Gerald V. Brown

Glenn Research Center, Cleveland, Ohio

Albert F. Kascak

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Ben Ebihara

University of Toledo, Toledo, Ohio

Dexter Johnson and Benjamin Choi

Glenn Research Center, Cleveland, Ohio

Mark Siebert and Carl Buccieri

University of Toledo, Toledo, Ohio

NASA Glenn Research Center Program in High

Power Density Motors for Aeropropulsion

NASA/TM—2005-213800

December 2005

ARL–MR–0628

U.S. ARMY

RESEARCH LABORATORY
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
Gerald V. Brown

Glenn Research Center, Cleveland, Ohio

Albert F. Kascak

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Ben Ebihara

University of Toledo, Toledo, Ohio

Dexter Johnson and Benjamin Choi

Glenn Research Center, Cleveland, Ohio

Mark Siebert and Carl Buccieri

University of Toledo, Toledo, Ohio

NASA Glenn Research Center Program in High

Power Density Motors for Aeropropulsion

NASA/TM—2005-213800

December 2005

National Aeronautics and

Space Administration

Glenn Research Center

ARL–MR–0628

U.S. ARMY

RESEARCH LABORATORY
Acknowledgments

Dr. Timothy Krantz developed the gear-box data and correlation. Jeffrey Trudell performed stress analysis for

the switched reluctance motor and the tip-drive rig. This work was sponsored by the Alternate Energy

Foundation Technologies Sub-Project of the Low Emissions Alternative Power Project at NASA

Glenn Research Center, David Ercegovic, Leo Burkardt, and Kim Pham, project managers.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

Available electronically at http://gltrs.grc.nasa.gov

This work was sponsored by the Low Emissions Alternative

Power Project of the Vehicle Systems Program at the

NASA Glenn Research Center.
NASA/TM—2005-213800 1

NASA Glenn Research Center Program in High Power 

Density Motors for Aeropropulsion 

Gerald V. Brown 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Albert F. Kascak 

U.S. Army Research Laboratory 

Glenn Research Center 

Cleveland, Ohio 44135 

Ben Ebihara 

University of Toledo 

Toledo, Ohio 43606 

Dexter Johnson and Benjamin Choi 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Mark Siebert and Carl Buccieri 

University of Toledo 

Toledo, Ohio 43606 

Abstract 

Electric drive of transport-sized aircraft propulsors, with electric power generated by fuel cells or 

turbo-generators, will require electric motors with much higher power density than conventional roomtemperature machines. Cryogenic cooling of the motor windings by the liquid hydrogen fuel offers a 

possible solution, enabling motors with higher power density than turbine engines. Some context on 

weights of various systems, which is required to assess the problem, is presented. This context includes a 

survey of turbine engine weights over a considerable size range, a correlation of gear box weights and 

some examples of conventional and advanced electric motor weights. The NASA Glenn Research Center 

program for high power density motors is outlined and some technical results to date are presented. These 

results include current densities of 5,000 A/cm2

 current density achieved in cryogenic coils, finite element 

predictions compared to measurements of torque production in a switched reluctance motor, and initial 

tests of a cryogenic switched reluctance motor. 

Introduction 

Pollution-free flight is one of NASA’s goals for the 21st Century. One method of approaching that 

goal is hydrogen-fueled aircraft that use fuel cells or turbo-generators to develop electric power that can 

drive electric motors that turn the aircraft’s propulsive fans or propellers (“propulsors”). Hydrogen fuel 

would likely be carried as a liquid, stored in tanks at its boiling point of 20.3 K (–423 oF). Figure 1 

compares a conventional propulsor drive system with an electric drive system with electric power shown 

produced by fuel cells, turbo-generators or, futuristically, beamed from earth or space. 
NASA/TM—2005-213800 2

Some benefits and possible technical challenges of electric drive are listed in table I. 

Table I. 

Advantages Challenges 

 

Cross coupling of prime movers and propulsors Potentially heavier if many components not redesigned 

Symmetrical thrust maintained with partial prime mover failure Possibly heavy electric power bus leads 

Decoupling of rpm and torque capability Possibly heavy power conditioning 

Possible prime mover shut-off during cruise 

Conventional electric motors, however, are far too heavy (for a given horsepower) to use on aircraft, 

as illustrated in figure 2. 

Fortunately the liquid hydrogen fuel can provide essentially free refrigeration that can be used to cool 

the windings of motors before the hydrogen is used for fuel. Either High Temperature Superconductors 

(HTS) or high purity metals such as copper or aluminum may be used in the motor windings. 

Superconductors have essentially zero electrical resistance to steady current. The electrical resistance of 

high purity aluminum or copper near liquid hydrogen temperature can be 1/100th or less of the room 

temperature resistance. These conductors could provide higher motor efficiency than normal roomtemperature motors achieve. But much more importantly, these conductors can carry ten to a hundred 

times more current density than copper conductors do in normal motors operating at room temperature 

(refs. 1 to 5). This is a consequence of the low (or zero) resistance and of good heat transfer coefficients 

to boiling LH2 or to flowing supercritical H2. Thus the conductors can produce higher magnetic field 

strengths and consequently higher motor torque and power. Pictorial comparisons of each type of 

conductor with room-temperature conductors are shown in figure 3. 

Designs, analysis and actual cryogenic motor tests show that such cryogenic motors could produce 

three or more times as much power per unit weight as turbine engines can, whereas conventional motors 

produce only 1/5 as much power per weight as turbine engines. 

