WO2004095680A1

WO2004095680A1 - Permanent magnet tubular homopolar linear motor

Info

Publication number: WO2004095680A1
Application number: PCT/IT2004/000228
Authority: WO
Inventors: Nicola Bianchi; Silverio Bolognani
Original assignee: Universita degli Studi di Padova
Current assignee: Universita degli Studi di Padova
Priority date: 2003-04-23
Filing date: 2004-04-22
Publication date: 2004-11-04
Anticipated expiration: 2005-10-23
Also published as: ITFI20030112A1

Abstract

The herein invention has as its scope an electric linear permanent magnet motor with a modified magnetic configuration such to result as a tubular linear homoplanar motor.

Description

PERMANENT MAGNET TUBULAR HOMOPOLAR LINEAR MOTOR

DESCRIPTION

Technical field

This invention pertains to the motor field and in particular to the permanent magnet motors.

Overview on the technique

The so-called "linear" motors are featured by a fixed part, usually indicated as stator, and by a translator in respect to it, indicated as cursor.

Linear motors offer a simple and efficient solution to the issue of generating mechanical force, from electric quantities. Indeed, they allow to remove mechanic connection parts such as gears, racks, screws, joints, necessarily associated with the use of traditional rotating motors when - instead of a torque - a force is needed.

Linear motors may be classified either on the basis of the operating principle or of the constructing modes. Accordingly, we can distinguish between stepper linear motors, asynchronous motors and permanent magnet synchronous motors, at their turn to be distinguished into unilateral motors (otherwise said flat), bilateral motors (otherwise said "U- channel") and cylindrical or tubular motors. In tubular linear motors the cursor and the stator are set one within the other, where usually the cursor is set within the stator, although the opposite is likewise possible.

The advantages of tubular motors, in respect to fiat linear motors, are several: - lower joule losses for given force generated because of the absence of end-windings,

- reduced dimension,

- higher force/volume ratios,

- higher force density values,

- under symmetry conditions, cursor and stator show no attraction forces on direction normal (perpendicular) to the motion.

It is thus clear how the use of tubular linear motors brings forth crucial advantages. The issues lie on the motors constructing technique, normally created by separating the permanent magnets from the electric windings, setting the former on the cursor and the latter on the stator or viceversa.

Indeed, with regards to the outer part, the application of laminations entails considerable difficulties. The said laminations are to be arranged radially, at equal intervals, this entailing that the mechanical hold in a topology as is the one herein described is difficult to accomplish. Furthermore, the space existing from a lamination stack to the next remains unused.

Should properly shaped non-laminated rings be applied, the above specified inconveniencies would be overcome, whereas other would rise, such as consistent losses due to eddy currents - when applying iron rings - or high costs, along with low mechanical resistance - when either sintered material, or soft composite material is used. Even the inner part construction shows difficulties.

Indeed, if permanent magnets are adopted - set on the surface of the inner cylinder - they shall necessarily have the same polarity on the entire circumference, reason why, by using anisotropic magnets (with higher residual flux density) magnet sectors shall be applied with additional difficulties. On the other hand, by adopting isotropic magnets, magnetic rings with radial magnetization may be used, being slid on the shaft of the tubular linear motor. In this case though; the inconvenience lies on the low residual flux density of the said magnets in comparison to that of the anisotropic magnets. When the permanent magnets are internal (buried), rings with axial magnetization shall be chosen. In such a case the cursor inner shaft shall be thin and non-magnetic, accordingly implying a higher cost and having a certain tendency to bend, thus compromising the motor stability. This is particularly true with motors having considerably long cursor. Also the electric winding coils of conventional tubular linear motors show some constructing difficulties. Indeed, in case of massive iron-core, their insertion into the ferromagnetic part requires sectors created on purpose to suitably house the said windings, being embedded upon a complex and time-consuming procedure. In case of laminated cores, semi-closed slots - difficult to assemble - shall be applied or rather open slots, simplifying the bundle assembly although showing very low magnetic flux density in the air gap, likely to cause wide ripple of the force generated by the motor.

