ELECTRIC LINEAR MOTOR

Abstract

The invention refers to an electric linear motor comprising a longitudinal stator beam; at least one mover adapted to move along the stator beam; which stator beam comprises at least two side faces located at opposite sides of the stator beam, each of the side faces carrying ferromagnetic poles spaced apart by a pitch, and which mover comprises at least two counter-faces facing the respective side faces of the stator beam, wherein the at least two side faces, as well as the at least two counter-faces facing the respective side faces, are inclined or offset with respect to each other.

Claims

1. An electric linear motor, comprising a longitudinal stator beam; at least one mover adapted to move along the stator beam; which stator beam comprises at least two side faces located at opposite sides of the stator beam, each of the side faces carrying ferromagnetic poles spaced apart by a pitch, and which mover comprises at least two counter-faces facing the respective side faces of the stator beam, wherein the at least two side faces, as well as the at least two counter-faces facing the respective side faces, are inclined or offset with respect to each other.

2. The electric linear motor according to claim 1, wherein the mover has in at least one of said counter-faces at least one rotor unit having at least one winding and at least one permanent magnet, which are arranged to co-act with the ferromagnetic poles of the respective side face of the stator beam.

3. The electric linear motor according to claim 1, wherein the mover has in each of said counter-faces at least one rotor unit having at least one winding and at least one permanent magnet, which are arranged to co-act with the ferromagnetic poles of the respective side face of the stator beam.

4. The electric linear motor according to claim 1, wherein the stator beam comprises at least four side faces located two by two at opposite sides of the stator beam, such that the four side faces substantially cover circumference of the stator beam, and wherein the mover comprises at least four counter-faces facing the respective side faces of the stator beam, and wherein the side faces located at opposite sides of the stator beam, as well as the counter-faces facing said side faces, are inclined or offset with respect to each other.

5. The electric linear motor according to claim 4, wherein each of the side faces carries ferromagnetic poles spaced apart by a pitch, and wherein the mover has in each of said counter-faces at least one rotor unit having at least one winding and at least one permanent magnet, which are arranged to co-act with the ferromagnetic poles of the respective side face of the stator beam.

6. The electric linear motor according to claim 1, wherein the ferromagnetic poles are teeth provided on a side face of a ferromagnetic stator rod, which teeth which are spaced apart by teeth gaps.

7. The electric linear motor according to claim 1, wherein the side-faces carrying ferromagnetic poles of the stator beam do not have any permanent magnets as well as no windings either.

8. The electric linear motor according to claim 1, wherein each of said rotor units comprises permanent magnets as well as motor winding, preferably three-phase motor winding.

9. The electric linear motor according to claim 1, wherein the mover has in each of said counter-faces at least two rotor units arranged consecutively in the travelling direction, each of said rotor units having at least one winding and at least one permanent magnet, which are arranged to co-act with the ferromagnetic poles of the respective side face of the stator beam.

10. The electric linear motor according to claim 1, wherein each of said rotor units contains at least two rotors having windings connected in series or in parallel.

11. A control apparatus of an electric linear motor according to claim 1, wherein the control apparatus comprises at least one drive unit configured to supply electrical power to the respective at least one rotor unit of the mover.

12. The control apparatus of claim 11, wherein the control apparatus comprises drive units configured to supply electrical power separately to the respective rotor units of the mover such that each rotor unit is supplied by a separate drive unit.

13. A transport system comprising an electric linear motor and a control apparatus according to claim 11, the transport system further comprising: a mobile load-receiving part coupled to the mover and arranged to travel along a trajectory defined by the stator beam by means of the propulsion force of the mover.