A rough understanding of why conventional motors are inferior in power density to turbine engines, 

and why cryogenic motors can be better, is provided by a look at the peak energy density or “pressure” in 

the machines. We postulate that energy density or pressure is a rough indication of the ability to do work 

(as gas pressure does work on turbine blades in a turbine engine). A magnetic energy density or pressure 

P (in atmospheres) can be defined in terms of the magnetic field strength B (in Teslas) by P ≈ 4 B2

. By 

using this equation, we see that the 25 to 40 atmospheres in the core of a turbine engine is much greater 

than the 10 or so atmospheres of “pressure” in a conventional electric machine operating at 1.6 Teslas in 

its working air gap. A cryogenic motor operating at 5 Teslas, on the other hand, would have an energy 
NASA/TM—2005-213800 3

density or pressure of 100 atmospheres in its working gap, a value far superior to existing turbine engine 

values. (In making this rough plausibility argument, we ignore the relative volumes in which these energy 

densities exist.) The comparison is shown graphically in figure 4. 

These relative energy densities are reflected in a comparison of typical power densities in various 

motors and engines, as shown in figure 5. But it must be emphasized that, although cryogenic motors look 

better than turbine engines in this comparison, turbine engines are prime movers, developing shaft power 

from fuel, whereas motors are not prime movers. A complete comparison requires the addition to the 

motor weight of the weight of components that produce the electric power from fuel and any electric 

power conditioning electronics. 

Two of the highest density electric machines so far demonstrated are an axial-gap permanent magnet 

machine developed for the Army Tank-Automotive Command (TACOM) and a cryogenic synchronous 

generator developed for the Air Force. (We make no distinction between motors and generators.) Figure 6 

pictures these machines. The TACOM machine, which operates at room temperature and was developed 
NASA/TM—2005-213800 4

 
NASA/TM—2005-213800 5

to propel ground vehicles, has a power density of around 2 hp/lb (3.3 kW/kg) (ref. 6). The Air Force 

cryogenic generator was operated at open circuit and short circuit conditions but was not run under 

full power conditions. It was designed for about 600 kW, but the short circuit and open circuit test 

results indicated that it could handle 1MW continuous (ref. 7). At that power level it would produce 

about 6 hp/lb (~10 kW/kg). 

Weight Comparisons of Motors, Engines, Fuel Cells and Gearboxes 

To help determine what improvements may be needed in various electric aircraft drive components, a 

number of data sets and trend lines have been developed. Plots of the weights of a number of motors and 

engines and some trend lines are shown in the next several figures. 

Comparing a turbofan engine with an electric-motor-driven-fan system requires some decisions on 

how much power the motor must produce, as well as how much of the turbine engine is actually replaced. 

We start with published (refs. 8 to 11) and proprietary data on turbofan engine weight and thrust. Data is 

shown in figure 7(a). The thrust shown is thrust at sea-level static take-off. Note that the power-law-fit 

exponent is about 0.9, indicating some economy of scale. 

The published weights of turbine engines usually include the “propulsor,” that is, the propulsive fan 

and related components such as the fan frame, brackets, supports, exit guide vanes and containment. 

These propulsor components would all be required with an electric motor drive. Our first step in making 

an appropriate comparison of motors and turbofan engines, therefore, is to subtract the propulsor weight 

from the total weight of a turbofan engine. We have some proprietary data for actual propulsor weights 

for large turbofan engines. The average fraction of total engine weight represented by the propulsor 

components is 30 percent. This factor was used to estimate the propulsor weight for all the “mid-range” 

engines in our survey, even though it would vary with bypass ratio and other variables. Actual values are 

used for the large engines for which we have data. 

A second issue is that the power delivered to the propulsive fan, which a motor would have to supply, 

is not typically published. Instead, the total turbofan engine thrust Ttot (sea-level static take-off thrust) is 

published. This total thrust is the sum of the thrust from the fan and the thrust from the jet. From some 

proprietary data we have derived a typical relation, for engines between 15,000 and 100,000 lb thrust, that 

the power Pfan,rot in horsepower, delivered to the fan through the fan shaft at take-off rotation (mach 0.25), 

is 0.97 times the total engine sea-level static thrust in pounds. 

 Pfan,rot = 0.97 * Ttot (Ttot in lb, Pfan in hp) 

Even though the factor would vary somewhat from 0.97 with bypass ratio and other variables, we 

have used that single value for all the engines in our survey. 

The remaining issue is how to deal with the jet thrust of a turbine engine, which does not exist for an 

electric motor driven fan. If an electric motor were to drive the fan used in a particular turbofan engine (at 

the same speed and with the same torque), the resulting thrust would be lower than the total thrust of the 

turbofan engine including its jet. We have chosen a factor of 0.8 to estimate fan thrust Tfan from total 

turbofan engine thrust Ttot, i.e., 

 Tfan = 0.8 * Ttot. 

Although the factor would be expected to vary from 0.8 with bypass ratio and other variables, we 

have used it for all the engines in our survey. Hence we will plot what we consider a “replacement 

effective power output” of the turbofan engine which is 1.25 times the fan horsepower. This power 

reasonably represents the power that a motor would have to produce to give the same thrust as a turbine 

engine. This would be accomplished with a somewhat larger fan than the turbine engine used. Figure 7(b) 

presents the weights (less propulsor weights) of the turbine engines versus their effective replacement 

power output at take-off rotation. 
NASA/TM—2005-213800 6

The power-law fit of the data in figure 7(b) is used in the next graph to represent the performance 

level that motors must match or exceed. Of course, the weight of the source of electric power and any 

power electronics must be added to motor weight for comparison with the turbine engine. 

