Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
1999-11-22
2001-11-06
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
Reexamination Certificate
active
06313552
ABSTRACT:
The following invention relates to permanent magnet linear electric motors, in particular those used for applications requiring extended travel.
Numerous types of linear motor exist for effecting powered travel over extended distances. Examples are a) the AC linear induction motor, b) the ‘sandwiched coil’ construction permanent magnet linear motor, and c) dc brushless motors of various configurations. Each of these examples suffers from various deficiencies. In the case of the AC induction motor, there are substantial electromagnetic and resistive losses inherent in both its construction and mode of operation, and the maximum velocity of movement is limited by factors such as the frequency of the AC supply used, and the practical limitations on the effective pole pitch of the armature coils. In addition, for systems in which the stator is used as the moving part, armature coils are required over the full length of travel, which is both expensive, and wasteful of energy. In the case of sandwich type constructions, in which the motor's armature coils are located between two facing rows of magnets, it is self evident that heat cannot escape conveniently from the said coils. The ability to dissipate heat is a key measure of the effectiveness of any linear motor design. In the case of flat dc brushless motors, these necessitate the use of a copious volume of permanent magnet material along their entire length, and are therefore expensive. Furthermore, the ways must be flat, and in many designs a very tight air gap must be maintained.
An ideal design is one in which a) a relatively limited volume of permanent magnet material is needed per unit length of travel, b) there is no need for a critical airgap between the stator and armature, and c) heat generated by the travelling armature coils can easily be dissipated. Such a motor, now in widespread use, is described in UK granted patent no. 2079068. Although a successful commercial design, an inherent disadvantage of this tubular type of construction, is however, the limited travel that may be realised. This is due to the tendency of the stator to bow under the effect of gravity, and thereby come into contact with the moving coils of the armature, which coaxially surround it. Travel is therefore limited, in practice, to no more than two metres. An ideal motor is one which combines the inherent advantages of the aforementioned invention, with the ability to travel extended distances.
According to the present invention, there is provided a linear motor comprising an armature and a stator coaxial with one another, the stator having a plurality of magnetic flux generators extending along the longitudinal axis of the motor over the required travel of the armature relative to the stator and providing a repeating sequence of magnetic poles along said axis, the armature having a plurality of phases of drive coils coaxial with the stator for providing, when appropriately commutated, thrust, the coils being wound such that they substantially surround the stator but leave a single gap extending transversely of the longitudinal axis of the motor to allow the presence of means, extending radially through the gap, for the mechanical support of the stator.
This gap means that unlike prior linear motors using coaxial drive coils, the stator and its flux generators are not restricted to being supported only at the opposite ends of the stator and can be supported at locations intermediate its ends and, indeed, over its entire length.
In the present motor, due to the coaxial arrangement of the drive coils, the thrust is produced by currents passing through the drive coils interacting with the lines of magneto motive force produced by the stator flux generators which are principally directed radially relative to the motor longitudinal axis. The parts of the coil conductors in which this thrust is generated are those which extend circumferentially around the motor longitudinal axis.
The requirement of the provision of the stator support gap is, on the face of it, in conflict with the winding of the armature coils from continuous electrical conductors (wire or tape) for this reason: the coil conductor cannot, because of the gap, extend 360° around the motor axis and must therefore after one part-turn (i.e. 360° minus a gap angle) turn back on itself around the motor axis. However, this would result in the current flow being in the opposite turning direction, cancelling out the thrust generated by the part-turn in question. In an embodiment of the invention, this problem is overcome as follows:
each coil is configured as two sub-coils which are spaced from one another longitudinally of the motor and have winding portions which extend clockwise and anti-clockwise respectively round the axis of the motor;
these sub-coils are longitudinally spaced apart by a distance, chosen in relation to the spacings of the stator poles, such that they are subject to radial lines of force from the stator poles which are of opposite polarities. Thus, the current flows in the sub-coils, being of opposite clock senses, produce thrust forces in the same longitudinal direction.
In order to assist visualisation of the way in which the coil is wound, each “turn” of the coil winding is made up of the following contiguous portions:
starting at one side of the gap, a portion which extends in a circular arc, in a plane perpendicular to the motor axis, around the motor axis to the other side of the gap (this circular arc subtends less than 360° around the motor axis to leave the mechanical support gap);
from there, a portion which turns perpendicular to that plane, i.e. now parallel to the motor axis, along the length of the armature to the other sub-coil of the coil;
from there a portion corresponding to the first portion, extending circumferentially around the motor axis through an arc corresponding to the first-mentioned one but counterclockwise to the starting side of the gap;
finally, a portion extending parallel to the motor axis, clear of the gap, returning to the first sub-coil.
In the illustrated embodiment of the invention, the coils of the respective phases of the motor overlap one another longitudinally of the motor and, to allow for this, the portions of the conductors which extend between the two sub-coils are not exactly parallel to the motor axis but are shaped to allow the overlapping of the coils. In a preferred embodiment of the invention, the stator flux generators are arranged such that successive flux generators have their magnetic poles facing one another i.e. in a NS . . . SN . . . NS . . . SN . . . sequence. Further, the armature sub-coils are contiguously overlapped so that there are no longitudinal spaces left between sub-coils of respective phases of the motor. The stator flux generators are conveniently constituted by a stacked sequence of permanent magnets and intermediate spacers so as to provide the required NS . . . SN . . . NS . . . SN . . . sequence.
Thus, in this arrangement, because the armature coils do not circumscribe the stator of the motor, the stator may be mechanically supported along its full length, so enabling the provision of a motor of whatever length is required to meet a particular application. At the same time, the maximum possible flux linkage between the armature coils and the stator magnetic stack is achieved, by virtue of their coaxial alignment and operation. A tubular linear motor results, with a number of salient advantages, as follows.
Firstly, and most significantly, the electromagnetic efficiency of the motor arising from the manner in which flux squeezing occurs due to the disposition of the permanent magnets. (By way of explanation, because like poles are facing, virtually all of the magnetic energy stored within the permanent magnets is caused to emanate radially, for direct linking with the coils of the armature.) Secondly, because the coils are arranged contiguously, all of the length of the armature is occupied by copper, so optimising the number of turns working against the flux emanating from the stator. Thirdly
Harness & Dickey & Pierce P.L.C.
Jones Judson H.
Linear Drives Limited
Ramirez Nestor
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