Note: Descriptions are shown in the official language in which they were submitted.
CA 02348228 2001-05-22
APPARATUS AND METHOD FOR REDUCING
MOTOR ELECTROMAGNETIC LOSSES
BACKGROUND OF THE INVENTION
This invention relates generally to an apparatus and method for
reducing electromagnetic losses and improving thermal dissipation in electric
motors,
and more particularly, to optimizing electromagnetic geometry for high
horsepower,
low voltage industrial motors to maximize power output capacity.
The size of an electric motor is typically dependent on an acceptable
winding temperature rise in operation, which, in turn, is dependent upon
heating
losses and the rate at which those losses can be dissipated from the motor.
Thus, if
heating losses can be reduced and/or the rate of cooling of the motor can be
increased,
a motor can deliver a given horsepower output with a reduced physical size or
more
horsepower output with the same physical size. The general purpose industrial
machine market has continually demanded higher horsepower, e.g. above 400 HP,
machines in low voltages, e.g., less than about 600 volts. Because of the size
of these
large motors, reducing the physical size of industrial motors is desirable,
especially in
the case of Totally Enclosed Fan Cooled ("TEFC'~ electric machines.
TEFC machines are constructed such that an externally mounted fan
moves air over the external surfaces of the frame and endshieids, thereby
removing
heat primarily by convection at the external surfaces, and by mixed convection
and
conduction within the motor. Therefore, the total cooling rate of the motor is
a
combination of internal and external heat transfer rates. Known methods of
increasing
external heat transfer rates include the use of external fins and/or
increasing an
external surface area of the motor, and increasing the air flow or velocity
along the
external surfaces. Likewise, known methods of increasing internal heat
transfer rates
include the use of internal fans and baffles to direct air flow through
internal motor
components.
Improvements in the internal heat transfer rate may also be obtained by
reducing electromagnetic losses, which affect both heat dissipation and the
output
power of the machine. If electromagnetic losses could be reduced, power output
for a
fixed sized machine would be increased, or a fixed output machine could be
provided
with a smaller physical size.
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Accordingly, it would be desirable to reduce electromagnetic losses in
high horsepower, low voltage industrial motors, such as TEFC motors.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention a high horsepower, low
voltage totally enclosed fan cooled AC induction motor includes a stator core
and a
rotor core each constructed to optimize electromagnetic geometry of the motor
and
reduce an operational winding temperature rise of the motor by reducing motor
losses
and improving internal heat transfer rates.
The stator core includes a plurality of stator laminations bonded to one
another and having a plurality of slots for receiving a first coil group and a
second coil
group. A leg of the first coil group and a leg of the second coil group are
placed into a
top and bottom of the stator core slots, respectively, so that each slot
contains one coil
leg of the first group and one coil leg of the second group. Each group has an
integral
number of turns per coil, and the second coil group has one more turn per coil
than the
first group. Therefore, a relatively simple stator winding arrangement
facilitates a
fractional, or non-integral, effective number of turns per coil that allows
fine tuning of
stator care magnetic flux levels for motors with windings having a relatively
low
number of turns, e.g., five or six turns. Thus, for example, performance of a
two pole
winding for stator core laminations having forty eight . slots can be fine
tuned while
avoiding relatively complicated conventional techniques and constructions
employing
a large number of coil connections that allow adjustment of flux produced by
windings having low numbers of turns. Motor reliability is improved by
minimizing
potentially defective connections and manufacturing of the motor is
simplified.
The first and second coil groups are formed in multiple passes to
further simplify manufacturing of the motor and to reduce a required inventory
of
spools of magnet wire used to form the coils. Portions of the coil groups are
successively wound to cumulatively form a complete group of coils with a
reduced
manufacturing setup time, thereby reducing manufacturing costs.
Each of the stator core laminations includes a stator bore defined by an
inner diameter of the lamination and a yoke having an outer diameter. A ratio
of the
inner diameter to the outer diameter is selected to maximize a cross sectional
area of
the stator lamination slots for reduced stator winding losses, as well as
improves heat
transfer rates in comparison to conventional AC induction motor systems.
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A rotor core is mounted within the stator core for rotational movement
within an air gap between the stator bore and a rotor outer diameter. A size
of the air
gap is selected to decrease selected components of losses typically
characterized as
harmonic losses.