A number of systems are compared in figure 7(c) on the basis of weight plotted versus power. The 

dark blue line is the power law fit to the weight versus effective power from the previous figure. The 

dashed green line shows the weight of fuel cell systems (stack plus accessories, not including output 

power conditioning), as represented by the 2020 goal of 6 hp/lb (10 kW/kg). Present automotivederivative fuel-cell technology produces on the order of 0.4 hp/lb (0.66 kW/kg), and government and 

industry laboratory demonstrations are indicating about 1.0 hp/lb (1.6 kW/kg) (ref. 12). If the 2020 goal is 

achieved, it can be seen that fuel cell weight will still exceed that of turbine engines. Hence fuel cells may 

likely be used only in some kind of hybrid combination. Their projected higher efficiency (~60 percent 

versus ~40 percent for turbine engines) may at least allow their use for cruise, with more conventional 

power augmentation for take-off. However, fuel cells are already lighter than reciprocating engines for 

small aircraft, one of which is plotted as a green square at 180 hp and 360 lb. Early electric flight 

demonstrations on small aircraft appear to be possible, even with near-term fuel-cell technology. 
NASA/TM—2005-213800 7

The dashed purple line in figure 7(c) is an approximation to the weight of a gearbox that might be 

used between a motor and a propulsor to allow different shaft speeds in the two components. This line, 

derived from a gearbox weight correlation discussed below, is for a three-to-one reduction in shaft speed 

from motor to propulsor. (Note that propulsor tip speed is limited to mach 1.5 or less, or even to only 

mach 0.8 for low noise propulsors, limiting large diameter fans to 2000 rpm or less.) Most motors would 

develop better power density at perhaps 6000 rpm, or even much higher. Figure 7(c) shows that the 

weight of a gearbox would not be a significant weight penalty, since a gearbox weighs much less than any 

cryogenic motors we can envision at present. 

A number of motor weights are shown in figure 7(c). A typical large industrial motor, shown at 

50,000 hp and 36,000 lb, weighs about seven times as much as a turbine engine producing the same 

thrust, as indicated by the turbine engine trend line. The two smaller commercial motors shown have 

superior power density to the reciprocating engine. Note that the projected fuel cell weights are an order 

of magnitude lower than both the reciprocating engine and the industrial motors shown for that power 

output range. 

The axial-gap permanent-magnet motor built for TACOM, shown in figure 7(c) at 1500 hp 

and 700 lb, though about 3.5 times as heavy as the turbine engine for the same power, is the best 

room-temperature motor, in terms of power density, for which we have data above 100 hp. 

Switched reluctance motors and generators appear to offer the best specific power levels in the 

power output range below 300 or 400 hp, and several advanced Integrated Starter/Generators (ISG’s) 

and Auxiliary Power Units (APU’s) have been designed and tested. Data for three of these are shown in 

figure 7(c). The point at 40 hp and 18 lb (electromagnetic weight only) is from reference 13. The point at 

100 hp and 22 lb (electromagnetic weight only) is from reference 14, and if a factor of 1.5 is used to 

convert from electromagnetic weight to total weight, it would correspond to a specific power of 3 hp/lb 

(5 kW/kg). This is the best non-cryogenic motor in terms of power density in any power range of which 

we are presently aware. 

Cryogenic motors, as indicated above, have the greatest specific power capability. Only one 

cryogenic electric machine known to us, the Air Force generator, was designed with a flight-weight 

objective. This is the 220 lb Air Force synchronous machine pictured in figure 6. At its potential 

power capability of 1 MW (ref. 6), this machine would have the highest power density of any actually 

built electrical machine known to us. (It is shown in figure 7(c) at 220 lb and the machine’s indicated 

power potential of 1 MW (1340 hp)). Note its power density is almost equal to that of an equivalent 

turbine engine. 

One point is shown (at 50,000 hp, 2600 lb) for a preliminary design of an LH2-cooled synchronous 

machine (ref. 15). This cryogenic machine approaches being light enough to power a transport-sized 

aircraft, being about half the weight of an equivalent turbofan engine. This leaves some room for the 

weight of other components (electric power source and power conditioning), if they are acceptably light. 

The switched reluctance motor point on the graph at 20 hp and 18 lb electromagnetic weight 

represents the initial tests on a NASA Glenn Research Center switched reluctance motor. This liquidnitrogen-cooled machine has been operated with available low-power electric drives and is expected 

to reach several times its present specific power with upgraded power electronics. It is discussed 

further below. 

Not shown in figure 7(c) is any data on power conditioning electronics. A major need is the 

development of power electronics that are much lighter due to utilization of cooling available from onboard cryogens. 

Gearbox Weight 

Data on existing and projected gearbox weights were gathered to let us evaluate whether a gearbox 

can be used between a motor and a propulsor. If the gearbox is light enough, the motor speed need not be 

constrained by the propulsor speed. The weights versus power outputs for these gearboxes are shown in 

figure 8. 
NASA/TM—2005-213800 8

A correlation that fits all the data rather well is shown in figure 9. Surprisingly, a single correlation 

reasonably fits both helicopter and fixed wing gearboxes, even though the output stage for helicopters 

must support very high torque. Curves appropriate for 1980, 2000 and future technology are shown. This 

correlation was used to derive the line in figure 7(c) for gearboxes, with the value of K chosen for year 

2000 technology). 
NASA/TM—2005-213800 9

NASA Glenn In-House Program 

The elements of our current program to advance high-power-density motors specifically for aircraft 

propulsion are shown in table II. Each of these is discussed in the subsections below. 

