High-field Self-starting Permanent-magnet Synchronous Motor - 04643350 - 1981

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High-field self-starting perm anent-magnet

synchronous motor

K.J. Binns, B.Sc, D.Sc, C.Eng., A.F.I.M .A., M.I.E.E., and M.A. Jabbar, B.Sc.(Eng.), Ph.D.

Indexing terms: Magnetic

ields,

Permanent

magnets,

Synchronous motors

Abstract:

A

number

of

configurations

of

self-starting permanent-magnet synchronous motor have been

developed in recent years. This paper describes a form of motor having a high efficiency/power-factor pro-

duct. Use can be made of ferrite, Alnico or rare-earth magnets. The machine is particularly suitable for

variable-speed drives involving the combination of an inverter and one or more synchronous motors, where

the speed of rotation needs to be accurately determed. The characteristics of the m achine are discussed an d

:

results indicating the performance are presented.

1 Introduction

Over recent years, number of forms of self-starting permanent

magnet machine have been developed [ 1 , 2 ] . They mainly use

ferrite or rare-earth magnets and they start by induction

motor action using permanent magnets to provide the syn-

chronous torque. Some use a com bination of reluctance and

permanent-magnet/reluctance motors give good pull-in charac-

teristics with low starting current

but do not

give

the

highest

values of efficiency/power-factor pro duc t at synchronism. The

teristics with low starting current but do not give the highest

values of efficiency-power factor produ ct at synchronism. The

optimum combination depends on the number of motors

supplied from each inverter.

If

a single motor is used

for

each

inverter, the starting characteristics dominate the maximum

kVA demand. If many motors are used for one inverter and

one or two are started while the remainder are synchronised,

the dominant factor tends to be the efficiency/power-factor

product

of

the synchronised machines.

If

the starting current

of each machine is excessive, some inverter manufacturers are

prepared to start motors on a ramp function of frequency

against time. This must surely involve extra cost. For some

applications,

for

example the processing of man-made fibres, in-

verters control the speed

of

a number

of

self-starting synchron-

ous motors to a very high degree of precision. The inverter

cost tends to be considerably higher than that of the motors.

There is also an energy saving in using motors having a high

efficiency.

This paper describes

a

particular family

of

motors which

can make use of any of the well known permanent-magnet

materials, including rare-earth magnets. The choice depends

mainly on the initial cost of motor and inverter for a given

duty and to some extent on the running cost. If a rare-earth

samarium-cobalt magnet

is

used, power factors

in

excess

of

90

are achievable for 3-phase mach ines.

The demagnetisation characteristics

of

some permanent-

magnet materials are shown

in

Table 1.

In

making use

of

par-

Table : Properties of some commonly available permanent magnet

materials approxim ate values)

Material and type

H

c

BH),

High remanence Alnico

High coercivity Alnico

High remanence ferrite

High coercivity ferrite

MnAIC

Polymer-bonded rare earth

Sm-Co

5

R

2

Co

17

T

1.32

0.88

0.38

0.34

0.58

0.55

0.88

1.10

kA/m

56

120

135

23 0

1 90

4 0 0

6 4 0

537

kJ/r

4 8

4 0

26

22

4 8

5 5

150

2 40

Paper 1293B, first received 7th November 1980 and in revised form

24th February 1981

The authors are with the Department of Electrical Engineering, The

University, Southampton, Hants. SO9 5NH, England

ticular material, one has to balance the performance achieved

against the total cost of the m achine and its inverter supply,

if necessary. There is no overall optimum. An expensive

magnet

may

produce

a

high overall performance

but its use

must be economical for the particular application. Com-

puter programs have been developed which enable the assess-

ment of the relative cost of different designs for a particular

duty. In the long term, the price of high-energy magnets may

reduce

in

comparison with

the

cost

of the

remainder

of

the

machine and its control. The results presented in this paper

should serve to indicate what may be achieved with the

particular design to be discussed.

