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2004-26

Soft Magnetic Composites for AC Machines -A Fresh Perspective

**University of WisconsinMadison WI 53706 USAPhone: (608)262-3889

Fax: (608)262-5559E-Mail: rwhite@cae.wisc.edu

WisconsinElectricMachines &PowerElectronicsConsortium

University of Wisconsin-MadisonCollege of Engineering

Wisconsin Power Electronics Research Center2559D Engineering Hall1415 Engineering Drive

Madison, WI 53706-1691

2005 Confidential

Research Report

T.A. Lipo, S.M. Madani*, R. White**

Work performed at WEMPEC sponsored by CPES.

University of WisconsinMadison, WI 53706 USAPhone: (608)262-0287

Fax: (608)262-5559E-Mail: lipo@engr.wisc.edu

http://www.wempec.org/users/lipo

*University of Puerto Rico-MayaguezMayaguez, PR 00681-9042 USA

Phone: (787)464-2087Fax: (787) 831-7564

E-Mail: madani@ece.uprm.edu

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Soft Magnetic Composites for AC Machines -

A Fresh Perspective

T.A. Lipo1, S.M. Madani2, R. White3

1,3University of WisconsinMadison WI 53706 USA

2University of Puerto Rico-MayaguezMayaguez, PR 00681-9042 USA

1Phone (608)262-0287 Fax (608)262-5559 E-Mail: lipo@engr.wisc.edu WWW:http://www.wempec.org/users/lipo2Phone (787)464-2087 Fax (787) 831-7564 E-Mail: madani@ece.uprm.edu3Phone (608)262-3889 Fax (608)262-5559 E-Mail: rwhite@cae.wisc.edu

Abstract - Soft magnetic composites (SMCs) have opened upnew possibilities for the design of both DC and AC machines.Since SMCs are isotropic, machine topologies such as claw

pole machines, transverse flux machines and the like, whichhave three dimensional flux density patterns have attractedthe most attention. Very little effort has gone into improvingthe performance of conventional AC machines such as cylin-drical rotor permanent magnet machines. This paper pro-

poses new options for design of conventional machines whichefficiently utilize the SMC material.

I. INTRODUCTION

Powder metallurgy is a well established process for produc-ing high volumes of magnetically hard material for a numberof applications such as magnets. Continued advances inmaterials research has realized the ability to also producehigh quality material with soft magnetic properties openingup the possibility of competing with steel laminations for asimilar cost [1]. Since this material, termed SMC (soft mag-netic composite), is magnetically isotropic, to date the major-ity of investigators exploring the use of this new materialhave focussed on utilizing it in applications where the lines ofmagnetic flux have or it is useful to add a significant non-pla-nar component. Machines of this type include claw-pole[2][4], transverse flux [3][4], universal motors [5][6] and dc

motors [7]. Attempts to replace conventional laminated per-manent magnet or induction machines by an SMC equivalentare far less researched with only three significant referenceshaving been identified [9][10],[11]. This paper provides someinsight into the design choices employed for these machinesand it is proposed that two phase machines as a possible via-ble alternative to these existing three phase designs.

II. BENEFITSOF SMC MATERIAL

The benefits of replacing conventional laminated machineswith powdered iron composites are considerable and include

Increased copper fill factor (66% vs. the typical 33%),

Essentially unity iron stacking factor,

Potential for reduced air gap length as a result of the tight

tolerances maintained in manufacturing SMC material, Reduced copper volume as a result of increased fill factor

and reduced end winding length,

Reduced copper loss as a result of the reduced copper

volume,

Reduced high frequency tooth ripple losses since the

SMC has essentially no eddy current losses,

The above two items suggest a potential increase in over-

all efficiency,

Modular construction allows the possibility of easy

removal of an individual modular unit for quick repair or

replacement,

Possibility of producing three dimensional flux patterns

in the SMC material,

Reduced axial length-over-end-winding dimension as a

result of the compact end winding, Absence of phase insulation as a result of using non-

overlapping windings,

Potential elimination of the ground wall insulation since

the SMC stator itself acts as an insulator,

No need to stress relieve the stator lamination after

punching and assembling the stack, a relatively costly

and time consuming task, (stress relief is, however,

included as part of the manufacture of the SMC part),

Reduced conducted EMI when machine is used with

inverter supplies since the stator SMC body acts as an

insulator and does not conduct current to ground,

Reduced bearing currents in the presence of PWM wave-forms again because of the use of SMC which acts as

insulation against this type of current flow,

Stator is easily recyclable since the stator can again be

compressed back into powered form with pressure and

the copper windings readily removed.

These advantages are, of course, accompanied by a num-ber of drawbacks. The most significant of these is the rela-tively low relative permeability of roughly 500 for the mostcommonly used SMC material SOMALOY 5001. Other con-cerns include relatively low rupture strength 100 MPa andhigher low frequency iron losses due to its increased hystere-sis loss. The low permeability of this material is most readilyovercome with design of permanent magnet machines since

the magnetic flux produced by permanent magnets are rela-tively insensitive to air gap length. Several new concepts forsuch machines will be introduced in this paper.

