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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to improvements in three-phase permanent magnet
rotary electrical machines such as motors and generators. More
particularly, the invention relates to such improvements which minimize
reluctance torque and electromagnetic torque ripple while maximizing
compactness, energy efficiency and motor starting torque per unit volume
of winding.
Permanent magnet motors having slotted armatures and multi-coil phases have
been produced in the past utilizing an odd number of slots and armature
poles and an even number of permanent magnet poles to reduce reluctance
torque and thus vibration, as exemplified by the motors shown in Ban U.S.
Pat. No. 4,437,029 and Aoki U.S. Pat. No. 4,532,449. However, the coils of
the windings for such motors are either superimposed upon each other or,
if not superimposed, require the use of more than three phases. In the
former case, the superimposed coils tend to maximize the amount of wire in
the winding, thereby maximizing both its volume and impedance and
minimizing its efficiency and torque (or emf) per turn. In the latter
case, the large number of phases is undesirable due to the need for a
correspondingly high number of phase-switching circuits which add
complexity and expense.
Three-phase permanent magnet motors, having multi-coil phases wherein the
individual coils are not superimposed upon each other, have been designed
as shown in FIGS. 2, 2A and 2B herein. However, even though the coils do
not overlap, the phases do overlap since the coils of one phase are
interstitially inserted between the coils of another phase. Such winding
configuration, although minimizing self-inductance which is beneficial in
high-speed applications, produces electromagnetic torque ripple and
reduced starting torque per unit volume of wire, both of which are
disadvantageous in many applications.
Conversely, motors having equal numbers of armature slots and permanent
magnet poles, as exemplified by Hahn U.S. Pat. No. 4,188,556, are
characterized by considerable reluctance, or "cogging", torque which
produces harmful vibration in many applications.
Accordingly, a need exists for a three-phase, permanent magnet rotary
electrical machine which compatibly satisfies all of the objectives of
compactness, minimal reluctance torque and electromagnetic torque ripple,
and maximum energy efficiency and starting torque per unit volume of wire.
SUMMARY OF THE INVENTION
The present invention provides a unique combination of features which
compatibly satisfies all of the foregoing competing objectives in a
three-phase permanent magnet rotary machine such as a motor or generator.
The machine may have an armature which is either internal or external
relative to the permanent magnet assembly, and may have either a radial or
an axial gap. Pursuant to the principles of the invention, an armature,
having a ferromagnetic core with 3(2n+1) protruding ferromagnetic poles
arranged in a circular array separated from each other by a like number of
slots, and a permanent magnet assembly having a circular array of
3(2n+1).+-.1 magnetic poles, are mounted for relative rotation with
respect to each other, n being an integer of one or more. This structure
enables the utilization of three phases, each having multiple coils, in
combination with permanent magnet poles of a slightly different number
than the ferromagnetic poles of the armature, so that the magnitude of the
reluctance torque is minimized while its frequency per revolution is
maximized. Compactness and high energy efficiency of the three-phase
winding is achieved by winding each coil of each phase about a respective
ferromagnetic armature pole so that each coil occupies a pair of slots
located immediately on each side of the respective armature pole. This
structure avoids any overlapping of the respective coils, thereby
minimizing the volume of coil wire and thereby also minimizing the
impedance of the winding while maximizing its efficiency and torque (or
emf) per turn.
The minimizing of electromagnetic torque ripple and the maximizing of
starting torque per unit volume of wire are achieved by concentrating the
coils of each phase into a limited sector of the armature so that the
phases, as well as their individual coils, do not overlap each other. This
is achieved by making the coils of each phase occupy 2n+2 armature slots
located within a sector of the armature poles which encompasses only 2n+1
armature poles, and winding the coils with alternating polarities.
Although such close placement of coils of alternating polarity increases
the self-inductance of each phase, the resultant increase in impedance is
insignificant except at unusually high motor speeds.
The foregoing and other objectives, features and advantages of the present
invention will be more readily understood upon consideration of the
following detailed description of the invention, taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of an exemplary radial-gap embodiment of the
invention.
FIG. 1A is a schematic diagram of one embodiment of a winding suitable for
the device of FIG. 1.
FIG. 1B is a schematic diagram of an alternative winding embodiment
suitable for the device of FIG. 1.
FIG. 2 is a schematic drawing, for comparison purposes, of a rotary machine
having a rotor and stator structurally identical to those of FIG. 1, but
with overlapping phases.
FIGS. 2A and 2B are schematic diagrams of alternative overlapping-phase
windings usable with the machine of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the exemplary embodiment of the invention shown in FIG.
1, the rotary electrical machine indicated generally as 10 comprises an
external permanent magnet rotor assembly 12 comprising a ferromagnetic
annular core or housing 14, on the inner surface of which is mounted a
circular array of radially magnetized permanent magnets 16 of ceramic
ferrite, rare earth cobalt or other suitable type. An armature 18 serves
as the stator and comprises a laminated, ferromagnetic core 20 having
protruding ferromagnetic poles 22 arranged in a circular array separated
from each other by slots 24 located interstitially between the poles 22,
and separated from the poles of the respective permanent magnets 16 by an
annular radial gap 26.