The above-described features facilitate a reduced physical size of a
motor necessary to generate a given horsepower output, which accordingly
reduces
motor cosh. per horsepower. Alternatively, increased motor efficiency allows a
given
physical size of a motor to produce more output horsepower, which reduces
motor
cost per horsepower. Also, decreased temperature rise of the motor windings in
operation ~ extends a working lifespan of motor components, such as electrical
insulation systems and motor bearing systems. These benefits are especially
advantageoi~~ fnr hish horsepower, low voltage industrial motors.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of an AC induction motor;
Figure 2 is a top plan view of a stator lamination for the motor shown
in Figure 1;
Figure 3 is a top plan view of a stator lap coil for the motor shown in
Figure l;
Figure 4 is a schematic illustration of a stator winding construction for
the motor shown in Figure 1;
Figure 5 is a schematic illustration of a winding assembly used to
manufacture the motor shown in Figure 1;
Figure 6 is a top plan view of a rotor lamination for the motor shown in
Figure 1;
Figure 7 is an exaggerated schematic illustration of the motor shown in
Figure 1 in use;
Figure 8 is a chart illustrating stator slot area versus a ratio of stator
core inner diameter ("1D") to outer diameter ("OD") for the motor shown in
Figure 1;
and
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Figure 9 is a chart illustrating a heat transfer rate the motor shown in
Figure 1.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 is a schematic illustration of an AC induction motor 20
including a frame 22 or housing having an exterior surface 24, a stator core
26 within
frame 22, and a rotor core 28 rotatably mounted within and extending through a
central bore 30 in stator core 26. Each of rotor core 28 and stator core 26 is
fabricated
from laminations (not shown in Figure 1) that include a plurality of slots
(not shown
in Figure' 1), and magnetic material is placed into the slots to form
conductive motor
windings (not shown) in rotor core 28 and stator care 26. Magnetic fields
venerated
by stator core 26 when a stator winding (not shown) is energized induces a
voltage in
the rotor windings and causes rotor core 28 to rotate within stator bore 30
upon a
bearing assembly (not shown) and to drive mechanical components and linkages
(not
shown) via a rotor shaft 32 extending from rotor core 28 and coupled to the
mechanical components and linkages. A radial space or gap 34 allows rotor core
28
to rotate within stator core 26.
Heating losses of an AC induction motor, such as motor 20 arc
typically segregated into one of five areas including stator winding loss,
magnetic
core loss, rotor winding loss, windage and friction loss, and stray load
losses. The
stator winding loss is primarily due to the resistance of the stator winding
which
generates heat as current flows through the stator winding. The magnetic core
Ioss is
dependent upon a flux density and material characteristics of stator core 26.
The rotor
winding loss is due to electrical resistance of the rotor winding which
generates heat
as current flows through the rotor winding. Windage and friction losses are
attributable to the power input to extema.l and internal motor fans (not
shown),
bearings (not shown), and other mechanical elements (not shown) of motor 20.
The
stray load loss includes gap harmonics and other factors related to an output
torque of
motor 20, and is otherwise defined as all losses unaccounted for by the stator
winding
loss, magnetic core loss, rotor winding loss, and windage and friction loss.
The relative significance of the above losses is a function of motor
design and performance characteristics. For example, in one common type of
motor
(e.g., NEMA design B, 300-500 HP, 3600rpm), the stator winding loss has been
found
to be about 20% to about 30% of the total motor losses, the magnetic core loss
has
been found to be about 10% to about 15% of the total motor losses, the rotor
winding
CA 02348228 2001-05-22
loss has been found to be about 15% to about 20% of the total motor losses,
the stray
load loss has been found to be about 20% to about 30% of the total motor
losses and
the remainder of the total motor Loss is attributable to windage and friction
loss. The
motor loss distribution may be significantly different for greater or lesser
horsepower
motors, and the loss distribution is dependent on the operating speed of the
motor.
The present invention optimizes electromagnetic geometry of motor 20
to improve an internal heat transfer rate of a high horsepower (e.g. 400 HP}
low
voltage (e.g., 460 V) TEFL motor to improve internal heat transfer by reducing
electromagnetic losses and thereby increasing motor efficiency and
performance.
Nevertheless, aspects of the invention are contemplated to be beneficial in
other sizes
and types of motors, and the methodology and principles set forth herein are
applicable in other contexts. The invention is therefore not intended to be
limited to a
specific motor size, type, or application.
