| Summary: | Classical direct torque control (DTC) suffers from problems such as high torque ripples, variable inverter switching frequency and flux drooping at low speeds. Traditionally, two-level inverters are employed in direct torque controlled motor drives. The torque and flux regulation in DTC drives can be improved if three-level inverters are used instead of two-level inverters; the increase in the degree of freedom for voltage vector selection means that the rotational speed of stator flux can be more precisely controlled to attain a superior regulation of torque and flux. At the same time, the broad review presented on the existing three-level inverter fed DTC (3L-DTC) methods at the beginning of this thesis reveal that the integration of three-level inverters into switching table based DTC drives is complicated and does not readily alleviate the shortcomings of classical DTC. Firstly, 3L-DTC drives are typically intended for use in higher power motors and therefore, the average inverter switching frequency has to be kept as low as possible to maximize the drive efficiency. However, if low switching frequencies are used, torque and flux ripples in 3L-DTC schemes can still be excessive and detrimental. Secondly, the inverter inherent switching constraints due to smooth voltage vector switching and neutral point voltage fluctuations, which are essential to reduce low order THD and ensure safe operation, must be respected.
This study focuses on the incorporation of three-level inverters into DTC drives. A number of 3L-DTC strategies incorporating a three-level neutral point clamped (3L-NPC) inverter, one of the most frequently used multilevel inverter in variable speed drives, are proposed and experimentally verified for the control of an interior permanent magnet synchronous motor (IPMSM). The torque variation rates or slopes of permanent magnet synchronous motors are considerably larger than the minimum rate required for the proper regulation of torque during steady state. Consequently, the application of a single voltage vector within one switching period in classical DTC leads to large ripples of torque. Duty cycle based DTC (DDTC) method, in which more than one voltage vectors is applied within each switching cycle, is an effective way to reduce the torque ripples. With the DDTC method, the biggest challenge is to determine the appropriate voltage vectors and their respective duty ratios. The handful of DDTC methods in literature for 3L-DTC drives uses parameter dependent and complicated techniques for duty ratio determination. Furthermore, no consideration is given to the aforementioned inverter switching constraints in selecting the voltage vectors.
In this thesis, a comprehensive analysis is carried out to investigate the typical variation rates of torque and flux in a 3L-DTC IPMSM. Based on this analysis, a novel DDTC method using two voltage vectors, one active and one passive within one switching cycle is proposed. For duty ratio calculation, torque ripple root mean square minimization (TR-RMSM) with minimal parameter dependency is developed, taking into account the dynamics of torque and flux characteristics in an IPMSM. In addition, proper switching techniques are introduced to overcome the problem of smooth voltage vector switching and large neutral point voltage fluctuations.
Ideally for optimal torque ripple reduction, the active and passive voltage vectors applied within each switching cycle in DDTC methods should have opposing variation rates of torque. For instance, the inclusion of a zero voltage vector as a passive vector is necessary for 3L-DTC drives to optimize the torque ripple reduction during medium to low speeds. However, this is not always possible considering the smooth voltage vector switching criteria, which limits the switching transitions to be between adjacent voltage vectors. Therefore, a new DDTC method employing three voltage vectors (one active and two passive) in one switching cycle is proposed in this thesis. The application of three vectors, however, complicates the direct application of TR-RMS method for duty ratio calculations. Consequently, a simplified duty ratio calculation technique which regulates the duty ratio of the non-zero passive vector according to the angular velocity of the motor is developed. Regulating the duty ratio of one passive vector means that aforementioned TR-RMSM method with minimal parameter dependency can be applied.
The low speed performance of 3L-DTC drives is typically affected by poor flux regulation, otherwise known as flux drooping. Flux drooping will increase the lower order harmonics in the output, and thus, affect the efficiency of the drive system. The back emf of the motor is low at low speeds and therefore, voltage vectors with smaller magnitudes are used more frequently. During heavy loads, the significant voltage drop across the stator resistance will cause the stator flux to droop. In order to alleviate the flux drooping issue during low speed, the use of virtual voltage vectors, which are synthesized from two adjacent vectors with smaller magnitudes, are proposed. The proposed method is evaluated through experiments carried out at 3% of the test IPMSMs rated speed. Results confirm that the effects of flux drooping at low speed are mitigated through the use virtual short voltage vectors.
Although the regulation of torque and flux is improved significantly by the proposed DDTC methods, they do not solve the issue of variable inverter switching frequency. The inconsistent variation of torque slope and the usage of fixed torque hysteresis bandwidth are the main causes for variable switching frequency in DTC drives. To attain a constant inverter switching frequency with improved torque and flux regulation, a simple torque regulator, consisting of a PI controller and triangular carriers, is proposed in place of the conventional torque hysteresis controller. The PI controller is used to negate the large variations in torque slope, which is desirable for reducing torque ripples. Then, the output of the PI controller is compared against triangular carriers to attain a constant switching frequency. Detailed analysis and design guidelines for the proposed torque regulator using small signal modeling are presented. In addition, the parametric robustness of the proposed method is experimentally verified.
Almost all of the existing DTC strategies used in multilevel inverter fed DTC (MLI-DTC) drives including the DDTC and constant switching frequency based methods proposed in this thesis are inverter specific. i.e., the level of hysteresis controllers and the proposed switching tables are designed specifically for inverters with a certain number of voltage levels. Therefore, these methods are not readily applicable to generic n-level multilevel inverters. In order to overcome this inconvenience, the proposed CSF strategy is extended to be generalized for inverters possessing any number of voltage levels by using a simple voltage vector decomposition technique. A comparative study with a prior art and parametric sensitivity analysis are presented to verify the effectiveness and robustness of the proposed MLI-DTC method.
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