14. A method of controlling the electric linear motor with a control apparatus according to claim 11, the method comprising obtaining position information of the mutual position of the ferromagnetic poles and the at least one rotor unit facing said ferromagnetic poles, the position information being obtained in the travelling direction of the rotor unit representing d, q-coordinate system of said at least one rotor unit by means of the position information such that the d-axis of said rotor unit is in the direction of the ferromagnetic poles facing the rotor unit and the q-axis is orthogonal to the d-axis obtaining information of length of air gap between the ferromagnetic poles and the at least one rotor unit facing said ferromagnetic poles supplying, by means of the at least one drive unit a d-axis current component to the at least one winding of the at least one rotor unit to adjust the length of air gap towards given reference value wherein the d-axis current component is established based on the difference between air gap reference value and obtained air gap length information.

15. The method according to claim 14, comprising: obtaining position information of mutual position of ferromagnetic poles located at opposite sides of the stator beam and the rotor units facing said ferromagnetic poles, the position information being obtained in the travelling direction of the rotor unit representing d, q-coordinate systems of said rotor units by means of the position information such that the d-axis of each said rotor unit is in the direction of the ferromagnetic poles facing the rotor unit and the q-axis is orthogonal to the d-axis obtaining information of length of air gap between the ferromagnetic poles and the rotor units facing said ferromagnetic poles supplying, by means of the drive units separate d-axis current components to the windings of the rotor units at opposite sides of the stator beam to adjust the length of air gaps towards given reference values, wherein the separate d-axis current components are established based on the difference between air gap reference value and obtained air gap length information.

16. The method according to claim 14, wherein the mover has in at least one of said counter-faces at least two rotor units arranged consecutively in the travelling direction, each of said rotor units having at least one winding and at least one permanent magnet, which are arranged to co-act with the ferromagnetic poles of the respective side face of the stator beam, and wherein the control apparatus comprises drive units configured to supply electrical power separately to the respective rotor units of the same counter-face, the method comprising: supplying by means of the drive units separate d-axis current components to the windings of rotor units of the same counter-face to straighten tilt of the air gap, the separate d-axis current components being established based on difference between air gap reference value and air gap length information.

17. The method according to claim 14, comprising: obtaining travel position information and/or travel speed information of the mover feeding with the at least one drive unit to the at least one winding of the at least one rotor unit a q-axis current component based on the difference between travel position reference and obtained travel position information and/or travel speed reference and obtained travel speed information to adjust the travel position and/or speed towards said position and/or speed reference.

18. The method according to claim 17, comprising: feeding with the drive units to the windings of rotor units separate q-axis current components based on the difference between travel position reference and obtained travel position information and/or travel speed reference and obtained travel speed information to adjust the travel position and/or speed towards said position and/or speed reference.

19. The method according to claim 14, comprising: changing at least one of d-axis current component and q-axis current component of a rotor unit responsive to change of at least one of travel position information, travel speed information and air gap length information when changing the at least one of d-axis current component and q-axis current component, providing at the same time a correction term to the other of the d-axis current component and the q-axis current component to compensate the effect of change to the attraction force and/or propulsion force of the mover.

20. The method according to claim 14, comprising: calculating a propulsion force reference value based on difference between travel position reference and obtained travel position information and/or between travel speed reference and obtained travel speed information of the mover calculating an attraction force reference value based at least on difference between air gap reference value and air gap length information changing at least one of d-axis current component and q-axis current component of a rotor unit responsive to change in at least one of propulsion force reference value, attraction force reference value and air gap length information of the rotor unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention is now described hereinafter with respect to the enclosed drawing. In this drawing

[0049] FIG. 1 shows a side view of an electric linear motor according to an embodiment

[0050] FIG. 2A shows a cross-section through a stator beam and a mover of FIG. 1,

[0051] FIG. 2B shows a cross-section through a stator beam and a mover of FIG. 1 of an alternative modification,

[0052] FIG. 2C shows a cross-section through a stator beam and a mover according to an embodiment of the invention,

[0053] FIG. 2D shows a detail of a cross-section through a stator beam and a mover according to an embodiment of the invention.

