PC-Opera European User Group Meeting Reluctance · PDF fileSWITCHED RELUCTANCE TRACTION MOTOR PC-Opera European User Group Meeting Reluctance Motors

1© Ricardo plc 2015April 2015Unclassified - Public Domain

RAPID DESIGN AND DEVELOPMENT OF A SWITCHED RELUCTANCE TRACTION MOTOR

PC-Opera European User Group Meeting

Reluctance Motors – Synchrel Design & OptimisationDr Eddie Wearing, Chief EngineerInnovate UK project 400233

June 2015

2© Ricardo plc 2015June 2015Unclassified - Public Domain


• Why Synchronous Reluctance?

• Design

• Optimisation

• FE Environment

• The End Result

Reluctance Motors – Synchrel Design & Optimisation



The wide variety of potential powertrain

architectures includes permanent magnet (PM),

AC induction, SR, wound field and others.

The anticipated duty cycle has a much stronger

effect on system design and selection in hybrid

and electric powertrains than for IC engines.

An optimal solution may look very different for

the same vehicle in different use cases.

A well crafted specification requires detailed

knowledge of the requirement and is the

cornerstone of good design.

The eDrive is far more efficient than the engine,

so it can operate away from its optimum

operating point to improve system performance.

Thermal challenges to balance heat flow

between the engine and generator.

The available envelope will always be small!

The selection of the eMachine topology is keyInduction motor:

Torque speed curve with

inverter control

IPM motor:

Performance trends

against load

Synchrel

motor:

Torque-speed-

efficiency map

There are many eMachine topologies. Rapid SR aimed to eliminate Permanent

Magnets by maximising reluctance torque through both SR & Synchrel motors.

Selecting the eMachine

260

Nm

110

6:4 SR Motor:

Torque ripple

variation by stator

& rotor pole

widths



Reluctance torque only, so fairly low torque

density, but controllable excitation.

Wide constant power speed range.

Efficiency OK at peak-, & good at part- loads.

Low iron loss on load and at high speed.

Notable rotor losses (hysteresis & eddy current).

Highest kVA/kW due to electrical excitation.

Overload limited by thermals and saturation.

Concentrated windings maximise saliency.

Requires dedicated PE converter design.

Pulsatile nature makes them vibrate noisily.

Better as traction motors or ISGs for (H)EVs.

Less good generators in APUs or series hybrids.

Key defining attributes

These attributes tend to imply

Switched Reluctance (SR) MotorsSequentially energised coils pull rotor round synchronously. Also known as Variable

Reluctance Motors (VRM), they have no magnets, are easily built, robust & fault tolerant.

260

Nm

110

6:4 SR Motor:

Torque ripple

variation by stator

& rotor pole

widths

SOURCES: SRDL website, Ricardo & others



Reluctance torque only so fairly low torque

density & moderate kVA/kW due to excitation.

Controllable excitation with a wide constant

power speed range using conventional inverter.

Efficiency moderate at peak- & good at part-

load at all speeds due to low rotor Fe losses.

Overload only limited by thermals & saturation.

Generally low pole count, typically 4 pole.

Usually distributed windings, same stator as IM.

Better as traction motors or ISGs for (H)EVs.

Similar design, manufacture & control to other

synchronous PM and IM machines.

Less good as generators in APUs or series

hybrids.


These attributes tend to imply

Synchronous Reluctance (SynchRel) MotorThey run synchronously, with a conventional AC stator. Rotor is laminated with non-

magnetic flux guides instead of magnets or rotor windings. Good alternative to IPM.

Synchrel motor:

Torque-speed-efficiency map



A number of gear ratios were explored to

examine how the trade-off in speed and torque

could be made between the machine and

gearbox.

To ensure that the selections made were valid

the gearboxes operational limits were examined.

It was decided to specify the peak power

requirement at top speed to be 45kW.


Agreeing The SpecificationInitial motor specification targets were developed using Systems Engineering techniques

and formed the starting point for the design stage.