TABLE II.—ELEMENTS OF NASA GLENN PROGRAM IN HIGH-POWER-DENSITY MOTORS 

 Performing Organization Type Status 

Cryogenic (non-superconducting) motor in liquid 

nitrogen NASA GRC -- Testing 

Tip-Drive Permanent-Magnet Motor NASA GRC -- Build-up 

Superconducting Synchronous Motor NASA GRC -- In Fabrication 

Systems Analysis of Heavy, Efficient Drives NASA GRC -- In-Progress 

450 hp Superconducting Motor in Liquid Hydrogen Long Electromagnetics, Inc. NRA In-Progress 

Optimized Motors with Novel Conductor Pennsylvania State 

University NRA In-Progress 

MgB2 Superconducting Coils for Synchronous 

Motors Hyper Tech Research, Inc. SBIR In-Progress 

Cryogenic (Non-Superconducting) Motor 

The switched reluctance motor or generator is rugged, very fault tolerant and has a good power 

density (refs. 16 and 17). As noted above, reference 14 reports achieving nearly 5 hp per pound of 

electromagnetic weight (~8 kW/kg), or 3 hp/lb (5 kW/kg) of total weight, if total weight is 1.5 times 

electromagnetic weight. The switched reluctance motor can be operated well into saturation of its iron 

core and can produce substantial extra torque after saturation occurs in parts of its poles. With cryogenic 

cooling of the coils, operation far into saturation is possible. 

We have designed and built a cryogenic switched reluctance motor that, in liquid nitrogen (LN2), can 

sustain current densities in its coils of over 50 A/mm2

 steady current, or 70 A/mm2 at a 50 percent duty 

cycle). This is 5 to 10 times the current density typically achieved in room temperature motor coils and 

about 6 times the level at which saturation sets in at pole alignment in our motor. This extra current 

density capability is due to the lower conductor resistance at liquid nitrogen (LN2) temperature, the good 

heat transfer of nucleate boiling of the nitrogen and to specially designed coils that expose extra surface 

area to the LN2. The high current density results in high torque and, with appropriate power conditioning, 

to high power at high speed. The rugged rotor (without coils or permanent magnets) permits high speed 

operation, as far as mechanical stress is concerned. 

LN2 may never be used in an actual application. Liquid hydrogen (LH2) would reduce the conductor 

resistance by a further factor of 15 or more, but with some reduction in peak nucleate boiling heat flux 

from the coils. (See ref (18) for nucleate and film boiling data and correlations for LN2 and LH2.) The 

peak nucleate boiling heat flux for LN2 is about 20 W/cm2

 and for LH2 about 10 W/cm2. The minimum 

film boiling heat flux is very similar for both fluids, at slightly less than 1 W/cm2

. Thus we might expect 

the attainable coil current density to increase by a factor of √(15/2) ≈ 2.7 in LH2. However, we can 

validate all our analysis models by experiments in LN2 at significantly less cost without adding LH2

infrastructure to our laboratory. Furthermore the motor can serve as a test-bed for improving coil 

geometries just as well in LN2 as in LH2. 

The elements of our cryogenic switched-reluctance motor program are shown in table III. 
NASA/TM—2005-213800 10

TABLE III. 

• Conductor current capacity measurements 

• Higher current from coil configuration improvements 

• Finite element calculations of torque 

• Locked-rotor torque measurements 

• Back EMF measurements 

• Flux linkage measurements 

• Torque and power measurements 

• Extension to higher power with power electronics upgrade 

The cryogenic switched reluctance motor is the farthest along of the motors in our development 

program, so some of our experimental results are presented. 

Coil Current-Carrying Capacity 

Current carrying capacity results for three types of coils for the motor are shown in figure 10. 

Photographs of the coils are shown in figure 11. Two of the 80-turn, 4-layer coils (number 18 magnet 

wire) have end turns configured like heat exchangers, spaced apart to provide greater heat transfer area to 

dissipate the ohmic heating from the coil. Such coils might be called “self-finned”. The number of turns 

was chosen to match the current capacity of the initially available power conditioning and will have to be 

greatly reduced to reach the motor’s maximum power potential. The bodies of the coils are epoxyimpregnated, but no epoxy is applied to spaced end turns. In the close-packed coil, heat generated 

internally must be conducted partially through copper and partially through epoxy to reach the LN2-

cooled surface. Most of the thermal drop occurs in the epoxy. In the coils with spaced end turns, the 

majority of the heat generated in the mid regions of the coil is conducted to the end turns, and dissipated 

from the large exposed surface. 

The room-temperature voltage-versus-current measurements in figure 10 show a thermal runaway 

condition beginning at about 6 to 8 A. The curves shown for the coils immersed in an LN2 bath (not 

mounted in the motor) show an initial slope (resistance) of 1/7th of the room temperature value. The 

approach to thermal runaway begins at much higher currents: around 50 A for the tight-wound coil, 

around 65 A for the coil with semi-spaced ends and above 70 A for the coil with fully spaced ends. 
NASA/TM—2005-213800 11

Mounted in the more confined slots of the motor, the coils are not as well cooled by LN2. 

Measurements of coil resistance installed in the motor in liquid nitrogen are shown in figure 12. The 

close-packed coil loses nearly half of its area exposed to liquid nitrogen when in the motor and begins to 

heat substantially above 20 A. The coils with fully spaced end turns are usable to 50 A or more. Note that 

this gives a current density in the motor slots of ≥50 A/mm2

 if the coils are wound with 100 percent dense 

square packing. Note that this is not far below the 63 A/mm2

 reported for superconducting rotor coils 

operating at 27 K (ref. 19). However, it must be noted that the rotor application is much more difficult 
NASA/TM—2005-213800 12

structurally and thermally than our stator application. All currents discussed so far have been steady direct 

currents. The ideal current wave form in actual motor operation is approximately rectangular at roughly a 

50 percent duty cycle for maximum torque or 33 percent for minimum torque ripple. Hence the maximum 

current during motor operation could be either √2 times as much, or about 70 A for maximum torque or 

√3 times as much, or nearly 90 A for minimum torque ripple. 

