X-ray tubes used in medical diagnostic imaging are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. This anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. This induction motor requires a source of alternating current which is normally supplied from the power mains or an inverter.
A typical state-of-the-art x-ray diagnostic system uses a high frequency inverter to supply alternating current to a step up transformer and rectifier/filter circuit that generates the high voltage DC power required by the x-ray tube. A separate inverter supplies AC voltage to the induction motor that rotates the anode. This second inverter is sized to quickly accelerate the anode to about 10,000 RPM and is typically rated about 10 KVA. Once the anode is up to rated speed, the load drops to a very small percentage of rated, due to absence of air friction in the vacuum inside the tube.
In U.S. Pat. No. 4,377,002, the anode drive induction motor is accelerated by using one leg of a full bridge inverter and an additional DC voltage source tapped at its midpoint to form a half bridge. During this time, the other leg of the full bridge is deenergized so as to prevent any voltage from being applied to the x-ray tube load. Once the anode is up to speed, the motor is disconnected and the anode is allowed to coast. Unfortunately, the switching means for switching from motor operation to x-ray generation and back again, is a mechanical switching means which creates time delays and mechanical wear problems in the system, as well as increasing the cost of the system.
It would be desirable then to have a rotor controller wherein the time delays and mechanical wear problems of electromechanical switching are overcome.