III. THREE PHASE PM MOTOR DESIGNS

Conventional AC stator windings typically incorporate eitherlap or concentric windings in which the coil span ranges

1. Hoganas AB, Hoganas, Sweden.

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between 2/3 and unity pole pitch in order to link asmuch flux as possible. While an SMC stator could beconstructed in a similar manner it would, inevitably, beaccompanied by a slot packing factor not far differentthan a laminated stator. Such a machine would almostcertainly be a poor performer since the low permeabil-ity of SMC would probably require a magnetizing cur-rent penalty which could not be overcome without the

use of additional copper. A high packing factor canonly be achieved if individual teeth are directly woundin so-called race-track fashion. Hence, an importantdistinction between conventional and SMC threemachines is the need to wind the coils around individ-ual teeth. This constraint, however, is not without itsdrawbacks.

Figure 1 shows the MMF produced by a simplethree phase winding having a pitch of 60 degrees (oneslot per pole per phase). In a three phase AC machine,the fundamental component of stator MMF per pole is[8]

(1)

whereNis the total number of series connected turns,Iis the peak AC current and P is the number of poles.The winding factor for the fundamental component ofthe MMF can be readily determined from conventionalmachine design formulas as [8]

(2)

where is the coil span in radians, in this case .Clearly a large penalty is encountered when short coilspans must be used. Also six toothed structures mustbe wound for each pair of poles making assembly ofthe stator structure relatively difficult.In this paper thevalue of 0.5NI/P is considered as a reference value of

fundamental ampere turns per pole for wound toothstructures rather than the more familiar value of unityampere turns per pole used for conventional windings.

Designs for practical SMC machines have chosenwider pitch angles in an attempt to reduce constructioncomplexity. References [9] and [10] have reported asix pole and an eight pole permanent magnet machines

which use a 120

o

pitch in order to reduce the numberof toothed elements. The winding layout for this choice

of pitch is given in Figure 2 It can be noted that thenumber of toothed structures per pole pair have beenreduced to only three. Since the winding factor for thefundamental component is normalized to a full pitchwinding with fundamental 4/, the winding factor forthis case can be calculated as

(3)

However, since the number of toothed elements arehalved, the number of ampere turns on the structurescan be doubled, keeping the total amount of active cop-per the same. In this case, the effective value of thewinding factor is

(4)

where Np are the turns per pole. Thus, no benefit isgained in changing the winding span although thenumber of elements has been halved. In addition onecan note the introduction of a significant amount ofsecond harmonic in the MMF waveform.

In another paper by the same group as [9],[10] awinding span of 240o is suggested [12]. This case isillustrated in Figure 3. Here it is interesting to note thatif the magnets are considered as the number of poles of

Fs1

3

2---

4

---

k1NI

P------------

=

k1

2---

sin

1

2---= =

3

Figure 1 MMF of a simple 3 phase PM machine

having one slot per pole per phase and 60o pitch, timeinstant shown when ia = I, ib = ic = I/2.

b -b a -a -c c b -b a -a -c c

nnss

pole pitch

Armature MMF

PM MMF

AngularPosition

AngularPosition

NI/P

NI/2P

Phase a MMF

AngularPosition

NI/P

b -b a -a c -c b -b a -a c -c

nn

nns

ss

s

Total Armature MMF

PM MMF

AngularPosition

AngularPosition

NI/P

-NI/2P

pole pitch

AngularPosition

Phase a MMF2NI/3P

-NI/3P

Figure 2 MMF of a 3 phase PM machine having 1/2

slots per pole per phase and 120o phase belt, timeinstant shown when ia = I, ib = ic = I/2.

k1

4---

2---

2

3---

x( )cos xd

0

6

1

3---

x( )cos xd

6

+ =

0.5=

k1NI P( )

ef f0.25( )2N

pI 0.5N

pI= =( )'

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the machine then the stator produces a subharmonic ofone half the number of poles. Torque is produced ononly half the stator surface of the machine with half themagnets not taking part in energy conversion at thisinstant. If the number of magnets are considered as thefundamental number of poles of the machine then thewinding factor corresponding to those poles involvedin energy conversion is

(5)

Since the number of coils of this case is one fourththe number of coils of the reference case (Figure 1) the

number of ampere turns can be increased by a factor offour. However, since the number of active poles is only1/2 the total number of poles the effective winding fac-tor is

(6)

Thus the 240o coil pitch arrangement is somewhatpoorer than the other two cases.

It is interesting to consider how one can progressbeyond the 0.5 winding factor which seems to be alimit for three phase machines. One possibility is toconstruct three single phase machines connected intandem each with active length 1/3 that of the referencemachine. Since each machine has only a single phasethe coils can now be pitched 180o resulting in a wind-ing factor of unity. For the reference machine with 60coil spans, the MMF available to produce torque isproportional to (3/2)NI*k1 = 3/4NI. If the magnetstrength is the same in all cases this quantity can also

be considered as proportional to the torque so that thetorque in a per unit form is 3/4. The ampere turns avail-able to each of the three single phase machines isNI.Being single phase, each of the three single phasemachines with ampere turnsNIwill produce (1/2)/3=1/6 per unit torque. The total torque produced by thethree machines will therefore be 1/2 per unit which isless than the 3/4 per unit produced by the reference

machine. Hence, playing such tricks with three phasemachines is of no benefit.

