Elevator with linear motor drive assembly

A multiple linear motor arrangement is provided having a secondary, a first primary, a plurality of second primaries, and a frame, in which the body of the first primary of the motor arrangement on one side of the secondary is offset from the bodies of the second primaries on the other side of the secondary. Magnetic flux emanating from the first and second primaries, therefore, does not appreciably overlap.

TECHNICAL FIELD 
This invention relates to an elevator linear motor drive assembly. 
BACKGROUND ART 
Linear motors having a flat secondary and a flat, mobile primary(s) may be 
employed as drive means for elevators. In one embodiment, a rail fixedly 
mounted in the hoistway acts as the secondary of the linear motor. The 
primary(s) attaches to and drives either the elevator car or the 
counterweight, using the rail as both secondary and guide. This elevator 
drive arrangement advantageously fits within the hoistway, thereby 
eliminating the need for a separate machine room. 
Linear motor drives are not without their problems, however. To begin, 
standard guide rails typically do not provide enough cross-sectional area 
to accommodate the magnitude of magnetic flux generated by the motor 
primary. As a result, the density of the flux in the rail generally 
exceeds acceptable limits, thereby negatively effecting motor performance. 
The problem is exacerbated in multiple motor configurations. For example, 
Japanese Patent Publication No. 63-117884 by Mitsui teaches a four-motor 
configuration having two pairs of motors symmetrically opposed to one 
another. Symmetrically opposed motors offer the advantage of each motor's 
attractive force balancing the attractive force of the other motor. The 
disadvantage of symmetrically aligned motors is that the cross-section of 
the guide rail (or secondary) must be wide enough to accommodate the 
magnetic flux generated from primaries on both sides of the rail. Wider 
secondaries are more expensive to both fabricate and to install. In some 
applications heavier rails may even require that the building be 
reinforced to accommodate the increased load. 
In addition, four-motor configurations as taught by Mitsui are inherently 
inefficient. All flat linear motors have motor windings which include 
numerous coil ends extending out from the metallic body of the primary. 
The shape of each coil end is determined by the smallest geometry possible 
which still permits the motor windings to be routed through the metallic 
body of the primary. Since the coil ends extend a length outside the body 
of the primary, they do not participate in the motor thrust and 
consequently do not increase the efficiency of the motor. In fact, they 
create joule losses thereby decreasing the efficiency of the motor. 
Mitsui's multi-motor arrangement, with four similar primaries, has the same 
ratio of primary body width to coil end length as a linear motor with a 
single primary of similar primary body width. In the four primary 
arrangement, the amount of body width and coil end length is just a 
multiple of that in the single primary linear motor. Therefore, whatever 
inefficiencies are associated with the ratio of primary body width to coil 
end length in the single primary are present in the four primary 
arrangement of Mitsui, assuming that all other variables such as the 
numbers of windings remain constant. 
In sum, what is needed is a new multiple linear motor arrangement. 
DISCLOSURE OF THE INVENTION 
It is, therefore, an object of the present invention to increase the 
efficiency of a multi-primary linear motor arrangement. 
It is a further object of the present invention to provide a linear motor 
which minimizes fabrication and installation expenses. 
According to the present invention, an elevator multiple linear motor 
arrangement is provided in which the body of the primary(s) of the motor 
arrangement on one side of the secondary is offset from the body of the 
primary(s) on the other side of the secondary, and therefore does not 
appreciably overlap. 
An advantage of the present invention is the orientation of the magnetic 
fields in the secondary due to the configuration of the primaries. 
Offsetting the primaries permits the magnetic field emanating from each 
primary to access a different region of the secondary, without appreciably 
overlapping each other. The flux density in each region is therefore 
attributable to a single primary. Consequently, design constraints on the 
secondary's cross-sectional area created by primary magnetic flux are 
effectively halved. The secondary, therefore, may be fabricated lighter 
and installed less expensively, since no hoistway reinforcement is 
required. 
A further advantage of the present invention is the increase in efficiency 
created by the present invention's increase in ratio of primary body width 
to coil end length. The present invention positions an equal amount of 
primary body width, using primaries of equal length, on each side of the 
secondary as is taught in the prior art. Equal amounts of primary body 
width on each side of the secondary creates a force balance across the 
secondary. The present invention creates the force balance, however, with 
an overall odd number of primaries. As a result, the ratio of primary body 
width to coil end length increases, thereby eliminating a percentage of 
the inefficiencies associated with end windings. The overall efficiency of 
the linear motor, therefore increases. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the detailed description of the best 
mode embodiment thereof, as illustrated in the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1 and 2, a linear motor comprising a secondary 14, a 
first primary 16, and two second primaries 18 provides motive power for an 
elevator (not shown). The elevator comprises an elevator car (not shown) 
and a counterweight assembly 10 attached to one another by a series of 
ropes in a hoistway (not shown), as is known in the art. The counterweight 
assembly includes the primaries of the linear motor and a frame 20 
attached to the primaries. The frame comprises a "U"-shaped body 17 and 
two flat primary supports 19 bolted to the ends 21 of the legs 23 of the 
"U". 
