2019 26th International Workshop on Electric Drives: Improvement in Efficiency of Electric Drives (IWED), Moscow, Russia.

Jan 30 – Feb 02, 2019

High-Torque Motor for a Gearless

Electromechanical Actuator
I.F. Sayakhov
F.R. Ismagilov V.E. Vavilov
Department of Electromechanics
Department of Electromechanics Department of Electromechanics
Ufa State Aviation Technical University
Ufa State Aviation Technical University Ufa State Aviation Technical University
(USATU)
(USATU) (USATU)
Ufa, Russia
Ufa, Russia Ufa, Russia
e-mail: isayakhov92@mail.ru
e-mail: s2_88@mail.ru

Abstract—The paper discusses the development of a high- based hydraulic actuator.

torque brushless permanent-magnet direct current (BLDC)
motor. This motor is designed to operate as a part of gearless In the most general case, the power channel of a
electromechanical actuator (EMA) in aircraft. Compared to its translational-motion EMA includes an electric motor, an
counterparts, the BLDC motor of our design has a 20.5% intermediate gear, a translational output gear, and a set of
greater specific torque. The paper presents the results of finite- electromagnetic clutches for backup and holding the control
element analysis for this BLDC motor. The specific power in an end or intermediate position.
reached stands at 0.77 kW/kg. The paper also shows a test
Modern EMA designs use high-speed motors that rotate
mockup of this BLDC motor. This paper describe the technical
solutions behind the BLDC motor, intended to reduce both the at up to 12000 rpm while having rather low torque; this
size and the losses. The contribution of this research to the solution keeps the motor size sufficiently small. However,
EMA motor design consists in making such a BLDC motor controlling a load requires rather low rotation speed but
design that has high specific torque, while featuring minimum considerable torque. In this case, an intermediate gear is used
mass and volume. The obtained results are important for the to bring the motor torques and speeds in line with the load.
efficient implementation of high-torque BLDC motors in The dimensions and weight of a mechanical gear depend on
aircraft industry as well as for the more electric aircraft its type, maximum torque, and gear ratio. The intermediate
concept. gear is the most problematic component in the EMA concept,
as it increases the actuator weight and dimensions while
Keywords—brushless permanent-magnet motor, reducing its reliability and kinematic precision.
electromechanical actuator, finite-element analysis, test mockup
From the standpoint of rational use of the limited space
available onboard an aircraft, gearless EMA designs seem to
I. INTRODUCTION be more appropriate. This reduces both the dimensions and
The more electric aircraft concept is one of the main the weight of an EMA compared to geared counterparts.
trends in the aircraft industry. Implementing this concept will
make aircraft operations eco-friendlier, reduce operating The distinctive feature of a gearless EMA design is that
costs, and expand their functionality. the motor shaft is directly connected to the translational
output gear, which greatly simplifies the design while also
Over the last decade, implementing this concept in reduced the moment of inertia applied to the motor shaft
various civilian and military aircraft has proven efficient and [9, 10]. An appropriate electric motor design helps minimize
highly recommendable [1]. the dimensions and weight of an EMA while also making it
sufficiently reliable.
The key idea behind this concept is to replace hydraulic
systems with electric systems [2, 3]. However, most modern As of today, several motor types have found broad use in
aircraft controls such as flaps, ailerons, elevators, rudders, aircraft: DC motors, asynchronous motors, switched
leading edges of wings, undercarriage and cargo reluctance motors, as well as brushless permanent-magnet
compartment shutters, etc. still use hydraulic systems. direct current (BLDC) motors. However, DC motors are too
heavy while not being sufficiently reliable. Asynchronous
The shock and the load-carrying capacity of
and switched reluctance motors are not as efficient and have
electromechanical actuators (EMA) cannot reasonably
low power factor in additional to considerable weight and
compete against those of hydraulic systems; however, their
dimensions. These disadvantages make such motors barely
response time is competitive. Besides, EMA are easier to
usable in EMA [11-14].
position, smaller in size, efficient, easier to maintain, and
have minimum environmental impact [4-6]. Beside these Modern trends show that BLDC motors have the best
advantages, application of EMA enables the aircraft to using performance-to-size ratio. The specific torque of high-torque
a single type of energy onboard, creating a unified and air-cooled BLDC motor can reach as much as 60 (kNm)/m3
highly-efficient energy system. [15, 16], which is sufficient for generating the required force
in an EMA.
Most of the aircraft controls use translational-motion
drives. This entails the most optimal use of free space for This paper presents a low-speed high-torque BLDC
controls that are tapered along their rotation axes [7, 8]. In motor. Compared to other known designs, the BLDC motor
this case, a translational-motion EMA is similar to a cylinder- of our design has a 20.5% greater specific torque. Also
This paper was supported by the Russian Science Foundation, the presents the design and specifications of this BLDC motor. It
project No. 17-79-20027.

