Apurva Somani

Direct-Matrix-Converter-Based Drive for a Three-Phase Open-End-Winding AC Machine With Advanced Features

This paper describes how matrix converters (MCs), one at each side of a three-phase open-end-winding ac machine, achieve the following features simultaneously: 1) machine phase voltage up to 1.5 times the input phase voltage in the linear modulation mode, therefore extending the rated torque operation region to 150% of the rated speed of the machine; 2) peak voltage stress across the slot insulation which is limited to the peak of input phase voltage, i.e., a factor of at least √3 lower as compared to the conventional back-to-back converter; 3) controllable grid power factor to be leading, lagging, or unity; and 4) elimination of the instantaneous common-mode voltage at the machine terminals, therefore eliminating the bearing current due to switching common-mode voltage and reduction in the conducted electromagnetic interference. To simultaneously achieve the aforementioned capabilities, a space vector pulsewidth modulation technique is described in which the MCs are modulated using only rotating space vectors. A hardware prototype of the drive system is built. Experimental results from this hardware prototype verify the operation and claims of the drive system.

On the Causes of Circulating Currents in PWM Drives With Open-End Winding AC Machines

Electric drives with open-end winding ac machines offer certain advantages over drives with star- or delta-connected machines. Such drives have been recently considered for some applications such as electric vehicles. Circulating currents have been experimentally observed in such open-end winding ac drives. These currents have the effect of increasing conduction losses in the stator winding resistance of the ac machine. Two major causes for these currents have been identified as device voltage drops in power semiconductor devices and dead time used for shoot-through current protection. These effects are analyzed, and a mathematical description for predicting their severity is provided. Experimental results are presented that support the theory. The theory developed should be useful for devising solutions for this problem such as the design of common-mode filters. Two methods of suppressing the circulating currents are also outlined, and their experimental results are presented.

Modulation strategies for direct-link drive for open-end winding AC machines

Switching common-mode voltages generated by conventional pulse-width modulated inverters are known to cause bearing currents in ac machines. These undesirable currents may result in bearing damage. A direct-link drive for open-end winding ac machines has recently been proposed. Some advantages of the drive include: 1) common-mode voltage suppression, 2) no storage elements, and 3) ability to achieve up to 1.5 times the peak input phase voltage across the machine phase windings. In this paper, pulsewidth modulation strategies for the drive are proposed. Two strategies are based on space vector modulation and suppress common-mode voltage at the machine terminals. One carrier-based strategy achieves 1.5 times the peak input phase voltage across the machine phase windings but causes switching common-mode voltage at the machine terminals. Simulation and experimental results are presented to verify the operation of the drive.

Space vector PWM for a direct matrix converter based open-end winding ac drives with enhanced capabilities

This paper introduces a space vector PWM technique for a direct matrix converter based three-phase open-end winding ac machine drive. With the proposed PWM technique, the following simultaneous capabilities of the open-end winding drive system are achieved: 1) Machine phase voltage up to 1.5 times the input phase voltage in the linear modulation, 2) Controllable grid power factor, and 3) Elimination of the high-frequency common-mode voltage at the machine terminals. Elimination of the high-frequency common-mode voltage results in elimination of the high-frequency bearing current and reduced conducted EMI. The proposed modulation scheme is implemented on a dSPACE and FPGA based control platform. Experimental results on a hardware prototype verify the abovementioned capabilities of the drive system.

A three-phase ac/ac power electronic transformer-based PWM ac drive with lossless commutation of leakage energy

This paper presents a novel three-phase ac/ac power converter topology with a high frequency ac-link for adjustable speed PWM ac drives. Such drives find applications in electric power generation from renewable energy sources like wind. This converter has a single power conversion stage with bidirectional power flow capability. The high frequency transformer provides voltage transformation, isolation, noise decoupling and high power density. This topology minimizes the number of switching transitions of the load with the transformer windings. This reduces the common-mode voltage switching and results in a better output voltage profile. This paper presents a lossless source based commutation of leakage energy. It also results in the soft switching of all the switches in the load side converter. The proposed topology is controlled with a carrier based PWM technique based on the direct modulation of matrix converters. This modulation results in controllable input power factor. The proposed converter has been analyzed and simulated. The simulation results confirm the operation and the advantages of the proposed topology.