An additional inconvenient, related to the tubular linear motors currently in use, lies on the impossibility to allow the cursor magnetic part to exit externally from the motor frame, unless the operating environment is absolutely clean and free from any whatsoever magnetic material near to the motor. Therefore, traditional tubular linear motors show external structures double the length of the cursor, so as to grant the nominal force in any and all positions of the cursor. This is extremely expensive both in terms of material and energy waste since, assuming - for instance - the stator to be external and the cursor to be internal, only the stator lamination part facing the magnetic part of the cursor shall be involved in the energy conversion, whereas the other half shall only waste.

Disclosure of the invention

The motor objective of this invention allows to overcome the inconveniences above described by introducing a new configuration of the tubular linear permanent magnet motor in which both the permanent magnets and the electric windings are set on the same part of the motor itself that - as herein described - shall be the external fixed part of the motor.

According to this invention, the permanent magnets applied in the electric motor are arrayed to obtain that the direction of magnetization is always from the stator to the cursor, (or the other way round), in that the electric windings shall be run by a magnetic flux density field having all-through the same direction, hence the name homopolar given to the motor, otherwise indicated by the acronym TL-HPM motor (Tubular Linear Homopolar Permanent

Magnet motor ).

Figure 1 shows a section of the motor applying the herein illustrated invention. The permanent magnets may be bent in the shape of a tile (11) and assembled on the surface, or flat (12) and assembled internally (buried). The use of the permanent magnets allows a magnetic field internal to the motor to be available, free from excitation currents, therefore having no losses thereof related.

In this fashion, the currents running within the motor contribute exclusively to generate the thrust force developed by the motor, by maximizing the motor force constant, defined as the ratio between the force and the current.

Figure 2 shows that the field lines due to the permanent magnets in the outer part are always in the plane perpendicular to the motor axis.

According to this invention, the external part of the motor may advantageously be constructed with axially-stacked magnetic laminations (13), thus entailing significant benefits from the viewpoint of the assembly easiness -practically equal to that of a common cylindrical rotating motor - allowing to use a well-consolidated technology and minimizing the relevant production costs. Furthermore, the use of magnetic laminations (13) allows to minimize any motor iron losses, also in case of wide magnetic flux variations.

With reference to Figure 2 - applying this invention - the motor electric windings (14) are made of coils having the same shape as those applied in cylindrical rotating machines, their insertion within the laminations (13) resulting considerably simplified when compared to circular coils of traditional tubular linear motors. To furtherly facilitate the insertion, the external part of the motor housing them may be split into two parts, to be subsequently rejoined once the windings have been housed. Because both the windings and the permanent magnets are set within the stator, the cursor (15) is totally passive, being made of magnetic material and having a peculiar anisotropic structure as shown in Figure 3. According to this invention, being the motor cursor thoroughly passive, it may freely come out from the stator without causing any whatsoever issues related to electromagnetic pollution. Therefore, the stator length may be maintained shorter to that of traditional tubular linear machines, thus limiting any Joule-effect losses related to the part of the stator that, in this kind of motors, does not take part to generating the thrust force.

According to this invention, the TL-HPM motor may also be produced without permanent magnets. Should this be the case, its functioning principle is likewise that of a reluctance motor. This would imply lower production costs but also a lower thrust force output, resulting - differently from the reluctance motor - proportioned to the injected current. With reference to the operating mode of the motor object of this invention, under no-load operation conditions there are no currents flowing through the stator windings. The magnetic flux is thoroughly due to the permanent magnets whose flux lines are shown in Figure 2. The flux linked by the windings is varying with the cursor position. In correspondence to the cursor polar expansions (15, Fig. 3), the magnitude of the magnetic flux density is high, being low the magnetic reluctance, whereas outside the cursor polar expansions, the magnetic flux density amplitude is low, being high the magnetic reluctance. The axial component of the magnetic flux density is present in correspondence to the polar expansion edges - in the air gap - whereas in the iron it is practically zeroed. According to this invention, in case of no-load operation, the motor shall behave likewise, both with laminated iron cores and with solid iron-cores.

