GB2164211A

GB2164211A - Direct current motor

Info

Publication number: GB2164211A
Application number: GB08521927A
Authority: GB
Inventors: Karl-Heinz Bernhardt
Original assignee: PFEIFFER VAKUUMTECHNIK; Arthur Pfeiffer Vakuumtechnik Wetzlar GmbH
Current assignee: PFEIFFER VAKUUMTECHNIK; Arthur Pfeiffer Vakuumtechnik Wetzlar GmbH
Priority date: 1984-09-07
Filing date: 1985-09-04
Publication date: 1986-03-12
Also published as: GB8521927D0; NL8502365A; BE903149A; GB2164211B; CH670339A5; JPS6166558A; IT1200724B; DE3432946A1; IT8521993A0; DE3432946C2; FR2570229A1

Abstract

In a collector-less direct-current motor with an ironless stator winding 4, within a rotor 1, and an air gap 11 with a two-pole ring magnet 3, the stator has a four-strand oblique winding. One end of each part-winding is connected to one terminal of a voltage supply and the other ends are connected via respective electronic switch elements to the second terminal of the voltage supply. With the help of two Hall- probes, pulses are produced for triggering the switching elements. <IMAGE>

Description

SPECIFICATION Direct current motor The invention concerns a direct-current motor for high rotational speeds and more particularly to a collector-less direct-current motor for driving a quickly-running rotor consisting of a bell-shaped or cylindrical member and a central rotor shaft connected thereto and forming a radial air gap with two 1800-wide magnetic poles, and an iron-less stator coil which projects into this air gap, two Hallprobes for sensing the position of the rotor and four electric switch elements, which, by means of electrical signals from the Hall-probes, are triggered one after the other. Such a motor may be used for driving for example turbo-molecular pumps, gyroscopes, and spinning- or grinding spindles, especially with rotors having magnetic bearings.

Most quickly running machines are today driven by three-phase motors. A disadvantage of these motors is the decreasing efficiency with increasing air gap between rotor and stator due to the increasing magnetization current. Collector-less direct-current motors with a wound flat stator and rotating permanent-magnetic rotor do not have this disadvantage. However there arise between rotor and stator radial attractive forces, which, in the case of a radial magnetic bearing, must be compensated by means of supplementary bearing forces. From German Patent Application P 3302 839 there is known a motor which does not have these disadvantages and projects into an iron-less winding in a multiple-pole magnetized air gap.Twopole arrangements can be realised with the help of this construction only with difficulty, since there is no room for the coil winding heads.

Four-pole arrangements necessitate a doubled running frequency, which leads to higher switching losses in the supply apparatus and higher eddycurrent losses in the wires of the winding. Moreover the manufacture of such a winding is very complicated and requires a large air gap.

Furthermore brush-commutated direct-current motors which iron-less oblique windings are known, as decribed in Faulhaber (Fritz Faulhaber: Einfache Berechnungsgrundlagen fir rasch anlaufende Gleichstrom-Stellmotoren, VDI-Z.Bd 107 Nr. 4, S.

149--152). These motors are indeed free of radial forces, however on account of the commutator are not suitable in vacuum or for very high rotational speeds.

The present invention seeks to provide a motor which avoids the above disadvantages and, especially in the case of a small air gap with a simple fixed winding without coil winding heads, permits drive of a shaft free of radial forces and with low losses.

According to the present invention there is provided a direct-current motor comprising a rotor with a central rotor shaft and a cylindrical portion defining an air-gap therewith, the rotor having magnetic poles, a stator coil which projects into the air gap, means for sensing the position of the rotor, and switch elements sequentially triggered by the sensing means, wherein the stator coil is obliquely wound and comprises a plurality of part-windings, of which first ends are all arranged to be connected to a first voltage supply terminal of which the second ends are arranged to be connected via a respective switch element to a second voltage supply terminal.

By means of the above construction of the oblique winding in four part-windings, of which the first ends are all connected to one pole and the second ends are connected via controllable electrical switch elements to the other pole of the operating voltage source, there is possible a use of commercial direct current drive devices (Betriebsanweisung fur TurbomolekularpumpeTPH 170,TPU 170 und Antriebs-ElektronikTCP 300 Nr. PM 800 093, BD,E,F der Firma Arthur Pfeiffer Vakuumtechnic Wetzlar GmbH).