For any design, demagnetisation must not occur during

normal runnings or fault conditions. The cost of remagnetis-

ation, including

the

loss

of

use

of

the drive system

is

almost

always prohibitive.

2 Machine configuration

A sketch of the rotor lamination is shown (not to scale) in

Fig. 1 for a 4-pole design. The flux passing from each r otor

pole emerges from

the

sides

of two

magnets which have

nonradial axes; some flux

can

also pass under

the

magnets,

X / _

Fig. 1 Schematic diagram of geometry of typical rotor configuration

1^. direction of magnetisation

a Magnet

b Rotor iron

c Cage bars

d Conducting material

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157


2/4

giving an additional path for induction motor action at starting.

Some of the flux from the magnets can pass through this thin

magnetic bridge,

but

this bridge

is

normally

in

saturation.

The

pole-arc to pole-pitch ratio is considerably smaller than has

been used in some designs. The width of the iron bridge below

the magnet has to be carefully chosen; it also serves to give

increased mechanical strength to the rotor, thereby raising

the possible maximum running speed. A cage winding is in-

corporated

in

slots

in the

rotor poles

and the

region

to the

side of the pole also containing conducting material con-

nected

to the end

rings.

3 Flux distribution

in the

rotor

In order to achieve a good synchronous performance, it is

essential to be able to design on the basis of computation of

the flux distribution, at least under steady-state conditions,

for the range of possible rotor configurations. The field com-

putation under steady-state conditions can be achieved by

discretising only a pole pitch of the machine as shown by

broken lines in Fig. 1. Load conditions can be simulated by

invoking symmetry and periodicity conditions. In this par-

ticular geometry, the effective pole centre lies at an angle of

64 degrees from the line OX. Fig. 2 shows a flux distribution

for strontium-ferrite magnets

and Fig. 3

gives

a

flux distri-

bution for samarium-cobalt magnets. The flux passing be-

tween adjacent lines is the same in the two field maps. A

computer program prepared

at

Southampton

was

used

in

Fig.

2

Flux map of permanent-magnet machine under load, using

ferrite magnets (Feroba III

the computation of these flux distributions. Results shown

in Figs. 2 and 3 are at conditions when the machine is under

load. Because of the difference of the magnet excitation, the

airgap flux axis has moved from the pole axis by 30 degrees

in Fig. 2 and 19 degrees in Fig. 3.

From the results of such a program, it is possible to deter-

mine the output torque for any given set of dimensional

paramaters, the demagnetisation properties of the magnet and

th e B/H characteristics of the rotor iron. The airgap flux

distribution can be computed by a method described earlier

[3 ,

4] and is shown in Fig. 4 for three magnetic materials —

ferrite, Alnico and rare earth

—

for the no-load co ndition. The

operating characteristics are those for a high value of load

angle.

40 60 80 100 120

airgap positions degrees

140 160 180

Fig. 4 Airgap flux distribution at no-load condition

a Sm-Co

5

b Ferrite

c Alnico

4 Exp erimen tal results

4.1 erformance

at 5

Hz

Tests were carried out on a development prototype using a

standard induction motor stator supplied initially at 50 Hz.

The synchronous performance is indicated in Table 2 for a

nominal rated voltage. The rotor length is 8.2 cm and its

diameter is 9.3 cm. The magnets used were of samarium-

cobalt. It is seen that, at low loads, the power factor is of the

order of 0.96, falling to 0.91 at a load corresponding to 70

of the pull-out torque. The efficiency is of the order of 85

over a wide range of loading and the pull-out torque is 2.6 kW.

This machine is not an optimal design, but represents a par-

ticular practical design. The balance of output parameters can

be varied, depending on the requirements of particular appli-

cations. The choice of supply voltage level (or stator winding

turns) is clearly an important one for any particular application.

Table 3 shows the performance parameters for a voltage of

0.96 p.u.

and

Table

4

shows

the

same

for a

voltage

of 1.04

p.u.

