force, have been proposed. Bearingless switched reluctance motors have combined characteristics of switched reluctance motors and magnetic bearings. In this paper, a method of determining advanced angle of square-wave currents in bearingless switched reluctance motors is proposed. Under every torque condition from no load to full load, the stable operation is realized by opportunely determining the advanced angle of square-wave currents with the proposed method. It is shown in experimental results that the proposed method is effective for stable operation.

Basic Characteristics of an Active Thrust Magnetic Bearing With a Cylindrical Rotor Core

Magnetic bearings, which can suspend rotor shafts by electromagnetic force without mechanical contacts and lubrication, have been developed and commercialized. In the various types of magnetic bearings, thrust magnetic bearings with thrust disks are generally used for active suspension control of thrust direction. However, the large thrust disks of the thrust magnetic bearings have caused various problems. Therefore, this paper proposes a novel structure of an active thrust magnetic bearing using a cylindrical rotor core without the large thrust disk, and it is shown with 3-D-FEA and experiment that the proposed thrust magnetic bearing with the cylindrical rotor core is effective in producing thrust force.

An air-gap-flux-oriented vector controller for stable operation of bearingless induction motors

A bearingless induction machine has combined characteristics of an induction motor and magnetic bearings. It is known that the magnetic suspension of the rotor becomes unstable at overload operation, particularly in transient conditions. A novel air-gap-flux-oriented vector control scheme has been proposed to operate the bearingless induction motor during the high torque acceleration period. It has been found that there is an optimal flux orientation for complete decoupling in radial force generation. Test results in a laboratory bearingless induction motor validates the performance efficacy of proposed controller at overload conditions.