Stator Design

Stator Design

Dunlee designs and manufactures rotating anode X-ray tubes. The target disc, or rotor, is mounted on a shaft extending from a rotor body, which can spin on internal bearings on the rotor shaft. This rotor shaft extends through the end of the insert to the outside of the insert vacuum for connection to the anode wire, and also is the mounting point for the insert inside the housing.

A stator frame surrounds the (glass) anode insulator over the rotor body. This is made of laminated sheet metal grooved on its inner surface to accept electrical coil wiring.

Most X-ray tubes use a single-phase stator with two coils and a rotor controller which splits the incoming power phase so that the two coils see their maximum current at different times. The result is that the magnetic fields from the coils, which are physically wound at 90 degrees to each other, provide a twisting, or starting, force to the rotor. These coils are wound in the slotted iron frame that surrounds the rotor. The iron concentrates magnetic fields through the insert glass and into the rotor body.

More recently, three-phase stator windings have been introduced. Three separate coil windings are arranged in the stator so that each phase independently creates a magnetic field through the rotor. Since the phases reach their maximum and minimum intensity at slightly different and sequenced times, the result is that the combined magnetic field rotates about the rotor and causes it to rotate.

Rotor speed depends on the frequency of the power applied to the stator windings. The maximum theoretical rotational speed of the anode is 3600 rpm if 60-cycle power is used, 7200 rpm if 120-cycle power is used, and 10,800 rpm if 180-cycle power is used. Some rotor controllers can supply several frequencies depending on the programmed tube loading to be used.

In 50-cycle countries, the theoretical speeds are 3000, 6000, and 9000 rpm. The actual speeds are about 10% less than the maximum because the spacing between the stator, which is outside the anode (glass) insulator, and the rotor, which is inside, degrades the magnetic coupling between the stator field and the rotor, resulting in less than the theoretical speeds. This spacing is required because the rotor body is at anode potential (perhaps 60 pkV) and the stator (generally) is at housing, or ground, potential.