Investigations On Dodecagonal Space Vector Generation For Induction Motor Drives

Abstract

Multilevel converters are finding increased attention in industry and academia as the preferred choice of electronic power conversion for high power applications. They have a wide application area in a variety of industries involving transportation and energy management, a significant portion of which comprises of multilevel inverter fed induction motor drives. Multilevel inverters are ideally suitable for high power drives, since the switching frequency of the devices is limited for high power applications. In low power drives, the switching frequency is often in the range of tens of kHz, so that switching frequency harmonics are pushed higher in the frequency spectrum thereby the size and cost of the filter are reduced. But higher switching frequency has its own drawbacks, in particular for high voltage, high power applications. They cause large dv/dt stress on the motor and the devices, increased EMI problems and higher switching losses. An engineering trade-o is thus needed to select the minimum switching frequency without compromising on the output voltage quality. The present work is an alternate approach in this direction. Here, new inverter topologies and PWM strategies are developed that can eliminate a set of harmonics in the phase voltage using 12-sided polygonal space vector diagrams, also called dodecagonal space vector diagrams. A dodecagonal space vector diagram has many advantages over a hexagonal one. Switching space vectors on a dodecagon will not produce any harmonics of the order 6n 1, (n=odd) in the phase voltage. The next set of harmonics thus reside at 12n 1, (n=integer). By increasing the number of samples in a sector, it is also possible to suppress the lower order harmonics and a nearly sinusoidal voltage can be obtained. This is possible to achieve at a low switching frequency of the inverters. At the same time, a dodecagon is closer to a circle than a hexagon; so the linear modulation range is extended by about 6.6% compared to the hexagonal case. For a 50 Hz rated frequency operation, under constant V/f ratio, the linear modulation can be achieved upto a frequency of 48.3 Hz. Also, the harmonics of the order 6n 1, (n=odd) are absent in the over-modulation region. Maximum fundamental voltage is obtained from this inverter at the end of over-modulation region, where the phase voltage becomes a 12-step waveform. The present work is developed on dodecagonal space vector diagrams. The entire work can be summarized and explained through Fig. 1. This figure shows the development of hexagonal and dodecagonal space vector diagrams. It is known that, 3-level and 5-level space vector diagrams have been developed as an improvement over 2-level ones. They Figure 1: Development of hexagonal and dodecagonal space vector diagrams have better harmonic performance, reduced dv/dt stress on the motor and devices, better electromagnetic compatibility and improvement of efficiency over 2-level space vector diagrams. This happens because the instantaneous error between the reference vector and the switching vectors reduces, as the space vector density increases in the diagram. This is shown at the top of the figure. In the bottom part, the development of the dodecagonal space vector diagram is shown, which is the contribution of this thesis work. This is explained in brief in the following lines. Initially, a space vector diagram is proposed which switches on hexagonal space vectors in lower-modulation region and dodecagonal space vectors in the higher modulation region. As the reference vector length increases, voltage vectors at the vertices of the outer dodecagon and the vertices from the outer most hexagon is used for PWM control. This results in highly suppressed 5th and 7th order harmonics thereby improving the harmonic profile of the motor current. This leads to the 12-step operation at rated voltage where all the 5th and 7th order harmonics are completely eliminated. At the same time, the linear range of modulation extends upto 96.6% of base speed. Because of this, and the high degree of suppression of lower order harmonics, smooth acceleration of the motor upto rated speed is possible. The presence of multilevel space vector structure also limits the switching frequency of the inverters. In the next work, the single dodecagonal space vector diagram is improved upon to form two concentric dodecagons spanning the space vector plane (Fig. 1). The radius of the outer dodecagon is double the inner one. It reduces the device rating and the dv/dt stress on the devices to half compared to existing 12-sided schemes. The entire space vector diagram is divided into smaller sized isosceles triangles. PWM switching on these smaller triangles reduces the inverter switching frequency without compromising on the output voltage quality. The space vector diagram is further refined to accommodate six concentric dodecagons in the space vector plane (Fig. 1). Here the space vector diagram is characterized by alternately placed dodecagons which become closer to each other at higher radii. As such the harmonics in the phase voltage are reduced, in particular at higher modulation indices. At the same time, because of the dodecagonal space vector structure, all the 6n ± 1, (n=odd) harmonics are eliminated from the phase voltage. A nearly sinusoidal phase voltage can be generated without resorting to high frequency switching of the inverters. The above space vector diagrams are developed using different inverter circuits. The first work is developed from cascaded combination of three 2-level inverters, while the second and third works use 3-level NPC inverters feeding an open end induction motor drive. The circuit topologies are explained in detail in the respective chapters. Apart from this, PWM switching schemes and detailed analysis on duty cycle calculations using the concept of volt-second balance are also presented. They show that with proper switching schemes, the proposed configurations can substantially reduce the overall loss of the inverter. Other operational issues like capacitor voltage balancing of 3-level NPC inverters and improvement of input current drawn from the grid are also covered. All the above propositions are first simulated by MATLAB and subsequently verified by an experimental laboratory prototype. Motor current waveforms both at steady state and transient conditions during motor acceleration show that the induction motor can be fed from nearly sinusoidal voltage at all operating conditions. Simplified comparative studies are also made with the proposed converters and higher level inverters in terms of output voltage quality and losses. These are some of the constituents for chapters 2, 3 and 4 in this thesis. Additionally, the first chapter also covers a brief survey on some of the recent progresses made in the field of multilevel inverter. The thesis concludes with some interesting ideas for further thought and exploration.