These measurements show the benefits of both the reduction in resistance due to the low temperature 

and the excellent nucleate boiling heat transfer environment, as well as the benefit of configuring the coils 

for maximum heat dissipation. Such high currents allow utilization of the switched reluctance motor’s 

ability to produce extra torque far into the magnetic saturation, as will be seen below. 

Motor Hardware 

The motor is a 12/8 configuration—twelve stator poles and eight rotor poles. It therefore has 

three independent phases. The stator O.D. is 7.25 in (18.4 cm); the rotor O.D. is 3.96 in (10.1 cm) 

and the axial length of the lamination stack is 1.97 in (5.08 cm). The radial air gap is 20 mils (0.051 cm). 

The stator-pole arc, the stator-pole-gap arc and the rotor-pole arc are all equal. The laminations are 6 Mils 

(0.152 mm) thick and are made of 49 percent Co, 49 percent Fe and 2 percent V. (Hiperco 50 HS). A 

photograph of the stator with one coil installed is shown in figure 13. 

Figure 14 shows a photograph of the motor, mounted with a vertical axis to simplify submersion in 

LN2, with six of its twelve coils installed. For simplicity the entire motor is submerged in LN2, not just the 

coils. This has the obvious disadvantage of considerable drag due to the rotor churning the LN2. 

Fortunately, LN2 has a relatively low viscosity. Liquid nitrogen viscosity is about 15 percent of that of 

water at room temperature. For this proof-of-principle test article, we avoided the complication of 

designing the motor so that LN2 bathes only the stator. 

 
NASA/TM—2005-213800 13

Calculated and Experimental Locked-Rotor Torque 

Finite element (F.E.) predictions of the locked-rotor torque were made using a commercial 2-D F.E. 

code (Maxwell 2D) for several values of rotor angle from unaligned to aligned and for five values of coil 

current. Both the calculations and experiments were for two opposite poles energized. Experimental 

locked-rotor torque was measured by using a lever arm, mounted on the rotor, which pressed on a load 

cell. Multiplying the F.E. code results by a “de-rating” factor of 88 percent (chosen somewhat arbitrarily) 

gives rough, but by no means uniform, agreement between calculated and measured torque, as seen in 

figure 15. Note that the qualitative shape of the curves is reasonably matched. 

It is beyond the scope and intent of this effort to try to determine or recommend a de-rating factor. 

But we do expect some de-rating to be required for three reasons. First, the actual length of our installed 

lamination stack was only 0.986 of the length assumed for the calculations. Secondly, the actual packing 

fraction of the laminations was 0.968, versus perfect packing assumed in the calculation. And thirdly and 

most importantly, the 2-D finite element calculations cannot take axial fringing of flux into account. (A 

fourth possibility would arise if the material B-H curve were for laminations not insulated with oxide. An 

oxide coating effectively reduces the packing fraction of the core. But we used data for 0.004 in. thick 

laminations with an oxide coating, and our actual laminations are 0.006 in. thick oxide coated. So we 

have, in effect done calculations based on a material with more oxide than we actually have.) We expect 

the fringing to become worse at higher currents, giving an increasing over-prediction of torque as exciting 

current rises. Indeed one can see from figure 15 that better agreement between experiment and calculation 

could be obtained with a smaller reduction of the calculated torque for low currents and a larger reduction 

for high currents. 

Running Torque and Power 

The motor has been run at 16,000 rpm at room temperature for short periods of time (on the order of 

15 to 20 seconds), without undue heating. It has run at various speeds up to 11,000 rpm in liquid nitrogen 

for longer periods. 
NASA/TM—2005-213800 14

Torque and power outputs were measured at room temperature and in liquid nitrogen. Because a 

dynamometer was not available for the tests, torque and power were derived from the speed as a function 

of time during a fast acceleration. One pulse-width-modulated power amplifier under PC control was used 

to power each of the 12 coils. Running in current mode, power amplifiers supplied up to 30 A peak 

current at up to 158 V. These peak values are much too low to approach the full torque and power 

potential of this motor, but were the best available for the initial tests. 

A typical speed-versus-time record for an acceleration test is shown in figure 16. The motor 

accelerates with no load, except its own inertia, friction and windage, from an 880 rpm idle to 8000 rpm 

in approximately 0.25 sec. Torque T and power P are found as functions of time from T = Ip dΩ/dt and 

P = T Ω, where Ω is shaft speed and Ip (0.0048 kg-m2

) is the polar moment of inertia. 

The maximum output torque achieved to date was 23.7 N-m at approximately 2700 rpm for 30 A 

maximum current. An upper bound for the low-speed torque, for optimum firing and commutation angles, 

can be derived by averaging locked-rotor torque from figure 15 over angle and projecting to 12 coils. The 

result for 30 A current is 28 N-m. We have achieved 85 percent of that locked rotor torque while running 

at low speed. Note that bearing friction and windage torques should be added to the observed net output 

torque for comparison to the locked-rotor torque. Further use of figure 15 data indicates that about 60 Nm low-speed torque should be obtainable at current levels shown feasible in figure 12. 

The maximum power output achieved for 30 A maximum current and 158 V was 13 hp (9.7 kW), 

which occurred at approximately 4900 rpm. The low-speed torque could be extended to much higher 

speed by a combination of fewer turns (hence higher current) and higher applied voltage. Planned 

upgrades of the power conditioning should permit an order of magnitude increase in power output. 

