axial flux plastic multi-disc brushless pm motors performance assessment

7/23/2019 Axial Flux Plastic Multi-Disc Brushless PM Motors Performance Assessment

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Axial Flux Plastic Multi-Disc Brushless PM Motors:

Performance Assessment

F. PROFUMO, A. TENCONI, M. CERCHIO

Politecnico di Torino

Dipartimento di Ingegneria Elettrica IndustrialeTorino, ITALY

[email protected]

J. F. EASTHAM, P. C. COLES

EnigmaTEC Ltd

Bath, [email protected]

Abstract A dedicated brushless PM motor prototype for a

stratospheric aircraft propeller drive has been manufactured and

experimentally tested. The motor has a non-conventional axial flux

multi-disc structure; the magnetic circuit is almost ironless and the

mechanical structure makes wide use of composite plastic materials.

The experimental tests demonstrate the motor functionality and

performances, basically confirming the expected motor

characteristics. The prototype performances are assessed

considering as reference the conventional radial flux motors.

Index Terms Axial Flux Machines, Performance Assessment,Synchronous Motors, Permanent Magnets.

I. INTRODUCTION

HELIPLAT is a stratospheric unmanned aircraft proposed for

use as telecommunication platforms in the HeliNet Project,

funded by the Information Society Technologies Programme

(IST), within the European Union Fifth RTD framework [1], [2].

HeliPlat employs eight propellers, each of them is directly

driven by a single motor, supplied by a solar cells - fuel cells

energy system. The aircraft must be able to take-off by itself,

climb to the destination altitude (1725 km) and than to keep a

constant height in a turning flight to achieve a geo-stationary

position: full power is required during take off and climbing to

the target altitude, but only about of the full power is enough

to maintain level flight. Since there are no reduction gears

between the motor and the propellers, the application requires a

low-speed/high-torque duty, the best possible specific torque and

high efficiency. In addition, the high efficiency is required over

the complete and wide power range. Furthermore the motor

dimensions are limited by space allocation constraints (Fig.1).

Hence, a dedicated machine design is needed to satisfy the

specific application requirements. Brushless permanent magnet

technology and an unconventional multi-disc arrangement using

an axial flux structure have been considered as the mostpromising solution [5], [6]. Furthermore, in order to increase the

motor efficiency, especially at low load, the magnetic circuit is

almost ironless to avoid iron losses; the structural function of the

iron in the conventional machines has been replaced by advanced

composite plastic materials, that also reduce the motor weight

[3], [4].

MDAFMMDAFM

Fig.1: HeliPlat motor allocation (Courtesy of DIASP).

The motor prototype has been manufactured and characterized

by means of laboratory experimental tests.

The performance assessment of the Multi Disc Axial Flux

Motor prototype is presented referring to brushless motors

commercially available.

II. 2. MULTI-DISC AXIAL FLUX MOTOR(MD_AFM)

A. Basic design considerationsWeight and energy efficiency of all the payloads and electrical

subsystems strongly affect the aircraft mission. For this reason

the propulsion system must maximize the torque density and

give a high efficiency over a wide working range.

To satisfy these fundamental requirements, a dedicated

machine has been developed starting from the following basic

design considerations:

Permanent magnet brushless machines - the high torque

density and efficiency of these motors is well known.

Axial flux multi-disc structure - this structure allows theincrease of the ratio between the torque producing surface

and the quantity of active material, particularly when

"flat" geometries are required [5], [7], [9]; furthermore

the modular construction makes possible to vary the

0-7803-8269-2/04/$17.00 (C) 2004 IEEE. 1117

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designed output torque by changing the number of discs.

Quasi-ironless magnetic circuit - the proposed solution

requires only two rotor end-discs made of iron for the

magnetic flux path closure [4] as shown in Fig.2. The

adoption of an almost ironless magnetic structure

eliminates the iron losses due to the main flux

(independent of load), hence it offers an efficiency

advantage, extending the range of optimal operation at

low loads (drawback: large magnet thickness are required

to push the flux through the long ironless path).

Plastic composite as structural material - advanced

engineering plastic materials provide the required

mechanical and thermal properties to realize the low

weight loss-less discs [3], [4], [5] (drawback:

unconventional construction difficulties have to be faced,

such as adhesive joints, machining problems, etc.).

