The invention relates to a linear synchronous motor.
Synchronous motors which are used as actuating motors should develop power in a way which is as uniform and free from interference as possible.
In the case of rotary synchronous motors, it is essentially the slotting of the stator which comes into consideration for causing periodic power fluctuations, also generally termed cyclic power variation. In order to compensate this cyclic power variation and other further effects, caused by the slotting, on the torque at the drive shaft, the rotor and/or stator poles are usually skewed over the width of a slotting.
It is also known, from U.S. Pat. No. 4,908,533, in the case of linear synchronous motors to bevel the poles over the width of a slot of the wound primary part in order to avoid the cyclic power variation. Since the edges of the end faces of the primary part run parallel to its slots in plan view, skewing results at the front and rear end edges of the poles in the case of the known slot skewing.
A further possibility, known from EP 0 334 645 A1, for reducing the cyclic power variation consists in designing the core of the primary part of a linear synchronous motor as a ferromagnetic plate, and arranging coils in the air gap of the linear motor such that the end regions of the plate project over the air gap coils and form a step in the region of the longitudinal mid line of the linear motor.
Unlike the rotary synchronous motors, which continue endlessly viewed in the circumferential direction, a linear synchronous motor has, as a particular feature, a start and an end. In the case of a linear synchronous motor, periodic motor end forces, which can have a disturbing effect on the continuous movement of the linear motor, are produced in the direction of movement at the transitions at the start and at the end.
The motor end forces are produced because the linear motor covers the magnetic poles differently depending on the motor position. There are preferred positions in this case, in which the stored magnetic energy of the linear motor is particularly high. An additional expenditure of force is required to move the linear motor out of such preferred positions. There is a preferred position over each magnet pole.
The pole force therefore varies periodically in relation to the magnet poles, and this leads to a disturbance in the motor power which is denoted as cyclic pole variation. Since the pole force is not a function of the motor current, it constitutes a passive force which is also present in the de-energized state. The pole force does not perform any work, since it acts alternately in the direction of movement and against the direction of movement of the linear motor. In operation, it is added to the force produced by the motor current. The pole force has nothing in common with the slot force with which the magnet pole edges and the stator slots act on one another.
The cyclic pole variation described leads to inaccurate movement of conventional linear synchronous motors, and this is particular undesirable when these motors are used as precision actuators.
Consequently, it is an object of the invention is to provide a linear synchronous motor which is capable to virtually completely suppress the cyclic power variation.
According to the invention, this object is achieved by means of a linear synchronous motor having the following features:
at least one primary part and at least one secondary part,
the secondary part has a series of poles formed by permanent magnets,
the length of the secondary part is greater than the length of the primary part in the direction of movement,
the primary part has slots which are suitable for holding monophase or polyphase windings,
the primary part has means which lead to a change in the magnetic force in the direction of movement of the linear motor in the region of the end pieces of the primary part, and
the end faces of the end pieces extend perpendicular to the direction of movement of the linear motor.
In the linear synchronous motor according to the invention, the air gap is formed in the region of the end pieces of the primary part in such a way that it varies from section to section. The end faces of the end pieces are designed in parallel and perpendicular to the direction of movement in each case. The cyclic power variation is substantially reduced hereby while yet maintaining the compact design of the linear synchronous motor virtually unchanged.
The air gap of the end pieces is formed in a further embodiment in such a way that the change in the magnetic force on the end pieces is continuous in the case of a relative movement of the primary and secondary parts. Because of the formation of the end pieces of the primary part in accordance with the invention, for each pole force contribution at the front side of the linear motor there is exactly one pole force contribution of equal and opposite magnitude at the rear side of the linear motor. The formed end pieces of the primary part are preferably not slotted and wound.
In a further embodiment, the parts of the end pieces of the primary part, that face the air gap, have a geometry which is selected in accordance with the following relationship,       y    ⁢          xe2x80x83        ⁢          (      x      )        =            δ      0        ⁡          [                        1                                    1              -                                                x                                      x                    0                                                  ·                                  [                                      1                    -                                          (                                              1                                                  1                          +                                                                                    y                              0                                                                                      δ                              0                                                                                                                          )                                                        ]                                                                    -        1            ]      
wherein xcex40 is the magnetically active air gap between the secondary part and the primary part, including the height of the permanent magnets,
x0 is the extent of the part of the end piece in the direction of movement of the linear motor having a non-constant air gap,
y0 is a height of the part of the end piece having a non-constant air gap at x0 and,
y(x) is the coordinate of the part of the end piece having a non-constant air gap at the point x.
In this case, the magnetic force on the end pieces decreases and increases linearly in the case of relative movement of the primary part and secondary part. The length of the end pieces in the direction of movement of the linear motor can thereby be kept short, so that the spatial extent of the primary part can be limited to the dimensions most necessary. The parameters are preferably selected as xcex40=5 mm, x0=5 mm and y0=4.2 mm.
In a further embodiment, the gaps, located between the poles, of the secondary part exhibit an angle differing from 90xc2x0 with respect to the direction of movement of the linear motor. In the following, the term xe2x80x9cpolexe2x80x9d will be understood as an arrangement of, for example, at least one permanent magnet which has a north pole and a south pole. The skewing is preferably selected in the region of the width of a slot of the primary part. The pole skewing is to be increased or to be decreased together with the end piece forming of the primary part, depending on the selection of the parameters in accordance with the above relationship with reference to the profile of the end piece.
In a further embodiment, the gaps located between the poles are designed essentially perpendicular to the movement direction, but have different gap widths so as to further contribute to the reduction of the cyclic power variation also in this case.
The gap widths are to be enlarged or reduced together with end piece formation of the primary part, depending on how the parameters are selected in accordance with the above relationship with reference to the profile of the end piece.
In a further embodiment, the stack of ferromagnetic laminations is subdivided into several partial stacks of laminations extending perpendicular to the direction of movement of the linear motor, in order to optimize assembly and stockholding.
The formed end pieces of the primary part can preferably be produced separately and fitted on the primary part such that, depending on the primary part and intended use, the end pieces as such can be produced and assigned to the respective primary part. All known positive and non-positive types of connections can hereby be used.