[0054] FIG. 3 shows a schematic drawing of the function of a switching permanent magnet motor (FSPM) according to an embodiment,

[0055] FIG. 4 illustrates schematically the control system according to an embodiment

[0056] FIG. 5 shows a side view of a multi-car elevator system according to an embodiment,

[0057] FIG. 6 shows a horizontal cross-section of the parts of the elevator motor and the guide rails in the area between the elevator car and the shaft wall of FIG. 5.

[0058] FIG. 7 shows a side view of an electric linear motor according to an embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] It is emphasized that identical parts or parts with the same functionality are designated by the same reference numbers in all figures.

[0060] FIG. 1 shows a side view of an electric linear motor. To facilitate understanding of the matter, only two opposite side faces 6A, 6B and respective counterfaces 7A, 7B of the motor are illustrated in FIG. 1. The linear motor comprises a longitudinal stator beam 1 and a mover 24, 26, which surrounds the stator beam 1. The stator beam 1 has four side faces 6A, 6B, 6C, 6D as illustrated in FIG. 2A and FIG. 2B. The side faces are located two by two at opposite sides of the stator beam 1, such that the four side faces 6A, 6B; 6C, 6D substantially cover circumference of the stator beam 1. Each of the side faces carries ferromagnetic poles 8, i.e. ferromagnetic teeth, spaced apart by a pitch 8, e.g. a gap or slot between the teeth 8.

[0061] The mover 24, 26 comprises four counter-faces 7A, 7B; 7C, 7D facing the respective side faces 6A, 6B; 6C, 6D of the stator beam 1.

[0062] The mover has in each of said counter-faces 7A, 7B; 7C, 7D rotor units 2,3,4,5; 2,3,4,5. The motor may be a flux-switching permanent magnet motor as depicted in FIG. 3. All the permanent magnets and three-phase motor windings are in the rotor units 2, 3, 4, 5. 3. In the embodiment of FIG. 3, the ferromagnetic poles 8 are teeth provided on a side face 6A, 6B; 6C, 6D of a ferromagnetic stator rod 50, which stator rod 50 is embedded into respective side-face of the stator beam.

[0063] The stator side of the motor is very simple, as the side-faces 6A, 6B; 6C, 6D of the stator beam carrying ferromagnetic poles 8 do not have any permanent magnets as well as no windings either. This simplicity is cumulative when the stator beam 1 becomes long to extend moving range of the mover 24, 26. When mover 24, 26 travels along the stator beam 1, there is an air gap 15 between the side faces 6A, 6B, 6C, 6D and the counter-faces 7A, 7B, 7C, 7D. This air gap 15 is maintained in a noncontact manner with levitation. The windings 74, 76 and permanent magnets 71 of the rotor units are arranged to co-act with the ferromagnetic poles 8 of the respective side faces 6A, 6B; 6C, 6D of the stator beam 1 to generate force components needed to levitate and drive the mover 24, 26 along the trajectory defined by the stator beam 1.

[0064] The expression at least two side faces 6A, 6B, 6C, 6D located at opposite sides of the stator beam 1 means that surface normals of said at least two side faces (n.sub.1, n.sub.2, n.sub.3, see FIG. 2C) both have a vector component such that said vector components are in opposite directions. Therefore, when attraction force is generated between said rotor units 2, 3, 4, 5 and the respective side faces 6A, 6B; 6C, 6D, the generated attraction forces have vector components in opposite directions relative to each other to enable adjustment of air gap length parallel to y-direction of FIG. 1 and therefore levitation of the mover.

[0065] Further, in some embodiments it may be necessary to control turning of the mover 24, 26 around the longitudinal axis (parallel to direction x of FIG. 1) of the stator beam. To enable this, stator and mover may be designed such that rotating torque is generated around the stator beam 1. Thus, as illustrated in FIG. 2D, at least some of the opposite side faces 6A, 6B may be inclined with respect to each other, that is, angled from parallel direction around the longitudinal axis of the stator beam. Of course the respective counter faces 7A, 7B of the mover have to be inclined in the same manner to face the side faces 6A, 6B.