B. Motor StructureThe multi-disc motor structure consists of 4 rotor discs

interleaved by 3 stator discs.

The rotor of this machine uses a number of non-magnetic

plastic discs that provides the support structure for an array of

permanent magnets of alternating polarity, forming a 16-pole

motor. Iron rings are located in the two end rotor discs, which

rotate with the magnets and close the flux path. The permanent

magnets have the same circumferential position on each disc.

The flux goes along the machine axially through a line of

magnets; then it goes circumferentially in the end iron ring to the

next pole and finally comes back along the machine through an

adjacent line of magnets. A drawing of the flux path is shown in

Fig.2 and a 3D section in Fig.3.

Permanent Magnet

with Iron Ring

N S

Composite Materials

CoilPermanent

Magnet

Axial

Flux Path

N

S

N SN SN S

S NS NS NS N

N

S

External Rotor

Discs

Internal RotorDiscs

Stator

Discs

Permanent Magnet

with Iron Ring

N S

Composite Materials

CoilPermanent

Magnet

Axial

Flux Path

N

S

N SN SN SN S

S NS NS NS NS N

N

S

External Rotor

Discs

Internal RotorDiscs

Stator

Discs

Fig.2: Axial Flux Machine Structure

Windings

Chassis

ShaftWindingsDisc (Stator)

Magnets

Disc (Rotor)

Iron for Flux

Path Enclosure

Permanent

Magnets

Fig.3: Axial Flux Machine 3D section

The stator discs interleave the rotating magnet discs. These are

again made of plastic and provide the support matrix for a set of

stator coils. The construction is therefore modular and one

module consists of a rotor and a stator disc as shown in Fig.4.

(a) (b)

Fig.4: Internal rotor disc (a) and stator disc (b)

The iron-less disc machine structure leads to air-gap slot-less

concentrated stator windings that should ideally be planar

without coil end winding crossovers. Among the different

possible arrangements, concentrated type windings have been

chosen for the relative simplicity of manufacture; in order to

improve the winding factor, the second harmonic of the m.m.f. is

used instead of the first harmonic. As a consequence, the three

coils of the winding occupy the same arc as 4 poles (3 coils 4

poles). In this way, the winding factor produced is 0.716, which

is rather high for concentrated air-gap windings. The

concentrated windings have a relatively poor copper exploitation

due to the long end connections but, on the other hand, there is

much room for copper due to the absence of iron, hence the

copper losses can be kept quite small.

Each one of the eight propellers requires the following motorperformance:

Rated power: 2250 W @ 1000 rpm;

Efficiency: 92 % @ full load - 90% @ 1/2 load;

External diameter:


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Case temperature: 90C (the motor is fixed to the wings

that are made of carbon fibber: this material can not withstand

temperature higher than 1000C).

The first motor prototype has been manufactured: the overall

machine is shown in Fig. 5.

Fig. 5: Multi-Disc Axial Flux Machine Prototype

C. Motor Construction DetailsThe motor has been manufactured adopting quite new

approaches in the material selection and design of the

mechanical structure. One of the most important novelties of this

motor consists of the application of a plastic material for the

manufacture of the motor discs. The choice of Ketron Peek

from a number of different kinds of advanced engineering plastic

materials has been described in [3], the table in Appendix A

summarize the characteristics of the materials used for the

prototype realisation.

The stator consists of three 290 mm diameter discs containing

the coils. Each stator is divided into two half discs so that the

rotor can first be completely constructed and balanced and thenthe stator discs can be inserted.

The strands of wire forming the coil conductors have been

made of small diameter wire to reduce the eddy currents, induced

by the main air-gap flux. The final choice of the 0.4 mm wire

diameter is the trade off between low eddy current losses and the

ease of hand manufacturing the coils (Fig. 6). Since the wire is

self-polymerising, the coils after winding have been baked in a

oven at 180 C

Fig. 6: The stator coil

The stator coils are glued into the plastic stator half discs that

provide the mechanical support. The glue used has been chosen

with particular care since Ketron Peek is quite difficult and a

good gap cure is required for this application (this means

practically that a medium viscosity glue must be used)

Furthermore the heat sources of the motor are concentrated in the

coils therefore the glue must also have the capability of

withstanding high temperatures.

