Dietz, Daniel ; Binder, Andreas (2018)
A Bearingless PM Synchronous Machine with a Zero- Sequence Current Driven Star Point-Connected Active Magnetic Thrust Bearing.
24th International Conference on Magnetically Levitated Systems and Linear Drives (MAGLEV 2018). St. Petersburg, Russia (05.-08.09.2018)
Conference or Workshop Item

Abstract

Common cylindrical bearingless drives require a separate thrust bearing, which is fed by a DC supply. Here, a technique is presented, which enables the feeding of the thrust bearing by an artificially generated zero-sequence current between the two star-points of the two parallel windings of the bearingless PM synchronous machine. This way, no additional DC supply for an axial active magnetic bearing is needed. It is replaced by two three-phase inverters as stator winding supply, which are needed in any case to generate torque and lateral rotor force in the motor. The principle is shown for the following configuration: A bearingless PM synchronous motor with a combined winding of two three-phase systems for torque and lateral force generation is fed by two asymmetrical three-phase current systems. Two standard three- phase PWM-inverters are used with field-oriented control. In a first step the time instants of the active stator voltage vectors, required for torque and lateral force generation, are calculated. The arbitrary zero voltage time spans can be arranged differently for the two star-connected three-phase current systems. This way, the difference of potential between the two star-points during the zero time instants cause a controllable current. This current feeds the thrust bearing. However, the function depends on the modulation index, which determines the zero voltage time span. Moreover, the resulting axial force depends on the relation between the bearing impedance and the zero-sequence motor impedance. Therefore, the aim is to illustrate the feasible inverter operation limits on the one hand and the special design requirements of the motor and the thrust bearing for efficient axial force generation on the other hand. Since each bearingless motor needs two three-phase current systems, either for a separate drive winding and a suspension winding or for the here considered two parallel three-phase windings, the principle of a zero-sequence current supply for an axial active magnetic bearing is generally applicable.

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