Freddy Gyllensten, Jörg Janning Challenges in Designs for Electric Propulsion

Challenges in Designs for Electric Propulsion Drivetrains in Aviation 

Dr. Freddy Gyllensten, Dr. Jörg Janning 

17 October 2019, e-flight Forum 2019 (Shijiazhuang)

Wolong – Introduction 

Founded 1984 

by Mr. Jiancheng Chen 

in Zhejiang 

18,000 

Employees 

61 

Holding subsidiaries 

5.6B 

Sales revenues 

(USD) in 2018 

4.7B 

Total assets 

(USD) in 2018 

© Wolong Electric 

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Company brands and history 

Wolong has achieved a rapid growth through global acquisitions, 

which have extended the company’s motor manufacturing 

experience up to 140 years. 

Founded in1878 

Founded in 1883 



Right to use GE mark for 10 years 







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Global Footprint 

Germany 

• 39 factories in 3 continents 

Vietnam 

CNE 

• Global Research Centre 

Explosion proof, special 

motors 

FHP/IHP Motor 

Explosion-proof motor, 

generator, synchronous 

motor 

UK 

Germany 

Poland 

Austria 

Serbia 

Italy 

China 

Headquarter 

Mexico 

Mexico 

Vietnam 

Appliance motors 

EVs and ind. motors 

UK 

Austria 

Jinan Plant 

Industrial motors 

Project motors, AC, DC and 

PM motors 

Power up to 800 MW 

Industrial, Special and 

Appliance motors 

Home Appliance Motor 


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Today – Typical installed power and mission energy for aircrafts 

More electric wide-body aircraft with highest generating capacity 

Typical installed power for different aircraft types (MW) 

Typical mission energy for different aircraft types (MWh) 

Boeing 787 Dreamliner 

Variable frequency starter generator 

1 MVA 

Medium range based on 2.5h flight 

Wide-body based on 13.5h flight 

4 x 250kVA 

(Source: Boeing) 

(Source: Rolls-Royce) 


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Tomorrow – Challenges for electric propulsion 

Gravimetric energy density of state-of-the-art Li-batteries (Wh/kg) 

Theoretical weight of a battery for different aircraft types of today (metric tons) 

>500 

280 

10 years of prediction 

Battery status 

• State-of-the-art batteries today use Lithium technology and 

have a gravimetric energy density of around 280 Wh/kg 

• It is predicted that the energy density will increase to around 

500 Wh/kg by the year 2030 

Assumptions 

• Batteries with energy density of 500 Wh/kg, as predicted available in 2030 

• Same mission energy as in fuel-driven engines (which is not fully true for 

more efficient electric propulsion, but exposes the challenges) 

Conclusions 

• It will be very difficult to replace long-haul wide-body aircraft engines with 

electric propulsion using batteries (hybrid concepts will develop first) 

• Lighter aircrafts with short range have the best characteristics for full 

utilization of electric propulsion and will take lead in commercial 

implementation 

(Source: Roland Berger) 


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Current performance of electrical machines 

Rated power density (kW/kg) 

Rated torque density (Nm/kg) 

(Permanent magnet machines only) 


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Source: A- EL-Refaie, M. Osama, ”High specific power electrical machines: a system perspective”, IEEE, 2017.

Basic performance factors of electrical machines 

The size and weight of an electrical machine is in principle governed by the required torque 

Increasing power and power density can be done by increasing speed 

‣ Machines with very high speed tend to have low pole numbers 

4 poles 

Machines with low speed have the potential to increase torque and 

torque density by having high pole numbers and large airgap diameters 

24 poles 


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Basic performance factors of electrical machines (2) 

The rated performance in electrical machines is ultimately limited by the maximum allowed 

temperatures in critical components (primarily for winding insulations and bearings) 

For a given rated speed, the torque and power of an electrical machine 

can be increased by: 

• Reducing the losses (copper, iron, friction, parasitic) 

• Increasing the operating temperature (if the design and materials 

allows) 

• Enhancing the cooling performance (gives temperature margin 

that can be utilized) 

Note: 

Industrial motors are 

normally run at winding 

temperatures below 120˚C 

(class B tempererature rise) 

For a given machine dimension the performance can be enhanced by: 

• Tighter packing of copper winding 

• Shortening end-windings 

• Lower loss materials (e.g. thinner electric steel laminations for 

SiFe, CoFe steel, amorphous steel, superconducting materials) 


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Design Requirements of Electrical Machines for Aerospace 

• High Efficiency ( > 96%); 

• Super Compact (4.5-6 kW/kg); 

• High Speed (up to 40,000 rpm); 

• Safety and reliability (Certifications); 