Figure 17 shows upper bounds for the power that would be produced at speeds up to 30,000 rpm (the 

rotor safe-speed limit from stress analysis) at two values of torque, 23.7 N-m (which has been produced) 

and 60 N-m (which is indicated possible at current densities achieved in static tests). Many factors will 

likely prevent achieving these upper-bound projected power levels, especially coil voltage limits, but also 

including controller sample rate and power converter switching rate, large electrical leads to the motor, 

conductor skin effect, LN2 use rate, LN2 pumping and sloshing, rotor and stator eddy currents and backiron saturation. 
NASA/TM—2005-213800 15

 

Tip Drive Motor 

A scaled blade-tip-drive test rig was designed and is being assembled. The rig is a scaled version of a 

DC brush-less motor which would be located in the shroud of a thrust fan, figure 18. This geometry is 

very attractive since the allowable speed of the armature is approximately the speed of the blade tips, 

(Mach 1 or 1100 ft/s). The magnetic pressure generated in the motor acts over a large area; and thus, 

produces a large force or torque. In addition, it has a large area to remove heat. This force multiplied by 

the large velocity results in a high power density motor. The downside is that this design results in large 

stresses on the rotating components. 
NASA/TM—2005-213800 16

The concept being studied has rotating back-iron and segmented permanent magnets, figure 19. The 

back-iron acts as structural support for the rotating magnets and also as a return path for the magnetic 

flux. The magnetic flux is fixed with the rotating back-iron and thus does not produce any eddy currents. 

The stator is between the permanent magnets and its coupling with the back-iron is minimized to result in 

a low inductance. This increases the response of the switched current in each phase. 

The goal of the test program is to maximize both the armature speed and the stator current density, 

thus producing a very high power density motor. The unique feature of the rig, (fig. 20), is that a test 

generator provides the electrical current and voltage to drive a test motor. The test motor and an auxiliary 

motor provide the torque to drive the generator. The auxiliary motor will also control the speed of the 

test rig. Later a PWM motor drive will be developed to drive the motor and the torque will be absorbed 

by the generator. 

An assembled view of the rig is shown in figure 21. The auxiliary motor drives the test motorgenerator through a pulley. The test motor- generator is supported on oil mist lubricated ball bearings, 

with a rub bearing backing up the ball bearings. The voltage difference between the test generator and the 

test motor is controlled by shims in the air gaps between the permanent magnets. The inductance of the 

test motor-generator is very low and most of the losses should be resistive losses in the stator wires. 

A small stationary test rig, shown in figure 22, was designed and is being assembled to test various 

stator cooling techniques. The objective is to improve the current density and thus the power density of 

the motor. Examples of cooling concepts to be investigated are edge cooling, surface cooling, etc. The 

permanent magnets are arranged north-south-north-south-etc.; and a back-iron is used outside of the 

magnets to form a return path. Each phase of square Litz-wire goes down one pole and up the adjacent 

pole. The stator wiring between the magnets is compressed tightly into a thin, flat shape with epoxy and 

fiberglass to maintain its plate-like structure. The stator moves relative to a fixed holder. The force and 

displacement, due to the current flow in the stator wiring, will be measured at the fixed holder end. 
NASA/TM—2005-213800 17
NASA/TM—2005-213800 18

High Temperature Superconducting Motor 

A four-pole synchronous motor with high-temperature-superconducting DC rotor windings and a 

standard three-phase AC stator has been designed and detailed for fabrication for a test bed facility. The 

motor will be operated in LN2 and possibly later in LH2. Initially the rotor windings will be commercially 

available BSSCO superconductor, which has only modest current-carrying capacity at LN2 temperature in 

an applied field of a few tenths of a Tesla. Hence the first version will have an iron core and fieldreducing “feet” at the ends of the rotor poles. When YBCO superconductor is available, the coils will be 

replaced with ones wound with that conductor and the iron core may be replaced with a non-magnetic 

core. Possible testing in liquid hydrogen is being considered. 

The motor is designed to run to approximately 6000 rpm. Its rotor coils will be tape-wound and racetrack shaped. The stator will be wound with copper conductor. Both rotor and stator coils will be cooled 

by LN2. An exploded drawing of some rotor parts is shown in figure 23. 

System Analysis of Heavy, Efficient Drive 

As noted previously, fuel cells appear to be too heavy to use as the power source for transport-size 

aircraft. However, they will likely be much more efficient than turbine engines. Similarly, the turbogenerator-motor drive system for a propulsor will likely be heavier than a straight turbine drive, because 

of the extra components. However, the turbo-generator-motor drive system may be more efficient than 

direct drive because the turbine main shaft is not constrained to the low speed of the propulsor and other 

factors. Because a more efficient system requires less fuel, higher weight can be offset to some degree. 

We are doing a preliminary system study to see how much heavier a candidate propulsor drive system can 

be for a given efficiency improvement yet still use the same amount of fuel. Any further efficiency 

improvement of the candidate system would save fuel, in spite of the heavier power plant. 
NASA/TM—2005-213800 19

For an initial simplified comparative study, we assumed both power plants are liquid hydrogen (LH2) 

fueled, and the Boeing 747-400 airframe and its characteristics were used for both systems. We ignored 

increased tankage weight or changes in airframe shape required to accommodate LH2. The simulation 

model, which consists of takeoff, climb, cruise, and landing modules, was developed using specifications 

and characteristics obtained from the aircraft and engine manufactures, internet websites, and other 

published sources. The CF6-80C2 engine that provides up to 63,500 lb of thrust at sea level and a range 

of approximately 8,400 statute miles (13,520 km) was chosen for the simulation model. 