Table 2: Synchronous performance at nominal rated volts at 5 Hz

Fig.

3 Flux map of per manent-m agnet mach ine under load using

rare earth magnets (Sm-CoJ

Current

A

0.84

1.10

1.48

1.82

2.39

2.89

3.36

3.91

4.72

6.00

7.84

10.20

/ „

=

0.6

Output

W

9 9

2 48

41 3

5 5 5

7 59

9 4 0

1121

1278

1514

1841

2206

2521

Power

factor

0.690

0.880

0.940

0.956

0.960

0.964

0.975

0.950

0.940

0.910

0.880

0.871

Efficiency

0.426

0.644

0.744

0.793

0.834

0.847

0.859

0.864

0.857

0.848

0.803

0.712

Pull-out =

Power facto r X

efficiency

0.294

0.567

0.699

0.758

0.801

0.817

0.838

0.821

0.806

0.772

0.707

0.620

2595 W

15 8

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Results show significant changes in efficiency, power factor,

no-load current and pull-out torque for relatively small changes

in supply voltage. It is not possible, of course, to control the

excitation level of the permanent magnets, and so the applied

voltage has a dominant effect on the operating power factor

as well as on the pull-out torque.

Table 3 : Synchronous performance at 0.96 p.u. rated volts at 5 0 Hz

Current

Output

Power

factor

Efficiency

Power factor X

efficiency

A

1.27

1.53

2.00

2.55

3.12

3.88

4.56

5.60

6.85

8.96

W

102

278

489

728

943

1212

1420

1710

1986

2316

0.512

0.712

0.820

0.911

0.934

0.954

0.952

0.951

0.935

0.887

0.433

0.670

0.782

0.823

0.850

0.859

0.858

0.843

0.814

0.765

/„ = 1.07 A

0.222

0.477

0.641

0.750

0.794

0.819

0.817

0.802

0.761

0.679

Pullout = 2500

W

Table 4: Synchronous performance at 1.04 p.u. rated volts at 5 0 Hz

Current

Output

Power

factor

Efficiency Power factor X

efficiency

A

1.01

1.34

1.93

2.56

3.28

4.10

4.76

6.00

7.82

9.87

/„ = 1.07 A

W

114

249

558

806

1058

1333

1533

1871

2245

2552

0.609

0.826

0.885

0.907

0.902

0.907

0.900

0.894

0.861

0.822

0.440

0.707

0.786

0.836

0.860

0.863

0.861

0.839

0.802

0.739

Pu

0.272

0.584

0.696

0.758

0.776

0.783

0.775

0.750

0.691

0.607

= 2784W

4.2 Operation over

a

range of frequencies

For variable-speed applications, it is important that a machine

performs well over the required range of frequencies. Tests

have been carried out over a range of frequencies, showing

good results and stable operation. Tables 5 to 10 show the

results of tests at different frequencies and m ain flux levels.

The results of test at 3 0 Hz for per-unit vo ltage of 0.94, 1.00

and 1.06 are shown in Tables 5 to 7. Comparative results at

75 Hz are shown in Tables 8 to

10.

It

is

apparent that the power

factor and efficiency of the machine remain high over this

frequency range. At 75 Hz, the supp ly voltage is increased

prop ortio nally . However, at 30 Hz the voltage is increased by

8.7 over such a value. This is necessary to keep the flux

level constant. Extensive experimentation has been done on

hybrid machines and an experimental relationship established

[4].

/„ is the noload current shown in Tables 2 to 8. In some

of the Tables it can be seen that /„ is larger than the first load

current. This happens when the motor is overexcited and

operating at a leading power factor.

Table 11 summarises the results at 70 of pull-out to rque

for three flux levels in each case. It is clear that the power

factor is critically dependent on the supply voltage. The

efficiency increases w ith frequency over this range, but has an

optimum related to flux level. Depending on the choice of

lamination, a supply frequency will be reached at which the

efficiency starts to fall owing to sta tor core loss.

