A brushless or commutator free DC- motor generally comprises a permanent magnet synchronous motor which is fed by an inverter controlled by magnetic or optical sensors arranged at the motor shaft, said sensors provide a pattern of output signals corresponding to the rotor position of said motor. The output signals are then processed in logic circuits and control the output bridge of the inverter. A synchronous motor with a feeding equipment of the type is normally self-commutating, that is, it operates basically in the same way as a DC -motor with mechanical commutation.
In conventional DC brushless motors designed for rotational speeds up to 10,000 rpm switching losses in the motor windings and slight unsymmetries in the commutation detection circuits ( which could be of type optic, magnetic or Back-EMF) normally do not cause any problems. However in a high speed motor a slight unsymmetry or displacement of the commutation points may cause very high losses e.g. in the magnitude of the rated power and might damage the motor. The design of the inverter has to be such that the majority of heat losses are dissipated in the inverter and not in the stator coils and core which are very sensitive to heating.
Thus the reference commutation points which have been obtained either by external shaft position encoders or internally by Back-EMF detection have to be displaced
a) individual angles with respect to unsymmetries of the rotor and stator and PA1 b) all with the same amount of angle when e.g. the armature reaction has to be compensated for. PA1 a) performing optimization based upon the logical position signals from the position detecting mechanism whereby the commutation points derived from said position signals are each individually displaced by a commutation control unit according to a certain scheme; PA1 b) detecting the input current level when controlling the commutation for minimum power dissipation in the permanent magnet motor to optimize motor performance by reducing motor vibrations, maximizing motor acceleration, and providing efficient operation at given velocity and load.
Various motor speeds as well as different motor species give different sets of commutation time points.
In order to keep the losses of the synchronous motor as small as possible, the waveform of the output voltage from the inverter should be adapted as well as possible to the waveform of the back- EMF generated by the motor, which normally is sinusoidal. By using switching technique in the inverter, the losses in this can be kept low, but consideration must also be taken to influence of harmonics in the stator winding which are generated as a result of the square wave of the inverter and which harmonics do not contribute to the torque of the synchronous motor.
When inverting at high operating frequencies corresponding to high rotational speeds (100,000 r.p.m) of the motor in question it is essential to keep the number of switchings per electric feeding period as low as possible in order to avoid substantial losses in the inverter. At high r.p.m. it is also important that the switching is performed in such a way that the output power per volume unit will be as large as possible. The difference between the calculated mechanical setting of the magnetic sensors (e.g. Hall sensors) or optical sensors and the actual setting which is influenced by unsymmetries of the rotor, the stator windings and stator iron is critical with respect to the efficiency of the motor especially at high rotor speeds. This increases the costs of production of such a motor since measures have to be taken for a final adjustment of the sensors. In certain cases also separate commutating magnets are used in order to avoid influence from the stator field, which involves an additional material and assembly cost in addition to the costs of a device for final adjustment and the sensors per se.