Such machines are used, for example, in printing machines and grinding machines. Furthermore, highly synchronous operation properties are necessary in servo applications. In order to achieve highly synchronous operation, so as, for instance, to be able to position exactly for servo applications, it is especially necessary for the motor to have a low torque ripple. Ideally, an electric motor should supply a constant torque at each point in time. Torque ripple is a measure of to what extent the torque of the motor deviates from the average torque of the motor at a point t. Most often, the torque ripple is stated with reference to the average torque of the electric motor as a function of the rotor angle. If a motor has too high a torque ripple, a desired rotor position may not be controlled exactly. Furthermore, an electric motor should have a low leakage inductance and not a high saturation behavior. A low leakage inductance may be implemented especially by as great a distance apart of the teeth in the stator. This comes about because the magnetomotive force behavior is a function of the motor geometry and the situation of the coils. A low leakage inductance results in a low saturation behavior of the motor, which, in turn, opens up to the motor a high maximum rotary speed, and thus, in relation to the speed, a broad field of applications.
Electrical machines have mostly (at least) one stationary and one mobile main element. In rotating machines these are stators and rotors. In linear machines, such a subdivision into a stationary and a mobile main element does not exist, and therefore, in this case, the designations primary part and secondary part are used, which are quite customary even for rotating machines. In this connection, the primary part is the element that has an electrical rotating field applied to it. The secondary part, on the other hand, is the element that is excited electrically or by permanent magnets. In the following, the designation primary part will include the stator and the designation secondary part will include the rotor. As a rule, these main elements are constructed of laminated iron, and bear windings made of insulated copper conductors. The torque formation takes place electromagnetically by the force action on current-carrying conductors in a magnetic field.
What is important for this is the electric loading of the winding that carries the load current, and the magnetic flux density in an air gap between the primary part and the secondary part. Such machines are, for instance, asynchronous and synchronous machines. In particular we shall discuss below synchronous machines, and particularly synchronous motors. In synchronous motors, the electrically generated excitation field circulates in the primary part as a function of the speed. In the secondary part, for the most part, permanent magnets (but also direct current magnets) are mounted, and in the primary part three-phase windings are mounted for generating the rotational field. The subdivision of the current running to the primary part windings is performed as a function of the secondary part positional angle: the secondary part and the rotating field of the synchronous motor have the same speed.
A stator for an electrical induction machine, especially for a synchronous machine, is discussed in German patent document DE 101 19 642 A1, the teeth of the stator being detachably fastened in a yoke, which allows for winding the teeth outside of the stator and to mount them subsequently in the stator, in the mounted state of the motor, directly adjacent teeth having a different winding direction. In addition, the stator includes unwound auxiliary teeth. These configurations make allow for a simple mounting of the coils, but the synchronous motor has a high leakage inductance, and thus also a high saturation behavior.
U.S. Pat. No. 5,642,013 discusses a synchronous motor whose slot width and tooth clearance are adjusted to each other, for reducing the torque ripple, in such a way that, in this connection, a ratio of 0.40 to 0.55 sets in. This ratio can only be achieved without pole shoes, which gives the synchronous motor a high leakage inductance, and thus also a high saturation behavior, the coils about the teeth being oriented in such a way and being supplied with current during the operation of the motor in such a way that on respective diametrically opposite teeth a different magnetic field polarity is created and in each case three adjacent teeth form one pole of the stator, which endows the motor with a high torque ripple. Winding the stator takes place in such a way that always three teeth are wound simultaneously, using one wire guide.
For the reduction of the torque ripple of a synchronous motor, European patent document EP 1 315 274 A1 discusses a stator whose teeth or tooth modules (plurality of directly mutually adjacent teeth) are wound with coils in such a way that directly mutually adjacent elements, when supplied with current, have a different magnetic field polarity, the number of teeth or the number of modules being exactly twice the number of current phases used for the motor. In this motor, its high saturation behavior is a disadvantage. Furthermore, this document teaches an appropriate winding rule for the teeth or tooth modules, and a ratio of the pole pitch to the tooth (slot) pitch which may be used.