5 Effects of cage win ding

on output

The design of the cage winding is important, since both the

synchronous and the asynchronous performance are affected

by the choice of the cage winding.

Table 5: Synchronous performance at 0.94 p .u. rated volts at 3 0 Hz

Current

Output

Power

factor


efficiency

A

1.22

2.05

2.88

3.83

4.77

6.03

7.40

W

204

393

581

779

958

1128

1326

0.896

0.935

0.967

0.980

0.986

0.954

0.967

0.770

0.844

0.860

0.856

0.840

0.810

0.764

/„ = 0.59 A

0.690

0.789

0.832

0.839

0.828

0.773

0.739

Pull-out = 1525W


Current

Output

Power

factor


efficiency

A

1.46

2.02

2.75

3.57

4.49

5.57

6.90

8.70

W

212

401

582

774

963

1157

1346

1534

0.778

0.934

0.966

0.965

0.969

0.967

0.948

0.938

0.717

0.819

0.847

0.865

0.853

0.826

0.791

0.724

/„ = 1.15 A

0.558

0.765

0.818

0.835

0.827

0.799

0.750

0.679

Pull-out = 1671 W

Table 7: Synchronous performance at 1.06 p.u. volts at 3 0 Hz

Current Output Power

factor


efficiency

A

2.46

2.61

3.07

3.70

4.50

5.48

6.65

8.06

10.00

W

218

407

596

784

973

1162

1346

1534

1723

0.484

0.719

0.840

0.902

0.918

0.928

0.922

0.916

0.908

0.660

0.782

0.833

0.848

0.850

0.824

0.791

0.750

0.685

/ „ = 2.62 A

0.319

0.562

0.700

0.765

0.780

0.765

0.729

0.687

0.622

Pull-out = 1812

W

Table 8: Synchronous performance at 0.94

p.u.

rated volts at 75 Hz

Current

Output Power

factor


efficiency

A

1.68

2.42

3.20

4.08

5.02

6.10

W

502

997

1469

1940

2436

2907

0.797

0.915

0.954

0.971

0.974

0.961

0.656

0.788

0.842

0.857

0.871

0.868

0.523

0.721

0.803

0.832

0.848

0.834

/„ - 0.98

A

Pull-out = 3 437 W


Current Output Power

factor


efficiency

A

1.42

2.25

3.07

3.92

4.84

5.95

7.15

/„

=

0.66

A

W

800

1295

1795

2235

2790

3400

4030

0.929

0.949

0.964

0.941

0.951

0.943

0.930

0.642

0.770

0.812

0.850

0.865

0.855

0.833

Pull out =

0.596

0.731

0.783

0.800

0.823

0.806

0.775

3804 W

IEE PROC, Vol. 128, Pt. B, No. 3, MA Y1981

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Table 10: Synchronous performance at 1.06 p.u. rated volts at 75 Hz

Current

Output Power

factor


efficiency

A

1.87

2.45

3.15

4.00

4.84

5.87

7.00

8.50

W

5 0 2

9 9 7

1445

1964

2412

2931

3379

3874

0.693

0.838

0.884

0.905

0.906

0.908

0.898

0.871

0.604

0.758

0.810

0.850

0.860

0.856

0.840

0.816

0.419

0.635

0.716

0.769

0.779

0.777

0.754

0.711

/„ = 1.50

Table 11

Frequency

Hz

30

50

75

Pul lout = 4323W

: Synchronous performance at 70 pull-out power over a

range of frequencies

Voltage

p.u.

