Electric Motor Design Processes in the Automotive

08/06/2014
Electric Motor Design Processes in the Automotive
Environment and the Importance of Software
Features
Cobham EUGM 2014
Team: Jose Soler Vizan (Lead E-Machine Development Engineer)
Istvan Kiraly (E-Machine Development Engineer)
Alex Michaelides (Technical Specialist Machines & Power Electronics)
3rd June 2014
Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Thermal design – heat rejection and temperature rise
• Mechanical design – mechanical integrity, NVH
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System constraints (I)
An electric drivetrain is, in general, composed of a High voltage battery, an
inverter and an electric motor (plus other small electrical loads).
The capability of each of these components needs to be correctly defined to
deliver the peak performance targets of the electric drive.
The peak performance of the e-drive will be dictated by:
- HV battery peak/continuous power capability
- Inverter peak/continuous power capability
- E-machine peak/continuous power capability
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System constraints (II)
The output power of an electric motor is proportional to the battery voltage.
The battery output voltage vs SOC is a characteristic that depends on the
chemistry used.
Battery voltage plot
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System constraints (III)
Limp home capability : this depends on the capacity of the cooling system to contain
the motor temperature, under an active short circuit condition.
Active short circuit
Open circuit voltage
The choice of rotor design, stator features and winding will affect these parameters and impact
performance
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Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Thermal design – heat rejection and temperature rise
• Mechanical design – mechanical integrity, NVH
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Targets definition
Automotive e-drives operate under a variety of load conditions, dependent on variable traffic and
road conditions as well as driver attitudes. The mix of engine and motor power also affects the
motor duty cycle
Driving cycles: these are sometimes defined to predict/represent the load variations. However,
others are only used for vehicle certification (eg. NEDC, UDDS).
Peak Accelerations: this is related to the peak performance capability (torque and power) of the emachine.
Steady state conditions - continuous power: Continuous loads, to a large extent, size the
e_machine; optimum drive efficiency areas need be optimised around these.
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Driving cycles and their role
Dedicated OEM drive cycles are time-speed diagrams which describe a typical driving cycle on
the basis of statistical data. They help determine the torque and speed demand of vehicle and, in
turn, define the required e-motor capability. OEMs also design systems to adhere to legislative
drivecycles.
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Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Thermal design – heat rejection and temperature rise
• Mechanical design – mechanical integrity, NVH
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Magnetic design
Machine topology
• Permanent Magnet Synchronous Machines (PMSM) exhibit the
highest torque and power density. Stator winding topology can be
concentrated or distributed.
• The magnets on the rotor can be surface-mounted or embedded.
• For each concept, the following parameters need to be optimized:
o Pole number
o Tooth number
o Magnet configuration (U-shape, bread loaf etc.)
o Winding configuration (number of phases, coils, turns…)
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PMSM - Magnetic design
Design optimisation:
• Peak torque
• Peak & continuous power & torque split
• Torque ripple
o
Minimise by
o
geometrical optimisation
o
Skewing (but watch out for loss of torque & demag issues)
• Back EMF harmonic peak reduction
• Mechanical integrity of rotor and stator & ease of
manufacture
o
This can limit torque output
• Thermal requirements
o
Continuous power requirement also important
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Electromagnetic Design (I)
Surface topologies:
• The magnets are placed on the rotor surface. This arrangement reduces motor inductance –
in “d” and “q” direction. Lower phase coil inductance helps deliver higher power
• Speed is limited by mechanical retention capability of magnets (unless sleeves are used), and
the magnets are less protected against mechanical damage and demagnetization. Reluctance
torque is negligible and high constant power / speed ratios (CPSR) are more difficult to obtain.
Embedded topologies:
• The magnets are embedded in the laminations, which protects them from mechanical damage
and reduce the demagnetization risk. The motor develops significant reluctance torque.
Usually the inductance in “q” direction is larger than in “d” direction.
• The variation in magnetic reluctance results in significant torque ripple and higher back-emf
harmonic content.
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E-motor Torque (reluctance and magnet torque)
In a PMSM with embedded magnets the
resulting motor torque can be divided in two
components:
Magnet torque: The result of the interaction of
permanent magnets and stator current. This
torque component is approximately proportional
to the motor current (if no saturation).
Reluctance torque : The result of the difference
in d and q axis reluctance values. This
component is proportional to the square of
motor current (if no saturation).
Example of varying
reluctance component
using a different rotor
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Concentrated Windings vs Distributed
Winding designs
•
•
Concentrated windings offer
• ease of manufacture
• lower manufacturing costs
• Better suited to short stack machines, as
coil ends are generally shorter
Distributed windings can offer
•
•
•
Example - concentrated
Example - Distributed
Higher voltage harmonics
Better heat rejection
Lower torque ripple
Better field weakening capacity
Higher Torque ripple
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Fault conditions
Design for Active symmetrical 3-ph Active Short Circuit (ASC)
This condition can be activated at any point during the e-motor
operation. The e-motor should be capable to survive to ASC without
any permanent demagnetization.