978-1-5386-9453-4/19/$31.00 ©2019 IEEE

also presents the results of finite-element analysis (FEA) for As can be seen in Fig. 1 and 2, the motor reaches the
this BLDC motor to prove it efficient. The 3.35 kW BLDC required output parameters (torque and power) at 800 rpm.
motor weighs only 4.33 kg, meaning it specific power only Fig. 3 presents the distribution of magnetic induction in the
0.77 kW/kg. The paper presents a test mockup of the BLDC active BLDC motor components. Fig. 4 shows the ohmic
motor and describes the technical solutions behind it. losses in permanent magnets as induced by the spatial
harmonics of the stator winding.
The first section of the paper presents the results of
designing and studying the parameters and characteristics of As can be seen in Fig. 3, the mean induction in the
the BLDC motor by FEA. The second section presents the BLDC motor stator yoke does not exceed 1.47 T; the
design of a BLDC motor test mockup. Described the average magnetic induction in the teeth is 1.64 T, which is
technical solutions intended to minimize the losses and below the saturation point of the 2214 electrical steel. The
dimensions. In conclusions, summarize the obtained results. ohmic losses (Fig. 4) in permanent magnets are modeled to
amount to 120-156 W. Given the dimensions and power of
This research contributes to the development of EMA the BLDC motor, these permanent-magnet losses are
mainly by designing a high-torque BLDC motor and apparently quite significant and may result in the
studying its properties. The obtained results are important for overheating of permanent magnets. It is known that
the efficient implementation of high-torque BLDC motor in
overheating the permanent magnets may result in altering
aircraft industry as well as for the more electric aircraft their magnetic properties, even in demagnetization. This is
concept. why the solution adopted when making calculations is to
make a pole segmentation [17] with each magnet having an
II. FEA OF BLDC MOTOR axial length of 17.5 mm. This reduces the losses in
In software package Ansys Maxwell a BLDC motor permanent-magnets from eddy currents by 150%.
computer model was created and made electromagnetic
calculations. Source data of the design are shown in Table I.
Complete numerical results of the BLDC motor
calculation are summarized in Table II and prove the design
fully functional. Fig. 1 presents a curve output-torque and
rotation-speed correlation. Fig. 2 presents a curve output-
power and rotation-speed correlation.

TABLE I. SOURCE DATA

Parameter Value
Power, kW 3.35
Torque, Nm 40
Rotation speed, rpm 800
Supply voltage, V 270
Cooling air

Fig. 3. Distribution of magnetic induction in the active BLDC motor

components

Fig. 1. Curve output-torque and rotation-speed correlation

Fig. 4. Ohmic losses in permanent magnets

Fig. 2. Curve output-power and rotation-speed correlation
 TABLE II. COMPLETE NUMERICAL RESULTS OF BLDC MOTOR III. BLDC MOTOR DESIGN
CALCULATION
Based on the FEA, a BLDC motor test mockup was
Parameter Value
Stator
made. It was designed for low speed and high torque, having
Number of slots 12 a number of advantages over conventional motors, such as
Outer diameter, mm 120 longer service life, greater ratio of torque to rotor moment of
Inner diameter, mm 80 inertia for a more rapid response; higher precision of
Active stator length, mm 70 rotation angle thanks to absence of elastic connections and
2214 backlashes. The use of such motors in EMA will in most
Steel grade
(SiFe) cases enable to get rid of the intermediate gear. Fig. 5
Number of phases 3
presents the design of the BLDC motor.
Supply voltage, V 270
Number of parallel branches 1 Should note a new trend in the design of motors, i.e.
Number of layers 2 designing and creating motors for direct embedding in the
Number of conductors per slot 110
Winding pitch 1
mechanism instead of using a versatile mechanism mount.
Conductor diameter, mm 0.63 Such motors do not use shafts, end shields and bearings,
Slot area, mm2 181.633 which greatly simplifies their design. Such motors have a
Slot fill factor, % 48 flat embeddable design consisting of a stator and a rotor.
Winding factor 0.866 The BLDC motor control system uses Hall sensors placed in
Winding-phase resistance, Ohm 1.12 stator slots.
d-axis inductance, mH 6.6
q-axis inductance, mH 6.7 To test and find the basic motor parameters outside an
Stator-tooth induction, T 1.64 EMA, a BLDC motor mockup that uses a versatile mount
Stator-yoke induction, T 1.47 have made. This version does include a shaft, end shields
Rotor-yoke induction, T 1.24 and bearings. Fig. 6 presents the BLDC motor design for
Air-gap induction, T 0.736 testing.
Current consumption, A 20.1
Current density, A/mm2 33.6 This version also uses a Hall sensor-based control
Rotor system, a mounting surface, and threaded-connection halls
Air-gap length, mm 2
on the cylindrical housing. Tests are conducted per
Outer diameter by magnets, mm 76
Inner diameter, mm 40
standards for electrical rotating machines. Parameters to be
Active rotor length, mm 70 controlled are the rotation speed, the output torque, the input
Steel grade Fe 360-C
Magnet embrace 0.77
Number of poles, 2p 8
Magnet grade SmCo
Magnet height, mm 10
Rotor moment of inertia, kg·m2 0.0017883
Weight indicators
Winding weight, kg 0.66
Magnet weight, kg 0.99
Stator core weight, kg 2.02
Rotor core weight, kg 0.66
Total weight of active materials, kg 4.33
Motor output parameters
Output power, kW 3.35
Torque, Nm 40
Fig. 5. Design of the BLDC motor
Rotation speed, rpm 800
Efficiency, % 64.1

