Measuring Speed and Temperature in a Large Air Gap Induction Motor

The origin of the present study is related to high-power X ray tubes [1]. Those consist of an anode and a cathode; the potential difference between them is 120 kV (Fig. 1). The electron beam intensity is 0.4A during the production of a computed tomography. In the case of vascular applications, those figures may be 100 kV and 1 A. To avoid excessive local heating, the anode has to rotate at high speed: 10800 rpm. To avoid excessive bearing wear, the anode rotates more slowly when no image is produced; 3600 rpm. Because of the vacuum inside the tube, the most natural drive to be chosen is a two-pole induction motor, whose stator is outside. As a result, the air gap must be very large. As for the rotor, it is made of a hollow solid iron cylinder, covered with a uniform sheet of copper. The bearings are inside the rotor (Fig. 2). The motor is supplied at variable frequency by an inverter.