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Field Weakening of

Permanent Magnet Motors -

Some New Possibilities

T. A. Lipo

Prof. Emeritus

University of Wisconsin

Madison Wisconsin

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Outline

• Brief Review of Field Weakening of PM Motors

• Field Weakening by Modification of the Motor Design

• Field Weakening by Modification of the Stator Circuit

• Issues and Opportunities

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Outline

• Brief Review of Field Weakening of PM Motors

• Field Weakening by Modification of the Motor Design

• Field Weakening by Modification of the Stator Circuit

• Issues and Opportunities

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Two major classes of PM synchronous

machines

Relatively low inductance Relatively high inductance

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dq model of a wound field synchronous

machine

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dq equivalent circuits for a PM machine in the

synchronously-rotating reference frame

Magnet acts as a

current source

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dq equivalent circuits for a PM machine in the

synchronously-rotating frame – steady state

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Reduced circuit during steady state

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Steady state – final result

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Vector diagram with lagging power factor at

the air gap – stator d-flux magnetizing

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Vector diagram assuming leading power factor

at the air gap – stator d-flux demagnetizing

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Traction Application Torque vs. Speed Curve

Trated

Torque

Power

Prated

= Trated

Rotor Speed max

(CPSR)

Constant Power Range

HighSpeedRegime

ConstantTorque

Range

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Phasor Diagram for the constant torque

range for Non-salient PM machine

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Phasor diagrams for constant power range with

increasing wE with constant VS and IS when EI > ISXS

e

e

e

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Phasor diagrams for increasing wE with constant VS

and IS when EI < ISXS for Xs = 1.1 and EI = 0.8

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Phasor diagrams for increasing we with

constant Vs & Is when EI = IsXs

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Power for increasing e with constant

VS and IS

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d–axis current for increasing speed with

constant Vs and Is

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Phasor Diagrams for Increasing wE with constant

VS and IS when EI=IS(XS+ XEXT)

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Field Weakening - The Problem

• The ratio of EI/Xs determines the maximum extent of

the field weakening range

• When Irated = EI/Xs the maximum power can be

extracted for all speeds

• Maximum power can be derived by designing the motor

with a proper value of Xs or by somehow achieving

a variable EI

• Well designed machines typically have a minimum

amount of copper and iron leading to a machine in

which Xs is to small to achieve the optimum EI/Xs

• Problem can be solved by reducing EI but at the cost

of enlarging the machine

• What to do??

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Solutions

• Modify the stator winding design (i.e. increase Xs)

Increase leakage by creating deeper stator slots

Increase leakage by running on non-fundamental

component space harmonic (i.e. fractional slot wdg.)

• Modify the stator or rotor of machine to vary EI

Stator Modifications

Rotor Modifications

Mechanical Modifications

• Modify the external circuit powering the PM machine

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Outline

• Brief Review of Field Weakening of PM Motors

• Field Weakening by Modification of the Motor Design

• Field Weakening by Modification of the Stator Circuit

• Issues and Opportunities

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The CPPM Machine (Tapia-Lipo 2002)

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Operating Modes of the CCPM

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SynPM Machine (Luo-Lipo 2000)

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Dual Rotor Machine (Naoe-Fukami 2001)

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DSPM Machine (Chau-Jiang-Wang, 2003)

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Rotor Magnet Adjustment (Zhou et.al. 2010)

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Rotating Iron Segment (Shibukawa 2006)

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Axial Flux Machine with Stator/Rotor Rotation

(Caricchi et.al. 2001&2006)

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Axial Air Gap Adjustment (Nakai et.al.)

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Flux Barrier Adjustment (Baoquan,

Chunyan, & Shukang-2005)

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Outline

• Brief Review of Field Weakening of PM Motors

• Field Weakening by Modification of the Motor Design

• Field Weakening by Modification of the Stator Circuit

• Issues and Opportunities

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a

b

c

a'

b'

c'

CcomVdc

INV1 INV2

Solution#1 Open Winding Machine Drive

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Field weakening of surface magnet PM machine

The concept presented in Lecture #2 can also

be applied to a PM machine

• Adjust the bridge effective reactance to be capacitive

and thus provide unity power factor as viewed

from the machine terminals below rated speed

• Adjust the bridge effective reactance to provide

a d-axis inductive reactance to allow field weakening

to occur above rated speed

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PI

MTPA/

Flux Weakening

Decoupling

Current Reg

a b c

a' b' c'

T(θ)-1

T(θ)

T(θ)-1

Lq_com

Ld_com

PI

ωr*

ωr

Limiter iq*

id*

vq1*

vd1*

θr

θr

θr

va1*

vb1*vc1*

Vdc

iaibic

iq

id

iq id

vqv*

vdv*

vq_com*

vd_com*

vcom* vq2*

vd2*

va2*vb2*vc2*

Vector Rotator

Vector Rotator

Vector Rotator

vcom

Field weakening of surface magnet PM machine-

Control block diagram

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Simulation results – below base speed

