Direct Torque Control Strategies for Interior Permanent Magnet Synchronous Motors Driven by a Three-level Simplified Neutral Point Clamped Inverter

Ngo, Hoang Tung
Foo, Gilbert
Baguley, Craig
Tjing Lie, Tek
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Doctor of Philosophy
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Auckland University of Technology

The three-level simplified neutral point clamped (3L-SNPC) was first proposed in 1993. Compared to the T-type and NPC inverters, it uses fewer switching devices to generate a multilevel waveform. This makes the 3L-SNPC inverter attractive for low-voltage motor drive applications. However, and to date, it has received comparatively little research attention, particularly in relation to suitable control strategies. The work described in this thesis addresses this shortcoming through proposing several direct torque control (DTC) strategies for 3L-SNPC inverter-fed interior permanent magnet synchronous motor (IPMSM) drives.

A duty-cycle-control-based DTC strategy with torque ripple reduction suited to the 3L-SNPC inverter is firstly proposed. It utilises hysteresis controllers to directly control electromagnetic torque and stator flux within hysteresis bands. To reduce the torque ripples the proposed strategy employs two voltage vectors within each sampling cycle. The duty cycle of each applied voltage vector is calculated from knowledge of the torque error and rotor speed. In addition, the proposed algorithm incorporates neutral point voltage balancing. The experimental results indicate that the proposed duty-cycle-control- based DTC strategy achieves smaller torque and flux ripples compared to a non-duty-cycle-based DTC strategy.

Another control method proposed in this thesis is predictive torque control (PTC). It uses stator current and rotor speed values to predict both the electromagnetic torque and stator flux. Then, the most appropriate voltage vector that minimises a predetermined cost function and, hence, the torque error is selected. To further reduce the torque error, duty-cycle-control and neutral point voltage balancing methods are incorporated. The combined duty-cycle PTC method necessitates variable switching frequency operation. To address this issue, torque-regulator-based DTC and space vector modulation (SVM) based deadbeat PTC (DB-PTC) strategies are proposed.

The proposed torque-regulator-based DTC strategy employs a PI torque controller and four triangular-carriers to attain torque ripple reduction at a constant switching frequency. The hysteresis torque levels are determined by comparing the output of the PI torque controller with four triangular-carriers. With knowledge of the torque and flux error levels, an appropriate voltage vector is selected from a switching table to regulate the torque and stator flux. In this method the PI torque controller is used to negate the large rate of torque variation, while periodic triangular-carriers are used to achieve a constant switching frequency.

On the other hand, the proposed DB-PTC strategy achieves a constant switching frequency through the adoption of a novel SVM. The desired voltage vector is determined on the basis of the deadbeat control and synthesized using SVM. The proposed SVM algorithm is developed specifically for the 3L-SNPC inverter and overcomes the problem of a lack of medium voltage vectors. The algorithm involves selecting the switching sequence in a manner to assure pole voltages can only change by one voltage level at most during each voltage vector transition. In addition, the algorithm assures that all reference voltage vectors confined to the hexagonal limit of the inverter can be synthesized. With the adoption of SVM, torque and flux ripples are significantly reduced.

All control strategies presented in this thesis are novel and described, modelled and experimentally verified under both steady-state and dynamic test conditions. A comprehensive comparative evaluation of the strategies is also carried out. All proposed strategies represent a contribution to knowledge, and are expected to improve the attractiveness of the 3L-SNPC inverter for future motor drive applications.

Direct torque control , Interior permanent magnet synchronous motor , Multilevel inverter , Torque ripple
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