In recent years, there has been a growing demand for high-speed cutting work in the field of machining. The fundamental performance of the processing use spindle is normally evaluated by the magnitude of a DN value (spindle diameter × number of revolutions). In the case of the magnetic bearing, it is possible to provide a spindle of which the DN value surpasses the DN value of the ball bearing by making the best use of the features of non-contact rotation. However, it was discovered that magnetic bearing, which was expected to have small loss because of the non-contact structure, unexpectedly caused a great friction loss as the result of striving for higher DN value. The principal factor of the above result is the eddy current loss of the radial bearing. We proposed a new radial magnetic bearing structure to satisfy following (1) (2) at the same time. (1) High stiffness and a large loading capability can be kept. (2) The eddy current loss can be greatly decreased. Wide magnetic pole teeth are formed in the radial electromagnet. And, the both ends of the magnetic pole teeth have "a long rising interval" and "a long decelerating interval" of the magnetic flux density curve. This electromagnet was installed in the developed magnetic bearing spindle, and the evaluation experiment was done under the condition of the spindle diameter: D =Φ 90 mm, rotational speed: N = 50000 rpm, DN = 4.5 million and rising interval: α =10deg, uniformity interval: β =25deg, decelerating interval: γ =10deg. As a result, the temperature of bearing gap have decreased to about 1/2, and the motor power have decreased to 2/3 with the same radial stiffness maintained.

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One of the significant problems in designing a magnetic bearing control system is that there are gyroscopic couplings between two rotational motions of the suspended rotor. Because of these couplings the optimal regulator system related to the rotational motions needs antisymmetric cross feedback compensation in addition to usual displacement and velocity feedbacks of individual motion. It has been shown that the optimal regulator system has better transient response characteristics than the independently controlled system, that is to say, the system with no cross feedback compensation which is obtained as a result of disregard of gyroscopic effects in the process of designing the control system. In view of applications of magnetic bearings it is important to investigate the dynamic behaviour of the system disturbed by external forces besides the transient natural motion. This paper shows an analytical method to study forced motions of the gyroscopic coupled systems when static or sinusoidal varying forces are applied to the rotor and clarifies differences between the optimal regulator system and the independently controlled system. Experimental tests are carried out to verify the theoretical results.

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New types of oil free Helium refrigerators, using centrifugal compressors are now under development to cool the superconducting electric power generators. Because of their low efficiency, not much attention has been paid to regenerative compressors up to now. But, their compression characteristics, for example, higher pressure ratio and no surging vibrations, are desirable for Helium refrigerators which require perfect reliability for cooling practical superconducting machines. This report describes the merits expected by adaption of regenerative compressors to new type Helium refrigerators, construction of the trial compressor equipped with 5-axis controlled magnetic bearings and results of the basic test operation of the magnetic bearings.

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