Circulating currents in open-end winding PWM ac drives

Circulating currents have been experimentally observed in pulse-width modulated ac open-end winding drives. These currents have the effect of increasing conduction losses in the stator winding resistance of the ac machine. Two major causes for these currents have been identified as device voltage drops in power semi-conductor devices and dead-time used for over-current protection. Analysis of these effects and a mathematical description for predicting their severity is provided. Experimental results are presented that support the theory.

A new sinusoidal input-output three-phase full-bridge direct power converter

A direct power converter based variable frequency drive with three-level input/output waveforms is presented. The proposed drive generates common-mode free output voltages while drawing sinusoidal input currents. Like the conventional matrix converter, the input and output quantities are controlled simultaneously with one controller. Unlike the conventional matrix converter, the circuit always provides a freewheeling path for the load and source currents, thus eliminating the clamp circuit. Further, zero voltage switching (ZVS) of the front-end is naturally achieved. Functional operation of the converter is demonstrated using a hardware prototype. The drive is evaluated for the output common-mode voltage as well as its performance to load and source transients. Extension of the presented drive to an open-end winding drive with enhanced voltage gain is also discussed in context of previous research.

Indirect matrix converter based open-end winding AC drives with zero common-mode voltage

Common-mode voltage (CMV) generated by semiconductor switching causes stray currents and mechanical failure in modern drive systems. Solutions employed to attenuate or isolate the common-mode voltage (CMV) require additional components, and may still fail to eliminate the detrimental effects. Matrix converter based open-end winding drives, when modulated using synchronous vectors, do not generate CMV to begin with. Additionally, these drives do not rely upon a large DC capacitor that is used in the state-of-the-art systems; and are therefore expected to be more compact and reliable. This paper will present prototypes of two distinct indirect matrix converter based open-end winding drives that eliminate output common-mode voltage, provide high voltage transfer ratio (up to 1.5), and allow input power factor control. These indirect drives have the additional advantages of clamp circuit elimination, lower voltage stress on the devices, naturally intelligent commutation, and natural low-voltage ride-through integration over their direct matrix converter counterpart. Experimental evidence of the voltage transfer ratio and input power factor control will be provided. Compared to 2-level and 3-level inverters, significant reduction in the CMV induced shaft voltage and ground currents will be shown. An optimal third-order grid filter applicable to all matrix converter based drives will also be discussed. This filter will be used with the presented drives to validate its superior performance.

Analytical evaluation of the peak-to-peak ripple current in the filter inductor for a space vector modulated grid-tied VSI

Two level voltage source inverters are extensively employed to integrate renewable energy sources and storage with the grid. The output voltage of the inverter, modulated with conventional space vector PWM, contains switching frequency components along with the desired grid frequency component. Usually a filter inductor or an LCL filter is used to remove these high frequency components resulting in low THD in the grid current. Either all, or a majority of the higher frequency components of the inverter output voltage appear across the inverter-side filter inductance, resulting in ripple current through it. This paper presents an analytical estimation of the peak-to-peak ripple current as a function of the modulation index, essential for the design of the filter. Simulation results confirm the analytical prediction of the ripple current.

A minimum-switch direct-link drive with common-mode voltage suppression and active filtering for open-end winding AC machines

Conventional pulse-width modulated inverters cause switching of common-mode voltage at the terminals of ac machines, which result in undesirable bearing currents and damage to the bearing. This paper presents a two-level indirect-matrix converter-based direct-link drive for three-phase open-end winding ac machines. The proposed topology suppresses common-mode voltage, outputs up to 1.5 times the peak input phase voltage and has controllable input power factor using a novel active filtering mechanism. Simulation results are presented that verify the operation of the proposed topology.