Improved scalar controller based four leg inverter for two 3-phase induction motor drives

A four leg inverter, having lower number of IGBTs as compared to six leg inverter, has been studied for controlling of two 3-phase induction motor drive. In this paper drift compensator has been avoided by the proper designing of v/f controller and simulation is done in MATLAB. A four leg inverter (FLI) consists of four legs having eight switches and two capacitors connected in series act as a DC link capacitors. A general pulse width modulation (PWM) technique is not directly applicable to FLI as it is applicable in the case of three leg six switch inverter, because modulation of third phase `B' is not possible. As phase `B' is shared between two induction motors and it is connected between neutral points of two split capacitors so it is not possible to modulate this phase. Hence two phases `R' and `Y' must be modulated of each motor. Phases R1, Y1, R2, and Y2 are connected to each leg of FLI. Mathematical analysis of neutral point potential of two split capacitors and its compensation is also done in this paper. A carrier based PWM is also shown in this paper for obtaining a balance three phase output voltage.