The control of the winding segments is achieved with the help of two Hall-probes which are preferably mounted on the inside or the outside of the winding on the same generatrix as the first ends of the partwindings.

With uniform air gap induction the back e.m.f.

induced by the oblique winding has a sinuous path, which is disadvantageous for the operating voltage source. Preferably one 180 -wide magnetic pole consists of two magnet segments which are each 90 -wide and are magnetized parallel to their planes of symmetry. With this construction of the pole surfaces the curve of the sinuous back e.m.f. can be flattened by means of the reduced air gap induction in the middle of a pole and thereby can be produced a constant backe.m.f. of the motor.

The use of a winding which is longer than the magnetized air gap and the projecting part of which is fixed to a carrier tube of good thermal conductivity permits the carrying off of heat from the winding, which is an advantage especially in vacuum.

A preferred embodiment of the present invention will now be decribed, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 shows a developed view of the winding of a motor in accordance with the present invention with the location of the Hall-probes and (schematically) the current flow; Fig. 2 shows a cross section through the motor in the plane of the Hall-probes; and Fig. 3 shows the longitudinal section of the motor.

The arrangement is located in a housing 9. In a rotor 1 with a steel rotor shaft 7, which is mounted in bearing end covers 10 with two magnetic bearings 8, there is installed an iron magnetic flux return sleeve 2 with a two-pole ring magnet in the form of radially magnetized magnets 3. In the air gap 11 formed between the rotor shaft and a cylindrical part of the rotor there is arranged a stator coil 4 fixed to the housing. One end of the stator coil projects out of the cylindrical part of the rotor and is surrounded by a carrier sleeve 6 of aluminium. This carries off the heat due to energy losses arising in the stator coil to the housing. The Hall-probes 5 are attached to the inside of the stator coil in the wall of a cylindrical thin-walled shell of insulating material.

Emergency bearings 12 and axial step bearings 13 are also provided.

Fig. 1 shows in its upper part the developed projection of the stator coil. Beginning at al the winding runs over half its circumference up to the opposite edge and from there goes on the reverse side back to the initial point. From there the next turn runs parallel to the first. By analogous continuation of this way of winding one obtains a continuous two-layer winding, of which the number of turns, length and diameter depend on the thickness of the wire. The coil is divided into four similar part-windings, of which the first ends are connected to a1 and ofwhich the second ends a2-a5 each lead to a collector of a switching transistor. If the pole separation line of the rotor passes over the Hall-probe Hl,transistor T1 is switched on. After a quarter rotation the pole separation line passes over the Hall-probe H2, and so tra nsistor T1 is switched off and transistor T2 is switched on etc. For the production of a control signal the output voltages of the Hall-probes are amplified by respective pre-amplifiers and the output voltages are fed to a known 1 from4 decoder, which, for the control of transistors T1 to T4, produces 90" shifted pulses lasting for 90" of rotation.

Claims

1. A direct-current motor comprising a rotor with a central rotor shaft and a cylndrical portion defining a air-gap therewith, the rotor having magnetic poles, a stator coil which projects into the air-gap, means for sensing the position of the rotor, and switch elements sequentially triggered by the sensing means, wherein the stator coil is obliquely wound and comprises a plurality of part-windings, of which first ends are all arranged to be connected to a first voltage supply terminal and of which the second ends are arranged to be connected via a respective switch element to a second voltage supply terminal.

2. A motor according to claim 1, wherein the magnetic poles each extend for 1800 of the circumference of the rotor.

3. A motor according to claim 2, wherein one or both magnetic poles consists of two magnet segments which each extend for 90" of the circumference of the rotor and are magnetized parallel to their plane of symmetry.

4. A motor according to any previous claim wherein the stator coil is iron-less.

5. A motor according to any preceding claim, wherein the rotor position sensing means comprises two Hall-probes.

6. A motor according to claim 5, wherein the Hallprobes lie on the inside or outside of the coil on the same generatrix as the first ends of first and second part-windings.

7. A motor according to any preceding claim, wherein there are four part-windings and four switch elements.

8. A motor according to any preceding claim, wherein the stator coil is longer than the air-gap and the projecting part of the coil is fixed to a carrier of good thermal conductivity.

9. As an independent invention the additional feature of any claims 2 to 8.

10 A direct-current motor substantially as herein described with reference to the accompanying drawings.