Electric drive train components (fuel cells, power conditioning and motors) were characterized by 

simple specific-power coefficients and efficiencies. The Power Specific Fuel Consumption (SFC, 

lb/hr/hp) and aircraft range were calculated. 

Figure 24 shows one example of the simulation results for total aircraft weight, as a function of time 

during flight, for the baseline turbine-powered system and a fuel-cell-motor-powered system from takeoff 

to landing during a 15-hr flight. As seen from the figure the fuel-cell-powered aircraft uses less fuel even 

though it is much heavier than the turbine-engine-powered aircraft. The heavier, more efficient system 

looks better for long flights, poorer for short ones. 

450 hp Liquid-Hydrogen-Cooled Superconducting Motor 

Sized to power a general aviation aircraft, a 450 hp wound-rotor synchronous motor is being designed 

and will be built by Long Electromagnetics Inc. (ref. 15 and 20). The motor will use technology similar to 

what would be used to power transport-sized aircraft, but will be greatly scaled down. Its rotor will have 

4 salient poles, wound with replaceable combinations of BSSCO superconductor and copper coils and 

possibly some MgB2 coils. The stator will be a standard three-phase AC stator with copper windings. 

Both rotor and stator will be cooled by LH2. The motor will run at about 6000 rpm. Initially the rotor 

current will be supplied through slip rings. The motor is intended to serve as a test bed for various types 
NASA/TM—2005-213800 20

of rotor and stator conductors, as well as for testing power sources and power conditioning electronics. 

With some modifications, it could actually be used as an early electric flight demonstration unit. 

Optimized Motors With Novel Conductor 

During Phase I of an NRA contract, Pennsylvania State University developed two-dimensional 

electromagnetic and structural continuum models to optimize ultrahigh-power density machines and 

evaluated performance envelopes of machine designs constructed using various electromagnetic and 

structural materials. In the Phase II contract (ref. 21), in progress at this writing, the coupled thermal, 

structural, and electromagnetic problem will be solved to include mechanical and thermal considerations, 

including the nonlinear, coupled constitutive properties of high-purity aluminum, implicit in the 

assumption of high current density. In addition, the manufacturing process using high purity aluminum 

and composite material will be investigated to minimize resistivity increases due to manufacturinginduced strains as well as operation-induced strains. Experimental tests will be conducted to determine 

the thermal, structural, and electrical properties of this structure at cryogenic temperatures. There is at 

present no commercial source of high purity aluminum conductor suitable for cryogenic motors. 

MgB2 Superconducting Coils for Synchronous Motors 

Magnesium diboride (MgB2) is possibly the lowest-cost and lightest superconductor that might 

be used in wound rotors of synchronous motors cooled by LH2. Under a Phase II SBIR contract 

(ref. 22), conductor and test coils will be produced that can be used in a motor like the 450 hp motor 

mentioned above. 

Concluding Remarks 

The benefits of electric drive that have been exploited in trains and ships may be extended to 

hydrogen-fueled aircraft. Cryogenic motors with either pure normal conductors or superconductors can 

likely produce the required specific power. Advanced designs of room-temperature motors will produce 

sufficient specific power density for smaller aircraft. Electric motor power densities greater than those of 

turbine engines are easily possible. 

The NASA GRC in-house program in this area is constructing and testing sub-scale models of several 

candidate motor types: switched reluctance (in testing), axial-gap permanent magnet (under construction) 

and superconducting synchronous (designed). Contracts support the development and construction of a 

motor large enough to power a general-aviation-sized aircraft, optimization studies to explore the limits of 

synchronous motor power density, the development of a novel composite conductor and the development 

of an MgB2 conductor suitable for synchronous motor rotors. Yet unexplored are ways to utilize 

cryogenic cooling to reduce the weight of power conditioning electronics. 

References 

1. Gerald V. Brown and Willard D. Coles, High-Field Liquid-Neon-Cooled Electromagnets, Cryogenic 

Engineering Conference, Houston, TX, Aug. 23–25, 1965. 

2. J.C. Laurence, G.V. Brown, Jacob Geist, and Kenneth Zeitz, “A Large Liquid-Neon-Cooled 

Electromagnet,” High Magnetic Fields, (Proceedings of the International Conference on High 

Magnetic Fields, Cambridge, MA, 1–4 Nov. 1961), ed. H. Kolm, B. Lax, F. Bitter and R. Mills, 

Wiley & Sons, New York, (1962) 170–179. 

3. C.E. Oberly and J.C. Ho, The Origin and Future of Composite Aluminum Conductors, IEEE 

Transactions on Magnetics, vol. 27, no. 1, Jan. 1991. 
NASA/TM—2005-213800 21

4. A.P. Malozemoff, W. Carter, S. Fleshler, et al., “HTS Wire at Commercial Performance Levels,” 

IEEE Trans. Appl. Superconductivity, vol. 9, no. 2, June 1999. 

5. American Super Conductor website, 

http://www.amsuper.com/products/htsWire/documents/4122GWhitePaper_v5.pdf 

6. Gus Khalil, TACOM: http://www.tacom.army.mil/tardec/electric_drive/motors 

7. Charles E. Oberly, AFOSR/AFRL Propulsion Directorate, Power Division, Power Generation 

Branch, WPAFB, private communication. 

8. Jane’s Aero-Engines, Edited by Bill Gunston OBE, FRAes, Jane’s Information Group Inc., Copyright 

2000, ISBN 0 7106 1405 5, http://www.janes.com. 

9. Jane’s Aero-Engines, Edited by Bill Gunston OBE, FRAes, Jane’s Information Group Inc., issue 15, 

March 2004, Copyright 2004, ISBN 0 7106 1405 5, http://www.jae.janes.com . 