Synchrel motor:

Torque-speed-efficiency map

3.5:1(11.6:1)

3.7:1(12.2:1)

4:1(13.24:1)

Gearbox maximum

efficiency assumptions96%

(98% x 98%)

96%(98% x 98%)

96%(98% x 98%)

Peak motor torque (Nm)

(2538Nm @ wheel

(6m/s2))

228 215 200

Peak motor speed (rpm)

(137kmh @ 4%)13,100 13,900 15,000

Base speed (85kW)

(rpm)

(JLR general target)

3560 3775 4058

Base speed (81kW)

(rpm)

(0-100 in 8s)

3392 3597 3867

Cont. torque @ max

speed (45kW)(Nm)

(137kmh @ 4%)

33 31 29

Max continuous torque

(Nm)

(28% grade GVW @

8kmh)

111 105 97

Speed condition for Max

continuous torque (rpm)768 810 876



0

50

100

150

200

250

0 2500 5000 7500 10000 12500 15000

Torq

ue (N

m)

Speed (r/min)

Peak

Continuous

The specification for the motor evolved as understanding improved.

The initial specification was based on information from JLR.

Design limits of the inverter & motor

The Requirement for Torque and Speed

Both the SynchRel and SR machines were

designed to a common target specification:

Base speed: 3,700 rpm

Torque at base speed: >200 Nm Peak,

>100Nm continuous

Maximum speed: 15,000 rpm.

Machine volume (including cooling)

comparable to similarly rated PM machine.

In practice the motor and inverter’s thermal limits

would limit torque and hence power, and would

be identified during testing.



Cobham’s needs

Cobham had developed a variant of their PC-

Opera software for designing SynchRel motors.

It was important that Cobham know the material

properties; the lamination, housing & winding

dimensions; the voltages, frequencies and

currents applied; and the resulting inductances

and torques produced at clearly defined rotor

positions.

The project also included verification of thermal

models, for which Cobham needed to know the

temperature rise against time; the resulting

winding resistance; the thermal performance at

steady state temperature; the thermal

resistances; and clearly identified interface

values wherever possible.

This information was required for a range of

values that included high running speeds and

max torque.

Ricardo also had a role to play in supporting Cobham’s work to develop and validate

a new variant of Opera specifically for designing synchronous reluctance motors.

Developing the FE Environment




• Design

• Optimisation

• FE Environment

• The End Result




The key aspect is to maximize the length and

minimize the width of the rotor bridges. Widening

the barriers also reduces q-axis flux, as does the

cut-out feature on the rotor OD.

However, the ratio of insulating to conducting

material should be 50:50 to maximize torque

production.

Key to reducing the q-axis flux

Design – Suppressing q-axis FluxTorque (per amp) is proportional to the ratio of the direct and quadrature axis fluxes.

This relates directly to the ratio of the reluctances through and across the rotor poles.

SOURCES: Ricardo

Minimize bridge

thickness or possibly

replace them with a

dia- or para- magnetic

retainer.

q-axis cut-out

increases reluctance

in the q-axis

Widen flux barriers to

achieve ~constant

thickness

Quadrature axis

(q-axis)

Direct axis

(d-axis)

This is a convention used in motor design to

simplify analysis.

The reluctance of a synchrel rotor will vary from

minimum in the d-axis to a maximum in the q-

axis, and torque production depends on the

difference between them.

Flux should pass easily along the pole through

the channels, but should struggle to move

across the pole through the bridges and barriers.

Direct & quadrature axes



Sustained effort and detailed work over many

projects results in profound understanding of the

measures necessary to achieve excellent heat

transfer and therefore high power density.

Surface finish, shape, manufacturing methods,

treatments, material properties, fits, corrosion &

wear are all important aspects of the process.

Design process

Thermal design starts in 1D using Motor-CAD

software or MATLAB & Simulink, then 3D FEA.

The key issue is the prediction of the thermal

resistance, which depends on many fine details.

The surface finish, fit, roundness etc of the stack

at the interface with the housing are important to

heat transfer and temperature rise.

Direct cooling reduces these problems and

allows a low cost, high quality design.

Correlating results & improving designs

Design – Thermal Design of eMachines in GeneralElectric motors are thermally limited devices – temperature control is vital.

50

150

Staircase graph

calc

°C

1 342 5

6

7

8

1 8



Thermal analyses showed that reducing the

conductor area had the biggest effect on

performance vs mass, & that stator temperature

was driven by J (A/mm2). Loss numbers were

taken from analyses for T-ω curve.

