Rotor Radial Position Control and its Effect on the Total Efficiency of a Bearingless Induction Motor With a Cage Rotor

This paper presents a control method for the rotor radial position control of a two-pole 30 kW 3000 r/m cage induction machine that has an additional four-pole stator winding to produce the radial force on the rotor. The controller is used to stabilize the machine, to remove vibration caused by mass-unbalance and rotation, and to produce static force to counter gravity. Both measurement results and finite element computations are shown. In the measurements, the rotor is on a long flexible shaft supported by external bearings. The proposed control strategy allows the machine to run at all operation points (speed, torque) up to the rated values, including at the first critical speed caused by the first natural bending frequency of the rotor shaft. The power consumption of the radial position control and the total effect on efficiency is analyzed. Both rotor eccentricity and the produced four-pole field affect the losses of the machine. The additional power consumption and the total efficiency are presented as a function of the radial force produced.