0.1 0.15 0.2 0.25 0.3 0.35 0.4-100

-50

0

50

100

time/s

va1,v

a2/V

; ia

/A

ia

va1

va2

•INV1 supply real voltage

•INV2 supplies imaginary voltage

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Simulation results – flux weakening

single inverter drive open-winding drive

Load: 20 Nm

Speed command: 100 rad/s

0.1 0.15 0.2 0.25 0.3 0.35 0.4-80

-60

-40

-20

0

20

40

60

80

time/s

va/V

; ia

/A

ia

va

0.1 0.15 0.2 0.25 0.3 0.35 0.4-80

-60

-40

-20

0

20

40

60

80

time/s

va1,v

a2/V

; ia

/A

ia

va1

va2

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Simulation results – flux weakening

0 0.1 0.2 0.3 0.4 0.50

20

40

60

80

100

time/s

m

/rad/s

single inverter drive open-winding drive

Load: 20 Nm

Speed command: 100 rad/s

0 0.1 0.2 0.3 0.4 0.50

20

40

60

80

100

time/s

m

/rad/s

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Rotor speed for ramp speed command

Dual converter:

Single converter:

Speed reference :

(Mech rad/s)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60

100

200

300

400

rm

t(sec)

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d- and q-axis currents during ramp speed increase

Dual converter:

id:

iq:

Single converter:

iq:

id:

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-30

-25

-20

-15

-10

-5

0

5

id &

iq (

A)

t(sec)

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Field weakening of surface magnet PM machine

Dual converter:

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

100

200

300

400

500

600

t(sec)

Vdc

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Dual converter:

Single converter:

Speed reference:

0 0.1 0.2 0.3 0.4 0.5 0.60

100

200

300

400

t(sec)

Rotor speed for step change in speed command

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Harmonics of open-winding drive

va1

va2

va

•Effective doubled switching frequency compared to

traditional open-winding drive

•The voltage reference assigned to the two inverters

are not 180 degrees apart

va1va2

va

regular INV2 compensation

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Harmonics of open-winding drive

0 0.005 0.01 0.015

-0.5

0

0.5

t

va

0 0.005 0.01 0.015

-0.5

0

0.5

t

va

0 0.5 1 1.5 2

x 104

0

0.2

0.4

0.6

0 0.5 1 1.5 2

x 104

0

0.2

0.4

0.6

0 0.005 0.01 0.015

-0.5

0

0.5

0 0.5 1 1.5 2

x 104

0

0.2

0.4

0.6

Single drive Open-winding 180 deg shift Open-winding 90 deg shift

m = 0.5

Vdc = 1

THD = 0.6574

m1 = 0.5, m2 = 0.5

Vdc1 = 0.5, Vdc2 = 0.5

Phase shift = 180 deg

THD = 0.3747

m1 = 0.5, m2 = 0.5

Vdc1 = 0.5, Vdc2 = 0.5

Phase shift = 90 deg

THD = 0.7696

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Harmonics of open-winding drive as function of

phase shift

80 100 120 140 160 180

0.4

0.5

0.6

0.7

0.8

phase shift

TH

D

open-winding drive

single drive

80 100 120 140 160 1800.2

0.3

0.4

0.5

0.6

0.7

phase shift

TH

D

open-winding drive

single drive

M = 0.5 < = 0.6

INV1:

Vdc = 1

THD = 0.6574

INV2:

M1 = 0.5, M2 = 0.5

Vdc1 = 0.5, Vdc2 = 0.5

INV1:

Vdc = 1

THD = 0.5631

INV2:

M1 = 0.5, M2 = 0.5

Vdc1 = 0.5, Vdc2 = 0.5

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Harmonics of open-winding drive THD as

function of modulation index M

0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

2.5

3

TH

D

m

open-winding

single drive

0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

2.5

TH

D

m

open-winding

single drive

180 degree phase shift 90 degree phase shift

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Adjustable Power Factor PM Machine

Features:

• Effective field weakening of a PM machine can be achieved

for any design (buried or surface magnet)

• Power factor control to unity power factor can be

achieved below rated speed

• Concept can be extended to PM machines operating

from the utility grid

• Concept can also be extended to PM generators

Issues:

• Rating of the auxiliary power converter

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Solution #2 Field weakening by winding switching

Stator winding currents of phase a1 and a2

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Normal operation – current directed into the dots

Thyristors turned on creating neutral

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Thyristors turned off reversing the current in phase a2,b2,c3

High speed operation – current reversed in half of the

three stator windings

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Standard lap winding – q = 2 slots/pole/phase

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Currents reversed in half the windings

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Phasor diagram for operation at 2 pu speed

before the switching event

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Phasor diagram for operation at 2 pu speed

after the switching event

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Currents as a function of speed in constant power region

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Outline

• Brief Review of Field Weakening of PM Motors

• Field Weakening by Modification of the Motor Design

• Field Weakening by Modification of the Stator Circuit

• Issues and Opportunities

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Issues and oppurtinities

• Rating of the inv2 inverter (variable impedance inverter

• Transients involved in changing inv2 controller from

and equivalent capacitor to an equivalent inductor

• What is the best winding switching strategy to extend

constant horsepower the highest?

• How do we design a concentric winding to accomplish

field weakening concept keeping the inverter ratings

equal?

• How serious are the losses resulting from the switched

winding strategy?

• Can the winding switching concept be extended to

fractional slot windings?

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Thank You for Attending!

Thanks to my student Di Pan

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