Download Field Weakening of Permanent Magnet Machines

Research Report
2004-13
Field Weakening of Permanent Magnet Machines – Design
Approaches
T.A. Lipo and M. Aydin*
Electrical and Computer Engineering Department
University of Wisconsin - Madison
1415 Engineering Drive
Madison, WI 53706-1691, USA
Email: [email protected]
*Caterpillar Inc.
Technical Center TC-G 855
P.O. Box 1875
Peoria, IL 61656-1875, USA
Email: [email protected]
isconsin
lectric
achines &
ower
lectronics
onsortium
University of Wisconsin-Madison
College of Engineering
Wisconsin Power Electronics Research
Center
2559D Engineering Hall
1415 Engineering Drive
Madison WI 53706-1691
© 2004 Confidential
Field Weakening of Permanent Magnet Machines – Design Approaches
‡
T. A. Lipo and M. Aydin
‡
†
†
Electrical and Computer Engineering Department
University of Wisconsin-Madison
1415 Engineering Drive
Madison, WI 53706-1691, U.S.A
Caterpillar Inc.
Technical Center TC-G 855
P.O. Box 1875
Peoria, IL, 61656-1875, USA
Email: [email protected]
Email: [email protected]
Abstract - Permanent Magnet (PM) machines have been developed for
numerous applications due to their attractive features especially after
the development of NdFeB magnets. However, their complicated
control lets the researchers develop new machine structures with easy
field control. New alternative PM machine topologies with field
weakening or hybrid excitation have been introduced in the literature
for years to eliminate the effects of problems associated with the
cumbersome field weakening techniques used in conventional PM
machines. This paper reviews the field weakening of PM machines
covered from machine’s perspective. Machine structures and features
of each structure are clarified for both radial and axial airgap PM
machines studied thus far.
magnet to return to its original operating point even after the
current is removed [1-2]. Thus, the torque capacity of the
machine is permanently diminished [3-4]. It is obvious that the
attainable speed range is limited by the largest tolerable
demagnetization current specified by the demagnetization
characteristics of the magnets. In addition, the capability of the
converter sets an additional limit to the flux weakening range of
the PM machine.
The search for a means to realize field weakening in PM
machines by eliminating the detrimental effects of d-axis current
injection has been of great interest to machine designers and
new machine structures are currently of great interest. There
presently exist a number of alternative solutions in order to
eliminate this problem in PM machines and the majority of
these solutions have been proposed in the 1990s. Advances in
material technology such as PMs, magnetic steel and powdered
iron composites have allowed researchers to arrive at new
machine configurations. A survey of these flux control capable
PM machine topologies is the subject of this paper.
I. INTRODUCTION
Demand for more compact, efficient and cheaper electric
machines has grown tremendously during the last decade.
Meanwhile, a great progress has been achieved not only in the
development of permanent magnets but in the area of electric
machine design and power electronics as well. Therefore, PM
machines have been drawing more and more attention.
Development of magnet technology has allowed increased
power/torque density and efficiency of the PM machines.
Especially with the use of NdFeB magnets, the PM machines
have reached the highest efficiency and power density levels in
the 90s. They are usually more efficient because of the fact that
field excitation losses are eliminated. In addition, copper losses
in general are reduced in PM machines compared to
conventional machines. In other words, due to lower losses,
heating of the PM machines will be less, which can result either
run the machine at low temperatures or to increase the shaft
power so that the maximum allowable temperature has been
reached. As far as the power electronics is concerned, less
power from the converter is required to deliver the same power
to the machine because of the high efficiency of the PM
machines.
