GB2373928A

GB2373928A - Electrical linear machine

Info

Publication number: GB2373928A
Application number: GB0126628A
Authority: GB
Inventors: Datong Wu; Oliver Zirn; Joachim Bruns; Hans-Werner Zimmermann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2000-11-07
Filing date: 2001-11-06
Publication date: 2002-10-02
Anticipated expiration: 2021-11-06
Also published as: GB2373928B; DE10055078C1; NL1019318A1; NL1019318C2; GB0126628D0

Abstract

An electrical linear machine has a stator 10 with stator poles 13 and 14 respectively wound with coils 21 and 22, and a coaxially arranged oscillator 11 made of magnetically permeable material, the oscillator 11 being displaceable relative to the stator 10. The oscillator 11 and the stator poles 13 and 14 are respectively crenellated in the axial direction. The machine contains no permanent magnets; coil 22 functions as a field excitation coil generating magnetic flux which passes across the air gaps 27 between the stator 10 and the oscillator 11. An electrical output is taken from coil 21. The machine may have a larger number of poles, for example four poles. The coils on successive stator poles in the circumferential direction are alternately arranged as a field excitation winding and an output winding. Field excitation coil 22 may be energised with either direct current or alternating current. When the field coil 22 is connected to a source of direct current, and the output coil 21 is connected to an a.c. mains network, the oscillator 11 must be driven, by for example a Stirling engine (not shown), at the mains frequency. When the field coil 22 is connected to the same alternating current mains supply (fig.4), the oscillator 11 must be driven at twice the mains frequency, which is desirable for a Stirling engine. Through suitable low-pass filtering, the fundamental component of the voltage generated in the output coil 21 can be fed into the a.c. network. The machine avoids the temperature limitations of permanent magnet machines and is robust and straightforwardly manufactured.

Description

- Electrical Linear Machine Prior art

The invention is based on an electrical linear machine according to the precharacterising clause of Claim 1.

In the case of a known electrical linear machine of this type (US 5 654 596) which can be operated selectively as a motor or generator, the magnetic flux, hereafter abbreviated to magnet flux, is generated by permanent magnets which are arranged next to one another on each of the stator poles with alternating polarization in the axial direction of the stator and define air gaps, through which the magnet flux passes, between themselves and the circumference of two mutually separated laminated stacks of the oscillator, referred to therein as the mover, which are fastened to a drive or thrust rod.

During the oscillatory movement of the driven oscillator, one laminated stack is respectively moved away by means of a pair of permanent magnets with opposite polarization or polarity. In the circumferential direction of the stator, a plurality of such stator poles occupied by permanent magnets are arranged offset by a constant circumferential angle.

To achieve a high efficiency of the linear machine, permanent magnets with high energy intensity, e.g. Nd-Fe-

B magnets, are used. At elevated temperatures, the efficiency of these permanent magnets decreases: rapidly,-.

and a temperature higher than e.g. 140 - 150 C leads to irreversible demagnetization of the permanent. magnets.

j ..

l Permanent magnets are furthermore expensive and the manufacture and maintenance of the linear motor is a constant problem Advantages of the invention The electrical linear machine according to the invention has the advantage over this that, by obviating permanent-

magnetic field excitation, a very robust design is

achieved which permits equally high power densities as a permanent magnetexcited linear machine. The linear machine according to the invention constitutes an electrically excited reluctance machine based on the transverse flux principle, in which the magnet flux hence extends across (transversely with respect to) the movement direction of the oscillator. The electrical field excitation avoids all the afore-mentioned

disadvantages associated with the permanent magnets. For instance, the linear machine is thermally stable and, with an appropriate winding configuration, can be operated in a wide temperature range.

In a two-poled configuration of the linear machine, the internal space of the stator can be fully utilised.

Furthermore, no lateral forces on the oscillator occur during operation of the linear machine, even when the position of the oscillator axis is off-centre or eccentric, so that the oscillator assumes a stable central position in its radial alignment.

Advantageous development and improvements of the electrical linear machine specified in Claim 1 are possible through the measures described in the other claims.

