Joint EC/EPoSS Workshop on Smart Systems for Full Electric Vehicle
Recent Development in Electric
Traction Drive Technologies
J. Wang
University of Sheffield, United Kingdom
The University of Sheffield
Electrical Machines & Drives Research Group
Basic electric traction configurations
Electric Traction
Single motor via
Mechanical
transmission
Independently controlled
In-wheel drives
Direct-drive
Via reduction gear
The University of Sheffield
Electrical Machines & Drives Research Group
Candidate Electric Machine Technologies
Permanent Magnet Machine
High power density & power factor
High efficiency
High cost
Induction Machine
Low cost
Robust rotor structure
Low power density
Inferior power factor and efficiency
Switched reluctance machine
Robust rotor structure
High torque ripple
Poor power factor
The University of Sheffield
Electrical Machines & Drives Research Group
Independently controlled In-wheel Drives
Key features
Great operational flexibility and mechanical simplicity
Optimal torque balance for great manoeuvrability and safety
Traction control and anti-skid built into each wheel
Regenerative braking recovers almost all energy
Fast acceleration and high top speed
The University of Sheffield
Electrical Machines & Drives Research Group
Hub-mounted Electric Traction Drive
Traction motor: 60Nm and 75kW, max speed: 20000rpm
12kg weight
Epicyclic gear 10:1, 13kg weight
Integrated hub-mounted traction drives
are commercially available
The University of Sheffield
Electrical Machines & Drives Research Group
Key technology for Hub-mounted Direct-Drive
External rotor brushless PM machines with modular windings
High packing factor
High power density
High efficiency
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The University of Sheffield
Electrical Machines & Drives Research Group
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Low manufacturing cost
Negligible cogging torque
Great fault-tolerance
Single motor vs Dual or Quad Electric Drive
Hub-mounted or in-wheel traction drive
Added weight increases unsprung mass which is not desirable
for suspension performance although this may be offset by the
removal of brake caliper/pads and drive shaft
More complex control and safety management are required
Single motor traction drive
Minimum modification to mechanical transmission and braking
systems
Use of high speed motor for weight and cost reduction
Ease of thermal and safety managements
Reliability, noise and vibration of gear box are often of concern
The University of Sheffield
Electrical Machines & Drives Research Group
High performance magnetic gears
Only 3 components
2 are free to rotate, the 3rd is earthed
Prototype 5.75:1 gear
• Torque density: 78kNm/m3
Zero wear and no lubrication
Low maintenance/high reliability
Inherent overload protection/no jamming
Transmitted torque density comparable to mechanical gear
The University of Sheffield
Electrical Machines & Drives Research Group
Pseudo direct-drive electrical machines
Mechanically coupled electrical machine and magnetic gear
System : Electrical machine + Magnetic gear
Electrical
machine
Magnetic
gear
Torque density of
electrical machine
The University of Sheffield
Electrical Machines & Drives Research Group
Pseudo direct-drive electrical machines
Mechanically and magnetically coupled electrical machine and magnetic gear
Stationary permanent magnets
Low speed rotor
with poles-pieces
High speed rotor with permanent magnets
The University of Sheffield
Electrical Machines & Drives Research Group
Pseudo direct drive electrical machines
• Torque density in excess of 60kNm/m3
(> transverse flux machines)
• Power factor higher than 0.9
• Very low current density
Prototype
International patent application PCT/GB2007/001456
The University of Sheffield
Electrical Machines & Drives Research Group
Pseudo direct drive electrical machines
External rotor topology for hub-mounted traction drive
Stator
The University of Sheffield
Electrical Machines & Drives Research Group
Magnetic CVT
4 pole-pair highspeed rotor (Ωh)
Input rotor with 28
ferromagnetic
pole-pieces (Ωs)
24 pole-pair low
speed rotor (Ωl)
phΩh + pl Ωl = ns Ωs
∴Ωl = (ns / pl )Ωs − ( ph / pl )Ωh
Output speed/torque/power can be controlled by regulating the
high speed rotor
The University of Sheffield
Electrical Machines & Drives Research Group
Magnetic CVT Based Hybrid Power Train
(Ωs,Ts)
(Ωl,Tl)
(Ωd,Td)
An arbitrary traction torque/speed combination can be achieved by
independent control of speed of MG1 and torque of MG2 while
maintaining the most fuel-efficient operation point of engine
Simpler
More efficient
Lighter and more compact
The University of Sheffield
Electrical Machines & Drives Research Group
Control of electric traction drives
Effective flux weakening control for maximum operation
capability with variable battery voltage
Sensorless operation for high reliability and low cost
Traction control/Anti-lock braking by detecting wheel
slippage
Torque vectoring --- improve driving dynamics and
manoeuvrability by controlling drive torque distribution
The University of Sheffield
Electrical Machines & Drives Research Group
Future Research
Novel high power/torque dense electric
traction devices
Drive health monitoring, fault diagnosis and
detection
Fail-safe and fault-tolerant operation
Optimisation, integration and cost reduction
Reliability improvement
The University of Sheffield
Electrical Machines & Drives Research Group