U.S. patent application number 13/748382 was filed with the patent office on 2014-01-09 for electric motor assisted turbocharger.
This patent application is currently assigned to Holset Engineering Company, Limited. The applicant listed for this patent is Holset Engineering Company, Limited. Invention is credited to Spooner Edward.
Application Number | 20140010669 13/748382 |
Document ID | / |
Family ID | 9952187 |
Filed Date | 2014-01-09 |
United States Patent
Application |
20140010669 |
Kind Code |
A1 |
Edward; Spooner |
January 9, 2014 |
ELECTRIC MOTOR ASSISTED TURBOCHARGER
Abstract
A turbocharger comprises a turbine wheel and a compressor wheel
mounted to a turbocharger shaft. An electric induction motor is
provided for assisting rotation of the compressor wheel in
predetermined circumstances. The motor comprises a fixed stator
having motor field coils which generate a rotating magnetic field
when energised by an AC control signal which induces eddy current
flow in a rotor to generate a rotor magnetic field which in turn
interacts with the stator magnetic field producing torque in the
rotor.
Inventors: |
Edward; Spooner; (Crook,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Holset Engineering Company, Limited; |
|
|
US |
|
|
Assignee: |
Holset Engineering Company,
Limited
Huddersfield
GB
|
Family ID: |
9952187 |
Appl. No.: |
13/748382 |
Filed: |
January 23, 2013 |
Current U.S.
Class: |
417/44.1 ;
417/420 |
Current CPC
Class: |
H02K 17/02 20130101;
H02K 7/14 20130101; F02B 39/10 20130101; F04D 13/06 20130101; F02B
37/025 20130101; Y02T 10/12 20130101; Y02T 10/144 20130101; F01D
15/10 20130101; F02B 37/10 20130101; F05D 2220/40 20130101 |
Class at
Publication: |
417/44.1 ;
417/420 |
International
Class: |
F04D 13/06 20060101
F04D013/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2003 |
GB |
0302235.7 |
Claims
1. A turbocharger comprising: a turbine wheel and a compressor
wheel mounted to a turbocharger shaft; an electric induction motor
for assisting rotation of the compressor wheel in predetermined
circumstances, the motor comprising a fixed stator having an array
of motor field coils which generate a rotating magnetic field when
energized by an AC control signal which induces eddy current flow
paths in a rotor in a plane generally perpendicular to the rotating
magnetic field, wherein the eddy current flow generates a rotor
magnetic field which interacts with the rotating magnetic field
producing torque in the rotor.
2. A turbocharger according to claim 1, wherein the rotor does not
include any closed conductor paths within which an electric current
circulates.
3. A turbocharger according to claim 1, wherein the rotor is
non-laminated.
4. A turbocharger according to claim 1, wherein the rotor is a
solid rotor.
5. A turbocharger according to claim 1, wherein the rotor is a
unitary member.
6. A turbocharger according to claim 1, wherein the rotor is
constructed from a ferromagnetic material.
7. A turbocharger according to claim 1, wherein the control signal
is a fixed frequency AC control signal such that the motor has a
predetermined synchronous speed.
8. A turbocharger according to claim 1, wherein the motor is
configured as an axial flux motor comprising an annular field coil
array positioned adjacent a generally disc shaped rotor.
9. A turbocharger according to claim 8, wherein stator coils are
adjacent opposing axial surfaces of the rotor.
10. A turbocharger according to claim 9, wherein the maximum
diameter of the rotor is smaller than the outer diameter of the
annular coil array such that the eddy current flow paths from one
axial side of the rotor to the other across the peripheral edge of
the rotor.
11. A turbocharger according to claim 10, wherein the rotor tapers
radially towards its outer peripheral edge.
12. A turbocharger according to claim 1, wherein the motor is
configured as a radial flux motor comprising a generally
cylindrical rotor surrounded by an annular stator.
13. A turbocharger according to claim 1, wherein the rotor is
mounted on the turbocharger shaft.
14. A turbocharger according to claim 1, comprising a bearing
housing located between the turbine wheel and compressor wheel and
housing bearings on which the turbocharger shaft rotates, wherein
the electric motor is located within the bearing housing.