The secondary 14 has a first side 22 and a second side 24 and extends 
throughout the hoistway 30. A web 26 conventionally attaches to the center 
of the secondary's second side and outwardly extends, perpendicular to the 
secondary. The end of the web opposite the secondary conventionally 
attaches to a base plate 28 which in turn is secured to the hoistway. The 
"T" shaped secondary acts as the guide rail for the counterweight 
assembly. 
A coating or layer 32 of highly conductive metal such as aluminum may be 
attached to the first 22 and second sides 24 of the secondary 14. The 
layer of highly conductive metal 32 does not cover the entire first or 
second side. Bare side margins 34 are left which permit braking or 
guidance on the secondary without damage to the highly conductive layer. 
Each primary 16,18 has a body 38 defined by a width 36 and a surface area 
37 (see FIG. 2). A plurality of windings 40 run through the body 38 of the 
primary, extending out on two sides of the body, as is known in the art. 
The windings outside of the body are called end windings or coil ends 42. 
Because the coil ends create inefficiencies in the motor, it is desirable 
to minimize the coil end length 44. Physical constraints, however, prevent 
the coil ends from being minimized beyond a certain point, thereby 
creating an inherent inefficiency in the flat linear motor. 
The counterweight assembly includes a set of rollers 46 attached to the 
frame 20 which act against the secondary 14. The rollers 46 maintain the 
frame, and therefore the primaries, a predetermined distance from the 
secondary. The distance is known in the art as the "separation distance" 
or the "air gap". A set of second rollers 50 acts against the outside 
edges 52 of the secondary. The second rollers keep the frame and the 
primaries in the correct position parallel to the secondary. 
When the counterweight assembly 10 is positioned within the hoistway, the 
first primary 16 mounts on an interior side of the frame 20, in close 
proximity to the first side 22 of the secondary 14. The second primaries 
18 mount on the opposite interior side of the frame, in close proximity to 
the second side 24 of the secondary. The bodies of the first and second 
primaries are offset from one another. In other words, if the bodies of 
the first and second primaries which are ordinarily on different sides of 
the secondary, were moved into the same plane on the same side of the 
secondary, the bodies would not appreciably overlap each other, if at all. 
If the primaries 16,18 overlapped or were aligned with one another on 
opposite sides of the secondary 14, the magnetic flux from each would 
enter the same cross-sectional area of the secondary. As a result, the 
magnetic flux density would equal the sum of the flux densities 
attributable to each primary. In that instance, the combined flux density 
may be greater than the saturation level permissible within the secondary. 
In the present invention, the primaries 16,18 do not appreciably overlap. 
The magnetic flux densities attributable to all three primaries, 
therefore, are not additive and the cross-sectional area of the secondary 
14 must only accommodate the magnetic flux emanating from a single primary 
in any given region. This allows the secondary to be designed in a more 
narrow, lighter, and less expensive configuration. Other design 
considerations, such as strength and rigidity, may also affect the design 
of the guide rail. 
A person of skill in the art will recognize that the offset configuration 
of the primaries 16,18 relative to the secondary 14 in the present 
invention may be used with secondary geometries other than the "T" 
geometry of the best mode. 
The configuration of the primaries 16,18 increases the ratio of the primary 
body width 36 to the coil end length 44 (see FIG. 2). In the prior art 
multiple linear motor arrangement, equal numbers of primaries were 
positioned opposite one another to balance the attractive forces created 
by the motors. Because the number of primaries were equal, the ratio of 
the body width 36 to the coil end length 44 was the same for "n" number of 
primaries as it was for a single primary. 
In the present invention, two "L" width primaries (the second primaries 
18), are positioned on one side of the secondary 14. A single "2L" width 
primary (the first primary 16) is positioned on the other side, thereby 
creating a force balance across the secondary. As a result, the ratio of 
body width 36 to coil end length 44 is four body widths to six coil end 
lengths, or in unitless terms, a ratio of two-thirds. In the prior art 
configurations with equal numbers of primaries, the ratio is one body 
width 36 to two coil end lengths 44, or a unitless ratio of one-half. 
In this example, the increase in the unitless ratio from one-half to 
two-thirds represents an increase in motor efficiency for two reasons. 
First, motor thrust increases as the primary body width 36 percentage 
increases. Second, coil end joule losses decrease as the coil end length 
44 percentage decreases. 
A person skilled in the art will recognize that the unitless ratios of 2/3 
and 1/2 discussed heretofore represent an illustrative example focusing on 
one embodiment. Other embodiments incorporating the teachings of the 
present invention may include primaries with different geometries which 
would therefore alter the exact ratios discussed. 
Although this invention has been shown and described with respect to the 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of the claimed invention.