A RELUCTANCE MOTOR

Abstract: A reluctance motor comprises a stator having two salient poles (11, 12) and a rotor (16) also having two poles (26, 27) and being of soft magnetic material. The stator poles (11, 12) support windings (13, 14) which when being supplied with current generate a magnetic field which strives to set the rotor in a stable position of equilibrium in which the rotor and the stator poles coincide. Either or both of the stator poles (11, 12) and/or either or both of the rotor poles (26, 27) are designed so as at high flux density to cause a constriction in a part of the cross section of the flux path in order to displace the position of equilibrium and when the flux density is moderate to distribute the flux generally uniformly over the said cross section. The constriction of the flux is formed by two recesses (17-20) or the like which are symmetrically located in a pair of stator poles and rotor poles (11, 26; 12, 27), one recess (19, 20) or the like being located in the rotor pole (26, 27) at its front or rear edge, as seen in the direction of rotation, while the other recess (17, 18) or the like is located in the stator pole (11, 12) at the opposite edge.

Description: A reluctance motor The present invention relates to a reluctance motor of the kind referred to in the preamble of the appending Claim 1. Such a reluctance motor has been described in Swedish Patent no 437203. The motor comprises a stator having two salient poles and a rotor also having two poles and being made of soft magnetic material. The stator poles support winding which, when supplied with current, generate a magnetic field which strives to set the rotor in a stable position of equilibrium in which the rotor poles and the stator poles coincide . The stator poles and the rotor poles are designated so as at high fl density to cause a constriction of the flux in a part of the cross section of the flux path in order to displace the position of equilibrium and at a moderate flux density to distribute the flux generally uniformly over the said cross section. Further, the motor is constructed such that during start the stator windings are supplied with current pulses of a magnitude which brings the part of the stator poles and the rotor poles, respectively, situated between the constriction of flux and the pole surface into saturation. In forming the constriction of the flux at least one stator pole at its front or rear edge, as seen in the direction of rotation of the rotor, can be provided with a recess or the like which at some distance from the pole surface restricts the cross section of the flux path in the pole. The object of the design of the reluctance motor as described is to offer a possibility to start the motor in the desired direction of rotation even if the rotor has stopped in a position wherein the stator poles completely coincide with the rotor poles. To that object, at start the stator windings can be supplied with current pulses of an amplitude and a duration such as to bring the rotor to pendulate about a displaced position of equilibrium at increasing amplitude and, finally, to turn into rotation. Starting from the reluctance motor thus described there is a desire to additionally improve the starting characteristics of the motor and particularly to make possible the control of the pendulating movements of the rotor which takes place before rotation. This desire will be achieved in a reluctance motor having the characterizing features stated in Claim 1. Preferred embodiments appear from the accompanying sub -claims. The invention will now be described in detail with reference to the enclosed drawings in which Fig. 1 shows a reluctance motor having stator and rotor poles of the same peripheral extension. Fig. 2 shows a motor in which the pole surfaces oi the rotor have two parts of the same dimensions, each having the same extension as the pole surfaces of the stator. Finally, Fig. 3 shows a motor according to Fig. 2 in which, however, the stator poles have a greater extension than the corresponding part of the pole surfaces of the rotor poles. In Fig. 1 a reluctance motor is shown having a stator 10 comprising two poles 11, 12 which carry windings 13, 14. In series, the windings are connected to a control device 15 arranged to supply to the windings suitable control pulses for the operation of the motor. A rotor 16 made of soft magnetic material is rotatably arranged in the air gap between the stator poles 11, 12. The cross section of the rotor is generally rectangular and the short sides which form the rotor poles 26, 27 are slightly bent to a form corresponding to the curvature of the pole surfaces 11a, 12a of the stator poles. To allow starting when the rotor is in the position shown, i.e. the rotor poles are exactly opposite the stator poles, the stator poles and/or the rotor poles are made so that a part of the pole surface, as seen in the direction of rotation of the rotor, will have greater reluctance than the remaining part of the pole surface. The change in reluctance can be achieved suitably by recesses 17, 18 in the stator poles and corresponding recesses 19, 20 in the rotor poles. The depth and location of the recesses are selected such that the high current pulses appearing during the start will bring the pole portion situated above the recess into saturation. The magnetic line of symmetry of the pole will thus be displaced relative to the physical line of symmetry of the pole, which causes the rotor, striving to adjust itself so as to be in the position of the flux path in which the reluctance is at its minimum, will be slightly turned out of the position shown. If the amplitude and the duration of the current pulses are appropriately chosen the rotor will pendulate about the displaced position of the equilibrium at an increasing amplitude so as to finally be caused into rotation. Tests have shown that frequencies of the magnitude 1 Hz are suitable starting frequencies. As shown in Fig. 1, in the two pairs of cooperating stator poles and rotor poles the recesses 17, 19 and 18, 20, respectively, are symmetrically located. Then, when the poles are driven into saturation portions in the respective pole are formed which are displaced in opposite direction with respect to the common physical line of symmetry of the poles. By this arrangement it will be achieved that when the stator and the rotor poles are exactly in front of each other, the rotor will always with great accuracy be turned out of its physical position of equilibrium to bring about the pendulating movements which are necessary for the starting process. Preferably, in the embodment shown in Fig. 1 the recesses in the rotor poles are located at the rear edges of the poles, as seen in the direction θι rotation of the rotor. In Fig. 2 an embodiment is shown using another type of rotor 21. Each rotor pole 22, 23 comprises two parts of the same size 22a, 22b, 23a, 23b each of which having a peripherical extension which equals that of the stator poles. However, the pole surfaces of the two parts are situated at a different distance from the rotor shaft so that the parts 22b, 23b are closer to the shaft than the parts 22a, 23a. The arrangement results in that the rotor can be driven during a greater part of the revolution than is possible in the embodiment of Fig. 1. As in the embodiment of Fig. 1 the recesses 19, 20 in the rotor poles are located at the rear edge, as seen in the direction of rotation, while the recesses 17, 18 in the stator poles are located at the opposite edge, as seen when the stator and rotor poles coincide. In Fig. 3 there is shown an embodiment according to Fig. 2, however, having stator poles 24, 25 of greater peripheral extension than the respective parts 22a, 22b, 23a, 23b of the rotor poles. The pole width of the stator poles that corresponds to the width of the respective rotor part, is designated b whereas the extended pole width is designated Δ hr. In this embodiment the depth of the recesses must be selected to be greater than Δ b if the intended displacement of the rotor is to be safely achieved. In this embodiment during a turning angle of the rotor corresponding to Δ b the stator pole can be emptied of magnetic energy without the development of any braking moment. As a result the rotor can be driven during a greater part of the revolution than would be possible in the embodiment of Fig. 2. By the recesses in the stator and the rotor poles being symmetrically located a controlled displacement of the rotor will result, the magnitude of which will be determined by the depth of the shallowest recess. As recesses of different depth only give the disadvantage of deteriorated conduction of flux in the part having the deeper recess, normally, said recesses are given the same depth.