• Subject to possible severe Environment; 

• Compact and Reliable Frequency Converter; 


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Stresses on e-Aircraft Propulsion Motors 

Electrical stress 

• Pulse voltage modulation 

• High frequency pulses 

• Fast voltage rise time 

Mechanical stress 

• Acceleration: Max. 10-20g 

• Vibrations 

Thermal stress 

• Operating temperatures above 200˚C 

Environmental stress 

• Oil cooling 

• Moisture 

• Metal particles 

• High altitudes 


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Comparison of Electrical Machine Topologies 

Source: 

Prof. Thomas Jahns 

IEEE Electrification Magazine 

MARCH 2017 

Stator 

Rotor 

Liquid-cooled 

PM Motor 


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Neodymium Iron Boron (NdFeB) Permanent Magnets 

• The magnets, once magnetized, continually deliver magnetic 

flux into the machine’s air gap without external excitation 

China has 65% of 

Neo Magnet Material 

reserve worldwide 

• No Rotor Joule Losses During Operation 

• Machine losses concentrated on stator side 

• Challenges: 

• Surface Treatment and Coating 

• Safety During Magnet Assembly 

• Demagnetization during operation 

• Temperature, 

• Short circuit current, 

• Environment 


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Winding Concepts 

Random winding Concentrated winding Hair-pin winding 

+ Standard in low voltage electric 

machinery for industrial applications 

+ Low AC resistance per default 

+ Produces sinusoidal flux for low 

parasitic effects 

− Relatively low copper slot fill factor 

and long end-windings result in high 

DC resistance, poor heat transfer 

and low torque density 

− Several manufacturing steps that 

can involve both automated and 

manual handling 

Litz wire 

+ Standard in low voltage electric 

machinery for servomotors/robotics 

applications 

+ Enables high level of automation 

+ Enables short end-windings and 

high copper slot fill factor for low 

DC resistance 

− Produces high amount of harmonic 

flux which is load dependent and 

can create severe parasitic effects 

like losses in core and permanent 

magnets 

− Topology faces added risk of 

vibration and noise for larger 

machinery and/or higher power 

+ Reletively new winding type in 

commercial use currently being 

explored in automotive applications 

+ Enables high level of automation 

+ Enables short end-windings and 

high copper slot fill factor for low 

DC resistance 

− Large rectangular wire dimensions 

may result in eddy-current winding 

losses at higher frequencies 

− Winding requires many termination 

points and very special and unique 

manufacturing tools 


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Insulation systems 

Effects of Inverters on Motor Life 


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High-End Power Electronics 

M 

~ - 

M 

~ - 

~ 

- 

M 

~ - Inverter 

M 

G/S 

FAN 

Battery 

G/S 

Turbine 

DC Bus 

Air 

- 

~ 

M 

M 

~ - 

~ - 

Motor 

Starter/Generator 

Combustor 

Hybrid electric aircraft 

• The propulsion system is mainly performed with 

electrically driven propellers 

• Gas generator stays the main energy source 

• Additional battery for energy buffering, efficiency 

increase, additional power boost, auxiliary devices 

Full electric aircraft 

• Gas generator gets replaced by larger battery and/or 

fuel cells 

• Low loss semiconductor devices (e.g. Silicon- 

Carbide (SiC) and Gallium-Nitride (GaN)) 


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Longer term potential – High Temperature Superconductors (HTS) 

HTS Trapped Field Magnets 

• Bulk YBCO Superconductor 

• Trap a magnetic field by the 

very incomplete Meissner Effect 

(VIME) 

• Pinning sites are created within 

the crystal lattice either chemically 

or using novel neutron activation 

Temp K Field (T) 

77 2.2 

65 5.1 

50 10.4 * 


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Press Release 


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Summary 

• The trend towards cleaner, greener and more reliable 

aircrafts continues 

Novel concepts of e-Aircrafts 

• The technology shift to more use of electric power for 

auxiliaries and propulsion is evident 

• The performance level of electric propulsion systems 

does not meet the target requirements for aircrafts today 

• Significant research efforts on electric propulsion and 

energy storage is needed and envisioned 

Novel concepts of eVTOLs 

• Commercial implementation will initially focus on lighter 

aircrafts (e.g. eVTOLs), before larger sized aircrafts 

• Automotive applications faces some of the same 

challenges and will also advance development 

• An exciting future for electric aviation lies ahead! 


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Thank you! 

Contacts: 

For technology: 

Allen (Liu Zhongqi) 

liuzhongqi@wolong.com 

For market: 

Lucas (Ye Chao) 

yechao@wolong.com

Freddy Gyllensten, Jörg Janning Challenges in Designs for Electric Propulsion

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