Air gap flux control of PM machines can generally be
accomplished by two means: control techniques and suitable
modification of the machine topology. Conventional PM
machines have a fixed magnet excitation which limits the
drive’s capability and becomes a significant limitation. The
machines are operated at constant volt/hertz operation up to
base speed and constant voltage operation which requires
weakening of the field at higher speeds to extend the speed
range. Above base speed, vector control techniques are typically
used to weaken the air gap flux. However, these techniques
cause large demagnetization current to flow in the machine daxis and results in high losses and demagnetization risk of the
magnets. Furthermore, the magnets may be forced to operate in
the irreversible demagnetization region which could
permanently demagnetize the magnets by not allowing the
II. FLUX WEAKENING OF PM MACHINES
The phasor diagram of a typical PM machine drive is shown
in Fig. 1 at base and high speeds. The equivalent circuit of this
kind of machine comprises the inductance and the back-EMF
voltage which is the product of magnet flux linkage ( m) and the
machine electrical speed ( ). The magnet flux lies along the daxis and the back-EMF phasor which is 90 degree phase
advanced lies along the positive q-axis. The machine torque is
generated both by the magnets and by the saliency and depends
jw1 Ld Id1
VS
VS
jw0 Lq Iq0
IS1
Id1
d-axis
Iq1 Is0 E0
S
N
jw1 Lq Iq1
E1
q-axis
q-axis
d-axis
Fig. 1. Phasor diagram of a PM machine
on the angle between the current phasor and the q-axis. The
current phasor must be aligned with the q-axis in order to obtain
maximum output torque for non-salient machines. As for the
salient pole machines, the current phasor is slightly shifted
towards the d-axis to achieve maximum torque for a given value
of current. At high speeds, flux weakening becomes necessary
since the machine back EMF can cause the stator voltage to
exceed the maximum inverter output voltage. Therefore, the
voltage drop j LdId becomes negative by adding a negative daxis current, which results in reduced total airgap flux and the
excess back-EMF compensation reducing the machine terminal
voltage.
III. REVIEW OF RADIAL AIRGAP PM MACHINES
CAPABLE OF FIELD WEAKENING
The development of relatively low cost rare earth magnets
opened a new era in PM machine design. One relatively early
development thrust was a novel Double Salient Permanent
Magnet (DSPM) machine seen in Fig. 2. DSPM machine
topologies can be realized by introducing high energy magnets
into doubly salient structure of a synchronous reluctance
machine. They are also good examples of flux control in PM
machines. The permanent magnets can be placed either in the
stator or in the rotor. The stator version is illustrated in Fig. 2.
In this case there exist both magnets and field winding in the
stator structure. Such DSPM machines can be used for
adjustable speed drive applications with improved efficiency
and power/torque density. It is one of the true field weakening
PM machine topologies which was developed at the University
of Wisconsin-Madison [5-7]. The stator is formed by laminated
steel, stator windings and high energy NdFeB magnets. Rotor
has a simple laminated structure. The machine flux can be
controlled by adjusting the reluctance path of the PM flux. One
important advantage of the DSPM machine is to utilize the high
energy NdFeB magnets. Required airgap flux can be provided
through this small size and small magnet thickness. In addition,
Stator
windings
Magnet
this structure introduces flux concentration principle. In other
words, airgap flux can be higher than magnet residual flux
density by introducing an increased magnet surface area.
Another type of DSPM machine is illustrated in Fig. 3. In this
case, PMs are introduced by using ferrite magnets on the inner
surface area of the stator and a circumferential DC field winding
is placed in the stator core [8]. Stator and rotor structures are
composed of laminated steel. The DC field winding produces
magnetic flux which is in the same trajectory of the magnet flux.
Flux boosting or weakening can be achieved simply changing
the direction of the current. One important advantage is that the
magnet cost is reduced dramatically in this structure. Also high
airgap flux density can still be obtained through the large
magnet surface area.
Field Winding
Stator
windings
Magnet
+If
A
-C
B
-B
C
-A
Rotor
-If
Inner Sator
Iron
Stator back
iron
Fig. 3. Double salient permanent magnet machine (DSPM 4/6)
capable of field weakening [8]
A different DSPM machine configuration suitable for traction
application is given in Fig. 4. This machine is the inside-out
version of the previous DSPM machine [9]. By reversing the
location of rotor and stator, airgap diameter is increased
resulting in increased torque capability. This type of PM
machines is already in use in the automotive industry.
