Study of an Induction Type Bearingless Motor

A control method that achieves stabilized operation under fluctuating torque load in an induction type bearingless motor using a squirrel cage rotor, is studied. When the squirrel cage rotor is adopted in the induction type bearingless motor, fluctuations of the driving flux and control flux upon the torque loaded condition, induce the secondary current in the rotor. And this alters the magnetic flux distribution in the air gap. The fluctuation of the flux distribution interferes the generated controlled magnetic force; therefore, maintaining the rotor to stably levitate has been difficult. This problem is caused by using current feedback to control the position control flux. Then it has to be needed another control method to control the posiotion control flux distribution in the air gap for using squirrel cage rotor. In this paper, a control method for stabilization under torque load condition is proposed. Magnetic flux feedback is used to control the position control flux by detecting magnetic flux distribution. Also, results of some experiments is introduced, describing behaviors of a rotor under fluctuated load with using a bearingless motor test unit having the two ordinary squirrel cage rotors. In addition, outlines a canned motor pump, which the control method to be presented in this paper is applied to. Furthermore, experimental results are shown when this method was applied to a test unit of the 15kW power output in order to confirm the feasibility to bearingless motors with the greater output. The results of the load tests using these test units show that the proposed method is effective to stabilize the levitated rotation against fluctuating torque loads.