electric motor design processes in the automotive ... · pdf file• electric motor nvh...

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Electric Motor Design Processes in the Automotive Environment and the Importance of Software Features

Cobham EUGM 2014

3rd June 2014

Team: Jose Soler Vizan (Lead E-Machine Development Engineer)Istvan Kiraly (E-Machine Development Engineer)Alex Michaelides (Technical Specialist Machines & Power Electronics)

2CONFIDENTIAL

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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• Magnetic design




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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 e-

machine.

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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• Magnetic design




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Magnetic designMachine 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

• Better heat rejection

• Lower torque ripple

• Better field weakening capacity

Example - concentrated Example - Distributed

Higher voltage harmonics

Higher Torque ripple

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15CONFIDENTIAL

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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• Magnetic design




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Electromagnetic Modelling

- Rotating Machine Modelling incl. fault injection: The motor characteristics (torque and Back-

EMF), 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=20h100 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

0

0.5

1

1.5

2

2.5

3

0 2000 4000 6000 8000 10000 12000 14000

P_ac/P

_d

c

RPM

AC_losses/DC_losses

160 degree

20 degree

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• Magnetic design




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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 DesignMotorCad Simulations

Thermal simulation vs test

Dyno test

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• Magnetic design




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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!

electric motor design processes in the automotive ... · pdf file• electric motor nvh...

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