The transient short circuit current is always higher than the steady
state current. The peak value of the transient ASC is generally around
twice of the steady state ASC
Short circuit current limitation Factors:
Magnet grade & rotor temperature : the magnet grade determines the
maximum temperature at which the motor can operate safely.
E-Motor Inductance: the short circuit current of the PMSM is mainly
controlled by the e-motor inductance. A higher inductance can reduce
the transient short circuit current.
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Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Mechanical design – mechanical integrity, NVH
• Thermal design – heat rejection and temperature rise
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Electromagnetic Modelling
-
Rotating Machine Modelling incl. fault injection: The motor characteristics (torque and BackEMF), and fault conditions are simulated by FE software. These characteristics significantly
depend on the non linearity of magnetic circuit – they cannot be determined by analytical
methods.
-
Skew Modelling: the torque ripple and the harmonics of Back-EMF can be reduced by
skewing the rotor.
- In PM motors, skewing is not continuous - this is usually simulated as a linear combination
of layers
-
Back EMF ,inductance: the Back-EMF is calculated as the rate of change of flux linkage. Apart
from the main flux linkage, leakage inductances exist and must be accounted for.
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2D vs 3D FEA
Three dimensional modelling is required when the aspect ratio (D/L) is large and
end-winding inductance needs to be calculated.
3D FEA is significantly more time consuming,
100 steps 3D transient calculation=20h
100 steps 2D transient=5min
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E-Machine Optimisation
There are different optimization algorithms available ( i.e. surface response method, Genetic
algorithm… ) . Some of them are more suitable for single objective functions (i.e surface
response) , whereas others (ie. Genetic algorithm) are more appropriate when multi-objective
functions are required (i.e Minimized losses and maximize torque)
Example:
Magnet loss reduction by shaping the tooth tip.
- Surface response optimizer method
- The optimum design has reduced the
magnet losses by 25%
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E-machine magnet weight/cost
minimization
Currently there are different approaches to reduce the weight or
cost of the magnets:
- Use of embedded rotor topologies (gain reluctance torque)
- Permanent Magnet Assisted Synchronous motor
- Embedded magnet designs generally increase winding
inductance, and hence affect peak power capability
- Reduce rotor temperature
- … and hence use a lower magnet grade.
- Use of non PMSM technologies :
- Induction, wound rotor, Switched reluctance motors.
The use of any of these technologies will be directly influenced by the requirements and
package constrains of the motor.
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Eddy current loss in stator coils
AC_losses/DC_losses
3
P_ac/P_dc
2.5
2
1.5
160 degree
1
20 degree
0.5
0
0
2000
4000
6000
8000
10000
12000
14000
RPM
Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Thermal design – heat rejection and temperature rise
• Mechanical design – mechanical integrity, NVH
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Thermal Design
A good thermal design is as important as a good electromagnetic design.
An optimized design could increase significantly the continuous performance of the e-motor
Thermal design aspects:
Cooling methods: i.e. Air-cooled motor, indirect liquid cooled , direct cooled, oil spray.
Water jacket : a good contact between the stator and the water jacket will improve the cooling
performance.
Impregnation methods : ie. trickle, vacuum process impregnation.
Slot liners : ie. nomex, kapton, plastics
Wire distributions: a good packing factor can improve the thermal conductivity between the wires.
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Thermal Design
MotorCad Simulations
Thermal simulation vs test
Dyno test
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Introduction & Contents
• System constraints
• Targets definition
• Magnetic design
• Modelling & simulation of machine performance
• Thermal design – heat rejection and temperature rise
• Mechanical design – mechanical integrity, NVH
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Mechanical design
• Stress calculation on the rotor is needed in
order to guarantee the robustness in case of
an over-speed event.
• The critical areas are usually the bridges that
hold the magnets radially.
• Significant trade-offs exist between the
optimum electromagnetic and mechanical
designs (thicker bridge increases magnet
leakage but improve the mechanical
performance)
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Noise, vibration and harshness (NVH)
•
Electric motor NVH needs to be taken into account during
the design process. Any noise generated in the motor might
be experienced by the driver especially in applications
where the target vehicle includes a pure EV mode.
•
The potential sources of noise include:
- Natural modes excited ( i.e frame, shaft, driveline…)
- High e-motor torque ripple.
- High and un-even electromagnetic stator radial force.
Natural mode excited
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E-Machine design Process
A mix of analytical and
FEA-based software tools
were employed during the
design of the machine.
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Conclusions
•
Permanent magnet machine design characteristics and cost can vary significantly depending
on factors including
• Rotor design
• Stator winding choice
• Cooling methods
• Mechanical design
Choice of topology will be target-driven.
A multi-physics design approach is necessary to get the most out of any given
topology. Fast, efficient simulation software is paramount for accurate evaluation of
potential designs:
- Coupled EM & Thermal Analysis (incl. dynamic thermal simulation during drivecycles)
- Coupled EM & Stress Analysis (is a magnetic design viable)
- Quick calculations on key design requirements
Need the right environment and the right solvers for he job!
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