A multilevel converter with a floating bridge for open-ended winding motor drive application

• Main Author: Chowdhury, Md Shajjad
• Published: University of Nottingham 2017
• Subjects: 621.31; TK7800 Electronics
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.719477

In this thesis, a dual inverter topology is considered as an alternative to a multilevel converter for the control of high speed machines. Instead of feeding to one end of the stator with a single power converter, this topology feeds from both sides of the stator winding using two converters, thus achieving multilevel output voltage waveforms across the load. A large amount of published work in the area of open end winding power converter topologies are focused on symmetrical voltage sources. This published research recognises the advantages of the converter system in terms of increased reliability, improved power sharing capability and elimination of common mode voltages when compared to traditional single sided three phase converter solutions. However isolated DC supplies come with the price of additional components thus increase size, weight and losses of the converter system. The aim of this project is, therefore, to investigate on reducing size, weight and losses of the open end winding motor drive by eliminating the need for isolated supply as well to achieve multilevel output voltage waveform. A traditional open-end winding induction motor drive has been analysed in terms of weight and losses and it has been clearly identified that the isolation transformer not only increases the size and weight of a drive system but also includes additional losses. A modified dual inverter system has then been proposed where one of the bridge inverters is floating, thus eliminated the need for isolated supplies. An asymmetric DC voltage sources ratio of 2:1 is utilised to achieve multilevel output voltage waveform across the load. The switching sequences are also analysed to identify the charging and discharging sequences to achieve control over floating capacitor voltage. This thesis describes the theoretical derivation of the modified converter model and algorithms as well as experimental results from an 11kW laboratory prototype.