15. A turbocharger according to claim 14, wherein the rotor is
mounted on the turbocharger shaft between bearing assemblies
located towards the compressor and turbine ends of the bearing
housing respectively.
16. An AC induction motor comprising a stator having motor field
coils which generate a rotating magnetic field when energized by an
AC control signal which induces electric current flow in a rotor to
generate a rotor magnetic field which interacts with the stator
magnetic field to rotate said rotor about an axis, the electric
flow induced in the rotor which generates said rotor magnetic field
comprising eddy currents, and wherein the field coils are arranged
in an annular array adjacent first and second axial surfaces of the
rotor and the maximum diameter of the rotor is smaller than the
outer diameter of the annular coil array such that eddy currents
are induced in the rotor which flow from one axial side of the
rotor to the other across the peripheral edge of the rotor.
17. An AC induction motor according to claim 16, wherein the rotor
tapers radially towards its outer peripheral edge.
18. An AC induction motor according to claim 16, wherein the rotor
does not induce any closed conductor paths within which an electric
current circulates.
19. An AC induction motor according to claim 16, wherein the rotor
is non-laminated.
20. An AC induction motor according to claim 16, wherein the rotor
is a solid rotor.
21. An AC induction motor according to claim 16, wherein the rotor
is a unitary member.
22. An AC induction motor according to claim 16, wherein the rotor
is constructed from a ferromagnetic material.
23. An AC induction motor according to claim 16, wherein the motor
field coils are energised energized by a fixed frequency AC control
signal such that the motor has a predetermined synchronous
speed.
24. A method, comprising: rotating a turbine wheel, a compressor
wheel, and a rotor mounted to a turbocharger shaft relative to a
stator, the stator including a number of field coils each wound
around a respective one of a number of pole pieces; providing an AC
control signal to the stator; generating a rotating magnetic field
with the stator in response to the AC control signal; inducing eddy
current flow in the rotor, the eddy current flow circulating in a
plane approximately perpendicular to the rotating magnetic field to
provide a rotor magnetic field; and in response to the rotor
magnetic field, causing the rotor to rotate.
25. A method according to claim 24, wherein the rotor is generally
cylindrical shaped and the fixed stator is radial relative to the
rotor and method shaft to form a radial flux motor
configuration.
26. A method according to claim 24, wherein the rotor is generally
disc shaped and the field coils of the fixed stator are arranged in
an axial direction relative to the turbocharger shaft such that the
field coils form an annular array adjacent the disc shaped rotor to
thereby form an axial flux type motor configuration.
27. A method according to claim 24, wherein the providing of the AC
control signal is at a generally fixed frequency.
28. A method according to claim 24, which includes turning the
rotor at a predetermined synchronous speed in response to the AC
control signal.
29. A method according to claim 24, which includes flowing the eddy
current flow from one axial side of the rotor to another across a
peripheral edge of the rotor.
30. A method according to claim 29, wherein the rotor tapers
inwardly towards an outer peripheral edge.
31. A method according to claim 24, wherein the rotor comprises a
solid non-laminated rotor constructed from a ferromagnetic
material.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/927,913 filed Nov. 29, 2010 and now
abandoned, which is a continuation of U.S. patent application Ser.
No. 11/894,265 filed Aug. 20, 2007 and now abandoned, which is a
continuation of U.S. patent application Ser. No. 10/768,358 filed
Jan. 30, 2004 and issued as U.S. Pat. No. 7,296,409 which claims
priority to British Patent Application No. 0302235.7 filed Jan. 31,
2003, the entire contents of each application hereby being
incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an electric motor assisted
turbocharger, and in particular to a turbocharger for an internal
combustion engine.
[0003] Turbochargers are well known devices for supplying air to
the intake of an internal combustion engine at pressures above
atmospheric (boost pressures). A conventional turbocharger
essentially comprises an exhaust gas driven turbine wheel mounted
on a rotatable shaft within a turbine housing. Rotation of the
turbine wheel rotates a compressor wheel mounted on the other end
of the shaft within a compressor housing. The compressor wheel
delivers compressed air to the engine intake manifold. The
turbocharger shaft is conventionally supported by journal and
thrust bearings, including appropriate lubricating systems, located
within a central bearing housing connected between the turbine and
compressor wheel housing.
