IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
3823
Design of a Superhigh-Speed Cryogenic Permanent
Magnet Synchronous Motor
Liping Zheng1 , Thomas X. Wu1 , Dipjyoti Acharya2 , Kalpathy B. Sundaram1 , Senior Member, IEEE, Jay Vaidya3 ,
Limei Zhao1 , Lei Zhou2 , Chan H. Ham4 , Nagaraj Arakere5 , Jayanta Kapat2 , and Louis Chow2
Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL 32816 USA
Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32826 USA
Electrodynamics Associates, Inc., Oviedo, FL 32765 USA
Florida Space Institute, University of Central Florida, Orlando, FL 32816 USA
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611 USA
This paper presents the design and simulation of a superhigh-speed permanent magnet synchronous motor (PMSM) that operates
in the cryogenic temperature of 77 K. The designed PMSM is used to drive a two-stage cryocooler for zero boil-off and long duration
storage of liquid hydrogen systems. The paper addresses electromagnetic and thermal finite-element analysis, selection of materials for
cryogenic applications, stress analysis, rotor dynamic analysis, and some tradeoffs used in the design. A prototype PMSM was built to
verify the design methodology.
Index Terms—Cryogenics, permanent magnet synchronous motor (PMSM), superhigh speed, V/f control.
I. INTRODUCTION
R
ECENTLY, superhigh-speed motor is becoming more
and more attractive in many applications such as machine
tools and centrifugal compressor drives [1]–[3]. It is also a
key component of the reverse turbo Brayton cycle cryocooler
(RTBC) [4], [5]. A multistage cryocooler will generally require
the motor to operate at cryogenic temperature. Permanent
magnet synchronous motor (PMSM) offers the advantage of
high efficiency compared to other types of motors since there is
no excitation power loss in the rotor.
In this paper, the design and simulation of a cryogenic PMSM
with an output shaft power of 2000 W at 200 000 rpm are provided. It drives a two-stage cryocooler for zero boil-off and long
duration storage of liquid hydrogen systems. Electromagnetic
and thermal finite-element analysis, material selections for cryogenic applications, stress analysis, rotor dynamic analysis, and
some tradeoffs are presented. The prototype has been fabricated
and tested to verify the design.
II. PMSM STRUCTURE AND DESIGN CONSIDERATION
A. PMSM Structure
The cross section of the designed radial flux superhigh-speed
cryogenic PMSM is shown in Fig. 1. Two groups of three-phase
windings were connected in parallel to meet the low-voltage
requirement. The permanent magnet is centrally located inside
the hollow shaft. The permanent magnet was inserted into the
hollow shaft by heating the titanium shaft to 570 K and cooling
the permanent magnet down to 77 K.
B. Design Considerations
The above PMSM structure was developed based on the following considerations.
Digital Object Identifier 10.1109/TMAG.2005.854983
Fig. 1. Cross section of the designed PMSM.
1) Two-Pole Rotor Structure: For the permanent magnet
motor, the magnetic excitation in the rotor is supplied by the
permanent magnets. The increase in the number of magnetic
poles will result in less copper loss in the end-turn windings
and better spatial sinusoidal distribution of the air gap magnetic
field. However, too many poles will increase the magnetic
leakage. Also, the required electrical frequency is proportional
to the motor speed and number of poles. A higher electrical
frequency will require a higher switching frequency of the pulse
width modulation (PWM), resulting in larger switching loss.
For superhigh-speed motor, the electrical frequency has already
been very high, hence it is necessary to make the number of
poles minimum.
2) Slotless Stator: The slot structure of the motor will increase the cogging torque, which will cause undesirable vibrations. It will also increase the eddy current loss in the rotor,
core loss in the stator teeth, and windage loss. These losses will
be significant at superhigh speeds. Various attempts have been
considered to reduce the cogging torque and losses associated
with the slot structure [6]. Slotless structure is one of the effective methods especially for superhigh-speed motors. By using
a slotless structure, the cogging torque due to the slots is eliminated and the eddy current loss in the rotor surface due to the
0018-9464/$20.00 © 2005 IEEE
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
TABLE I
KEY DIMENSIONS OF THE DESIGNED PMSM
harmonics of the air gap flux density is also greatly reduced. A
slotless structure can also reduce the loss in the stator. Therefore, the stator is chosen to be slotless and made of laminated
low loss silicon steel.
3) Multistrand Winding: A slotless structure is very effective to reduce cogging torque. However, it will increase the
eddy current loss in the winding that is mainly due to the proximity effect caused by the rotating permanent magnet. Therefore, a multistrand twisted Litz wire was used to effectively reduce the eddy current loss in the winding. The Litz wire was
constructed using 75 strands of AWG 36 (0.125-mm diameter),
coated with heavy 200 polyesterimide, and overcoated with
polyamide-imide to meet IEC MW 35. After that, it was further
wrapped with sofimide to withstand temperature down to 77 K.
The constructed Litz wire was tested at 77 K, and no insulation
degradation problem was observed.