The glue used for the assembly of the stator discs belongs to

the two component Polyurethane family. This glue has good

thermal properties since it can operate continuously up to 120C.

The rotor is made of 4 plastic discs, the two internal discs

support the PMs, while the two external support the PMs and the

iron rings for the flux path closure.

The permanent magnets have a typical Brem of 1.2T and a

coercive force of 890kA/m, they can withstand a maximum

working temperature of 150C and have zinc coating. The

magnets have been made with stepped sides to give better fixing

into the plastic disks; this solution leads to a more reliable

assembly of the discs because the integrity is not based entirely

on glue joints.Fig. 7 shows the assembly of the internal rotor disc: in

particular it can be seen that the stepped NdFeB permanent

magnets are inserted between two half plastic discs to ensure

mechanical strength.

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Fig. 7: Rotor Disc

To join the magnets to the two plastic discs a two-component

low-density epoxy resin has been used.

The external discs were manufactured using Ketron Peek

glass fibre reinforced plastic with better mechanical properties

with respect to the natural Ketron Peek

used in the internalrotor discs. In these discs NdFeB stepped permanent magnets

were inserted together with iron ring for flux closure. A careful

design of the mechanical structure of the external discs has been

made to withstand the unbalanced attractive action of the

permanent magnets on the external discs. In the prototype the

estimated force of more than 700N has not lead to measurable

deformation of the discs.

To limit the deformation effects of the permanent magnets on

the plastic discs, the shaft has been made from a Titanium alloy

with a two-diameter structure (Fig. 8).

Fig. 8: Rotor Assembly

From the initial design the electromagnetic structure has been

slightly changed, increasing the 1 mm clearance air gap to1.5mm to reduce the uncertainty level about mechanical

interference due to possible heat deformation of the plastic

materials under thermal load. This has modified the initial 51mm

(clearance + magnets + windings) to 54mm resulting in the

reduction of about 6% in the flux and induced emf. The

compensation for this by increasing the current by about 6%

gives the same torque and output with an increased loss of about

12% and a reduction in the efficiency of about 0.6%.

Since the prototype has to prove the basic design ideas, (it will

be not embedded on the aircraft) the non-critical parts of the

prototype have been realized with conventional material and

solutions: steel bearings, aluminum flanges and housing etc.

III. EXPERIMENTAL TESTS

The motor prototype has been bench tested in "normal"

environmental conditions to assess the performance against

analytical forecasts.

The tests can be subdivided as follows:

Functional tests to verify the basic technical solutions

adopted in the motor construction;

Motor parameter estimation tests to validate the FEM

analysis and the parameter computation to tune the design

procedure for further optimisation;

Thermal tests to verify the motor rated performance and thethermal behaviour of the mechanical properties of the plastic

materials and the resins used to assembly the stator and rotor

discs.

A. Functional TestsBeside the routine tests on winding resistance and isolation

the generator test (back-emf waveform) shows the pure

sinusoidal nature of the motor, according to the 5/6 magnet pitch

design. The analysis of the waveform acquired has revealed a 3%

content of third harmonic. The result of the acquisition for the

phase back emf at 400 rpm is showed in Fig.9.

Fig. 9: Acquired phase back emf

B. Motor parameter estimation testsThe MD_AFM has no saliency so that the d and q values are

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almost identical and the equivalent circuits in the conventional d-

and q-axes representation use the same phase impedance.

m

Rph Ld=Lq

vq

iq +

m

Rph Ld=Lq

vq

iq +

Fig. 10: MD_AFM q axis model

Hence the parameters to be experimentally determined are:

the constants KV andKT (whereKT = 3 KV);

the stator resistance;

the stator inductance.

The proportionality constant measured at 20 C is KV=1.73

Vs/rad (referred to line-to-line rms voltage), whilst the

measurement with the MD_AFM operating as a generator at

thermal steady state (about 100C in the windings) has gave

KV=1.6 Vs/rad (7% less).