0.94

1.00

1.06

0.96

1.00

1.04

0.94

1.00

1.06

Current Power

A factor

5.59

5.56

6.16

5.87

5.98

6.477

5.00

5.40

6.11

0.965

0.960

0.925

0.948

0.910

0.885

0.974

0.955

0.906

Efficiency

0.821

0.824

0.805

0.827

0.840

0.821

0.871

0.860

0.853

Power factor X

efficiency

0.792

0.790

0.745

0.784

0.764

0.727

0.848

0.821

0.772

Table 12 : Effects o f changes in the size of cage bars on pull-out torque

a function of pole arc and magnet material*

Pole arc

Size

of

cage bars

- 1 0

Standard

design

+ 10

Magnet

Type

- 1 0

Fe- l l l

5.95

5.82

5.75

Standard design

RE Fe -ll l RE

15.17 6.30 15.95

14.45 6.

14.15 6.

.23 15.70

.15 15.44

i + 10

Fe- l l l RE

6.15 16.40

6.10 16.18

6.03 15.96

*The standard design represents a reasonable balance between starting

and running conditions

Being a high-field machine, the accelerating torque due to

the cage is opposed by the generation of a speed-dependent

EMF due to the magnets [5]. The voltage level needs to be

sufficiently high to pull the machine into synchronism in

many applications, but if the voltage exceeds the back EMF

by a significant amount, the power factor starts to fall. The

output from a given frame size increases rapidly with supply

voltage, as does the maximum demand from the inverter.

The design of the cage winding on the rotor presents some

interesting problems. When rare-earth magnets are used, the

flux density in the rotor poles is such that the iron operates

in the nonlinear regime. As the area devoted to the cage in-

creases, the rotor flux is reduced and, for a design having an

effective cage winding, a balance between good ind uctio n

motor action and high synchronous torque has to be achieved.

Table 12 shows the variation in maximum torque at synchron-

ism as the area of the rotor slots is varied for both ferrite and

rare-earth magnets.

The torque is clearly dependent of the pole arc since the

level of saturation depends on the iron area for a given magnet

flux. It is apparent that, as the pole arc decreases, the cage de-

sign becomes more critical.

It is also important for good asynchronous torque that the

cage bars are positioned at an appropriate depth below the sur-

face.

In this particular design, the depth at which the con-

ducting bars are buried also affects the available space for the

synchronous flux and so changes the saturation level in that

part of the rotor.

6 Conclusions

The configuration of a high-field perm anent magnet synchro n-

ous motor is discussed. Performance characteristics for a

particular machine are presented and an assessment of the

problems encountered in designing such machines is given. The

balance between synchronous and asynchronous performance

characteristics is sensitive to fairly small changes in design

parameters. A computer program has been developed for the

computation of the synchronous performance. The efficiency/

power-factor product can be high for such machines compared

with that achieved using an induction motor of the same frame

size.

7 Acknowledgments

The authors wish to thank the UK Science Research Council

for financial support for this work, and also to acknowledge

the fruitful collabo ration with W alter Jones and Co. in the

development of permanent-magnet machines.

8 References

1 BINNS, K.J., BA RNARD, W .R., and JABBAR, M.A.: 'Hybrid

permanent-magnet synchronous motors', Proc. IEE, 1978, 125,

(3), pp. 203-2 08

2 SIEMENS, A.G.: 'An electric machine having permanent m agnets

mounted in the rotor between its pole segments', British Patent

1177247

3 BINNS, K.J., JABBA R, M.A., and BARNARD, W.R.: 'Comp utation

of the magnetic field of permanent magnets in iron cores', ibid.,

1975,

122, (12), pp. 1 377-138 1

4 JABBAR, M.A.: 'Analysis of the performance of a perman ent-

magnet a.c. machine'. Ph.D thesis, Southampton University, 1977,

p.

268

5 HONSINGER, V.B.: 'Permanent-magnet machines: asynchro nous

operation', IEEE Trans., 1980,

PAS-99,

pp. 1 503-1509

16 0

IEE PROC,

Vol.

128, Pt. B, No. 3, MA Y 1981

High-field Self-starting Permanent-magnet Synchronous Motor - 04643350 - 1981

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