According to alternative embodiments of the invention, the magnetic field of the single-phase linear machine

connected to an AC voltage mains supply is generated by supplying the coils of the field-excitation winding with

direct current or with alternating current. In the case of AC excitation, the linear machine operates as a synchronous machine, the driver, preferably a Stirling motor, having to oscillate with twice the excitation frequency. The higher working frequency is advantageous for the driver. By combining the line-commutated excitation with the excursion movement of the driver, a voltage with a mains-frequency fundamental oscillation can be achieved with an excursion frequency corresponding to twice the mains frequency. Synchronisation of the driver with the mains can furthermore be assisted or forced by the line- commutated excitation.

Drawing The invention is explained in more detail in the description below with the aid of an exemplary embodiment

represented in the drawing. In respective schematic representations: Fig. 1 shows a longitudinal section of a linear machine according to the section line I - I in Fig. 2, Fig. 2 shows a section on the line II - II in Fig. 1, Figs 3 respectively show a diagram of the functions: and 4 excitation current, driver speed and armature voltage against time for excitation with

constant direct current (Fig. 3) and for excitation with alternating current (Fig. 4).

Description of the exemplary embodiment

The linear machine represented in longitudinal and cross section in Figs 1 and 2 has, in the known way, a stator 10 and an oscillator 11 which is arranged coaxially in the stator 10, can be moved to and fro along the stator axis 101 and, in the case when the linear machine is implemented as a linear generator, is driven by a driver machine (not represented here), preferably a Stirling motor. The stator 10 has a lamellate stator body 12, assembled from stamped sheet-metal sections by stamp-pressing, with two protruding stator poles 13, 14 which are diametrically opposite on the stator axis 101 and are integrally supported radially inwards by a magnetic-

return yoke or magnetic-return ring 15. In the exemplary embodiment, the magnetic-return ring 15 is formed by two ring halves which are assembled by means of two joints 16 (Fig. 2). A pole shoe 17 and 18, respectively, which has a concave surface in the form of a lateral cylinder surface portion, is formed integrally on the free ends of the two identically designed protruding stator poles 13, 14. Teeth 20, which are designed with axial symmetry relative to the stator axis lOl, are equidistantly sequential in the direction of the stator axis lO1 and -

as viewed in the direction of the stator axis 101 - have a constant tooth width, are formed on the stator poles 13, 14 by cutting constant-width grooves 19 in the pole shoes 17, 18. In the exemplary embodiment of Figs 1 and 2, there are three such teeth 20. A coil 21 forming a

single-phase armature winding is wound on the stator pole 13, and a coil 22 forming a field-excitation winding is

wound on the stator pole 14.

The oscillator 11 is formed by a cylindrically shaped laminated stack 24 of assembled stamped sheet-metal sections 25, which is supported in a rotationally fixed fashion on a drive or thrust rod 23. Like the stator 10, the oscillator 11 is crenellated in the direction of the stator axis 101, a number of teeth 26, in this case three, equal to the number of teeth 20 on the stator poles 13, 14, being provided. The teeth 26 are designed with rotational symmetry, have a constant axial separation and a constant axial tooth width. The teeth 26 are produced by alternately joining together assembly segments of stamped sheet-metal profiles 25 which have a diameter corresponding to the tooth diameter, and assembly segments of stamped sheet-metal profiles 25 which have a smaller diameter than this, during the assembly of the stamped sheet-metal profiles 25. The teeth 26 on the oscillator 11 respectively enclose, with the teeth 20 on the stator poles 13, 14, an air gap 27 in the form of a circular arc portion with the length l of the circular arc portion.

The voltage Ui induced in the coil 21 of the armature winding when the oscillator 11 is driven in oscillation depends on the turns number w of turns of the coil 21 and the magnetic-flux variation d ldt in the air gaps 27, and, and is calculated as: U. = W. dew = w BA - I -

where BA is the air-gap infliction which can be controlled by the excitation current in the coil 22 and l is the aforementioned air-gap width. By corresponding adjustment of the excitation current delivered to the coil 22, the voltage produced by the linear machine in generator mode can be adjusted continuously.