[0066] As illustrated in FIG. 2C, at least some of the side faces 6A, 6B, 6C and the respective counter faces 7A, 7B, 7C may be curved.

[0067] As illustrated in modification of FIG. 2B, at least some of the opposite side faces (and the respective counter faces 7A, 7B, 7C, 7D) may be offset with respect to each other. The side faces 6A, 6B, 6C, 6D (and the respective counter faces 7A, 7B, 7C, 7D) may form a parallelogram. Also this modification may enable generation of rotating torque around the stator beam 1.

[0068] The mover frame 25 may be made of any suitable rigid, preferably light-weight material, such as glassfiber composite, carbon fibre composite or aluminium.

[0069] As FIG. 1 shows, the mover 24, 26 has in each counter-face 7A, 7B two rotor units 2, 3; 4, 5 arranged consecutively in the travelling direction, which is parallel to direction x in FIG. 1. Two consecutive, rotor units are needed to straighten tilt of air gap 15. Each rotor unit is supplied with its own inverter 9, 10, 11, 12. In an alternative embodiment the mover 24, 26 has in each counter-face 7A, 7B three rotor units arranged consecutively in the travelling direction, and each rotor unit is supplied with its own inverter. In some other embodiments there may be even more than three rotor units per counter-face/inverters for supplying the same. Still in another embodiment, as illustrated in FIG. 7, the mover 24, 26 has in one counter-face 7A two rotor units 2, 3 arranged consecutively in the travelling direction, whereas the other counter-face 7B at opposite side of the stator beam 1 has only one, longer rotor unit 4. Each rotor unit 2, 3, 4 has an inverter 9, 10, 11. Also this kind of solution may be adequate to straighten tilt of air gap 15 with control of the rotor units. Further, to achieve uniform force distribution, each rotor unit has two (or even more than two) commonly controlled rotors 2A, 2B; 3A, 3B; 4A, 4B; 5A, 5B with windings. To achieve common control, windings of the different rotors of same rotor unit are connected in series or in parallel to be supplied with the same inverter 9, 10, 11, 12.

[0070] FIG. 4 depicts a control architecture used to control levitation and travel of the linear motor of FIG. 1. The control architecture shows control elements which are implemented in the control software of the processing units of each inverter 9, 10, 11, 12.

[0071] According to FIG. 4, each inverter 9, 10, 11, 12 receives position information X.sub.act of the mutual position of the three-phase windings of the rotor unit controlled with corresponding inverter, and the ferromagnetic poles facing/co-acting with said three-phase windings. The mutual position X.sub.act is measured in the travelling direction, parallel to direction x in FIG. 1, by means of one or more position sensors 16A, 16B, 16C, 16D, which may be hall sensors or inductive proximity sensors, for example. Each inverter 9, 10, 11, 12 controls current supply of the rotor windings in a d, q-coordinate system of its own. The d, q coordinate system is synchronized by means of the position information X.sub.act to the position of ferromagnetic poles of the stator beam facing the rotor windings. The d-axis is referenced to the direction of the ferromagnetic poles 8 such that it is in the direction of the center line of the co-acting ferromagnetic pole. This direction may be the same as center line of the stator teeth (see FIG. 3); on the other hand it may also differ from that, e.g. due to saturation of the stator teeth. d-axis direction may also be defined otherwise: for example, to be in position wherein flux linkage of R-phase of the rotor unit has its maximum.