Figure 2 is a top plan view of a stator lamination 40 including a
plurality of slots 42, a plurality of teeth 44, a yoke 46, and central opening
or bore 30
therethrough. Each slot 42 includes a top portion 48 located adjacent a
periphery 50
of central bore 30, and a bottom portion 52 located opposite slot top portion
48. A
plurality of laminations 40 are stacked and bonded together by known
techniques,
such as welding and fitting keys (not shown), to form stator core 26 (shown in
Figure
1 ). Each lamination 40 is fabricated from ferromagnetic materials known in
the art,
and aligned slots 42 form passages (not shown) through stator core 26 for
receiving a
stator winding (not shown).
Figure 3 is a top plan view of an exemplary stator lap coil 60 fabricated
from a number of strands of magnet wire of pre-selected sizes, and when
connected to
other similar coils forms a stator winding (not shown). Coil 60 includes
opposite coil
legs 64 and a number of turns 62 between a coil beginning 66 and a coil end
68. A
leg 64 of each successive coil 60 is placed in adjacent lamination slots 42
(shown in
Figure 2) of stator core 26 (shown in Figure 1 }, and each coil 60 spans a
number of
lamination slots 42 and/or teeth 44 (shown in Figure 2) when wound inside
lamination
slots 42. More specifically, the stator winding is formed by placing a first
leg 70 of a
first coil, such as coil 60, in top portion 48 (shown in Figure 2) of a stator
slot 42, and
a second leg '72 of the first coil is placed in a bottom portion 52 (shown in
Figure 2) of
a different stator slot 42 located a number of stator slots away from coil
first leg 70.
A second coil, similar to coil 60, is connected in series to the first coil
and is placed in
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stator core 26 in the same fashion as the first coil so that a first leg of
the second coil
is placed in a top portion 48 of a stator slot 42 adjacent, or one slot
removed from the
stator slot containing the first leg 70 of the first coil. The second, leg of
the second
coil is placed in a bottom portion 52 of a different stator slot 42 located
adjacent, or
one slot removed from, the slot containing the second leg 72 of the first
coil.
Successive coils are connected in series to the first and second coils and
placed in
adjacent stator slots 42 as described above to complete the stator winding.
When wound into stator core 26, the plurality of coils 60 are generally
divided into groups (not shown), and each coil group is connected in series to
form a
pole (ndt shown). Each coil group is wound at the same time to simplify
manufacturing steps. When voltage is applied to each of the coil groups or
poles, a
corresponding magnetic pole is created. Each pole is characterized by a pole
pitch,
which those in the art will recognize as a circumferential division of stator
laminations
40 (shown in Figure 2) intp equal parts by the number of poles in the winding.
A ,flux of the stator winding, which produces output power of motor 20
(shown in Figure I), is dependent upon the effective number of coil turns per
pole.
The effective number of turns per pole is dependent upon the coil span, i.e.,
the
number of lamination slots 42 separating Legs 64 of coils 60 in the stator
winding. A
pitch factor can be calculated based upon the coil span and the pole pitch,
and the coil
turns are more effective the closer the coil pitch is to the pole pitch. When
a number
of turns 62 is relatively high, such as in relatively smaller horsepower, low
voltage
(less than about 600V) machines having forty or more turns, the flux of the
winding
can be optimized in small amounts, e.g., I % or 2% by changing the number of
turns
62 in the coil by a small number of turns, e.g., 1 turn. However, in higher
horsepower, low voltage machines (e.g., 400 HP, 460V) the number of turns 62
is
relatively low, e.g., five or six turns, which limits optimization of the flux
level. In
this case, changing the number of turns by one (e.g., from five turns to six
turns)
results in a 10% to 15% difference in the flux of the stator winding.
Figure 4 partially illustrates a stator winding construction that
overcomes flux optimization limits due to a small number of coil turns in
higher
horsepower low voltage motors, such as motor 20 (shown in Figure 1 ). The
stator
winding coils are divided into two groups, for example, an "S" group,
represented by
an "S" coil 80 and an "L" group, represented by an "L" coil 82. Legs 84 of the
S coils
80 and legs 86 of the L coils 82 are placed in stator slots 42 (shown in
Figure 2) so
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that a leg 86 of an L coil 82 is placed in the same slot 42 as a leg 84 of an
S coil 80.