Rotor
Magnets
Rotor
Stator core
Stator
AC
winding
Fig. 4. Outer rotor double salient permanent magnet machine
(DSPM 8/12) [9]
Fig. 2. Double salient permanent magnet machine with flux
control [5-7]
Another PM machine topology with flux weakening
capability developed at UMIST in the UK is shown in Fig. 5
[10]. In this machine, the rotor structure is composed of two
sections, one of which is surface mounted part and the other is
axially laminated reluctance section, and they are both
connected to the same shaft. The main objective of such a
design is that the two rotor sections can be design independently
so as to acquire a desired ratio of Ld /Lq.
Stator winding
Rotor
magnets
Rotor
Stator
Fig. 5. Two part rotor synchronous PM machine [10]
A new radial flux PM machine with airgap flux weakening is
shown in Fig. 6 [11]. This machine has an annular iron mounted
on the surface of the magnets. There exist four iron sections and
eight flux barriers as seen in the figure. The stator structure is
the same as conventional radial flux PM machine. In this
structure, the control of airgap flux is achieved by applying Id
current, which is not used to lessen the magnet flux but to
modify the flux path. The magnet flux linked by the armature
winding is decreased with this approach while the flux from the
magnets is preserved.
Permanent
magnet
Flux barrier
Stator
Iron
Fig. 6. Cross section of the radial flux PM machine for airgap
flux weakening operation [11]
One of the attractive radial flux PM machine structures with
easy flux weakening feature is the Consequent Pole Permanent
Magnet (CPPM) machine developed at the University of
Wisconsin-Madison [12-13]. The actual machine picture
including a zoomed stator view and the machine view is given
in Fig. 7. The machine stator and rotor have two sections. The
stator is composed of a laminated core, iron yoke and 3 phase
conventional winding. A circumferential DC winding is placed
in the middle of the stator core. The rotor pole is divided into
two sections, one of which has radially magnetized magnet and
the other has laminated iron pole. This machine structure has
several advantages in comparison with conventional PM
machines. Firstly, an easy and a wide range of flux control can
be achieved with this machine using airgap flux control
technique. The ampere-turn requirement of the field winding is
claimed to be low. Secondly, the magnetic configuration of the
machine permits airgap flux control with no demagnetization
risk of the rotor magnets because the control is realized by the
iron pole pieces. Moreover, a simple DC field current control is
used in this machine and there is no need for brushes or slip
rings. However, the extra DC winding reduces the power
density of the machine. The space required for the field winding
increases the machine volume. Additionally, airgap surface
associated with the field winding does not contribute the energy
conversion. Also, 3D flux distribution introduces extra losses.
Fig. 7. CPPM machine structure (Source: J.A.Tapia) [12-13]
A new hybrid electric machine proposed is illustrated in Fig.
8 [14-16]. The PM machine is formed with a stator and rotor
which is composed of two sections called first and second field
magnets. Both field magnets are opposing with the magnet
stator pole with a mechanism for varying a phase of magnetic
pole. The two rotor concept could be applied to any surface
magnet or interior magnet structures. The first field magnets of
the rotor is alternately arranged with opposite magnetic poles
and the second one has the same structure and is capable of
causing relative angular displacement relative to the first one in
order to achieve field weakening. It should be mentioned that
the same concept was proposed in [17] for surface magnet
machine in 1998.
N
N
N
S
N
N
S
S
N
N
N
N
S
S
N
N
S
S
S
N
Fig. 8. Dynamo electric machine (US Patent 6,462,430) [14-16]
In addition to the techniques mentioned above there exists
some mechanical methods to accomplish field control in radial
flux machines. A mechanical technique was introduced in [18].
A brushless PM machine with a fixed radial airgap is operated
to a higher speed than the normal speed by reducing the magnet
strength or average flux per pole. This is achieved by increasing
the amount of axial misalignment of the PM rotor resulting in
providing axial misalignment between the rotor poles and stator
reducing the effective flux over a rotor pole or flux entering the
stator as seen in Fig. 9. An integral constant velocity linear
bearing is used to couple the moveable rotor and fixed position
machine shaft. The constant velocity linear bearing lets the
machine shaft, radial bearing, cooling fan, position encoder and
output coupling remain in a constant position.