[0072] Each inverter 9, 10, 11, 12 receives also information of length of air gap (Y.sub.act) between side-face 6A, 6B carrying the ferromagnetic poles 8 and the counterface 7A, 7B containing the rotor unit 2, 3, 4, 5. Air gap length information (Y.sub.act) may be received from sensors 16A, 16B, 16C, 16D or, additionally or alternatively, from separate air gap sensors, such as eddy current sensors, which may be disposed at same locations as sensors 16A, 16B, 16C, 16D or which may replace one or more of the sensors 16A, 16B, 16C, 16D. To measure air gap length as well as air gap tilt in longitudinal direction of stator beam 1, at least two sensors are needed for example at opposite ends at opposite sides of the mover, for example at sensor positions 16A and 16D of FIG. 1.

[0073] Further, to measure turning of the mover 24, 26 around the longitudinal axis of the stator beam, two parallel air gap sensors 16, 16 may be disposed in transverse direction of air gap 15, as illustrated in FIG. 2D.

[0074] A reference value for the air gap Y.sub.ref is memorized in the processing unit of the inverter 9, 10, 11, 12. Air gap controller 40 calculates a difference between the air gap reference value Y.sub.ref and the air gap length information Y.sub.act and generates a reference value for the attraction force F.sub.yref, e.g. the force component parallel to the y-direction of FIG. 1, to adjust the length of air gap Y.sub.act towards the reference value Y.sub.ref. Air gap controller 40 is a state controller which uses observer 42 to obtain simulated position y and velocity y (in the y-axis direction of FIG. 1) of the mover 24, 26 mass under the effect of the attraction force estimate F.sub.yref.

[0075] In a first embodiment air gap controllers 40 of inverters controlling rotor units at both opposite sides of the stator beam are used to adjust air gap length. In a second alternative embodiment, on one side of the stator beam the reference value for the attraction force F.sub.yref, is kept constant and air gap controller is used only in connection with rotor units of the other side of the stator beam to adjust attraction force reference value F.sub.yref. This means the one or more rotor units of one side provide a constant attraction force against which the air gap controllers act at the other side of the stator beam. In a further alternative, no inverter/motor windings are used to generate a constant attraction force F.sub.yref, Instead, at one side of the stator beam the rotor units of the counter face are replaced with permanent magnets only, which permanent magnets generate an attraction force towards the side face of the stator beam. On the other side of the stator beam rotor units with windings are controlled with air gap controllers of the inverters to act against the attraction force of said permanent magnets. With this solution no motor windings/inverters are needed for those counter faces with permanent magnets only.

[0076] Further, at least one of the inverters 9, 10, 11, 12 of a common mover receives travel position information x.sub.act and travel speed information v.sub.act of the mover. In this connection travel position information x.sub.act and travel speed information refers to position/speed information of the mover in the direction parallel to the x-axis direction of FIG. 1. In the current embodiment the same position information x.sub.act is used to define mutual position between rotor unit and respective ferromagnetic poles to synchronize d, q-axis of the drive unit/inverter to said ferromagnetic poles 8. This information is also used to control position x.sub.act/speed v.sub.act of the mover along the stator beam 1. In this embodiment the travel position information x.sub.act is be received from the one or more sensors 16A, 16B, 16C, 16D but alternatively a separate sensor may be used. The travel speed information v.sub.ea may be received from a separate speed sensor, such as an encoder or tachometer, or it may be obtained from timely variation of the travel position information x.sub.act (e.g. time derivative of the travel position information) which is the case in this embodiment. One of the inverters of a common mover acts as a master which performs position/speed control in the travelling direction of the mover and outputs a propulsion force reference value F.sub.xref (i.e. reference force component parallel to the x-axis direction of FIG. 1) to the other inverters 9, 10, 11, 12. Other inverters of the common mover then act as slaves, which do not perform position/speed control but propulsion force control only. If two or more movers are coupled to a common load-receiving means, such as to a common elevator car, it is also possible that only one inverter of only one mover acts as a master and all the other inverters/movers act as slaves to avoid interference of position/speed controllers.

