IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007
1619
Safety Torque Generation in HTS Propulsion Motor for General Aviation Aircraft Philippe J. Masson, Member, IEEE, Pascal Tixador, and Cesar A. Luongo, Member, IEEE
Abstract—As part of the development of all-electrical transportation systems, superconducting technology is strongly considered in propulsion systems as it enables implementation of very compact and efficient motors. However, superconductors bring a new type of possible failure mode, as they have to operate at cryogenic temperature and be stable against quench. While a failure of the cooling system or a quench may not be a critical issue in many ground-based applications, it could be fatal in airborne applications. We designed a high temperature superconducting motor to drive a general aviation aircraft and developed an auxiliary torque generation system ensuring thirty percent of the nominal torque needed for safe landing in case of quench or failure of the cooling system. The motor uses magnetized bulk superconducting plates and field coils to generate excitation field and provides 150 kW at 2700 RPM to drive a propeller. Safety torque is generated either from the electromagnetic shield or permanent magnets located in the inductor. The armature design has also been modified in order to accommodate the current increase needed to generate the required safety torque. This paper describes the design modifications done to the HTS motor in order to generate safety torque based on the minimum power needed for the aircraft to land safely. Index Terms—All-electric aircraft, electrical propulsion, safety torque generation, superconducting motor.
I. INTRODUCTION
PPLICATION of high temperature superconducting (HTS) motors in aircraft propulsion is considered as it would enable the development of quieter, less polluting airplanes [1]. Due to their increased power density, HTS motors could provide thrust when mechanically connected to a propeller without adding too much weight to the overall system. However, even though HTS motors have been proven to be highly reliable, their operation is linked to that of the cooling system used to keep superconducting windings at cryogenic temperature thus bringing a new failure mode. While failure of the propulsion system in ground-based applications does not usually induce life-threatening scenarios, it can be catastrophic in airborne vehicles. Generally, in case of loss of cryogenic
A
Manuscript received August 27, 2006. This research was supported by the NASA Vehicle Systems Program and the Department of Defense Research and Engineering (DDR&E) division under the URETI on Aeropropulsion and Power. P. J. Masson and C. A. Luongo are with the FAMU-FSU College of Engineering and the Center for Advanced Power Systems, Tallahassee, FL 32310 USA (e-mail:
[email protected];
[email protected]). P. Tixador is with CNRS-CRTBT/LEG, Grenoble, France (e-mail: pascal.
[email protected]). Digital Object Identifier 10.1109/TASC.2007.898114
Fig. 1. Cessna 172.
power, the machine’s superconducting excitation system remains at cryogenic temperature during several hours during which the motor can sustain partial loading, however, in the case of rupture of thermal insulation, the machine would warm up very rapidly to room temperature leading to a complete failure of the propulsion system. The work presented here deals with design modification to an HTS motor developed to power a general aviation aircraft enabling generation of 30% of nominal power in case of loss of superconductivity. The designed motor [2] exhibits 150 kW at 2700 RPM and is coupled to a two-blade propeller. Safety torque generation can be achieved in two ways; the first is to use the mechanical damping system to generate asynchronous torque, the second is to add permanent magnets conveniently located in the inductor and keep operating in synchronous mode. II. AIRCRAFT PROPULSION REQUIREMENTS A. Propulsion Power and Configuration As part of the development of all-electric aircrafts, the Cessna 172, shown in Fig. 1, has been chosen as a first benchmark for an aero-vehicle to use electrical propulsion. It is a four-seat aircraft powered by a 160 HP four-cylinder engine rotating at 2700 RPM. The maximum propulsion power is needed for take-off and is used as reference for the mission profile of the aircraft. So while 100% power is needed for take-off only 55% is needed during cruise and about 30% for loitering and landing. When turning the airplane into all-electric configuration, one has to keep these numbers in mind and develop a propulsion system able to generate the safety torque (30%) in case of failure. B. Electric Propulsion System The onboard electrical system consists of power generation based on solid oxide fuel cells fed with conventional fuel through a reformer. The superconducting propulsion motor is connected to and controlled through an inverter taking power from a regulated DC bus [4]. Of course, the total weight of the
1051-8223/$25.00 © 2007 IEEE
1620
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007
Fig. 2. Diagram of HTS motor showing Bi2223 coils and YBCO plates.