[0004] In known turbochargers, the turbine stage comprises a
turbine chamber within which the turbine wheel is mounted, an
annular inlet passageway arranged around the turbine chamber, an
inlet arranged around the inlet passageway, and an outlet
passageway extending from the turbine chamber. The passageways and
chambers communicate such that pressurised exhaust gas admitted to
the inlet chamber flows through the inlet passageway to the outlet
passageway via the turbine chamber and rotates the turbine
wheel.
[0005] Under steady state conditions of engine speed and load a
conventional turbocharger can supply the required amount of air to
the engine for efficient combustion. However, there are other
conditions, such as at engine start up or transient conditions such
as a sudden requirement for a high load from the engine, in which
the energy in the exhaust gas is not sufficient to enable the
turbocharger to deliver the required air supply to the engine
quickly enough. Modern engines are designed to reduce engine
fuelling in such circumstances to avoid high levels of exhaust
emissions through incomplete combustion, and accordingly engine
response suffers during such transient or other conditions.
[0006] It is known to address the above problem by providing a
turbocharger with an integral electric motor to assist rotation of
the compressor to improve the response of the turbocharger and thus
the engine performance. An example of such an electric motor
assisted turbocharger is disclosed in U.S. Pat. No. 5,604,045. The
electric motor is essentially a synchronous motor located within
the turbocharger bearing housing, and comprising a magnetic rotor
assembly mounted to the turbocharger shaft surrounded by a fixed
stator comprising field coils wound on magnetically permeable pole
pieces. The operation of the synchronous motor is essentially
conventional in that the field coils are energised with an AC
supply to create a rotating magnetic field around the shaft which
couples with the magnetic field of the magnetic rotor. The motor
may be energised whenever the turbocharger requires power
assistance to ensure optimum air supply to the engine.
[0007] With the above known form of electric motor assisted
turbocharger, a synchronous motor comprising a magnetic rotor is
used to avoid the need for commutation. A disadvantage of the
synchronous motor is that relatively complicated control
electronics are required as the excitation frequency of the stator
coils must always be matched to the rotational speed of the
turbocharger, so that both a variable frequency control signal and
means for monitoring the speed of the turbocharger are required.
See for instance the control system disclosed in PCT patent
application WO98/16728.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to obviate or
mitigate the above disadvantages.
[0009] According to a first aspect of the present invention there
is provided a turbocharger comprising:
[0010] a turbine wheel and a compressor wheel mounted to a
turbocharger shaft;
[0011] an electric induction motor for assisting rotation of the
compressor wheel in predetermined circumstances, the motor
comprising a fixed stator having motor field coils which generate a
rotating magnetic field when energised by an AC control signal
which induces electric current flow in a rotor to generate a rotor
magnetic field which interacts with the stator magnetic field
producing torque in the rotor;
[0012] wherein the electric current flow induced in the rotor which
generates said rotor magnetic field comprises eddy currents.
[0013] Since induced eddy currents are relied upon to produce the
rotor magnetic field, the rotor may have an advantageously simple
structure and may for instance comprise a unitary solid member.
Since the motor is an induction motor commutation is not required.
Moreover, since the motor is asynchronous there is no requirement
to provide a variable AC control frequency in order to vary the
rotational speed of the motor. These and other advantages of the
present invention are described in more detail below.
[0014] The present invention also provides an AC induction motor
comprising a stator having motor field coils which generate a
rotating magnetic field when energised by an AC control signal
which induces electric current flow in a rotor to generate a rotor
magnetic field which interacts with the stator magnetic field to
rotate said rotor about an axis, the electric flow induced in the
rotor which generates said rotor magnetic field comprising eddy
currents, and wherein the field coils are arranged in an annular
array adjacent first and second axial surfaces of the rotor and the
maximum diameter of the rotor is smaller than the outer diameter of
the annular coil array such that eddy currents are induced in the
rotor which flow from one axial side of the rotor to the other
across the peripheral edge of the rotor.