Variable
airgap
Magnet
Slotted
Stator
Torque
airgap-1
airgap-2
Shaft
airgap-3
Rotor
stator
Speed
Fig. 10. Axial flux PM machine with variable airgap (a) and torque
speed characteristic of variable airgap AFMs
rotor
Moveable
shaft
magnet
stator
Moveable
shaft
rotor
One of the axial flux machines for flux weakening operation
is developed at the University of Torino in Italy [19]. The
machine structure over two poles is displayed in Fig. 11. This
work deals with the design of a new Axial Flux Interior PM
(AFIPM) machine with flux weakening capability by the use of
soft magnetic materials. The machine is composed of two
slotted stators and a single rotor. The slotted side of the stator
has tape wound core with series connected stator windings. The
rotor structure has axially magnetized magnets, rotor disc and
main and leakage poles. There exist two flux barriers in between
the leakage and main poles. The position and size of the flux
barriers can be designed in such a manner that d-axis and q-axis
stator inductances can satisfy the required torque in the flux
weakening region.
magnet
Fig. 9. Brushless PM machine or alternator with variable axial
rotor/stator alignment to increase speed capability (WO 03/077403
A1) [18]
Leakage pole
Main pole
Rotor disc
S
Stator 1
Stator 2
IV. REVIEW OF AXIAL AIRGAP PM MACHINES
CAPABLE OF FIELD WEAKENING
Axial flux PM machines have drawn a lot of attention for
more than a decade. They provide certain advantages over
conventional PM machines such as higher power/torque density
and efficiency, easily adjustable airgaps, low noise and
vibration levels etc. By the virtue of its structure axial flux
machines can have a variable airgap which may be suitable for
some flux weakening applications such as electric traction.
Axial flux design and rotor-stator arrangement allow the varying
airgap to optimize the machine performance as shown in Fig.
10. This feature affects the machine torque and speed range and
makes this technology promising for many applications
requiring flux weakening. The other important advantage of this
technique is to be able to change the torque constant of the
machine which results in variable rotor and stator losses. This
technique can be applied to double-rotor-single-stator machines
too.
N
Flux
barrier
Fig. 11. Axial flux interior PM synchronous machine realized with
powdered soft magnetic materials [19]
Another interesting axial flux machine with flux control
feature is proposed in [20-21]. This machine uses a field
weakening coil to achieve field weakening by directly
controlling the magnitude and polarity of a DC current of the
field weakening coil. The machine structure and the rotor are
displayed in Fig. 12. The rotor is formed by magnet and iron
pole pieces which are mounted in holes in a non-magnetic rotor
body. The machine has two slotted stators and AC windings,
and each stator has a yoke providing a flux return path. Two
field weakening coils in toroidal form are mounted on a
machine frame as seen in the figure. The coils encircle the shaft
and the frame is made of mild steel in order to provide a flux
path for the DC coils. It should be mentioned that it is not
necessary to control the d-axis or q-axis current components of
the PM machine. In addition, under normal control range,
demagnetization of the magnets is not an issue by any means.
Field coil
Housing
Stator 1
Stator 2
Shaft
alternate north and south iron poles which are made of steel.
The north poles of the disc-1 are located opposite of the north
poles of the second rotor disc which are steel poles. The
excitation of the steel poles is provided by the DC excitation
coil which surrounds the shaft as seen in the figure and is fixed
to the inner side of the stator. NdFeB magnets provide high
magnetic loading and creates a compact design. There exists
ferrous shim under each magnet in order to reduce the interpolar
leakage. The stator is formed by a strip of magnetic steel sheet
and slots are punched by index punching machine. Toroidal
windings are used in the stator slots. The main advantage of this
machine is the capability of the field control via DC field
excitation which is achieved with a low reluctance path through
the rotor discs, the pole pieces and the shaft. It should be
mentioned that the axial length of both stator and rotor is bigger
because of the shim under the magnets and the stator yoke to let
the flux travel in the stator. Also, the loss mechanisms are more
complicated than the conventional and other axial flux PM
machines.
