Operational constraints of zero-sequence current feeding in bearingless PM synchronous machines

For magnetically levitated drives with a 3-phase stator winding, one degree of freedom in the rotor motion can be controlled through a zero-sequence current. For example, an axial active magnetic bearing or a single-phase motor winding can be fed by this zero-sequence current. The zero-sequence current feeding is advantageous due to the saving of power electronic switches. This way, the 4-quadrant chopper can be replaced by connecting the coil to be fed with the star point of the 3-phase winding. The field of application for this principle is limited by non-linearities at high inverter modulation degrees since the maximum possible voltages of the zero-sequence current system and the 3-phase current system are mutually dependent. Therefore, a pulse width modulation is discussed, which prioritizes the 3-phase voltage system over the zero-sequence voltage system, in contrast to a modulation without prioritization. These two methods are evaluated by the maximum possible voltages and the voltage harmonic content. If only one of the two current systems demands high-voltage values, such as in the combination of a magnetic bearing and a motor winding, the zero-sequence current feeding is well-suited.