IV. SINUSOIDALLY SHAPED POLES

Recall that the flux over a differential surface ofa cylindrical rotor is given by [8]

(7)

where , l, and rare the air gap length, stack lengthand radius at the air gap respectively and is theangular position along the air gap locating the flux ele-ment. In order to produce a sinusoidal variation alongthe air gap it is traditional to vary the number of turns

Nsinusoidally. Alternatively the (effective) gap g or air

gap radius rcan be made to vary sinusoidally resultingin reluctance torque. A third alternative, not normallyconsidered is to make the length l to be sinusoidallyvarying. This is clearly only possible of SMC is used toform the stator since a sinusoidal variation in lengthwould imply using a series of laminations of a continu-ously varying shape. The differential flux becomes

(8)

where denotes the maximum length of the statorstack. If the poles are wound with concentrated coilswith alternating polarities of the same current the dif-ferential flux with then spatially vary sinusoidally. Asketch of the pole arrangement is shown in Figure 4.

Since the net MMF is sinusoidally distributed thewinding factor for this layout is the same as for a per-fect sinusoidally distributed winding,

(9)

A two phase machine can now be constructed byplacing two sets of sinusoidal poles back-to-back asshown in Figure 4(b). One can express the differentialfluxes of the two windings as

(10)

(11)

When averaged over the total length of the machine ,the flux densities of the two stator phases is

Figure 3 MMF of a 3 phase PM machine having 1/4

slots per pole per phase and 240o phase belt, timeinstant shown when ia = I, ib = ic = I/2.

-a c -c b -b a -a

Total Armature MMF

PM MMF

AngularPosition

AngularPosition

NI/P

-NI/2P

pole pitch

AngularPosition

Phase a MMF2NI/3P

-NI/3P

ab -b

k1

4---

1

---

2

3---

x( )cos xd

0

2 3

1

3---

x( )cos xd

23

------

2

+=

3

8------- 0.216= =

k1N

pI( )

ef f

3

8-------

4( )1

2---

Np

I3

4-------N

pI 0.433N

pI= = =

d

d BdA N pIdP

oN

pI

g--------------- lrd( )= = =

g

d

oN

pI

g----------------l

mr sin d( )=

lm

k1 ef f,

4---

2

--- sin d

0

4--- 0.785= = =

da

orl

msin d=

db

orl

mcos d=

ls

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(12)

(13)

If the poles of the machine are consecutively numberedand if and , in poles 1,3,5,..,

p1 of the p pole machine and the negative of thesevalues in poles 2,4,6,...,p, the total effective flux den-sity in the air gap becomes,

. (14)

Hence, the effective air gap flux density created by thismachine is ideally sinusoidal in both space andtime.Here, the effective flux density is defined at theflux density as experienced by a magnet or rotor coil of

active length ls. The machine has only four toothedstructures per pole which compares favorably with thethree phase machines discussed previously. Being atwo phase machine, the amplitude of the flux densityis, as expected, 2/3 that of the three phase machine.

However, with ampere turns NIthe amount of copperused for this machine is also 2/3 that of the three phasemachine. The same flux density and torque can be pro-duced by simply increasing the number of turns of eachpole by 50%.

It is now useful to compare the winding factor forthis machine versus the three phase machines. Neglect-

ing the small spacing between sets of poles (as hasbeen assumed up to this point for the other windingpatterns) so that the poles of the two phases are justtouching, the overall length of this machine is

. Normalizing to the same active length the effec-tive ampere turns become

(15)

Thus, the two phase sinusoidal pole machine is capableof producing 11% more flux linkage for the sameamount of active copper, and thus at least 11% moretorque than any of the three phase machines of SectionIII.

It should be mentioned here that the major issue

with this type of structure lies not in torque ripple(which is ideally zero) but in lateral (axial) forces pro-duced when the alternate sides of the machine producethe rotating flux density. It would be necessary to solvethis problem before such a machine can be successfullyapplied.

V. TWO PHASE MACHINESWITHTRAPEZOIDALLYSHAPEDPOLES

Other two phase pole shapes have been developedwhich can improve the fundamental winding factor. Itis useful to also consider the effectiveness of a trape-zoidally shaped pole as shown in Figure 5. Because thetwo phases are interdigitated, the top and bottom sidesof the trapezoid are constrained such that the sum of

the top and bottom lengths is one pole pitch. Thuswhen = 0, the poles form triangular poles while if = /2, rectangular poles are formed. Figure 6 showshow the fundamental and third harmonic winding fac-tors vary with . Since the machine is two phase, thethird harmonic represents the lowest undesirable com-ponent, (a positive sequence component in a two phasesystem). The value ofk1 reaches a maximum near =60o but little change is observed over the range 0