10. Jane’s All The World’s Aircraft, Editor-in-Chief: Paul Jackson, MRAeS, Deputy Editor: Kenneth 

Munson, AMRAeS, Assistant Editor: Lindsay Peacock, Ninety-fourth year of issue 2003–2004, 

Jane’s Information Group Inc., Copyright 2003, ISBN 0 7106 2537 5, http://www.jawa.janes.com 

11. The Engine Handbook, Propulsion Product Group Manager, San Antonio Air Logistics Center, Kelly 

Air Force Base, Texas, 1995 Edition. 

12. Jeffrey J. Berton, Joshua E. Freeh, and Timothy J. Wickenheiser, “An Analytical Performance 

Assessment of a Fuel Cell-Powered, Small Electric Airplane,” NASA/TM—2003-212393, Prepared 

for the Symposium on Novel and Emerging Vehicle and Vehicle Technology Concepts, organized by 

the Applied Vehicle Technology Panel of the North Atlantic Treaty Organization Research and 

Technology Agency, Brussels, Belgium, April 7–11, 2003. 

13. Ferreira, C.A., Jones, S.R., Heglund, W.S., and Jones, W.D., “Detailed Design of a 30-kW Switched 

Reluctance Starter/Generator System for a Gas Turbine Engine Application,” IEEE Ind. Appl., vol. 

31, issue 3, pp. 55–561 (1995). 

14. Arthur V. Radun, High-Power Density Switched Reluctance Motor Drive for Aerospace 

Applications,” IEEE Transactions on Industry Applications, vol. 28, no. 1, Jan./Feb. 1992. 

15. Final Report, Phase I NRA Contract NAS3 02112, “Design of Lightweight High Power Motors 

and Generators for Electric Aircraft Propulsion,” Long Electromagnetics, Inc, 700 Technology 

Drive, Room 5107, Pittsburgh, PA 15219; Larry Long, P.I.; Albert F. Kascak, Technical Monitor. 

March 31, 2003. 

16. T.J.E. Miller, Switched Reluctance Motors and Their Control, Magna Physics Publishing and 

Clarendon Press, Oxford, 1993. 

17. Rama Krishnan, Switched Reluctance Motor Drives, CRC Press, Boca Raton, FL, 2001. 

18. E.G. Brentari and R.V. Smith, “Nucleate and Film Pool Boiling Design Correlations for O2, N2, H2, 

and He,” vol. 10, pp. 325–341, International Advances in Cryogenic Engineering, Proceedings of the 

1964 Cryogenic Engineering Conference, K.D. Timmerhaus, Ed, Plenum Press, New York, 1965. 

19. J.P. Voccio, B.B. Gamble, C.B. Prum, and H.J. Picard, 125 HP HTS Motor Field Winding 

Development, IEEE Transactions on Applied Superconductivity, vol. 7, no. 2, pp. 519–522, 

June 1997. 

20. Phase II NRA Contract, NAS3–03106, Long Electromagnetics Inc., Larry Long, P.I., 700 Technology 

Drive, Room 5107, Pittsburgh, PA 15219, Albert F. Kascak, Technical Monitor. 

21. Phase II NRA Contract, NAG3–03101, Pennsylvania State University, Prof. Heath Hofmann and 

Prof. Charles Bakis, P.I.’s, 110 Technology Center, 200 Innovation Blvd., University Park, PA 

16802, Jeffrey J. Trudell, Technical Monitor. 

22. Phase II SBIR Contract No. NNC05CA04A, Hyper Tech Research Inc., Dr. Mike Tomsic, P.I., 110 

E. Canal St., Troy, OH, 45373, Jeffrey J. Trudell, Technical Monitor, Dec. 1, 2004 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Technical Memorandum

Unclassified

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

and

U.S. Army Research Laboratory

Adelphi, Maryland 20783–1145

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

Available electronically at http://gltrs.grc.nasa.gov

December 2005

NASA TM—2005–213800

ARL–MR–0628

E–15158

27

NASA Glenn Research Center Program in High Power Density Motors

for Aeropropulsion

Gerald V. Brown, Albert F. Kascak, Ben Ebihara, Dexter Johnson, Benjamin Choi,

Mark Siebert, and Carl Buccieri

Aeropropulsion; High power density motors; Aircraft propulsion motors; Cryogenic

motors; Superconducting motors; Switched reluctance motors;

Unclassified -Unlimited

Subject Category: 07

WBS–22–066–30–02

1L161102AF20

Gerald V. Brown, Dexter Johnson, and Benjamin Choi, NASA Glenn Research Center; Albert F. Kascak, U.S. Army

Research Laboratory, NASA Glenn Research Center; Ben Ebihara, Mark Siebert, and Carl Buccieri, University of

Toledo, 2801 W. Bancroft Street, Toledo, Ohio 43606–3390. Responsible person, Gerald V. Brown, organization code

RSS, 216–433–6047.

Electric drive of transport-sized aircraft propulsors, with electric power generated by fuel cells or turbo-generators, will

require electric motors with much higher power density than conventional room-temperature machines. Cryogenic cooling

of the motor windings by the liquid hydrogen fuel offers a possible solution, enabling motors with higher power density

than turbine engines. Some context on weights of various systems, which is required to assess the problem, is presented.

This context includes a survey of turbine engine weights over a considerable size range, a correlation of gear box weights

and some examples of conventional and advanced electric motor weights. The NASA Glenn Research Center program for

high power density motors is outlined and some technical results to date are presented. These results include current

densities of 5,000 A/cm2 current density achieved in cryogenic coils, finite element predictions compared to measurements

of torque production in a switched reluctance motor, and initial tests of a cryogenic switched reluctance motor.