[0015] This particular rotor and stator coil configuration enhances
the efficiency of the motor in the manner described below. This
improved motor may be used in applications other than the power
assistance of turbochargers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Specific embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0017] FIG. 1 is a cross-section through a conventional
turbocharger schematically illustrating the major components;
[0018] FIGS. 2a and 2b schematically illustrate two alternative
configurations for the motor assisted turbocharger according to the
present invention;
[0019] FIG. 3 is a cross-section through the turbine and bearing
housings of a turbocharger including an electric motor in
accordance with the present invention; and
[0020] FIGS. 4a, 4b, 5a and 5b schematically illustrate motor
assemblies in which exemplify a further aspect of the present
invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0021] Referring to FIG. 1, this shows a cross-section through a
conventional turbocharger without electric motor assistance and is
included to illustrate the major components of a turbocharger. The
turbocharger comprises a central bearing housing 1 which
interconnects a turbine housing 2 and a compressor housing which
comprises a compressor cover 3 secured to an end flange 4 of the
bearing housing 1. A turbine wheel 5 and a compressor wheel 6 are
each mounted to opposite ends of a turbocharger shaft 7, which
extends through the bearing housing 1, for rotation within the
turbine housing 2 and compressor housing 3/4 respectively. The
turbocharger shaft 7 is mounted on bearing assemblies 9 located
within the bearing housing 1. Oil is supplied to, and drained from,
the bearing assemblies 9 via oil passages 10 and appropriate oil
seal arrangements 11/12 are included at the compressor and turbine
end of the bearing housing respectively. The turbine housing
defines a volute or inlet chamber 13 to which exhaust gas from an
internal combustion engine (not shown) is delivered. The exhaust
gas flows from the inlet chamber 13 to an outlet 14 via an annular
inlet passageway 15 defined around the turbine wheel 5.
Accordingly, gas flowing from the inlet chamber 13 to the outlet 14
passes over, and thus rotates, turbine wheel 5 which as a result
drives the compressor wheel 6 via the turbocharger shaft 7.
[0022] Rotation of the compressor wheel 6 draws in air through a
compressor inlet 16 and delivers compressed air to the intake of
the engine (not shown) via an outlet volute 17.
[0023] Referring now to FIGS. 2a and 2b, these schematically
illustrate two alternative configurations of an electric motor
assisted turbocharger in accordance with the present invention. In
both figures, the turbine is illustrated by reference 18 and the
compressor is illustrated by reference 19. The turbocharger shaft
is represented by line 20 and the blocks 21 represent the bearing
assemblies.
[0024] The electric motor of FIG. 2a comprises a rotor 22 mounted
to the turbocharger shaft 20 between the bearing assemblies 21. In
accordance with preferred embodiments of the invention the rotor
may be a unitary solid member of a ferromagnetic material. The
rotor 22a is surrounded by a fixed stator 23a, located within the
bearing housing (not shown) which comprises an array of field coils
wound around respective pole pieces which when appropriately
energised with an AC supply will generate a rotating magnetic field
in a conventional way.
[0025] With the configuration of FIG. 2a, the rotor 22a is
generally cylindrical and the stator is arranged to generate
magnetic flux which is radial relative to the rotor 22a and
turbocharger shaft 21. In other words, this is a radial flux type
motor. On the other hand, with the arrangement of FIG. 2b, the
rotor 22b is again mounted for rotation with the turbocharger shaft
20, but in this case the rotor 22b is generally disc like and the
stator field coils 23b are arranged to generate magnetic flux in an
axial direction relative to the turbocharger shaft 20. This is
achieved by arranging the stator coils in an annular array adjacent
the disc shaped rotor 22b. This is an axial flux type motor and the
basic arrangement of stator and rotor is again known.