The field excitation of the linear machine can take place

both with direct current and with alternating current. In the former case, a constant direct current is applied to the coil 22. If the armature winding (coil 21) is connected to an AC voltage mains supply, then line-

commutated, rectified excitation, called pulsed excitation, can also take place, the alternating current taken from the AC voltage network being rectified. In the case of DC excitation, the excursion frequency of the oscillator 11 needs to be synchronized with the mains frequency. If the armature winding (coil 21) is connected to an AC voltage mains supply and if the field-excitation winding

(coil 22) is supplied with alternating current, then this line-commutated AC excitation offers the advantage that the oscillator 11 has to oscillate at twice the mains frequency, which is desirable for a Stirling motor used as a driver machine for the oscillator 11. Through suitable low-pass filtering, the fundamental oscillation of the induced voltage Ui can be fed as a generator voltage into the AC voltage network.

Fig. 3 represents, in three diagrams arranged below one another and respectively as a function of time, the excitation current Iex in the coil 22, the speed dx/dt of the oscillator 11 and the voltage Ui induced in the coil

21 (armature winding). It is clear to see that, with constant DC excitation, the oscillator 11 oscillates at mains frequency and a voltage with the same frequency is induced in the armature winding.

Fig. 4 represents the same diagrams for AC excitation of the coil 22. The oscillator 11 oscillates at twice the mains frequency and, through suitable low-pass filtering, the fundamental oscillation (indicated by dashes in the lower diagram) of the induced voltage Ui can be fed as a generator voltage into the mains. The upper limit frequency of the filter is in this case slightly above 50 Hz. The invention is not restricted to the aforementioned exemplary embodiment of the linear machine. For instance, the number of stator poles, which are respectively wound with a coil 21 of the armature winding and a coil 22 of the field-excitation winding, may be increased. For

example, there may be four of them, in which case the stator poles are each then mutually offset by 90 at the circumference of the magneticreturn ring 15 and are successively wound alternately with a coil 21 and a coil 22. The number of teeth 20 and 26 on the stator lO and the oscillator 11 can also be arbitrary, and the tooth distribution and the tooth width can be varied.

Claims

Patent Claims 1. Electrical linear machine with a stator (10) which has a

plurality of stator poles (13, 14) which are arranged mutually offset by a constant circumferential angle and are respectively wound with a coil (21, 22), with a coaxially arranged oscillator (11) made of magnetically permeable material, which is enclosed by the stator (10), can be displaced along the stator axis (101) relative to the stator (10) and, with the stator poles (13, 14), encloses air gaps (27) through which a magnet flux passes, Characterized in that the oscillator (11) and the stator poles (13, 14) are erenellated in the axial direction arid, with their teeth (26, 20), define air gaps (27) mutually separated in the axial direction, and in that the coils (21, 22) on successive stator poles (13, 14) in the circumferential direction are alternately associated with a single-phase armature winding and with a field- excitation winding which generates the magnet

flux.
2. Linear machine according to Claim 1, characterized in that the teeth (20, 26) sequentially arranged in the axial direction on the stator poles (13, 14) and the oscillator (11) have a constant mutual spacing.
3. Linear machine according to Claim 1 or 2, characterized in that the width of the teeth (20, 26) on the stator poles (13, 14) and the oscillator (11) as viewed in the axial direction is of constant size.
4. Linear machine according to one of Claims 1 - 3, characterized in that the teeth (26) on the oscillator (11) are designed with rotational symmetry and the teeth

(20) on the stator poles (13, 14) are designed with axial symmetry relative to the stator axis (101).
5. Linear machine according to one of Claims 1 - 4, characterized in that the armature winding (21) is connected to an AC voltage mains supply and the field-

excitation winding (22) is supplied with a constant direct current or with line-commutated rectified current pulses, and in that the oscillator (11) is driven at an oscillation frequency corresponding to the mains frequency.
6. Linear machine according to one of Claims 1 - 4, characterized in that the armature winding (21) is connected to an AC voltage mains supply and the field-

excitation winding (22) is supplied with alternating current, and in that the oscillator (11) is driven at an oscillation frequency corresponding to twice the mains frequency.
7. Linear machine according to Claim 6, characterized in that the voltage induced in the armature winding (21) is low-pass filtered.
8. Linear machine substantially as herein described with reference to the accompanying drawings.