Fig. 3. Flux distribution in a fourth of the cross section of the machine. Calculations are done using ANSOFT Maxwell3D.
power system will exceed the weight of the original engine [5], however, the gain in terms of noise and emissions is dramatic as emissions can be completely canceled and noise is limited to the friction of the propeller blades in air. Since the propulsion motor, which is quite insensitive to harmonics, will consume all the electrical generated power, power quality can be greatly decreased and therefore power converters can be considerably lightened. C. Safety Torque for Safe Landing At least 30% of the take-off power has to be generated for safe landing. This implies that torque generation should not only rely on the superconducting state of the excitation coils and auxiliary torque generation needs to be included in the motor design. The original motor configuration [2] briefly described in the next paragraph can be modified in several ways to address this need. The damper system can be used to provide asynchronous torque, due to the large air gap of HTS motors; a squirrel cage can be judiciously placed in the air gap to maximize the coupling with the armature. Another solution would be to place permanent magnets as part of the excitation system to generate synchronous torque. The armature will be oversized to accommodate a higher electrical loading and maximize the available safety propulsion power. Both of the proposed solutions are discussed later. III. HTS MOTOR CONFIGURATION The proposed motor is based on flux trapping and flux concentration [2] and generates an average of 1.3 T in the air gap which, when associated with the 300 kA/m of the armature leads to an output mechanical power of 150 kW for a weight of 28 kg. The motor exhibits and very high power density of 5.3 kW/kg, more than an order of magnitude improvement over conventional motors in the same power range. Bi2223 Helmoltz type coils fed with opposite currents generate the excitation field (Fig. 2). The radial field is then used to trap magnetic flux in the YBCO plates and then after inversion of the current in the coils, eight magnetic poles are generated as shown in the flux distribution in Fig. 3. The field generation is based on a two-step cooling process currently under development and to be presented in future publications. The armature is based on an air-gap configuration in which the forces are supported by G10 spacers [6] mechanically
Fig. 4. Diagram of the air-gap armature.
bonded to the backiron, thus increasing the allowable electrical loading (Fig. 4). The armature is cooled down by forced convection using air. Electrical loading can be as high as 300 kA/m during take off in the conductors. Thermal with a current density of 10 simulation showed an acceptable temperature in steady state [8] when cooled by airflow. In case of failure, the armature is the part of the motor that will have to be modified to accommodate a higher electrical loading and compensate the decreased excitation provided by the safety system. IV. ASYNCHRONOUS TORQUE GENERATION Due to its fairly large air gap the designed motor exhibits a synchronous reactance of about 0.27 p.u. An electromagnetic shield is inserted in the air gap; its main purpose is to protect the superconducting windings from flux variation and thus acts as a filter which attenuation can be expressed as follows:
(1)
With R the radius of the shield, e its thickness, its resistivity, and f the frequency. This shield is usually operating at cryogenic temperature, and in some cases can be removed from the design. Another purpose for the shield can be electromechanical damping, this time; the shield is warm and located in the air gap. It can be either a squirrel cage or a hollowed resistive cylinder.
MASSON et al.: SAFETY TORQUE GENERATION IN HTS PROPULSION MOTOR
1621
Fig. 6. Insertion of permanent magnets to generate synchronous torque (TFM=Trapped Field Magnets).
the case of the machine described here, theoretical optimum radius for the cage of:
Fig. 5. Squirrel cage placed in the motor air gap to generate torque.
Although it is not its main purpose, the shield can be used to generate electromagnetic torque in case of failure of the superconducting windings. However, because of the ironless configuration of the machine, coupling between armature winding and shield is not good enough, and therefore the theoretical maximum asynchronous torque generated should be fairly low. Let us consider a shield composed of 20 rods as shown in Fig. 5. between armature and the The coupling coefficient squirrel cage with no iron is:
(2) With the mutual inductance between armature and shield, , and , respectively the self-inductance and the radius of the armature and the shield, p the number of pair of poles. The shield is considered to be composed of a three-phase system to allow for a simpler inductance calculation. The number of rods can be chosen to adjust the self inductance of the cage, thus also modifying the mutual inductance between the cage and the stator, however, the coupling coefficient will remain constant as shown in (2). Using a two-phase d-q system, it is easy to show that the maximum torque can be written using the following equation:
(3) (4) With the line frequency and the phase voltage, the the dispersion coefficient, the number of pole pairs, and d-axis reactance. The maximum torque can be expressed as a function of the phase current as follows:
(5) The torque presents a maximum for a dispersion coefficient of about 0.2 corresponding to a coupling coefficient of 0.89. In
thus leading to a
(6) is almost non achievable as it would lead This value of to too small of a distance between the shield and the armature can be brought close to 0.9. windings. However the ratio The number or rods and their thickness are chosen so that the squirrel cage can accommodate the induced current during asynchronous operation with a reasonable temperature rise. If the nominal current is maintained, the asynchronous torque generated is limited to around 7% of the nominal torque. Output torque can be increased by increasing the line current, however, it is limited by the heat generated in both the armature and the rotor. Achieving 30% of the nominal torque would require doubling the phase current, the rods composing the squirrel cage can be designed to sustain the temperature increase. If this solution were to be chosen, the armature would have to be designed so that it could accommodate twice the nominal current for duration to be determined by aircraft manufacturers. The overall weight of the motor would then be greatly increased as doubling the current carrying capability of the winding would require to almost double the volume of conductor. V. PERMANENT MAGNETS TO GENERATE SYNCHRONOUS TORQUE Another way of creating electromagnetic torque is to use permanent magnets to generate the excitation field in case of a failure of the superconductors [9]. The proposed configuration allows us to place NdFe magnets in between the YBCO plates as presented in Fig. 6. Rare earth magnets exhibit magnetization in excess of 1.2 T and are pretty insensitive to demagnetization when operated at cryogenic temperature [7]. The magnets see a demagnetizing field during the flux-trapping step of the two-step cooling procedure needed to generate the desire excitation field [2]. The flux density during this phase at 150 kW power level do not exceed 2.5 T, therefore, the magnetization of the permanent magnets should remain unaffected. Due to the large air gap of the machine, the flux density generated by the magnets on the armature is very low with amplitude peak to peak of about 0.15 T. The generated torque represents then 11% of the nominal torque. The generation of the 30% of nominal torque needed for safe landing would require an overloading of the armature that would probably need to be
1622
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007
TABLE I COMPARISON OF THE METHODS
Fig. 7. Flux lines generated by one magnet system, superconducting elements are not represented.