As can be seen from these results, for creating a high

torque of 40 Nm creates a high current density of
33.6 A/mm2. For this reason, the BLDC motor can perform
an S3 mode (intermittent duty cycle), which is in line with
EMA operations. The specific power of the BLDC motor
equals 0.77 kW/kg. The specific torque of the BLDC motor
totals 72.3 (kNm)/m3, whereas the specific torque of high-
torque motors generally equals 60 (kNm)/m3 [18].
Therefore, the specific torque of the BLDC motor under
development is 20.5% higher than that of its known
counterparts, which enables more optimal use of the limited
space an aircraft can provide for such equipment. The FEA-
based electromagnetic calculations enable creating a BLDC
motor that takes weight, dimension, and energy performance
to a completely new level.
Fig. 6. BLDC motor design for testing
power, the current consumption, the supply voltage, and the IV. CONCLUSIONS
stator-phase winding inductance and resistance. Based on the developed model and electromagnetic
A. Stator design calculations, a high-torque BLDC motor for a gearless EMA
have created. In order to verify the output parameters, a
The stator of BLDC motor is placed in a cylindrical BLDC motor mockup with a versatile mount was made.
housing and is made of 0.5 mm sheets of 2214 silicon steel
alloy. The three-phase stator winding is a two-layer The required low rotation speed with a high specific
concentric winding, also referred to as “toothed” winding, torque (more than 70 (kNm)/m3) is ensured by using an
see Fig. 7. There is one coil per stator tooth, and every phase eight-pole rotor with permanent magnets. Ohmic losses in
winding consists in 4 series-connected coils made of 55 turns permanent magnets are reduced by segmenting the magnetic
each. Winding conductors consist of two parallel wires with poles of the rotor along the axis. The BLDC motor presented
copper nickel-plated circular cross-section strands, the herein boasts short response time and high rotor rotation
insulation being based on polyimide varnish. These wires are angle precision thanks to the high torque to rotor moment-of-
designed for electric-machine windings operated at up to inertia ratio. Besides, the design of this BLDC motor enables
220 °C. its direct embedding in the EMA. The results prove the high-
torque BLDC motor to be an efficient solution for a more
Using concentric windings reduces the weight and electric aircraft.
dimensions of the BLDC motor thanks to rather small
overhang of the frontal parts. At the same time, using such
windings increases the ohmic losses in permanent magnets, REFERENCES
which is why the rotor is designed to minimize such losses. [1] P.W Wheeler, J.C. Clare, A. Trentin, et al. "An overview of the more
electrical aircraft", in Journal of Mechanical Engineering Science,
2013
B. Rotor design
[2] C. Sciascera, P. Giangrande, L. Papini, C. Gerada and M. Galea
The rotor is based on semi-circular copper-alloyed "Analytical Thermal Model for Fast Stator Winding Temperature
permanent magnets Sm2Co17 grade, see Fig. 8. To attain high Prediction," in IEEE Transactions on Industrial Electronics, vol. 64, n.
torque and low speeds, the rotor of BLDC motor has eight 8, pp. 6116-6126, March 2017.
poles. In order to reduce the ohmic losses in permanent [3] V. Madonna, P. Giangrande, and M. Galea, "Electrical Power
Generation in Aircraft: review, challenges and opportunities," in press
magnets, each rotor pole consists of permanent magnets on IEEE Transactions on Transportation Electrification,
segmented along the axis. Permanent magnet segments are DOI:10.1109/TTE.2018.2834142, 2018.