The rotor and stator may be mounted for relative rotation with respect to
each other by any suitable bearing assembly, such as that shown in Blom
U.S. Pat. No. 4,540,906, which is incorporated herein by reference.
Although the permanent magnet rotor assembly enables brushless
commutation, the permanent magnet assembly could, alternatively, serve as
the stator in cooperation with a mechanically commutated armature. As a
further alternative, the armature could be the exterior element, serving
either as stator or rotor, with the permanent magnet assembly located
interior thereof.
The respective locations of the three phases A, B and C of the winding for
the device of FIG. 1 are indicated in FIG. 1 by the respective sets of
ferromagnetic armature poles A1, A2, A3; B1, B2, B3; and C1, C2, C3. As
shown in FIG. 1A, which is a schematic radial view of the respective
armature poles extended into a straight line, a typical phase such as A
has three coils, each wound about a respective armature pole such as A1,
A2, A3 and occupying a pair of slots located immediately adjacent to each
side of the respective armature pole so that none of the coils overlaps
any other coil. Phases B and C, respectively, are wound on their
respective poles B1, B2, B3 and C1, C2, C3 identically to phase A,
although the direction of the current depends on the commutation as is
well-known to those skilled in the art. FIG. 1B shows an alternative
configuration for phase A, the other two phases B and C being wound
identically.
In both embodiments of the winding shown in FIGS. 1A and 1B, not only are
the individual coils not superimposed upon each other, but the phases
likewise are not superimposed upon each other. Rather, each phase A, B, C
is limited to a respective exclusive sector, as shown in dashed lines in
FIG. 1, of the circular array of armature poles 22. The avoidance of
overlapping coils provides maximum compactness and efficiency by
minimizing the volume of wire needed, while the avoidance of overlapping
phases minimizes electromagnetic torque ripple and maximizes starting
torque per unit volume of wire. Thus each pole is wound with a coil of a
single phase and each coil therefore occupies the pair of slots 24 located
immediately on each side of the respective armature pole 22.
On the other hand, the combination of eight permanent magnet poles (of the
eight magnets 16) and nine ferromagnetic armature poles 22 minimizes the
amplitude of the reluctance torque by permitting only a single alignment
of permanent magnet and ferromagnetic poles at any one time. The numerical
closeness of permanent magnet poles and armature poles also maximizes the
frequency of the reluctance torque at 72 cycles per revolution, which is
much higher than any of the significant frequencies of the rotary machine
of FIG. 1. Both of these factors contribute significantly to minimizing
the vibration of the machine.
The same principles apply to other embodiments of the invention featuring
different numbers of permanent magnet poles and ferromagnetic poles, as
long as the number of ferromagnetic poles equals 3(2n+1) and the number of
permanent magnet poles equals 3(2n+1).+-.1, where n is an integer of 1 or
more. The multicoil phases likewise satisfy the principles of the
invention if the coils of each phase occupy 2n+2 slots and are located
within a sector of the circular array of ferromagnetic poles encompassing
2n+1 ferromagnetic poles, and are wound with alternating polarities. It
will be recognized that the device of FIG. 1 satisfies these relationships
where n equals 1. By way of example only, rotary electrical machines
having the following relationships would satisfy the invention:
______________________________________
Slots Ferromatnetic
Ferro- Permanent Occupied Poles in
magnetic Magnet By Single
Single Phase
n Poles Poles Phase Sector
______________________________________
1 9 8 4 3
1 9 10 4 3
2 15 14 6 5
2 15 16 6 5
3 21 20 8 7
3 21 22 8 7
4 27 26 10 9
4 27 28 10 9
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and so on for higher values of n.
For comparison purposes, FIG. 2 depicts a rotary machine having the same
number of permanent magnet poles and ferromagnetic poles as in FIG. 1 and
having winding coils which do not overlap each other, but having multicoil
phases A', B', C' which do overlap each other. FIGS. 2A and 2B show
alternative embodiments of phase A', phases B' and C' being wound
identically to phase A' in each case. Phase A' in each case has three
coils each wound about a respective armature pole such as A'1, A'2 and
A'3. Phases B' and C' respectively are wound on their respective poles,
B'1, B'2, B'3 and C'1, C'2, C'3 identically to phase A'. FIG. 2 shows how
the respective phases, because of their overlapping relationships, occupy
sectors shown in dashed lines which are not limited to the narrow,
mutually exclusive sectors of the invention of FIG. 1. Rather, the
majority of each sector is overlapped by other sectors which, although
minimizing the self-inductance of each phase because the respective coils
thereof are all wound with the same polarity, increases torque ripple and
reduces starting torque per unit volume of wire. This trade-off would be
advantageous only in exceptionally high-speed motors where the
self-inductance would introduce significant impedance into the winding.
For most applications, however, the present invention is the superior one
because of the minimized torque ripple and maximized starting torque per
unit volume of wire which it achieves. The higher self-inductance of each
phase does not outweigh these advantages at motor speeds which are normal
for high-volume applications such as computer disk drive motors and fan
motors.
The terms and expressions which have been employed in the foregoing
specification are used therein as terms of description and not of
limitation, and there is no intention, is the use of such terms and
expressions, of excluding equivalents of the features shown and described
or portions thereof, it being recognized that the scope of the invention
is defined and limited only by the claims which follow.
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