Ferrous
pole
rotor plate
Fig. 12. An axial flux PM machine with direct control of airgap
flux (US Patent 6,057,622) [20-21]
Same principle of DC field coil is applied to another axial
flux PM machine as seen in Fig. 13 [22]. This axial flux
machine comprises two stators and one rotor which has
permanent magnets and pole portions. The magnets in the rotor
generate a first magnetic flux and the consequent rotor poles
generate a second magnetic flux. A field coil, which is mounted
to the housing and located very close to the rotor, is very
effective to vary the second magnetic flux mentioned and
therefore the machine provides a controllable output voltage.
slotted
stator
excitation
coil
shaft
magnet
Fig. 14. A new brushless axial synchronous alternator [23]
Housing
S NS N
Stator 2
Stator 1
Shaft
Field
coil
S NS N
Fig. 13. An axial PM machine with flux control (US Patent
6,373,162) [22]
Fig. 14 shows an axial flux PM brushless synchronous
alternator [23]. This machine combines a variable DC coil
excitation in addition to PM excitation. The rotor has two discs
mounted on a common shaft. Each disc carries magnets and
Recently, a new axial flux PM machine topology with a DC
field winding has been introduced in order to accomplish easy
and inexpensive control at the University of Wisconsin-Madison
[24-26]. This new Field Controlled Axial Flux surface mounted
PM (FCAFPM) machine concept has been proposed not only to
offer a solution to field weakening operation but also to improve
the features of the conventional PM machines by introducing a
new axial flux machine concept with flux weakening capability.
Modifying the multiple-rotor-multiple-stator conventional axial
flux PM structures by adding one or two DC field windings
depending on the machine type to control the airgap flux and
providing a path for the DC flux results in different new axial
flux machines with field control capability. Some of these new
structures are illustrated in Fig. 15. Both NN and NS type
double-rotor-double-stator FCAFPM machine concept are
shown Fig. 15 (b) and (s) while the double-stator-single-rotor
and MULTI stage concepts are displayed in Fig. 15 (d) and (e).
One derivation of the new concept which is called doublerotor-single-stator NS type FCAFPM machine is used as an
example to describe the structure and an actual prototype
machine built and tested is illustrated in Fig. 16.
S
S
S
N
S
S
Inner ring
N
Iron pole
S
N
N
(a)
S
N
(b)
outer ring
N
DC field
(c)
SN
S
NS
N
NS
N
SN
S
(d)
(e)
Fig. 15. 2D views of the FCAFPM machines [24-26]: (a) Singlerotor-single-stator FCAFPM machine, (b) NN type double-rotorsingle-stator FCAFPM machine, (c) NS type double-rotor-singlestator FCAFPM machine, (d) double-stator-single-rotor FCAFPM
machine and (e) MULTI stage FCAFPM machine
The new NS type FCAFPM structure is composed of a two
part tape wound disc type slotted stator structure one
incorporated into another, two rotor discs with axially
magnetized surface mounted magnets and iron pieces mounted
on the rotor surface, two sets of 3 phase AC stator windings and
a DC field winding which is the main difference between the
axial flux PM machine and the new concept FCAFPM machine.
In other words, there exist two sources in the machine: constant
magnet excitation and variable DC field excitation. Excitation
of the DC coil of one polarity tends to increase the consequent
poles on both inner and outer portions of the rotor thus
strengthening the field. Excitation of the DC coil with opposite
polarity decreases the field in the consequent poles in both inner
and outer portions of the rotor disc thereby weakening the
airgap flux. This topology eliminates the demagnetization risk
of the magnets since the DC field Aturns do not directly oppose
the magnet Aturns and airgap flux can be controlled in a wide
range with the FCAFPM machine. More detailed information
about the FCAFPM concepts are provided in [26].
The same flux weakening principle can be applied to singlestator-single-rotor structures seen in and double-stator-singlerotor machines seen in Fig. 17. The new Field Controlled
double-stator-single-rotor Axial Flux Internal Rotor (FC-AFIR)
PM machine has two stators with two sets of 3 phase stator
winding and 2 sets of DC field winding. The basic principle of
the FC-AFIR machine is the same as FCAFPM machine.