An S coil leg 84 is placed in a top 48 of a stator slot 42, and an L coil leg
86 is placed
in a bottom 52 of the same stator slot 42, and all stator slots 42 are
likewise filled with
an S coil leg 84 and an L coil leg 86. Both the S coils 80 and the L coils 82
contain an
integral number of turns 62 (shown in Figure 3), and the L turn coils 82
include one
more turn 62 than the S group coils 80. Because the coil groups 80, 82 have
different
numbers of turns 62, a fractional change in the effective number of turns may
be
obtained. Because the L coils 82 contain one more turn 62 than the S coils 80,
the
effective number of turns is the average of the number of turns 62 of the two
coil
groups 80, 82, or more specifically, the number of turns 62 of an S coil 80
plus one
half turn. Therefore, finer adjustments may be made to the flux level when the
number of turns 62 in the coil groups 80, 82 is relatively small. As an
example,
changing the effective number of turns from five to five and a half would have
a
smaller effect on the resultant flux level than would changing the number of
turns
from five to six. Consequently, finer optimization of the winding flux level
may be
achieved with fractional components of effective turns while at the same time
avoiding an additional increase in winding resistance that, fox example, a
change of a
whole turn, or an integer change in effective toms, would produce. Because
stator
winding loss is proportional to stator winding resistance, a fractional change
in
effective number of turns not only allows a finer tuning of stator winding
flux, but
also decreases stator winding loss.
Arrangement of S coils 80 and L coils 82 is dependent upon the coil
span and the number of stator slots 42, and a coil arrangement can be
optimized to
minmize stator winding harmonics and hence rcduce total losses of motor 20.
One
such exemplary coil arrangement for a stator core 26 fabricated from stator
laminations 40 with twenty four slots 42 is as follows:
Slot No. Slot Bottom Ton of Slot
Ja Lb
2 Sa Lb
3 La Sb
4 La Sb
5 Sa Lb
6 Sa Lb
7 La Sc
8 La Sc
Sb Lc
10 Sb Lc
'l l Lb Sc
12 Lb Sc
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13 Sb Lc
14 Sb Lc
I S Lb Sa
16 Lb Sa
17 Sc La
18 Sc La
19 Lc Sa
20 Lc Sa
21 Sc ' La
22 Sc La
23 Lc Sb
24 Lc Sb
where "Sa" and "La" represent S and L coils, 80, 82 respectively, in phase "A"
of a
three phase power supply knot shown), "Sb" and "Lb" represent S and L coils,
80, 82
respectively, in Phase "B" of the three phase power supply, and "Sc" and "Lc"
represent S and L coils 80, 82 respectively, in Phase "C" of the three phase
power
supply_
Depending on the coil span and number of stator lamination slots 42 of
a particular motor, such as motor 20, a coil arrangement according to the
invention
may not be possible. Further exemplary arrangements of coil groups 80, 82 for
a
stator core 26 fabricated from laminations 40 having forty eight slots and for
a variety
of coil spans for a two pole winding are as follows:
No. of Slots Coil Span 2 Pole Coil Arrangement
48 1-13 SSSSLLLL, SSLLLLSS, SLLSLSSL, or SLSLLSLS
48 I-14 SLSLSLSL
48 I-I S SLLSSLLS or SLLSSLLS
48 1-16 SLSLSLSL
48 I-17 Combination Not Possible
48 I-18 SLSLSLSL
Coil arrangements such as those described above allow optimization of
winding flux while decreasing stator winding harmonics, and further while
reducing a
number of potentially defective winding connections in comparison to known
techniques for increasing the effective number of turns per coil, such as
split-
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CA 02348228 2001-05-22
interleaved poles. Manufacturing difficulties associated with known techniques
increasing the effective number of turns per coil are further avoided.
It is contemplated that in alternative embodiments of the invention,
more than two groups of coils are used to produce an effective number of turns
with
fractional components such as 1l3, 1/4 etc. in lieu of the above-described 1/2
turn
embodiment, subject to potential assembly problems and increased winding
losses
because, unlike the 1/2 turn embodiment, only a portion of stator lamination
slots 42
would be filled, or partially filled, with coils 60.
For optimum performance each stator lamination slot 42 is filled with
as much magnet wire as possible to form coils 60. Because the number of
required
effective coil turns decreases as motor size increases, a larger motor 10
requires more
parallel strands of magnet wire. Further, to enhance energy effciency, large
low
voltage machines often include an increased amount of magnetic steel. The
additional
magnetic steel demands a greater flux of the stator winding, which further
requires
fewer coil turns 62 and an. increased number of parallel strands of magnet
wire. For
high horsepower low voltage motors (e.g., 400 HP, 460 V) the large number of
required strands of magnet wire requires a large number of spools (not shown
in
Figure 4) of magnet wire (e.g., 30 or more) of different sizes, and an
associated
lengthy setup time in manufacturing of coils 60. One aspect of the invention
simplifies manufacturing of coils 60 by reducing the number of required magnet
wire
spools to produce them.