[0077] Going back to FIG. 4, processing unit of the master inverter 9, 10, 11, 12 calculates travel position reference value x.sub.ref to establish an intended motion profile for the controlled mover(s). Position controller 44 calculates travel speed reference value v.sub.ref from the difference between travel position reference x.sub.ref and travel position of the mover x.sub.act in the travelling direction x of the mover. Speed controller 45 calculates a propulsion force reference value F.sub.xref from the difference between the travel speed reference v.sub.ref and the travel speed information v.sub.act.

[0078] Propulsion force reference value F.sub.xref, attraction force reference value F.sub.yref and air gap length information Y.sub.act are inputted into magnetic model 43, which calculates d-axis and q-axis current reference components I.sub.dref, I.sub.qref for the rotor windings. In case of slave inverters, each slave inverter calculates its own attraction force reference value F.sub.yref by means of the air gap length information Y.sub.act, but receives propulsion force reference value F.sub.xref from the master inverter. With these reference values as well as the air gap length information from air gap sensor 16A, 16B slave inverter calculates the d-axis and q-axis current component reference values with the magnetic model 43.

[0079] The magnetic model may consist of algorithms, which represent how attraction force and propulsion force of the motor depend on d-axis and q-axis currents as well as air gap length. This representation may be based on the following motor equations:

[00001]$\begin{array}{cc}{i}_d=\left(a_{\mathrm{d0}}-b_{\mathrm{dm}}.Math.y\right).Math.\left({}_d-{}_r\right)+\left(b_d.Math.y+a_{\mathrm{dd}}.Math.{.Math.{}_d.Math.}^S+\frac{\mathrm{dq}}{V+2}.Math.{.Math.{}_d.Math.}^U.Math.{.Math.{}_q.Math.}^{V+2}\right).Math.{}_d& \left(1\right)\\ .Math.i_q=\left(a_{\mathrm{qo}}+b_q.Math.y+a_{\mathrm{qq}}.Math.{.Math.{}_q.Math.}^T+\frac{\mathrm{dq}}{U+2}.Math.{.Math.{}_d.Math.}^{U+2}.Math.{.Math.{}_q.Math.}^V\right).Math.{}_q& \left(2\right)\\ F_x=\frac{3.Math..Math.}{r}.Math.\left({}_d.Math.i_q-{}_d.Math.i_d\right)& \left(3\right)\\ F_y=-\frac{3}{2}\left[b_d.Math.{}_d^2+b_q.Math.{}_q^2-{b_{\mathrm{dm}}\left({}_d-{}_r\right)}^2\right]-\frac{f_{}}{{\left(1+c_{}.Math.y\right)}^2}& \left(4\right)\end{array}$

wherein i.sub.d and i.sub.q represent current components in d, q coordinate system, a.sub.d0, a.sub.dd, a.sub.dq, a.sub.q0, a.sub.qq, a.sub.dq, b.sub.dm, b.sub.d, b.sub.q, c.sub., f.sub., .sub.r, S, T, U, V are motor-specific constants. They are derived based on reluctances, which depend on motor geometry. .sub.d and .sub.q are d and q-axis components of the motor flux linkage, is pole pitch of the motor (2), y is air gap length between rotor and stator, and F.sub.x is propulsion force reference value and F.sub.y is attraction force reference value.

[0080] in view of the above equations, F.sub.x may be represented to be dependent only on magnetic flux linkage and air gap length y:

F.sub.x(.sub.d,.sub.q,y)

also F.sub.y may be represented to be dependent only on magnetic flux linkage and air gap length y:

F.sub.y(.sub.d,.sub.q,y)

[0081] Thus magnetic flux linkage components .sub.d and .sub.q may be solved by means of the representations (3) and (4) when the (reference) values of propulsion force F.sub.xref and attraction force F.sub.yref are received from the speed controller 45 and the air gap controller 40. Reference current values I.sub.dref, I.sub.qref may then be calculated with the equations (1) and (2) by means of the magnetic flux linkage components .sub.d and .sub.q.