redesigned to accommodate the higher electrical loading and almost three times the nominal current. The increase of electrical loading can be achieved by increasing current density in the conductors leading to the need of a more powerful cooling system, or by increasing the volume of conductors in the armature. The main advantage of safety torque generation using permanent magnets is the absence of heat generation in the rotor, which reduces the thermal management needs. The magnet system produces torque even during normal operation, which could potentially lead to a decrease of the required volume of superconductor needed for a given power. In the proposed case, the torque generated by the magnet system would enable and operation at lower current in the field coils around therefore stability would be improved (Fig. 7).
VII. CONCLUSION One of the major drawbacks of superconducting propulsion motors is their dependence on a cryo-cooling system. In case of failure of the cooling system or failure of a superconducting element, the motor would be totally disabled, which is not acceptable in airborne vehicles. We have presented two approaches to generate safety torque independent from the superconducting excitation system; both of the solutions lead to a great increase in reliability at the expense of an increased motor weight. The power density of the fail-safe motor would range around 4 kW/kg, which represents 55–65% of the power density of the original machine without a fail-safe feature, but is still very much higher than that of conventional motors.
VI. RESULTS AND DISCUSSION Both methods presented here to provide torque in case of failure of the superconducting excitation are practical. Control is needed in both cases to adjust the rotation speed of the propeller to the thrust required to maintain the aircraft in the air. They both require modification of the armature design along with its cooling system; the increase in electrical loading needed is higher with the permanent magnet excitation but stability is improved due to the lower current needed in the field coils. The major drawback of the asynchronous system is that the squirrel cage has to be located very close to the armature in order to increase coupling; this can pose a mechanical problem and lead to an even larger air gap for the machine. An increased air gap would require redesign of the excitation system and possibly more superconducting material needed. The methods are compared in Table I. The gain of weight caused by the armature modification is between 35 to 45%, hence a weight between 38 and 40 kg, the best case being the asynchronous system. The new designs would exhibit lower power densities respectively 3.5 kW/kg for the permanent magnet system and 4 kW/kg for the asynchronous system. These power densities are still much higher than that of conventional motors and can still be used to power a Cessna 172 type aircraft. The increase of reliability is dramatic, which is a very important feature in airborne application.
REFERENCES [1] P. J. Masson, D. S. Soban, E. Upton, J. E. Pienkos, and C. Luongo, “HTS motors in aircraft propulsion: Design considerations,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 2218–2221, June 2005. [2] P. J. Masson and C. A. Luongo, “High power density superconducting motor for all-electric aircraft propulsion,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 2226–2229, June 2005. [3] J. E. Pienkos, P. J. Masson, S. V. Pamidi, and C. A. Luongo, “Conduction cooling of a compact HTS motor for aeropropulsion,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 2150–2153, June 2005. [4] N. R. Brooks and T. L. Baldwin, “Utilization of magnetic fields for highly advanced aircraft flight control,” IEEE Trans. Appl. Supercond., vol. 14, no. 2, pp. 1878–1881, June 2004. [5] NASA/DoD URETI on Aeropropulsion and Power Technology Annual Review 2004 [Online]. Available: http://www.asdl.gatech.edu/teams/ ureti [6] S. S. Kalsi, K. Weeber, H. Takesue, C. Lewis, H.-W. Neumueller, and R. D. Blaugher, “Development status of rotating machines employing superconducting field windings,” Proc. IEEE, vol. 92, no. 10, pp. 1688–1704, October 2004. [7] P. Tixador, C. Berriaud, and Y. Brunet, “Superconducting permanent magnet motor design and first tests,” IEEE Trans. Appl. Supercond., vol. 3, no. 3, pp. 381–384, March 1993. [8] J. C. Ordonez, A. M. Morega, M. Mathur, and P. J. Masson, “Thermal model for the AC armature winding of a high temperature superconducting motor for electric ship propulsion,” in ESRDC Workshop on Simulation Based Design for Electric War Ships, Tallahassee, FL, Feb. 14, 2006. [9] P. Tixador, Invention Disclosure FAMU Office of Technology transfer, 2004.