Achieved class F (155°C) hot spot limit, an

improvement over specified class H (180°C),

conferring increased life or later cost reduction.

Results: 178°C peak & 146°C average winding

temperatures reduced to 140°C & 123°C.

4kPa pressure drop achieved at 8 l/min or less.

The thermal design

Numerous cooling options were modelled. The design was optimised in terms of weight

and size and various cooling jackets considered.

Design – Thermal Design of the SynchRel Motor

Axial water jacket

MotorCAD thermal model

Effect of wire ø on T (°C)

Specified values (RD.13/167301.1)

target design units

Ambient air temp. 50 50 °C

Coolant inlet temp. 90 90 °C

Pk wdg ins. temp. 200 200 °C

Avge wdg ins. temp. 180 180 °C

Coolant nom. flow

rate

8 8 litres/

min

Coolant mix(water : ethylene glycol)

50:50 50 : 50 ratio

Max pressure drop 50 50 kPa

The thermal design aimed to use conventional

automotive methods to: maintain the average-,

and limit the maximum- winding temperatures;

prevent bearing overheating; and minimise

cooling pump requirements & thus reduce cost

Thermal design criteria




• Design

• Optimisation

• FE Environment

• The End Result




Design process

Designs are derived using automated software

that charts trends in performance as the design

changes.

The work includes simulations of candidate

motors’ torque – speed – efficiency maps

matched against real and specified drive cycles.

Knowledge of the real short time and continuous

torque requirements is important.

The temperatures of different parts of the motor

can be modelled over time.

Trend charting means that, instead of doing one

design that meets the requirement, automation

allows thousands to be produced and discussed

with the customer.

The design will be optimised to the exact

requirement as defined in the specification,

which needs to be thoroughly understood.

Results and benefits

Optimisation – Optimising eMachine DesignsTrend charting allows the options to be considered more thoroughly but relies

on thorough, detailed knowledge of the application to be of maximum value.

2

3

4

5

6

7

8

9

10

-100 -96 -92 -88 -84 -80 -76 -72

Machine efficiency [%]

To

tal m

oto

r w

eig

ht

[kg

]

30krpm 4pole -15 slot

40krpm 4 pole -15 slot



Candidate 3

Candidate 1


Candidate 2

30krpm 6 pole 9 slot

7

Wt

(kg)

2100 Eff (%) 84

0.4

0.4 0.4 0.4

0.4

0.4 0.4 0.4

0.5

0.5 0.5 0.5

0.5

0.5 0.5 0.5

0.6

0.6 0.6 0.6

0.6

0.6 0.6 0.6

0.7

0.7 0.7 0.7

0.7

0.7 0.7 0.7

0.8

0.8 0.8 0.8

0.8

0.8 0.8 0.8

0.83

0.83 0.83 0.83

0.83

0.83 0.83 0.83

0.84

0.84 0.84

0.84

0.84 0.84

0.8

5

0.85 0.85

0.85

0.85 0.85

0.85

0.85 0.85

0.85

0.85 0.85

0.86

0.86

0.8

6

0.86

0.86

0.86

0.87

0.87

0.87

0.87

0.88

0.88

Speed (rpm)

Torq

ue (

Nm

)

0 500 1000 1500 2000 2500 3000 3500 4000 4500

-200

-150

-100

-50

0

50

100

150

200 Efficiency (-)

Peak Motoring Load

Peak Regen Load

0 time (s) 1200

260

Temp

(°C)

200

140

120.0

140.0

160.0

180.0

200.0

220.0

240.0

260.0

280.0

0 200 400 600 800 1000 1200 1400 1600

Time [s]

Te

mp

era

ture

[d

eg

C]

Magnet

EWdg_F

EWdg_R

Winding_Layer1

Winding_Layer2

Winding_Layer3

Winding_Layer4

Winding_Layer5

Winding_Layer6



Varying axial length (lax) allows scaling of

inductance to meet required value.

L is inversely proportional to airgap length.

Winding configuration varies quadratically so the

chording (or ‘throw’ of a coil) is used to fine tune

inductances. e.g. 3 slots / pole / phase permits

89% chording, improves MMF harmonics but

only reduces kw1 to 95%.

Saliency ratio is inversely proportional to pole

count, pushing designs to low pole numbers.

