And two switches remain turned on at a time one from

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and two switches remain turned on at a time, one from positive group and one from negative group. But no two switches of the same leg should be turned on simultaneously in both cases as this condition would short circuit the dc source. In 120º conduction mode the chances of short circuit of the dc link voltage source is avoided as each switch conduct for 120º in one cycle, so there is an interval of 60º in each cycle when no switch is in conduction mode and the output voltage at this time interval is zero. In general there is a 60º interval between turning off one switch and turning on of the complimentary switch in the same leg. This 60º interval is sufficient for the outgoing switch to regain its forward blocking capability. The standard three-phase VSI topology has eight valid switching states which are given in Table. 1. Of the eight valid switching states, two are zero voltage states (0 and 7 in Table.1) which produce zero ac line voltages and in this case, either through the upper or lower components, the ac currents free wheel. The remaining states (1 to 6 in Table .1) are active states which produce non-zero ac output voltages. The inverter switches from one state to another in order to generate a given voltage waveform. Thus the ensuing ac output line voltages consist of discrete values of voltage for the topology shown in the figure 2.5.In 180 degree mode of conduction, the waveform of ac phase output voltage is stepped one and line voltage waveform is quasi square wave.
M.TECH THESIS NIT ROURKELA Page 16 Table 2.1 Switching states of a three phase VSI 2.4 Conclusion In this chapter IM detailed dynamic model is discoursed in stationary reference frame. In objective to design and understand vector controlled drives the machine dynamic model to be commanded should be understood which would be a good assessment of the true model. For the formulation of the model space phasor notations and two axis theory has been utilized. It is proved that notation of space phasor is easier and compact to deal with.
M.TECH THESIS NIT ROURKELA Page 17 CHAPTER-3 INDUCTION MOTOR - DIRECT TORQUE CONTROL 3.1 Introduction In the last decade “induction motor control techniques” have been the field of involvement of so many scientists to put out distinctive answers for control of induction motor having highlights of fast and precise torque response, and decrease in the multifaceted nature of field orientating control [3]-[7]. The DTC procedure has been perceived as the basic and practical answer to accomplish this needs.DTC is a standout among the most incredible and productive control techniques of IM. This strategy is in view of decoupled control of stator flux and torque and as of now it is a standout amongst most effectively researched methods where the objective is to control perfectly the flux and torque.

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