[0082] Alternatively or additionally, the magnetic model 43 may comprise a table, having d-axis and q-axis current components memorized and indexed by means of propulsion force reference values F.sub.xref, attraction force reference values F.sub.yref, and air gap length information Y.sub.act. To get more accurate values for the d, q-axis current reference components, it is possible to use interpolation between the memorized values of the table. Table values may also be determined with simulation, for example by using Finite Element Method (FEM).

[0083] In the magnetic model 43 at least one of d-axis current reference component I.sub.dref and q-axis current reference component I.sub.qref of the motor windings is changed when a change in at least one of the propulsion force reference value F.sub.xref, attraction force reference value F.sub.yref and air gap length information Y.sub.act of the rotor unit 2, 3, 4, 5 takes place. Therefore magnetic model 43 may speed up adaptation of the rotor units and thus the mover to variable operation conditions, making operation of the mover 24, 26 more stable and responsive.

[0084] d-axis and q-axis current component reference values I.sub.dref, I.sub.qref are communicated to current controller 41, which calculates d-axis and q-axis voltage references U.sub.d, U.sub.q for the windings of the rotor unit based on the difference between d- and q-axis current reference values I.sub.dref, I.sub.qref and measured d-axis and q-axis current components I.sub.d, I.sub.q. Transformation from d, q coordination system to three phase voltage components U.sub.R, U.sub.S, U.sub.T, as well as transformation from three-phase current measurements i.sub.R, i.sub.S, i.sub.T to d, q-axis component values I.sub.d, I.sub.q takes place with Park and Clarke transformations, which transformations as such are known in the art. For the synchronization of the d, q coordinate system travel position information X.sub.act is used as disclosed above.

[0085] The three-phase voltage components of the rotor unit U.sub.R, U.sub.S, U.sub.T are communicated to state vector PWM modulator 46 (pulse width modulator) of the inverter, which creates the control pulses for controlling the solid state switches of the inverter power stage to introduce modulated three-phase voltage components to the windings of the rotor unit. These solid state switches may be, for example, igbt-transistors, mosfet-transistors, silicon carbide transistors and/or gallium nitride transistors.

[0086] In an alternative embodiment, an adequate performance level may be achieved with a simplified control architecture, wherein the speed controller 45 of the master inverter outputs directly q-axis current reference components I.sub.qref to the slave inverters. Each slave inverter generates d-axis current reference component I.sub.dref of its own, by means of the air gap controller 40. These d, q-current reference components I.sub.dref, I.sub.qref are then directly communicated to current controller 41, thus avoiding use of magnetic model 43, i.e. bypassing it. This may reduce processing power needed for levitation/speed control of the mover 24, 26.

[0087] Instead of one inverter 9, 10, 11, 12 acting as a master, it is possible to use a separate master control unit which may perform function of at least one of air gap controller 40, position controller 44 and speed controller 45 for one or more of the inverters 9, 10, 11, 12 and output the required reference values to the inverters 9, 10, 11, 12 to control current supply to the rotor units.

[0088] FIG. 5 shows a multicar elevator system according to an embodiment of the third aspect of the invention. Elevator system comprises plurality of elevator cars 16 each coupled to parallel stator beams 1, 1 (see FIG. 6) by means of the movers 24, 26. The cars are in a circular motion within two parallel elevator shafts 17. To each car four movers 24, 26 are installed, 2 per stator beam 1, 1. The linear motors are similar to those disclosed in the embodiments above, thus there are 32 rotor units per car and 32 inverters per car. All the inverters of the same car 16 are connected to common DC link, such that regenerative energy returning from one inverter back to DC link may be shared with/supplied to the other inverters. Each car 16 has a battery, which is connected to the common DC link.

[0089] The invention can be carried out within the scope of the appended patent claims. Thus, the above-mentioned embodiments should not be understood as delimiting the invention.