Figure 5 schematically illustrates a winding assembly 90 including a
dereeler machine 92 and a winding arbor 94. A plurality of magnet wire spools
96 are
coupled to dereeler 92 and the respective magnet wires are threaded through
wire
guides 98 and attached to winding arbor 94. Winding arbor 94 is formed into a
selected shape of magnet wire coil 60, and when winding arbor 94 is rotated
about
axis 100, magnet wire is pulled from dereeler 92 and turns 62 in coil 60 are
formed.
To reduce the large number of spools 96 of wire and the ensuing large number
of
threaded connections to dereeler 92 and winding arbor 94, a portion of stator
coils 60
may be formed using a relatively fewer number of spools 96 and another portion
of
coils 60 could be subsequently formed also using the relatively few number of
spools
96 to form a complete coil 60. 'that is, coils 60, and more specifically S
coils 80 and
L coils 82, are formed in multiple passes through dereeler 92 and winding
arbor 94 to
cumulatively form single coils 60 with a specified number of magnet wire
strands.
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In one embodiment of the invention, half of the magnet wire spools
otherwise required to form a complete coil 60 (shown in Figure 3) is used to
form a
first pass (or first half) of the coil, and the same spools are used to form a
second pass
(or second half] of the coil to form a complete (or whole) coil 60. For
'example, in a
specific high horsepower low voltage (e.g., 400 HP, 460 V) motor 20, twenty
eight
strands of 15.0 gauge wire and six strands of 14.0 gauge wire are required to
manufacture a winding wish four turns to produce a su#~cient magnetic flux.
'thus,
thirty four spools of magnet wire would be required to manufacture complete
coils 60
at once. In this example, fourteen strands of 15.0 gauge wire and three
strands of 14.0
gauge wire, or a total of seventeen strands (or seventeen spools of wire) are
connected
to dereeler 92 and to winding arbor 94 to form a first portion of a coil 60.
The
seventeen strands are then cut and reattached to winding arbor 94 to form a
second
portion of coil 60 over the previously formed first portion. This method of
manufacture not only reduces by one half the number of required magnet wire
spools
in the first instance, but also reduces manufacturing times by requiring that
only half
the number of magnet wire spools be threaded through dereeler 92 in
manufacturing
setup.
Multiple pass construction of coils 80, 82 (shown in Figure 4) requires
randomization of the magnet strands used in each successive pass or the
different
passes of strands may have a different slot leakage reactance, which could
undesirably
lead to a circulating or unbalanced current between the passes when the stator
winding
is energized. Circulating current will create an additional losses in motor 20
(shown
in Figure 1 ). It has been observed, however, that typical operations in
removing coils
80, 82 from winding arbor 94 and inserting coils 80, 82 into the stator
lamination slots
42 sufficiently randomizes the strands of the multiple passes.
It is contemplated that in alternative embodiments, more than two
passes could be used to successively form a complete coil 80 or 82 from a
smaller
number of magnet wire spools. Any number of passes may be used, provided that
the
number of strands of the complete coil 80 or 82 is evenly divisible by the
number of
passes to be used, and further provided that the strands of the passage are
sufficiently
randomized to prevent circulating currents.
Figure 6 is a top plan view of a rotor lamination 110 including a
plurality of slots 112 separated by a plurality of teeth 114 around an outer
perimeter
116 of a lamination yoke I 18. A plurality of laminations 110 are stacked and
bonded
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CA 02348228 2001-05-22
to one another to form rotor core 28 (shown in Figure 1), and slots 112 are
filled with
magnetic wire coils (not shown) similar to stator core 26 (shown in Figure 1
and
described above), or electrical conducting bars (not shown), such as, for
example,
cast-in-place aluminum or copper bars.
Figure 7 is an exaggerated schematic illustration of motor 20 wherein
rotor core 28 is offset within stator core bore 30, thereby creating an
eccentric air gap
34 between rotor core 28 and stator core 26.