4 poles can meet the cardinal requirements for

peak power of 85kW at base speed and >45kW

at maximum speed, VDCmin.

8 pole design worse due to a high level of q-axis

saturation, leading to low Ld/Lq at peak load.

Setting the airgap radial length

Pole count affects torque profoundly

Lots of work was done to decide the details of the Synchrel motor. Two of the many

studies are shown here to illustrate the process.

Optimisation – Selecting the Airgap and Pole Count

0 2000 4000 6000 8000 10000 12000 140000

50

100

150

200

250

Speed (rpm)

Torq

ue (

Nm

)

Tmax@VDCmin

Tmax@VDCnom

Trequested

g = 0.6mm

"2

2

,03

gp

NklDL

p

sphwaxbore

0 2000 4000 6000 8000 10000 12000 1400020

40

60

80

100

120

140

160

180

200

220

Speed (rpm)

Tor

que

(Nm

)

Tmax@VDCmin

Tmax@VDCnom

Trequested

g = 1.2mm

Equation for

inductance L

0 2000 4000 6000 8000 10000 12000 1400040

60

80

100

120

140

160

180

200

220

240

Speed (rpm)

Torq

ue (

Nm

)

Tmax@VDCmin

Tmax@VDCnom

Trequested

4 poles 8 poles

0 2000 4000 6000 8000 10000 12000 140000

50

100

150

200

250

Speed (rpm)

Torq

ue (

Nm

)

Tmax@VDCmin

Tmax@VDCnom

Trequested



The end result of the electromagnetic design work was a good torque – speed curve, but

turning this into a successful mechanical design was very challenging.

Optimisation – Predicted Performance

CAD rendering of

the motor.

Total mass: 84 kg including casing.

Dimensions: 348 mm long (excluding shaft)

360 mm diameter

Low speed torque requirements met, further

optimisation required.

Torque limited by available voltage at high speed

Efficiency (peak): 94 % at 140 Nm & 4,000 rpm.

Outline performance prediction

0

50

100

150

200

250

0 2500 5000 7500 10000 12500 15000

Torq

ue (N

m)

Speed (r/min)

Peak (Target)

Predicted (Peak)

Torque – speed map.



The motor design has evolved significantly. Two rotor designs emerged: the

‘bridged’ and ‘unbridged’ versions. The ‘bridged’ version was built.

Calculation showed that the original ‘unbridged’

design would produce plenty of torque (shown

left) although this would, naturally, depend on

the current provided by the inverter.

The design had poor mechanical properties,

producing a maximum speed of ~8000 rpm.

Attempted solutions included a mechanical

holding structure; modifying the properties of the

laminations to make them impermeable but there

was not enough budget available to develop this.

A second ‘bridged’ design was developed to

achieve the 15 krpm design maximum speed. In

the end this design was adopted.

Limitations of the inverter & motor designs

‘Bridged’ version‘Unbridged’ version

Optimisation – The Rotor’s Mechanical Design

Unbridged flux barrier (750 A current limit)

Unbridged flux barrier (650 A current limit)

Bridged flux barrier (750 A current limit)

Bridged flux barrier (650 A current limit)



Optimisation – Six Sigma Production QualityDesigning a motor to meet a specification is not simply about the nominal

dimensions. The tolerances strongly affect performance and cost in production.

These issues can produce a wide range of peak

torques from one nominal design, but to

maximise the stated performance & control cost

& weight the tolerance of the performance must

be narrow

The mechanical, magnetic & thermal properties

can be modelled & combined with production

data to predict 6 sigma results

Modelling production tolerances

Induction & Synchrel motors need small airgaps,

perhaps 0.25mm, so the motor’s tolerances and

the wear of bearings are very important.

The thermal performance of any motor is also

strongly dependent on tolerances.

Position, temperature & current sensors, & their

mountings, have significant tolerance issues.

Tolerance issues




• Design

• Optimisation

• FE Environment

• The End Result




In parallel with the motor design Ricardo worked with Cobham to develop a

version of PC-Opera for Synchrel machines.

The assessment was based on the existing

ME3D environment in V15R3, assumed to be the

preferred environment for development.

Could develop more templates from existing

PMSM scripts.

Can utilise existing stator geometry modeller.

Rotor PMs could be turned off to make Synch

Rel or adapted to make assisted Synch Rel.