4) Samarium
Cobalt: The
neodymium–iron–boron
(Nd–Fe–B) magnet is widely used in electrical machines
and other applications. It has the highest energy product
compared to other types of permanent magnets. However,
when the temperature is below 140 K, Nd–Fe–B will change
from a uniaxial material to an easy-cone anisotropy material,
and this makes the magnet easily demagnetized. Therefore,
Nd–Fe–B is generally not considered for applications below
140 K, although operation at 60–64 K has been reported [7].
Samarium–cobalt (Sm–Co) is very stable at low temperature
and has a very low temperature coefficient of coercivity and
remanence. It also has a very high energy product. The Curie
and operating temperatures of Sm–Co are very high, which can
prevent demagnetization during assembling and welding.
III. SIMULATION AND ANALYSIS
Sizing equations are generally used in motor design to optimize
the dimensions to achieve the best power density performance [8],
[9]. However, for superhigh-speed applications, the sizing equations cannot predict machine performance. Electromagnetic finite-element analysis, thermal analysis, rotordynamic analysis,
and some tradeoffs are used to finalize the dimensions and optimize the performance. Some key dimensions of the designed
PMSM are shown in Table I. The oval structure of a permanent
magnet can avoid slip caused by the expansion of shaft due to
thermal effect and the centrifugal force at superhigh speed.
A. Electromagnetic Finite-Element Analysis
It is necessary to reduce the harmonics of the air gap flux
density to reduce losses in the rotor and stator. Magnetostatic
Fig. 2. Simulated air gap flux densities in normal and tangent directions.
TABLE II
SIMULATED AND ESTIMATED LOSSES AT 200 000 rpm
solver is used to analyze the air gap flux density. Fig. 2 shows
the air gap flux densities in normal and tangential directions. The
flux density in tangential direction has no use but increases the
eddy current loss. Fast Fourier transform (FFT) analysis shows
that the dominant harmonics is the third harmonics, which is
extremely low compared to that of the slot structure.
To accurately simulate the dynamic behavior of the PMSM,
two-dimensional (2-D) transient time-stepping solver with motion was used. The equation dealing with coupled time-varying
electric and magnetic fields is [10], [11]
(1)
where is the velocity of the moving parts, is the magnetic
vector potential, is the time,
is the electric scalar potential,
is the current source density,
is the coercivity of the
permanent magnet, and is the conductivity of the material.
Table II shows the simulation results. The copper eddy current
loss and iron losses in the rotor and stator will all increase at 77
K due to the increase of conductivity. The back electromotive
force (EMF) at 77 K is slightly larger than that at room temperature since the flux remanence is larger.
B. Mechanical Analyses
Mechanical analyses include rotor stress analysis, thermal
analysis, and rotor dynamic analysis. Stress analysis shows that
the combined thermal and centrifugal stress developed in the
shaft is 814 MPa at 200 000 rpm and 77 K. The yield strength of
the selected titanium grade is about 1400 MPa at 77 K. Thermal
analysis shows that when the motor is working at 77 K, no additional cooling is required. When the motor is working at room
ZHENG et al.: DESIGN OF A SUPERHIGH SPEED CRYOGENIC PERMANENT MAGNET SYNCHRONOUS MOTOR
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temperature is about 186 W when rotating at 200 000 rpm. This
meets our design specifications.
V. CONCLUSION
A superhigh-speed permanent magnet synchronous motor
(PMSM) operating at 77 K was designed. The hollow shaft with
a permanent magnet inside is proved to be a good option for
miniature superhigh-speed motor. The cooling of the miniature
rotor is a real challenge. It is found that the slotless structure
combined with a multistrand Litz wire can reduce iron loss in
the rotor significantly without increase in eddy current loss in
the winding.
ACKNOWLEDGMENT
Fig. 3. (a) Stator with winding. (b) Integrated rotor.
This work was supported by the Florida Solar Energy
Center and the National Aeronautics and Space Administration (NASA) under the research program “NASA Hydrogen
Research at Florida Universities.” The authors wish to sincerely thank D. Block and A. Raissi of the Florida Solar
Energy Center; D. Chato and J. Burkhart of NASA Glenn
Research Center; and D. Bartine and H. T. Everett of NASA
Kennedy Space Center for their unstinted and continued support throughout the project.
REFERENCES
Fig. 4. Measured input power to the controller at no load.
temperature, water cooling keeps the temperature of the winding
below 45 . Rotor dynamic analysis shows that the first critical
speed is 245 290 rpm, which is well above the operating speed
of 200 000 rpm.
IV. MOTOR PROTOTYPE AND TEST
The first prototype employing ceramic ball bearings was fabricated. Fig. 3(a) and (b) shows the stator with winding and the
integrated rotor, respectively. The rotor was welded and well
balanced before assembling. A stable and high-efficient V/f control method, based on Texas Instrument DSP F2407A, was used
to drive the PMSM. The spin down and free spin tests were performed at room temperature. Fig. 4 shows the input power to the
controller at no-load condition. The total no-load loss at room
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Manuscript received January 31, 2005.