A power factor (PF) of an AC electrical system is the ratio of a power
applied to the system over the volt-amperes of the system. For a given motor,
increasing the size of air gap 34 tends to increase the magnitude of current
in the
stator windings and lower the PF. The increased current in stator core 26
increases
the stator loss and therefore would appear to contribute to increased losses
~of motor
20. Accordingly, conventional motors include air gaps 34 as small as possible
to
reduce current levels in the stator. To the contrary, however, an aspect of
the present
invention reduces total motor losses by increasing the size of air gap 34.
Induction motor theory is based upon an assumption that a radial
component of the magnetic field created by the stator winding in air gap 34 is
sinusoidally distributed and forms a single wave that rotates around air gap
34. Due
to manufacturing limitations, the conductive windings and conductive material
are not
sinusoidally distributed in either stator core 26 or rotor core 28, and
consequently the
magneto-motive force (mmf) and flux density distributions contain a series of
harmonics in addition to the fundamental wave in air gap 34. The harmonics may
be
classified as either winding harmonics or slot harmonics. Winding harmonics
are
dependent upon the winding type, the winding pitch, and the number of stator
slots 42
(shown in Figure 2). Slot harmonics are dependent upon the number of stator
slots 42
per pole. In smaller motors, winding harmonics and slot harmonics, as well as
the
associated losses, are reduced by using a large number of stator slots 42
(shown in
Figure 2), skewing rotor laminations 110 (shown in Figure 7), and optimizing a
winding pitch or span to reduce winding harmonic effects. However, for larger
high
horsepower motors, such as motor 10, stray load losses attributable to current
leakage
between the rotor bars, and manufacturing limitations of stator teeth 44
{shown in
Figure 2) renders these methods impractical or ineffective in reducing winding
harmonic effects.
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In an aspect of the present invention, winding and slot harmonics are
reduced by increasing the size of air gap 34. Increasing gap 34 decreases the
flux
level of harmonic mmf waves. While increasing gap 34 also increases the no
load
current of motor 10, thereby increasing the winding loss and decreasing the
motor PF,
S reduced stray load losses from harmonic effects offsets the winding loss and
the total
motor loss is decreased.
In addition, increasing the size of air gap 34 reduces vibration and
noise. In operation, and as seen in Figure 7, rotor core 28 is rarely centered
with
respect to stator core 26. Rather rotor core 28 is offset with respect to
stator core 26,
thereby creating an uneven air gap 34 around rotor core 28 that creates a side-
to-side
force imbalance around rotor core 28. The force imbalance creates a radial
decentering force that causes vibration in combination with the rotating flux
field in
gap 34. The force imbalance is proportional to the percentage offset of rotor
core 28
from a centered position, or to an average length of air gap 34 around rotor
core 28.
Hence, by increasing a size of air gap 34, a given offset of rotor core 28
produces a
smaller percentage offset than the same offset would produce in a motor with a
conventional minimized air gap 34. The reduced percentage offset produces a
smaller
decentering force, and hence produces less vibration and ensuing noise losses.
Increasing a size of air gap 34 also reduces another type of noise that
contributes to motor losses. As rotor core 28 rotates within stator core 26,
rotor
lamination teeth 114 (shown in Fig. 6) experience high flux levels emanating
from
stator teeth 44 (shown in Fig. 2) and low flux levels emanating from stator
core slots
42 (shown in Fig. 2) when the stator winding is energized. Thus, rotor
lamination
teeth 114 experience a pulsating flux as rotor core 28 rotates within stator
core 26.
The pulsating flux produces noise in rotor lamination teeth 114 and leads to
associated
motor losses. A larger air gap 34 more evenly distributes the flux over stator
core
slots 42 and reduces pulsation levels, thereby reducing the associated noise
and
reducing motor loss.
A larger air gap 34 also simplifies motor assembly by providing an
increased clearance between stator core central bore 30 and rotor core 28.
Therefore,
a risk of damaging stator, teeth 44 and stator windings during. motor assembly
is
reduced.
Conventional AC motor design methods tend to minimize stator bore
30 diameters to reduce total motor losses. In high horsepower low voltage TEFC
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CA 02348228 2001-05-22
motors, however, this methodology increases temperature rise of the motor,
which
counteracts heat dissipation measures and reduces output power of the motor.
Another aspect of the invention reduces total motor losses and improves motor
heat
transfer of such 'f)rFC motors by enlarging a stator bore 30 diameter.
