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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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