insulated from each other with a 0.1 mm polyimide film. [4] C. Gerada and K. J. Bradley, “Integrated PM machine for an aircraft
EMA”, IEEE Trans. Ind. Electron., vol. 55, no. 9, p.p. 3300–3306,
The magnetic rotor system in use minimizes the harmonic Sep. 2008.
distortion factor as well as the losses attributable to higher
[5] S. E. Lyshevski, V. A. Skormin, and R. D. Colgren, “High-torque
harmonics. The operating temperature of the permanent density integrated electro-mechanical flight actuators” IEEE Trans.
magnets in use is 450 °C. Gaps between magnet poles are Aerosp. Electron. Syst., vol. 38, no. 1, p.p. 174–182, Jan. 2002.
filled with an epoxy adhesive for greater rotor rigidity. The [6] M. Montanari, F. Ronchi, C. Rossi, and A. Tonielli, “Control of a
rotor magnetic core is hollow to enable direct with EMA camless engine electromechanical actuator: Position reconstruction
component connection. and dynamic performance analysis,”IEEE Trans. Ind. Electron., vol.
51, no. 2, p.p. 299–311, Apr. 2004.
[7] X. Giraud, M. Sartor, X. Roboam, B. Sareni, H. Piquet, M. Budinger,
S. Vial “Load allocation problem for optimal design of aircraft
electrical power”, International Journal of Applied Electromagnetics
and Mechanics, Volume 43, Number 1-2, p.p. 37-49, 2013.
[8] J. Liscouet, J-Ch. Mare, M. Budinger, “An integrated methodology
for the preliminary design of highly reliable electromechanical
actuators: Search for architecture solutions”, Aerospace Science and
Technology, Volume 22, Issue 1, October–November 2012, p.p. 9–18.
[9] S. E. Lyshevski, “High-performance Electromechanical Direct-drive
Actuators for Flight Cehicles”. Proc. Of the ACC, Arlington,
p.p.1345-1350, June 2001.
[10] S. E. Lyshevski, “Electromechanical Flight Actuators for Advanced
Flight Vehicles”, IEEE Trans. On Aerospace and Electr., Sys. Vol.35,
No.2, p.p.511-518, Apr. (1999).
[11] M. Melfi, S. Evon, and R. McElveen, “Induction versus permanent
Fig. 7. Stator magnetic core of BLDC motor (left) and “toothed” winding
magnet motors,” IEEE Industry Applications Magazine, vol. 15, no. 6,
stator (right) p.p. 28–35, Nov. 2009.
[12] D. G. Dorrell, M.-F. Hsieh, M. Popescu, L. Evans, D. A. Staton,and
V. Grout, “A Review of the Design Issues and Techniques for Radial-
Flux Brushless Surface and Internal Rare-Earth Permanent-Magnet
Motors,” IEEE Transactions on Industrial Electronics, vol. 58, no. 9,
p.p. 3741–3757, Sep. 2011.
[13] P. E. Kakosimos, E. M. Tsampouris, A. G. Kladas, and C. Gerada,
“Aerospace Actuator Design: a Comparative analysis of Permanent
Magnet and Induction Motor configurations,” in 20th IEEE
International Conference on Electrical Machines, 2012, p.p. 2538 –
2544.
[14] M. Galea, C. Gerada, and T. Hamiti, “Design considerations for an
outer rotor, field wound, flux switching machine”, in 20th IEEE
Fig. 8. Rotor of BLDC motor International Conference on Electrical Machines, 2012, p.p. 171–176.
[15] W. Gu, X. Zhu, L. Quan, Y. Du “Design and Optimization of
Permanent Magnet Brushless Machines for Electric Vehicle
Applications”. Energies 2015, 8, p.p. 13996-14008.
[16] C.H.T. Lee, C. Liu, K.T. Chau “A Magnetless Axial-Flux Machine
for Range-Extended Electric Vehicles”. Energies 2014, 7, p.p.1483-
1499.
[17] S. Spas, G. Dajaku, and D. Gerling, “Eddy Current Loss Reduction in
PM Traction Machines Using Two-Tooth Winding,” in 2015 IEEE
Vehicle Power and Propulsion Conference, VPPC 2015 -
Proceedings, 2015, pp. 1–6.
[18] K. Atallah, J. Rens, S. Mezani, and D. Howe, “A novel “pseudo”
directdrive brushless permanent magnet machine,”IEEE Trans.
Magn., vol. 44, no. 11, pp. 4349–4352, Nov. 2008.