2D modelling capability was the first priority for

supporting machine development although the

ability to switch the model to 3D is of course

desirable once the base design has been

established.

Note - SynchRel can be analysed on the same

basis as PMSM machine in d-q reference frame

and therefore all improvements can be carried

into PM machine environment.

Initial thoughts

Existing rotor modellers

Existing stator modeller

FE Environment – Synchrel Requirements



One aspect of this would be to review the way that windings are modelled in

Opera, as Synchrel machines use standard AC winding layouts.

Consider introducing the ability to import a

winding data table from third party software,

such as SPEED.

Could also provide a graphical check of the

winding setup before going to full 3D mesh.

Review winding options

Existing OPERA winding interface

SPEED winding GUI

FE Environment – Winding Editor



The first step was to consider what parameters to model, using the same GUIs

as existing modellers.

Key parameters for synchrel:

Number of barriers or n-poles

Thickness of barriers & channels.

Barrier shape function.

V, arc and \_/ shapes to be modelled.

Bridge thickness (circumferential and

radial).

Material anisotropy (not available in other

packages).

Magnet size (for PM assist hybrid type).

2D rotor geometry definitionExisting IPM rotor model

FE Environment – 2D Modelling Requirements

Sample Synchrel rotor

Synchrel rotor model



The different ways that electromagnetic performance could be analysed, prior

to FE analysis, were considered for both static and rotating cases.

Torque vs. speed definition (advance angle

optimisation routine).

Loss analysis (for defined operating conditions).

RM (Rotating Machine) analysis

FE Environment – Analysis Options

St: Static analyses

d-q analysis (parameterised model)

Flux linkage (also Ld & Lq) v Id & Iq

Torque v adv. angle v current (fixed rotor).

Torque v rotor angle v current (defined

advance angle).

Air gap flux v rotor angle & current

Key parameter calculations sheet:

Current density

I2R loss estimation

EM analysis options

d-q reference frame: Source SPEED manual




• Design

• Optimisation

• FE Environment

• The End Result




0.0

0.5

1.0

1.5Power (pu)

Torque @3600rpm (pu)

Volume (pu)

Active Mass (pu)

Cost (pu)

Efficiency (pu)

Ricardo SyncRelSpecification

Relatively low cost due to elimination of

magnets.

Good efficiency across a wide speed range

makes this type of machine attractive.

Performance compromised by conflicting

mechanical and magnetic requirements.

Conventional style of stator design makes the

performance similar to induction or PM AC

stators, which is good.

Key design features

The End Result – Ricardo’s Design SolutionRicardo investigated several options & chose to develop a water cooled

Synchronous Reluctance machine adapted for high speeds.

Direct water cooling of the stator

Robust rotor design for high speed

Novel rotor versions planned to allow improved

performance at speed

Ricardo’s design concept



The End Result – Synchrel Motor AssemblyRicardo investigated several options & chose to develop a water cooled


Internal structure of rotor Assembled rotor

Stator and WindingsThe fully assembled machine

The motor has been built from scratch with laser

cut laminations, cast housing and end caps and

with several parts machined from solid.

The rotor laminations were cut to the ‘bridged’

design, formed into a stack and fitted to the

shaft, which was machined from solid, before

being balanced.

The housing was cast and machined, and the

end caps machined from solid aluminium.

The stator stack was also laser cut and bonded,

and was wound and varnished before being hot

dropped into the housing.

Then the rotor, bearings and end caps were

fitted to it to complete the basic assembly before

the encoder, lifting eyes, water jacket manifold,

terminals and thermocouples were fitted.

The new Rapid SR motor



Rotor angle set by very stiff external motor drive

(servo motor and harmonic gear), incremented

by 1 degree (mechanical) per test step.

Current in all 3 phases (star connected to fix

current angle) controlled by lab power supply.

Torque measured by in-line transducer for each

combination of current and rotor angle.

Testing automated using LabView.

Key points

The End Result – Synchrel TestingRicardo investigated several options & chose to develop a water cooled


SynchRel on static torque testSynchRel static torque test result

Electromagnetic FEA torque prediction



Documents

PC-Opera European User Group Meeting Reluctance · PDF fileSWITCHED RELUCTANCE TRACTION MOTOR PC-Opera European User Group Meeting Reluctance Motors