[0026] In both radial and axial flux configurations, operation of
the motor is essentially the same. The magnetic flux produced by
the stator coils induces eddy currents in the surface of the solid
ferromagnetic rotor which in turn generate a magnetic field which
interacts with the stator field. In particular, as the stator
magnetic field rotates, the interaction of the magnetic fields
generated by the stator and the rotor causes the rotor to rotate.
In other words, the motor is essentially an asynchronous AC
induction motor, the operating principles of which are well known.
Having said that, the motor in accordance with the present
invention differs from that of a typical AC asynchronous induction
motor in that the rotor is a simple "solid" rotor and the induced
currents are eddy currents. With a more conventional AC
asynchronous motor the rotor comprises a more complex coil
assembly, such as the well known "squirrel cage" rotor
assembly.
[0027] The electric motor assisted turbocharger according to the
present invention thus avoids the control difficulties associated
with the prior art asynchronous motor assisted turbochargers,
whilst retaining the advantage of a commutator free rotor.
Moreover, since the induced currents are eddy currents, the rotor
of the present invention can be of a very simple construction, and
could for instance be a unitary solid member such as a mild steel
cylinder or disk (depending upon whether the motor is a radial or
axial flux motor respectively). This has further advantages over
the relatively complicated prior art magnetic rotor assemblies as
for instance disclosed in U.S. Pat. No. 5,604,045 mentioned above,
as well as typical coil rotors of conventional AC asynchronous
motors (such as "squirrel cage" rotors).
[0028] Throughout this specification the rotor of the present
invention is referred to as a `solid` rotor. The term `solid` is to
distinguish the nature of the rotor from a typical laminated rotor
of a conventional AC asynchronous electric motor. However, it will
be appreciated that the rotor of the present invention does not for
instance need to be a unitary member or indeed strictly solid (in
the sense that it could have apertures or voids formed therein). A
solid unitary rotor is however preferred for simplicity of
construction and to provide minimum resistance to formation of eddy
currents (see below).
[0029] The generation of eddy currents is a well known phenomenon.
Eddy currents are swirling currents which are established in a
block of conducting material placed in a changing magnetic field.
The eddy currents swirl in a plane perpendicular to the magnetic
field producing them and in turn produce a magnetic field of their
own. The magnetic field produced by the eddy currents tends to
oppose the change producing the eddy currents which generally leads
to the generation of heat in the rotor. For this reason, the rotor
cores of conventional AC induction motors are typically laminated
to provide an increased resistance to the generation of eddy
currents to minimise this heating effect. However, with the present
invention the eddy currents are the only currents which flow in the
rotor and provide the magnetic field for interaction with the
rotating stator field.
[0030] With the electric motor assisted turbocharger of the present
invention, the potential disadvantage of increased heat generation
in the rotor, compared with a conventional induction or magnetic
rotor, is more than compensated for by the advantages gained. The
turbocharger is itself a high temperature environment and the solid
rotor construction has much greater tolerance to heat flow from the
surrounding turbocharger components than a conventional rotor
structure. The solid rotor also has high resistance to rotational
fatigue stresses and in particular can have fatigue characteristics
to match those of other rotating components of the turbocharger
such as the compressor wheel. The solid rotor is also much cheaper
and simpler to manufacture and assemble than other conventional
rotor structures.
[0031] It should also be noted that the heat generation in the
rotor is proportional to the motor `slip` speed. The term `slip`
will be well known to the person skilled in the art of AC motors
and refers to the difference between the rotor rotational speed and
the synchronous speed at any given time. With the electric motor
assisted turbocharger of the present invention it is envisaged that
the motor will generally only be require to assist the turbocharger
when air flow, and thus turbocharger speed, is low. Thus relatively
low synchronous speeds will generally be required with
correspondingly low maximum slip speeds which will inherently avoid
excessive heat generation.
[0032] Aside from the construction of the rotor together with the
reliance on eddy currents to generate a magnetic field to interact
with the stator field, and the difference in performance
characteristics these features provide (as described below),
operation of the present motor is essentially the same of any AC
induction motor. In particular, the arrangement, constructions and
excitation of the stator field coils can be entirely conventional.