The locked rotor current and torque test measures the torque

against angular position at zero or very low speed [5], feeding

the prototype stator windings with a DC voltage; the test has

been made for the two supply configurations (1), (2) depicted in

Fig.11.

( 2 )

q

d

b c

a

VIDC

q

d

b c

a

V

IDC

IDC/2

( 1 ) DCpeak II =DCpea k I3

2I = ( 2 )

q

d

b c

a

VIDC

q

d

b c

a

VIDC

q

d

b c

a

V

IDC

IDC/2

q

d

b c

a

V

IDC

IDC/2

( 1 ) DCpeak II =DCpea k I3

2I =

Fig. 11: Phase motor connections for the locked rotor torque

test

The measurements have been made at 0.9 rpm in order to have

a negligible production of back EMF.

Fig. 12 shows the torque measurements: trace 2 represents the

DC current (8.7 A), trace 1 is the measured torque (torquemeter

constant = 20).

Fig. 12: 8.7Amps - Torque vs. angular position

The small deformations near the zero crossing are due to a

non-perfect mechanical connection (caused by the cone keys)

and are not caused by a lack of motor symmetry.

The results (Fig.13) allow the determination of the Torque

constant KT=3 Nm/Arms; the value is coherent with the constant

KV measured at no load. In the picture is showed the comparison

between the experimental measurements made with both the

supply configurations described in Fig 11 and the FEM results

obtained for the final prototype configuration.

Torque vs Peak Current

0

5

10

15

20

25

0 2 4 6 8 10 12

Peak Current [A]

Torque[Nm]

MEGA 1.5mm 1.17T (2) (1)

Fig. 13: Torque vs. Peak Current

The d- and q-axes equivalent circuit parameters for the

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MD_AFM motor have been evaluated by performing the

standstill time response test (STR). During the test the motor is

energized with a low DC voltage step using the two supply

configurations shown in Fig.11 with stationary rotor [6]. The

recorded current and voltage response for d-axis is shown in

Fig.14.

Fig. 14: d-axis STR test acquisitions (trace 3: voltage step,

trace 1: current response)

The fitting method applied to the measured currents leads to

the identification of the d- and q-axes inductances and

resistances. For comparison, the inductance has been calculated

using a 3D finite element analysis and the resistance by a

conventional geometric method. The results of the calculations

are 3.65 mH and 1.73 ohms respectively and agree well with the

measured values of 3.4 mH, 1.74 ohms at 20C (average of the

two axis results).

To summarize, in Table I are reported the three values of the

motor parameters:1. the value computed starting from the motor design;

2. the value computed considering the prototype realisation,

that is, taking into account the construction modification

respect to the initial design;

3. the value experimentally determined.

TABLE I:MOTORPARAMETERS

Parameter Computed

(design)

Computed

(prototype)

Experimental

Rphase @ 20C [] 1.73 1.73 1.74

Lphase [mH] 3.75 3.65 3.4

Kv [Vs/rad] 2.02 1.83 1.73

KT [Nm/Arms] 3.5 3.17 3.00

The differences between the two "computed" columns are

basically due to the increased airgap in the prototype respect to

the initial design. The experience of building the prototype has

given the necessary confidence in the mechanical properties of

the material to permit future machines to use the original air-gap

dimension. The last two columns are in satisfactory agreement

showing the validity of the computation method.

C. PERFORMANCE ASSESSMENTThe prototype has been equipped with several temperature

sensors making possible the thermal monitoring of the most

critical parts such as the coils and stator plastic discs. Than the

prototype has been tested in several different loaded conditions

at different speeds; in particular, the rated torque and speed have

been successfully verified.

The measured efficiencies that are lower than those

forecasted; in particular the no load loss is higher than the

computed value are mainly due to constructional problems that

require further investigations.

Finally, the prototype torque density is 1Nm/kg, which is

comparable with the torque density range of conventional

industrial machines. However in making the comparison, the

torque density of the motors for industrial applications must be

de-rated to take into account that in industrial applications there

is usually no constrains on maximum external temperature, and

part of the losses are exchanged through the mechanica

interface. Furthermore, it must be considered that the prototype

does not optimise all the parts of the mechanical structure; the

CAD analysis and the experience gained in the prototype

construction suggest a possible weight reduction of about 30%.