According to this invention, in case of operation under load, the coils of the motor windings link a magnetic field, as shown in Figure 2. When electric current flows through the windings, an electrodynamic force is generated, due to the interaction between the said current and the permanent magnet magnetic field. The greater the magnetic flux density value, the greater the force shall result; therefore the force for given current shall reach its peak in correspondence to the winding parts facing the cursor polar expansions, whereas it shall reach its lowest in correspondence to the winding parts far from the said polar expansions. With reference to Figure 4, we may see how the resulting force is made of two contributions, corresponding to the two sides of the motor coil. Because the direction of the magnetic flux density vector is the same, whereas the current directions on the two sides of the coil are opposed one to the other, accordingly the two force contributions in axial direction have opposite direction. In any case, as the magnetic flux density is higher in correspondence to the cursor polar expansions, the force output shall meet the component generated in correspondence to the cursor polar expansions. When the cursor moves, the currents are synchronised with the cursor motion, so as to constantly obtain a force with the same direction.

Under load operation, the currents flowing through the winding coils generate magnetic voltages within the motor core, producing - at their turn - reacting magnetic fluxes. The reluctance of the magnetic circuit affected by the reacting fluxes changes depends on the type of core selected. In case of cores produced with laminations, the magnetic flux density shall solely have the component directed perpendicularly to the axis and, accordingly, the field lines shall have the same direction as those generated by the permanent magnets. In case of solid iron-core, the magnetic flux density also shows an axial direction within the core itself.

According to this invention, the tubular linear homopolar motor offers several advantages:

- Possibility to build the stator magnetic circuit with laminations perpendicular to the cursor motion ,

- The use of magnetic laminations for the supplied part simplifies the motor assembly, - The use of magnetic laminations for the supplied part simplifies the insertion of electric winding coils,

- The use of magnetic laminations for the supplied part reduces iron losses,

- To furtherly simplify the insertion process of electric winding coils, the stator structure may be produced in two parts, separated by an axial plane - Possibility to use both superficial and interior permanent magnets,

- In case of permanent magnets application, the force density is extremely high,

- Under symmetry conditions, there are no attraction forces perpendicularly to the motion between the fixed part and the moving part, - According to this invention, the entire active part of the motor generates thrust force,

- According to this invention, the motor reaches greater forces than the conventional ones at equal stator length, or it has inferior stator lengths, at equal force generated,

- The cursor is totally passive, therefore unaffected by electromagnetic pollution phenomena,

- According to this invention, the motor - as is constructed - is modular.

Brief description of the drawings

FIGURE 1 : sections of the outer part of the motor applying the herein illustrated invention, FIGURE 2: section of the motor with indications both of the electric winding positions and of the magnetic field lines, FIGURE 3 : motor cursor applying the herein illustrated invention, FIGURE 4: illustration of the interaction between the magnetic fields generated by permanent magnets and winding currents.

Claims

1. Tubular linear homopolar permanent magnet motors, that is featured by the fact that the electric windings link a magnetic flux density field with constant direction (single polarity),

2. As per claim in point 1 above, motors where both permanent magnets and electric windings are set on the same part of the motor,

3. As per claims in points 1 and 2 above, motors where the external part is made of axially- stacked magnetic laminations (13),

4. As per claims in points 1 and 2 above, motors where the external part is made of solid iron- core,

5. As per claims from point 1 to 3 above, motors where the permanent magnets (11) are surface mounted with respect to the lamination stack,

6. As per claims from point 1 to 3 above, motors where the permanent magnets (12) are interior with respect to the lamination stack,

7. As per claims from point 1 to 6 above, motors where the external part is split in two parts assembled - during the assembling phase - once the electric windings have been inserted,

8. As per claims from point 1 to 7 above, motors where the electric windings (14) are made of circular-shaped coils,

9. As per claims from point 1 to 8 above, motors where the cursor (15) is totally passive and made of magnetic material.