For instance, the AC power supply to the motor may have a fixed or
variable frequency. However, a significant advantage of the use of
an asynchronous motor (rather than a synchronous motor with a
magnetic core as in the prior art mentioned above) is that a fixed
frequency control signal may be used. This avoids the requirement
to monitor the rotational speed of the turbocharger and vary the AC
supply frequency accordingly, which greatly simplifies the control
electronics required. The control system may therefore be much
simpler and cheaper than the variable frequency control systems
used in the prior art.
[0033] In common with any asynchronous AC motor, the torque
generated in the rotor drops to zero as the rotor speed reaches the
motor synchronous speed, i.e. the rotational speed of the stator
magnetic field (which is a well known function of the AC supply
frequency and coil arrangement, namely 120 times the AC supply
frequency divided by the number of stator poles). However, unlike a
conventional AC induction motor with a laminated cage rotor (e.g.
squirrel cage) the torque vs. speed characteristic of the solid
rotor motor provides a high starting torque which is maintained
over a broad speed range before dropping sharply to zero as the
speed of the rotor reaches the synchronous speed (at which point
the motor may be de-excited to avoid generation of a retarding
force). The motor is therefore able to provide a high accelerating
torque over most of its torque curve which is available for
assisting in rotation of the turbocharger.
[0034] Where the motor is excited by a fixed frequency AC supply
the frequency (and thus synchronous speed of the motor) may be
determined to correspond to a desired turbocharger speed below
which the electric motor assistance is required. The synchronous
speed can for example be selected to provide significant boost
pressure for the engine corresponding to 60% load at full engine
speed. The appropriate motor synchronous speed, and thus AC supply
frequency, may be determined for each given engine type and
application.
[0035] Referring now to FIG. 3, this is a cross-section through
part of an electric motor assisted turbocharger in accordance with
the present invention. Conventional features of the turbocharger
are identified by the same reference numerals used above in
relation to FIG. 1. Thus the illustrated turbocharger comprises a
central bearing housing 1 which interconnects a turbine housing 2
and a compressor housing. In this illustration only the end flange
4 of the bearing housing which forms part of the compressor housing
is shown--other components of the compressor are omitted but may be
entirely conventional. A turbine wheel 5 and a compressor wheel
(not shown) are each mounted to opposite ends of turbocharger shaft
7 which is mounted on bearing assemblies 9a and 9b located at the
turbine and compressor ends of the bearing housing respectively.
Other conventional features of the turbocharger will not be
described in detail.
[0036] In accordance with the present invention, a solid disk
shaped rotor 24 is mounted to the turbocharger shaft 7 for rotation
therewith. Annular stator rings 25/26 surround the turbocharger
shaft 7 and are fixed to the bearing housing 1 on either side of
the disk shaped rotor 24. The stator rings carry conductor coils
27/28 for generating a rotating magnetic field when excited by an
AC current. As such, the stator rings may be entirely
conventional.
[0037] Together, the rotor 24 and stator rings 25/26 comprise an
axial flux solid rotor AC induction motor. In this embodiment, the
motor is located between the bearing assemblies 9a and 9b within
the bearing housing 1, but in other embodiments the motor could be
located between one or other of the bearing assemblies and the
turbine/compressor end of the beating housing respectively.
[0038] Operation of the electric motor is as described above. The
motor is energised by supply of an AC signal in accordance with a
control scheme which determines when the turbocharger requires
electric motor assistance. Such a scheme may be entirely
conventional, and could for instance be programmed into the normal
engine management electronic system. Typically assistance will be
required during transient conditions, such as gear changes, or at
start-up. The determination of when to activate/deactivate the
motor does not form part of the present invention, rather this
determination may be entirely conventional.
[0039] When the motor stator coils 27/28 are excited eddy currents
are induced in the rotor 24 which generate magnetic fields which
interact with the stator field. Accordingly, as the stator field
rotates the rotor 24 rotates in accordance with normal induction
motor principles.
