METHOD FOR REDUCING THE COGGING TORQUE PRODUCED BY BRUSHLESS ELECTRIC MOTORS USED SIMULTANEOUSLY

Abstract

The invention relates to a method for reducing the cogging torque (C1, C2) produced by at least two brushless electric motors used simultaneously, said motors comprising a rotor connected to an output shaft and composed of at least one permanent magnet, and a stator comprising at least two receiving volumes for at least three coils generating a magnetic field, characterized in that it comprises at least the following steps: a) determining the period of the cogging torque (C1, C2) for each motor, b) putting into phase opposition the periods for each motor (1, 2) determined in step a).

Claims

1. A method for reducing the cogging torque (C1, C2) produced by at least two brushless electric motors (1, 2) used simultaneously, said motors (1, 2) comprising a rotor connected to an output shaft and composed of at least one permanent magnet, and a stator comprising at least two receiving volumes for at least three coils generating a magnetic field, characterized in that it comprises at least the following steps: a) determining the period T of the cogging torque for each motor (1, 2), b) putting into phase opposition the periods for each motor (1, 2) determined in step a).

2. A method according to claim 1, characterized in that during step a), the period T is determined by the relationship T=360/Na, where Na=NpNs (GCD Np, Ns), Na being the number of alignments of each magnetic pole Np on the rotor with each receiving volume Ns in the stator.

3. A method according to claim 1, characterized in that during step b), the phase opposition is achieved by phase shifting the periods T of each motor by a value of (T/2).

4. A method according to claim 3, characterized in that during step b), the phase shift is achieved by offsetting the teeth on the output shaft of the motors relative to each other.

5. A method according to claim 1, characterized in that the number of motors in phase opposition is even.

6. A method according to claim 1, characterized in that said method is associated with another solution for reducing cogging torque (C1, C2).

Description

[0024] The invention will be better understood, and further advantages thereof will become more clearly apparent, upon reading the following description of a number of embodiments of the invention, provided by way of a non-exhaustive example and with reference to the accompanying drawings, in which:

[0025] FIG. 1 is a perspective view of two brushless motors assembled on the same component, according to one embodiment of the invention,

[0026] FIG. 2 is a simplified curve, on another scale, illustrating the implementation of the method according to one embodiment of the invention applied to the motors in FIG. 1 and

[0027] FIGS. 3A and 3B are simplified diagrams illustrating, respectively, the absence and the presence of a cogging torque.

[0028] FIG. 1 illustrates, schematically, an example of simultaneous use of two brushless motors 1, 2. Each motor 1, 2 is engaged, in this case directly, by its rotor with a rotating shaft 3 or a gear in a device known per se. In an advantageous embodiment, the motors 1, 2 are identical. Alternatively, they are not identical. The number of brushless motors can be greater than two, for example four, six or more, provided that, within the context of the invention, the number of motors is even. Similarly, the motors 1, 2 can be arranged as illustrated or in series, i.e. one behind the other on the shaft 3.

[0029] Regarding the relative position of the motors in relation to the rotating shaft, it is the same when the number of motors is greater than two. In all cases, it is understood that the direction of rotation, in this case represented by the arrow F, is identical for all the motors, this in view of the fact that the direction of rotation of the motors can be reversed.

[0030] Each motor 1, 2 generates a cogging torque. As can be seen from FIGS. 3A and 3B, the cogging effect occurs every time a magnetic pole, north or south, of the rotor is in front of a saliency of the ferromagnetic material on the stator, whether the rotor is inserted into or surrounds the stator. It should be borne in mind that the electromagnetic field propagates along the path wherein the distance to be covered in the air, which is an electromagnetic insulator, is as short as possible.

[0031] In FIG. 3A, a magnet 4 of the rotor is positioned in front of a metal part 5 without constituent saliency of the stator. In this case, the force of attraction, according to the arrow FA, of the magnet 4 on the metal part 5 is constant and smooth, regardless of the angle of rotation of the rotor. In FIG. 3B, the magnet 4 is in front of a saliency 7 due to the presence of a slot 6. In this case, the force of attraction, according to the arrow FA, of the magnet 4 on the metal part 5 is no longer constant and smooth, regardless of the angle of rotation of the rotor. As indicated previously, the electromagnetic field propagates along the path wherein the distance to be covered in the air, which is an electromagnetic insulator, is as short as possible, i.e. in this case following the arrow FB.

[0032] As is apparent from FIG. 2, each motor 1, 2 has a cogging torque C1, C2 for a given period T. According to the invention, if the periods T of the cogging torques C1, C2 of the motors 1, 2, expressed in Newtons as a function of the angular position of the rotor, are in phase opposition, the resulting torque Cr, which is zero, is observed.

[0033] For a given motor 1, 2, it is understood that, during a complete turn of the rotor, each magnetic pole of the rotor magnets is aligned once with each ferromagnetic part provided on the stator and materialized by the slots. In other words, knowing the number of alignments Na per turn, hence per complete turn of the rotor, for each motor enables the cogging torque to be characterized.

[0034] Thus, in function of the number of magnetic poles Np and in function of the number of slots Ns, it is possible to determine the number of alignments per turn Na for each motor. It should be borne in mind that a number of poles can be aligned simultaneously with a slot. In this case, the number of alignments per turn Na is given by the relationship: [0035] Na=NpNs/GCD (Ns; Np) where: [0036] Np: number of poles; [0037] Ns: number of slots; [0038] GCD: Greatest Common Divisor between Ns and Np.

[0039] From this number of alignments Na, it is then possible to determine a period T of the cogging torque for each motor 1, 2 via the relationship: [0040] T=360/Na

[0041] Each motor 1, 2 therefore has a cogging torque for a period T determined by the previously specified relationship.

[0042] According to the invention, if the periods T of the cogging torques for the motors 1, 2 are shifted by a given value, in this case, when there are two motors, a phase shift value D equal to one half period, i.e. D=T/2, then, for identical motors 1, 2, opposing values are obtained for the cogging torques. It is understood that with a number of motors greater than two but even, the value of the phase shift is adapted to the cogging profile. Indeed, the cogging curve is not a perfect sinusoid. Moreover, the cogging profiles between the motors may be different if said motors are not identical but have an identical period.

[0043] Thanks to the invention, it is therefore easy, when there are two or an even number greater than two, of identical brushless motors, or at least with an identical period T when the motors are not the same, to reduce or even eliminate the cogging effect.

[0044] The invention thus has applications both on assemblies with new motors and on existing assemblies. In addition, no structural modification of the motors is necessary, except to optimize the positioning of the rotor relative to the rotating shaft of each motor.

[0045] The phase shift is achieved for each motor, for example, by indexing the teeth on the rotor gear relative to a given fixed point of the stator for the motors 1, 2. Thus, by a purely mechanical adjustment, without adding any component and without changing the overall dimensions and/or location of the motors, cogging is definitively canceled for the assembly of motors concerned.

[0046] It is understood that, depending on the motors, the size of the cogging torques varies and therefore, de facto, that the phase shift to be achieved must vary in size. For this purpose, it is possible, but not mandatory, to couple the invention with another technical solution enabling the cogging effect to be minimized as much as possible.