While increasing the stator bore 30 diameter reduces the cross sectional
area of the ferromagnetic stator laminations 40 (shown in Figure 2), thereby
increasing flux density and stator core loss, increasing the stator bore 30
diameter has
several beneficial effects.- For example, a larger stator bore 30 allows a
larger
diameter rotor core 28 to be used with an increased surface area for heat
dissipation
inside the motor. A larger stator bore 30 also provides an increased cross
sectional
area for conductive winding material in the rotor lamination slots 114, which
decreases rotor resistance and therefore reduces the rotor loss. In addition,
a larger
stator bore 30, along with manufacturing limits such as stator tooth thickness
and
stator tooth depth to width ratio, provides an increased cross sectional area
for
conductive winding material in the stator lamination slots 42. The increased
cross
sectional area decreases stator resistance and therefore reduces the stator
winding loss,
as well as reduces thermal resistance of stator core 26 and improves heat
transfer from
stator core 26 to motor frame 22 (shown in Figure 1) or exterior. An
acceptable
balance between the increased stator core loss and overall heat transfer rates
of motor
20 can be determined as follows.
Figure 8 is an empirically derived chart illustrating stator slot area
versus a ratio of stator core inner diameter ("ID") to outer diameter ("10")
for
approximately equal tooth%yoke densities for a forty eight slot, two pole AC
motor I 0.
Due to motor manufacturing limitations that require, for example, a 0.25 inch
stator
tooth width, and stator slot depth of 6.5 to 7 times the tooth width to
provide sufficient
mechanical stiffness of stator teeth 44 for stator winding operations,
approximately
equal stator teeth flux density and stator yoke flux density is desirable to
achieve
maximum stator slot cross-sectional area.
It is seen from Figure 8, that a stator inner diameter to outer diameter
ratio of about 0.6 to 0.62 provides the maximum increase in stator. slot cross
sectional
area to reduce motor winding losses of a forty eight slot, two pole motor, and
improves heat transfer rates from stator core 26 (shown in Figure 7) to the
exterior
frame 22 (shown in Figure I ) of the motor. For purposes of comparison,
conventional
criteria for selection of stator diameter ratio requires a ratio of 0.58 or
smaller for two
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CA 02348228 2001-05-22
pole machines. It is observed from Figure 8 that ratios of 0.58 or smaller
offers a
smaller slot cross-sectional area that accordingly retards heat dissipation
and increases
stator winding loss in relation to diameter ratios of about 0.6 to about 0.62.
Figure 9 is an empirically derived chart graphically illustrating an
improved heat transfer rate for stator diameter ratios of about 0.6 to about
0.62
relative to conventional stator diameter ratios of 0.58 or lower.
While one exemplary stator diameter ratio is provided herein for an
exemplary forty eight slot, two pole AC motor, in alternative embodiments
other
diameter ratios, slot configurations, and winding conf gurations can be
selected to
provide a reduced winding temperature rise for a given motor physical size and
horsepower output.
In combination, the above-described motor offers many advantages
over known high horsepower, TEFC, AC induction motors or other thermally
limited
motors with limiting performance parameters relating to temperature effects.
For
1 S example, reduced motor losses allows reduction of motor size for a
selected
horsepower and reduces the resulting motor cost per horsepower. Increased
motor
e~ciency allows greater output power for a given motor size or for a lower
cost per
horsepower output of the motor. A lesser temperature rise of the motor
windings
increases motor components lifespans, such as the electrical insulation system
and the
bearing system Finer tuning of magnetic flux levels in large, high horsepower
machines is achieved, and an easier assembled and more reliable motor with
lesser
manufacturing setup times is provided. Unlike conventional motor practices,
the
foregoing advantages achieved are the product of balancing thermal dissipation
rates
and electromagnetic dissipation sates to yield the lowest tempetature rise,
rather than
traditional approaches which address thermal dissipation rates and
electromagnetic
rates isolated from one another.
It is contemplated that in certain types of motors, fewer than all of the
above-described fractional effective number of stator coil turns, multiple
pass
construction of stator windings, increased air gap size, and optimization of
stator
diameter ratio may be employed to optimize, or at least improve,
electromagnetic
geometry of other selected motors to enhance motor ef~iciericy and performance
within the scope of the present invention.
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CA 02348228 2001-05-22
While the invention has been described in tenors of various specific
embodiments, those skilled in the art will recognize that the invention can be
practiced
with modification within the spirit and scope of the claims.
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