The NASA Glenn Research Center has been investigating non-combustion-based aircraft propulsion through experimental and system tradeoff studies. Conventional electric motors are too heavy for use in large aircraft propulsion. To address this deficiency, a team at Glenn designed an ironless motor to evaluate flight applications.
In this design, the rotating magnetic flux is generated by two sets of axial Halbach arrays using high-energy-density permanent magnets. The magnetic flux is a maximum 1.2-tesla flux density at the center of a 0.5-in. gap (ref. 1) and approximately sinusoidal with respect to angle. The arrangement of the magnets eliminates the need for backiron in the motor, which is heavy and lossy.
Experimental results and one-dimensional predictions of torque as a function of current in one phase of the ironless motor at an angle of maximum torque. The one-dimensional predictions are for 1.0-, 1.1-, and 1.2-tesla maximum flux density.
In fiscal year 2006, one stator design was evaluated for static torque as a function of current and current-carrying capacity with and without forced air convection. Static torque measurements were made as a function of angle and current up to 104 A. A one-dimensional prediction of the torque was compared with the experimental data at 1.0-, 1,1-, and 1.2-tesla maximum flux in the gap. The preceding graph shows good agreement between the experiment and the analytic one-dimensional prediction. Forced-air convection was demonstrated to increase the steady-state current-carrying capacity of the stator by 140 percent, from 42 to 104 A.
The effect of stator packing factor on the specific power of the motor was determined using the motor setup in the photograph. The packing factor is the percentage of the stator that is occupied by conductors.
Ironless motor setup for evaluation of static torque as a function of current and rotor angle. The current stator has a ~10-percent packing factor.
The electromagnetic (EM) weight is defined as the weight of just the EM components (i.e., stator copper and permanent magnets). The total weight of a prototype motor is taken to be 50 percent more than the EM of the motor to account for structural support. The current stator (ref. 1) has only a 10-percent packing factor. As shown in the following graph, the specific power of the current motor is 0.6 hp/lb using total weight and increases to 5 hp/lb for a 75-percent packing factor. A packing factor of 50 percent is more realistic and indicates that 3.4 hp/lb is realizable for this ironless motor running at room temperature and high speed. For comparison, the estimated horsepower per pound of the 98-percent-efficient Halbach motor designed by the Commonwealth Scientific and Industrial Research Organization (CSIRO) that was used in a solar-powered car in the World Solar Challenge race across Australia is shown. This work was supported by the Subsonic Rotary Wing Project.
Horsepower per pound as a function of packing factor (pf) in the stator and motor speed.
Last updated: December 15, 2007
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