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9 CHAPTER 2 CURRENT SOURCE INVERTER FOR IM CONTROL 2.1 INTRODUCTION AC drives are mainly classified into direct and indirect converter drives. In direct converters (cycloconverters), the AC power is fed directly to the AC motor. The maximum output frequency of the cycloconverter is about one third of the supply frequency. It has principally been used for applications in which motor size is large and the speed of the operation is low. In indirect converters the AC power variation process is carried out in two stages. In the first stage, the AC power, using either a diode or SCR bridge rectifier, is rectified and in second stage rectified power is converted to a variable AC power by means of an inverter. This connection between the converters is known as a DC link. DC link AC converters are divided into VSI and CSI. They are in fact duals of each other. CSI supplies a controlled current to the motor whereas VSI controls, voltage at the motor terminals. A VSI consists of a controlled rectifier, an inductor, a capacitor which forms a filter, and an inverter. Because of the large capacitor, VSI provides a constant voltage to the load and the output voltage waveforms are not affected by the load. CSI on the other hand, consists of a rectifier, an inductor and an inverter. The presence of inductor ensures that CSI provides constant torque to the load and the output current waveforms are not affected by the load. CSI has a stiff DC current at its input source, so, it has received considerable interest in the applications which require control over the torque. 10 This chapter contains a review of theoretical concepts of CSI fed IM drives. In the first section, mathematical model of IM is described. When an electrical motor is viewed as a mathematical system with inputs and outputs, it can be analyzed and described in multiple ways, considering different reference frames and state-space variables. New mathematical models have to be implemented for the three-phase IM in order to analyze its operation both dynamically and in steady-state. In the second section, the DC link frequency converters, VSI and CSI, commonly used in IM drives are reviewed. The distinction is based on the inverter switching constraints; in VSIs, the DC voltage is converted into AC output voltage, whereas in CSIs, DC current is converted into AC output current. In this work, CSI is preferred to control IM. So, CSI topologies are presented in detail. In the third section, a brief description of the four control methods such as V/f, IOL, FOC, and DTC are presented with their salient features. 2.2 MATHEMATICAL MODEL OF IM In general, IM is used to transform electrical energy to mechanical energy. It consists of electric circuitry, electromagnetic circuitry and electro mechanic circuitry. The main objective of the motor modeling is to build a simple but competent model that describes these circuits and their interconnections. The main path in the model is in relation between the voltage and/or current or the stator phases in input where as the magnetic flux inside the motor and electromagnetic torque in the output. As mentioned earlier, the squirrel cage IM is prepared for industry applications. The IM can either be supplied by a voltage source or a current source. These sources are assumed to be ideal; this means that they can supply any desired voltage or current without losses. According to Krause et al (1965), the basic equations of the IM are the stator voltage equation (2.1), the rotor voltage 11 equation (2.2), the equations of the flux linkages of the stator and the rotor winding systems, (2.3) and (2.4), and the torque equation (2.5). = + (2.1) =0= + (2.2) = + (2.3) = + (2.4) = (2.5) 2 The equations are given in vector format. Every vector variable has two components: The first component is parallel to the reference frame axis or ‘d’ axis and the second component is perpendicular to the reference axis or ‘q’ axis. The symbols are -voltage, -current, -flux linkage, -resistance and - leakage inductance; subscript ‘s’ indicates the stator winding system, ‘s’ ‘r’, the rotor winding system; superscript indicates the stat

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