The torque of an alternating-current machine is gener- ated by means of an armature circuit, normally the stator, a supply voltage and magnetization of an exci- tation circuit, normally the rotor. The supply voltage is applied to stator windings fitted in stator slots.

Excitation is developed using permanent magnets or ex- citation windings fitted in the rotor. The uniformity of the torque depends on how far the field developed in the air gap of the motor approaches the sinusoidal form. A curve form deviating from the sinusoidal form produces torque variations at a frequency determined by the harmonic components, giving rise to mechanical vibration and noise.

In addition to the supply voltage frequency, the form of the air gap field also depends on the form of the field generated by the excitation. A permanent magnet synchronous motor typically produces a torque charac- terized by so-called"cogging", which is a result of the reluctance variation caused by the stator teeth as the angle between the rotor and stator changes.

The object of the invention is to achieve a solution for substantially reducing the vibration and noise generated by harmonic moments in the motor. To imple- ment this, the method of the invention is character- ized by the features presented in the characterization

part of claim 1. Correspondingly, the apparatus of the invention is characterized by the features presented in the characterization part of claim 6. The features of some preferred embodiments of the invention are presented in the subclaims.

According to a preferred embodiment of the invention, a correction signal dependent on the angle between the rotor and stator is summed with the output of a torque regulator to compensate the torque ripple produced by the motor.

According to a preferred embodiment, a signal process- ing circuit comprises a storage element from where a correcting torque component is read. The stored compo- nent is preferably generated by first operating the motor without a correction signal while at the same time measuring the torque ripple component. In normal motor operation, a corresponding signal in opposite phase is summed with a torque reference. This results in compensation of the torque ripple.

Further, according to a preferred embodiment, the cor- rection signal is generated using a neural network comprised in a signal processor.

According to another preferred embodiment, the fre- quency of the correction signal is the same as the slot ripple frequency and its phase is locked to the rotor angle. The locked phase and the amplitude pref- erably form two parameters which are kept constant during operation or which are made to be variable in accordance with the operating point.

The invention provides a cheap and reliable solution for eliminating the torque ripple caused by slot har- monics in a permanent magnet synchronous motor with an axial air gap. The invention is especially applicable to an elevator drive motor.

In the following, the invention will be described by the aid of an embodiment example with reference to the drawing, which presents a motor regulator according to the invention.

The motor 2 is controlled using a torque regulator 4.

The angle between the stator and rotor of the motor is measured using angle measuring equipment 6 and the mo- tor speed is measured by means of a velocimeter 8 con- nected to the shaft of the motor 2. The speed data 28 is fed into a differential element 18, which is also fed by a speed reference 20. The speed reference and the speed measurement are used to control the speed regulator 12 in a manner known in itself. The output of the speed regulator is taken to an adder 14, to which also a correction signal 22 according to the in- vention is input. The output of the adder 14 is passed to the torque regulator 4 controlling the motor.

The angle measurement data (16,26) is input to the torque regulator 4 and to the signal processor 10. In the signal processor 10, a correction signal 22 is generated, to be input to the adder 14 and summed with the torque reference. As shown in the figure, the speed signal is also input to the signal processor 10.

According to a preferred embodiment, the signal proc- essor comprises storage elements in which the correc-

tion signal is stored. By operating the motor without a correction signal, the torque ripple can be measured and the signal corresponding to it is stored in mem- ory. During normal operation, a corresponding signal in opposite phase is input to the adder. Naturally, it is also possible to store a correction signal gener- ated in the manner described above directly in memory.

According to an embodiment, the frequency of the cor- rection signal is the same as the slot ripple fre- quency of the motor and its phase is locked to the ro- tor angle. The phase and amplitude form two parame- ters, which are either kept constant during operation or made to be variable in accordance with the operat- ing point. The parameters are suitably stored in mem- ory.

In the foregoing, the invention has been described by the aid of a few of its embodiments. However, the presentation is not to be regarded as constituting a limitation of the sphere of patent protection, but different embodiments are possible within the scope of the inventive idea defined in the claims.