IV. CONCLUSION

A dedicated brushless PM motor with unconventional plastic(quasi) ironless structure for stratospheric aircraft propeller drive

has been manufactured and tested. The experimental results

basically confirm the expected motor characteristics and

performance predicted during the design stage.

These results position the proposed solution (compared with

conventional PM brushless motors in terms of torque density and

efficiency) in the range of premium motors. Further

improvements are expected by extending the use of the advanced

plastic materials. Weight reductions will be possible by

optimising the structural design and by also adopting plastic

materials for the external enclosure and the shaft.

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APPENDIX A

TABLE II:PLASTIC COMPOSITE MATERIALS PROPERTIES

KetronPEEK Glass fibre

Reinforced Ketron

PEEKOverall Chem. Resist. Very Good Very Good

Moisture Absorption Very Good Very Good

Steam Resistance Good Good

Wear Resistance (dry) Very Good Very Good

Specific Weight [kg/dm3] 1.31 1.51

Cont. Service Temperature (250C) (250C)

Heat Deflection

Temperature under Load

(160C) (230C)

Coefficient of Linear

Thermal Expansion [K-1]

5.00E-05 3.00E-05

Thermal Conductivity

[W/mK]

0.25 0.43

Tensile Strength [N/mm2] 110 90

Low temperatureresistance [C] -60 -20

Relative Machinability

(1=easy 10=very difficult)

5 7

REFERENCES

[1] G.Romeo, G.Frulla, E.Cestino, G.Corsino: "HELIPLAT: Design,Aerodynamic and Structural Analysis of Very-Long Endurance Solar

Powered Stratospheric UAV". AIAA JOURNAL OF AIRCRAFT, Vol. 40,No. 6, Nov-Dec 2003

[2] E.Lavagno, R.Gerboni "Energy Subsystem for HeliNet: Hydrogen asStratospheric Application Propellant", Conf. Rec WHEC '02, Montral

(Canada), C1.8.[3] F.Profumo, A.Tenconi, G.Gianolio, M.Cerchio, R.J.Hill-Cottingham,

P.C.Coles, J.F.Eastham, A Plastic Structure Multi-Disc Axial Flux PMMotor, Conf. Rec. IEEE-IAS02, Pittsburgh (USA), pp. 1274 -1280 vol.2

[4] R.J.Hill-Cottingham, P.C.Coles, J.F.Eastham, F.Profumo, A.Tenconi,G.Gianolio, Novel Axial Flux Machine for Aircraft Drive: Design andmodeling IEEE Transactions on Magnetics, Vol. 38, N. 5, September

2002.

[5] Cavagnino, A.; Lazzari, M.; Profumo, F.; Tenconi, A., A comparisonbetween the axial flux and the radial flux structures for PM synchronous

motors, IEEE Transactions on Industry Applications, Volume: 38 Issue: 6,

Nov/Dec 2002, pp: 1517 1524[6] K. Sitapati, R. Krishnan, Performance Comparisons of Radial and Axial

Field, Permanent Magnet, Brushless Machines, Industry Applications,

IEEE Transactions on, Volume: 37 Issue: 5 , Sept.-Oct. 2001Page(s): 1219 1226

[7] R.J.Hill-Cottingham, P.C.Coles, J.F.Eastham, F.Profumo, A.Tenconi,

G.Gianolio, Multi-disc Axial Flux Stratospheric Aircraft Propeller Drive,Conf. Rec. IEEE IAS, '01 Chicago, USA 01, 2001, pp 1634-1639.

[8] Cavagnino, A.; Lazzari, M.; Profumo, F.; Tenconi, A., Axial flux interiorPM synchronous motor: parameters identification and steady-state

performance measurements; Industry Applications, IEEE Transactions on,

Volume: 36 Issue: 6 , Nov/Dec 2000 pp: 1581 1588

[9] S. Huang, J. Luo, F. Leonardi, T. A. Lipo, A Comparison of PowerDensity for Axial Flux Machines Based on General Purpose Sizing

Equations, IEEE Trans. on Energy Conversion, Vol.14, n.5, June 1999

pp.185-192.

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axial flux plastic multi-disc brushless pm motors performance assessment

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