Induction Motor Drives Based on Multilevel Dodecagonal and Octadecagonal Volatage Space Vectors
Abstract
For medium and highvoltage drive applications, multilevel inverters are very popular. It is due to their superior performance compared to 2level inverters such as reduced harmonic content in the output voltage and current, lower common mode voltage and dv=dt, and lesser voltage stress on power switches. The popular circuit topologies for multilevel inverters are neutral point clamped, cascaded Hbridge and flying capacitor based circuits. There exist different combinations of these basic topologies to realize multilevel inverters with modularity, better fault tolerance, and reliability. Due to these advantages,
multilevel converters are getting good acceptance from the industry, and researchers all over the world are continuously trying to improve the performance of these converters. To meet such demands, three multilevel inverter topologies are proposed in this thesis.
These topologies can be used for highpower induction motor drives, and the concepts
presented are also applicable for synchronous motor drives, gridconnected inverters, etc.
To get nearly sinusoidal phase current waveforms, the switching frequency of the
conventional inverter has to be increased. It will lead to higher switching losses and
electromagnetic interference. The problem with lower switching frequency is the intro
duction of low order harmonics in phase currents and undesirable torque ripple in the motor. The 5th and 7th harmonics are dominant for hexagonal voltage spacevector based low frequency switching, and it is possible to eliminate these harmonics by dodecagonal switching. Further improvement in the waveform quality is possible by octadecagonal voltage spacevectors. In this case, the complete elimination of 11th and 13th harmonic is possible for the entire modulation range. The concepts of dodecagonal and octadecagonal voltage spacevectors are used in the proposed inverter topologies.
The first topology proposed in this thesis consists of cascaded connection of two
Hbridge cells. The two cells are fed from unequal DC voltage sources having a ratio
of 1 : 0:366, and this inverter can produce six concentric dodecagonal voltage space
vectors. This ratio of voltages can be obtained easily from a combination of stardelta transformers, since 1 : 0:366 = (
p 3 + 1) : 1. The cascaded connection of two Hbridge cells can generate nine asymmetric pole voltage levels, and the combined threephase inverter can produce 729 voltage spacevectors (9 9 9). From this large number of combinations, only certain voltage spacevectors are selected, which forms dodecagonal pattern. In the case of conventional multilevel inverters, the voltage spacevector diagram consists of equilateral triangles of equal size, but for the proposed inverter, the triangular
regions are isosceles and are having different sizes. By properly placing the voltage spacevectors in a sampling period, it is possible to achieve lower switching frequency for the individual cells, with substantial improvement in the harmonic spectrum of the output voltage. During the experimental veri cation, the motor is operated at di erent speeds using open loop v=f control method. The samples taken are always synchronised with the start of the sector to get synchronised PWM. The number of samples per sector is decreased with increase in the fundamental frequency to limit the switching frequency.
Even though many topologies are available in literature, the most preferred topology for drives application such as traction drives is the 3level NPC structure. This
implies that the industry is still looking for viable alternatives to construct multilevel inverter topologies based on available power circuits. The second work focuses on the development of a multilevel inverter for variable speed mediumvoltage drive application with dodecagonal voltage spacevectors, using lesser number of switches and power sources compared to earlier implementations. It can generate three concentric 12sided polygonal voltage spacevectors and it is based on commonly available 2level and 3level inverters. A simple PWM timing computation method based on the hexagonal spacevector PWM is developed. The sampled values of the threephase reference voltages are initially converted to the timings of a twolevel inverter. These timings are mapped
to the dodecagonal timings using a change of basis transformation. The voltage space
vector diagram of the proposed drive consists of sixty isosceles triangular regions, and the dodecagonal timings calculated are converted to the timings of the inner triangles. A searching algorithm is used to identify the triangular region in which the reference vector is located. A frontend recti er that may be easily implemented using standard stardelta transformers is also developed, to provide nearunity power factor. To test
the performance of the inverter drive, an openloop v=f control is used on a threephase induction motor under noload condition. The harmonic spectra of the phase voltages were computed in order to analyse the harmonic distortion of the waveforms. The carrier frequency was kept around 1.2 KHz for the entire range of operation.
If the switching frequency is decreased, the conventional hexagonal spacevector
based switching introduce signifi cant 5th, 7th, 11th and 13th harmonics in the phase currents. Out of these dominant harmonics, the 5th and 7th harmonics can be completely
suppressed using dodecagonal voltage spacevector based switching as observed in the first and second work. It is also possible to remove the 11th and the 13th harmonics by using voltage spacevectors with 18 sides. The last topology is based on multilevel octadecagonal (18sided polygon) voltage spacevectors, and it has better harmonic performance than the previously mentioned topologies. Here, a multilevel inverter system capable of producing three octadecagonal voltage spacevectors is proposed for the fi rst time, along with a simple timing calculation method. The conventional threelevel inverters are only
required to construct the proposed drive. Four asymmetric power supply voltages with
0:3054Vdc, 0:3473Vdc, 0:2266Vdc and 0:1207Vdc are required for the operation of the drive, and it is the main drawback of the circuit. Generally frontend isolation transformer is essential for highpower drives and these asymmetric voltages can be easily obtained from the multiple windings of the isolation transformer. The total harmonic distortion of the phase current is improved due to the 18sided voltage spacevector switching. The ratio of the radius of the largest polygon and its inscribing circle is cos10 = 0:985. This
ratio in the case of hexagonal voltage spacevector modulation is cos30 = 0:866, which means that the range of the linear modulation for the proposed scheme is signifi cantly higher. The drive is designed for openend winding induction motors and it has better fault tolerance. It any of the inverter fails, it can be easily bypassed and the drive will be still functional with reduced speed. Open loop v=f control and rotor flux oriented vector control schemes were used during the experimental verifi cation.
TMS320F2812 DSP platform was used to execute the control code for the proposed
drive schemes. For the entire range of operation, the carrier was synchronized with the fundamental. For the synchronization, the sampling period is varied dynamically so that the number of samples in a triangular region is fi xed, keeping the switching frequency around 1.2 KHz. The average execution time for the v=f code was found to be 20 S, where as for vector control it took nearly 100 S. The PWM terminals and I/O lines of the DSP is used to output the timings and the triangle number respectively. To convert the triangle number and the timings to IGBT gate drive logic, an FPGA (XC3S200) was used. A constant deadtime of 1.5 S is also implemented inside the FPGA. Optoisolated gate drivers with desaturation protection (M57962L) were used to drive the IGBTs. Halleffect sensors were used to measure the phase currents and DC bus voltages. An incremental shaft position encoder with 2500 pulse per revolution is also connected to the motor shaft, to measure the angular velocity. 1200 V, 75 A IGBT halfbridge module is used to realize the switches. The concepts were initially simulated and experimentally verifi ed using laboratory prototypes at low power. While these concepts maybe easily extended to higher power levels by using suitably rated devices, the control techniques presented shall still remain applicable.
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