S
(a)
Iron pole
N
Iron pole
(b)
(c)
Fig. 16. NN type FCAFPM machine prototype [26]: (a)Disc type
stator structure with both DC field and AC windings, (b) surface
mounted PM disc rotor with iron poles and (c) complete FCAFPM
machine prototype
Permanent
magnets
Stator-2
Iron poles
Epoxy
DC coil-1
Stator-1
Fig. 17. Field controlled axial flux internal rotor external stator
PM machine [26]
The FC-AFIR machine has the same advantages as its tworotor counterpart. Moreover, it offers higher torque per inertia
ratio than two-rotor FCAFPM machine, which makes this
topology attractive for certain applications in addition to its easy
and cheap field control feature. Furthermore, cooling of this
machine is easier due to the more stator surface area. However,
it is expected that the efficiency will be lower because of the
iron losses of the two stators. Gramme type windings are not
suitable for this structure. Therefore, some kind of a lap winding
should be used which results in longer end windings.
V. CONCLUSIONS
[11]
Both radial and axial flux novel PM machines with flux
weakening features reported in the literature have been reviewed
in this paper. Machine structures, features and advantages are
discussed. Finally, a detailed and complete reference section
about the flux weakening PM machines has been provided.
L. Xu, L. Ye, L. Zhen and A. El-Antably, “A new design concept of
permanent magnet machine for flux weakening operation”, IEEE
Transactions on Industry Applications, Vol.31, No.2, March/April
1995, pp.373-378.
[12]
J. A. Tapia, “Development of the consequent pole permanent
magnet machine”, PhD Thesis, University of Wisconsin-Madison,
2002.
[13]
J. A. Tapia, F. Leonardi and T. A. Lipo, “Consequent pole PM
machine with field weakening capability”, IEEE International
Conference on Electrical Machines and Drives, Boston, 2001,
pp.126-131.
[14]
H. J. Kim et al. “Hybrid car and dynamo-electric machine”, United
States Patent, Patent Number: 6,462,430 B1, 2002.
[15]
H. J. Kim et al. “Wind power generation system”, United States
Patent, Patent Number: 6,541,877, 2003.
[16]
H. J. Kim et al. “Hybrid car and dynamo-electric machine”, United
States Patent, Patent Number: 6,577,022 B2, 2003.
[17]
M. Masuzawa et al. “Brushless motor having permanent magnets”,
United States Patent, Patent Number: 5,821,710, 1998.
[18]
P. Lawrence “Brushless PM motor or alternator with variable axial
rotor/stator alignment to increase speed capability” World
Intellectual Property Organization: WO 03/077403 A1, 2003.
[19]
F. Profumo, A. Tenconi, Z. Zhang and A. Cavagnino, “Novel axial
flux interior PM synchronous motor with powdered soft magnetic
material”, IEEE Industry Applications Society Annual Meeting,
1998, pp.152-158.
[20]
John S. Hsu et al. “Direct control of airgap flux in permanent
magnet machines”, United States Patent, Patent Number:
6,057,622, 2000.
[21]
J.S. Hsu, “Direct control of air-gap flux in permanent-magnet
machines”, IEEE Transactions on Energy Conversion, Vol.15 No.4,
Dec.2000, pp.361-365.
[22]
F. Liang, et. al., “Permanent magnet electric machine with flux
control”, United States Patent, Patent Number: 6,373,162 B1, 2002.
[23]
N. L. Brown and L. Haydock, “New brushless synchronous
alternator”, IEE Proceedings of Electric Power Applications,
Vol.150, No.6, November 2003, pp.629-635.
[24]
M. Aydin, S. Huang and T. A. Lipo, “A new axial flux surface
mounted permanent magnet machine capable of field control”, IEEE
Industry Applications Annual Meeting, Oct 2002, pp.1250-1257.
[25]
M. Aydin, S. Huang and T. A. Lipo, “Performance evaluation of an
axial flux consequent pole PM motor using finite element analysis”,
IEEE International Conference on Electrical Machines and Drives,
Madison, WI, 2003.
[26]
M. Aydin, “Axial flux surface mounted PM machines for smooth
torque traction drive applications”, PhD Thesis, University of
Wisconsin-Madison, 2004.
ACKNOWLEDGMENT
The authors would like to thank Wisconsin Electrical
Machines and Power Electronics Consortium (WEMPEC) for
the financial support of this research.
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