[0040] As mentioned above, a preferred and particularly
advantageous feature of the present invention is that the motor can
be energised by a fixed frequency AC signal. This is not possible
with a synchronous motor, and is problematic with conventional
induction motors having laminated core coil rotors (such as a
squirrel cage) in which the torque generated varies more widely
with speed and with the solid rotor motor, and thus is much more
dependant on the difference between the instantaneous rotor (and
thus turbocharger) speed and the motor synchronous speed
(determined by the excitation frequency).
[0041] The above described embodiment of the invention is an axial
flux motor which is the preferred arrangement for a turbocharger as
it has a more compact axial dimension than a radial flux motor.
However, radial flux motors can be used and the particular
configuration/location of the motor can vary widely to suit
different turbocharger sizes and constructions.
[0042] Whereas the rotor is preferably a solid unitary member, the
rotor could be comprised of a number of components, e.g.
interconnected segments or annular sections. The term `solid` rotor
is used primarily to distinguish the rotor from a conventional
induction motor rotor comprising conductor coils or equivalent
arrangements in which the induced current flow in closed paths and
which are generally constructed (laminated) so as to suppress
induced eddy currents.
[0043] As is clear from the above description, the generation of
eddy currents in the rotor is fundamental to operation of the
present invention FIG. 4 schematically illustrates a motor
arrangement with a modified rotor adapted to further reduce the
resistance to eddy current flow. An aspect of the present invention
which reduces eddy current resistance will now be described with
reference to FIGS. 4 and 5.
[0044] FIG. 4a illustrates an axial flux solid rotor motor of
structure similar to the axial motor arrangement described above.
As such, the motor comprises a disc shaped rotor 29 mounted to a
shaft 30 for rotation within a motor housing 31 on bearing
assemblies 32. Two fixed stator rings comprising stator coils 33
are mounted on either side of the rotor 29. Upon excitation of the
stator coils 33 eddy currents are induced on either side of the
solid rotor and the rotor 29 operates as described above. FIG. 4b
is an axial view of a section of the rotor 29 which schematically
illustrates the swirling eddy currents 34 which are induced by
three adjacent stator coils. From this it can be seen that the eddy
currents flow is closed loops on each side of the rotor, flowing in
a plane perpendicular to the axial flux generated by the stator
fields. Each eddy current path has a section which generally
follows the curvature of the radial outer periphery 29a of the
rotor, the sections identified by references 34a. It will be
appreciated that eddy currents on the opposite side of the stator
ring to that illustrated it FIG. 4b will be formed in exactly the
same way, and will have exactly the same configuration.
[0045] Referring now to FIG. 5a, the illustrated motor corresponds
to that illustrated in FIG. 4a, and like reference numbers are used
to identify like components, but has a modified rotor
configuration. Specifically, the modified rotor 35 axially tapers
towards its radially outermost edge, so that the axial dimension at
the outer periphery of the rotor is significantly less than that of
the rotor of FIG. 4a. Furthermore, the rotor 35 has a smaller outer
radius than that of the stator coils 32.
[0046] Referring now to FIG. 5b, this is an axial view of one side
of the rotor illustrating the path of the induced eddy currents in
a similar way to FIG. 4b. Here it can be seen that because the
rotor 35 has a smaller radius than the stator ring radius, some of
the eddy current paths ate interrupted by the peripheral edge 35a
of the rotor (dotted lines indicate the paths which would form if
the rotor were of an increased radial dimension corresponding to
that of the motor of FIGS. 4a and 4b. What in fact occurs is that
the "interrupted" eddy currents circuits are completed by similar
eddy currents on the opposing side of the rotor. In other words,
rather than the formation of separate eddy currents on each side of
the rotor, eddy currents are formed which flow across the outer
peripheral edge of the rotor from one side of the rotor to the
other. The effect of this is that the overall eddy current path
length is reduced with a corresponding reduction in resistance thus
increasing the efficiency of eddy current induction and thus the
efficiency of the motor, with reduced heat generation.
[0047] The skilled person will appreciate that the modified rotor
described above in relation to FIGS. 5a and 5b could have
applications other than providing motor assistance to
turbochargers. Thus, this aspect of the present invention provides
a novel AC induction motor of wider application than the electric
motor assistance of turbochargers.
* * * * *