12-8 switched reluctance motor with motor coil windings only--no magnetic bearing coil windings.
Bottom view of motor with x- and y-axis position probes. Motor coils generate torque and radial force at the same time.
Before a super-high-power-density motor can be developed for the next-generation all-electric propulsion system, a noncontact rotor bearing system is necessary to circumvent the poor bearing life that ordinarily accompanies cryogenic operation. The Self-Levitation Team at the NASA Glenn Research Center developed and successfully demonstrated a bearingless motor control technology for the vertical-axis, 12-stator-pole, 8-rotor-pole (12/8) switched-reluctance motor (ref. 1). This motor does not have separate magnetic bearing coils, but only motor coils as in a conventional motor configuration (see the top photograph). This novel self-bearing control technology generates the required torque (motor function) and radial force (bearing function) by using the motor winding coils only (see the bottom photograph). This is believed to be a world’s first for this motor type.
Two controllers were developed and tested for the bearingless motor. The first one is based on the mathematical expression of radial force using Takemoto’s method (ref. 2). The other controller is relatively simple because it does not require a mathematical equation of radial force. Those two controllers are working very well, and details will be published later. As shown in the following photograph on the left, the rotor was quite stable in the operation speed range from levitation up to 6500 rpm (maximum allowable speed at that time). At the operating speed, the rotor orbit has 10 percent of back-bearing clearance. Furthermore, the required levitation current is less than 10 percent of the operating motor current (see the following photograph on the right). In the near future, a controller will be developed for a horizontal-axis bearingless motor, where we will need to generate a levitating force to counteract the rotor weight as well as for motor and bearing functions.
Left: Control screen showing rotor orbit at 6000 rpm and less than 10 percent of backup bearing gap (±10 mils). Right: Computer screen showing motor and control currents. Motor speed, 6000 rpm; motor current, 4.0 A; levitation current, less than 10 percent of motor current.
This technology can significantly reduce overall system weight by eliminating separate mechanical or magnetic bearing systems and the associated plumbing and electrical subsystems. It can also prepare the way for an all-electric, quiet, pollution-free aircraft propulsion system. This work is supported by the Alternate Energy Foundation Technologies Project of the Low Emissions Alternative Power Project.
Last updated: October 16, 2006
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