Claims: L A I M S 1. A reluctance motor comprising a stator 10 having two salient poles 11,12, and a rotor 16 also having two poles and being of soft magnetic material, the stator poles 11,12 supporting windings 13,14 which when current is being supplied generate a magnetic field which tends to set the rotor 16 in a stable position of equilibrium in which the rotor poles and the stator poles coincide, wherein the stator and the rotor poles are designed so as at high flux density to cause a constriction of the flux in a part of the cross section of the flux path in order to displace the position of equilibrium and when the flux density is moderate to distribute the flux generally uniformly over the said cross section, the motor further being designed such that during start the stator windings are supplied with current pulses of a magnitude which brings the part of the respective pole which is located between the constriction of flux and the pole surface into saturation, c h a r a c t e r i z e d in that the constriction of flux is formed by two recesses (17,19;1820) or the like which are symmetrically located in a pair of stator and rotor poles, one recess (18,20) or the like being located in the rotor pole (26,27) at its front or rear edge, as seen in its direction of rotation while the other recess (17.19) or the like is located in the stator pole (11,12) at the opposite edge. 2. A motor according to Claim 1, c h a r a c t e r i z e d in that the stator and rotor poles (11,12;26,27) have the same extension, as seen in the direction of location of the rotor, the recess (19,20) or the like in the respective rotor pole (26,27) being located at the rear edge of the pole, as seen in the direction of rotation. 3. A motor according to Claim 1, c h a r a c t e r i z e d in that the respective rotor pole (22,23) comprises two parts (22a,23a;22b,23b) of the same peripheral extension corresponding to the peripheral extension of the stator poles (11,12), the pole surface (22b, 23b) of one part being located at a shorter distance from the rotor shaft than the pole surface (22a,23a) of the other part, the recess (19.20) in the respective rotor pole (22,23) being located at the rear edge of the rotor pole, as seen in the direction of rotation of the pole. 4. A motor according to Claim 1, c h a r a c t e r i z e d in that the respective rotor pole (22,23) comprises two parts (22a,23a;22b,23b) of the same peripheral extension being less than the corresponding extension of the stator poles (24,25), the pole surface (22b, 23b) of one part being located at a shorter distance from the rotor shaft than the pole surface (22a,23a) of the other part, and the recess (19,20) in the respective rotor pole (22,23) being located at the rear edge, as seen in the direction of rotation of the rotor pole, and having a peripheral depth which is greater than the difference between the peripheral extension of the stator pole and the corresponding extension of either part of the rotor pole. 5. A motor according to any of the preceding claims, characterizeu in that the recesses (17-20) in the stator and the rotor poles are of equal depth.

Inventors: MOREN LARS GUNNAR, MOREN, LARS, GUNNAR
Applicants: ELECTROLUX AB, AKTIEBOLAGET ELECTROLUX
Application year: 1986
Main classification: H02K19
IPC codes: H02K19