Jorge Juliet

Control Scheme for an Induction Motor Fed by a Cascade Multicell Converter Under Internal Fault

Cascade multicell (CM) converters are characterized by their high modularity, allowing their power to be easily increased. This also allows any faulty module (cell) to be isolated, so the load can be fed by the remaining operative cells. This leads to operation with an unbalanced number of cells. The use of modified voltage references to avoid the unbalanced operation in steady state has been proposed in the literature (denoted as fundamental phase-shifted compensation). This paper presents the transient operation from normal to faulty operation when such voltages are used and an induction motor is driven by the converter. In the first place, a simple way to include fundamental phase-shifted compensation (FPSC) in closed-loop operation, specifically with a field-oriented control (FOC) scheme, is presented. Then, the use of two supervisor controllers is proposed to modify the FOC speed and flux references. These new references avoid hard torque pulsation while automatically determining the maximum power available from the converter and applying it to the load. Experimental results obtained with the proposed technique in a low-power CM converter are shown for two different cases, constant torque load and fan curve load, showing a significant improvement on the signal behavior.

Indirect sensorless speed control of a PMSG for wind application

In this paper, the sensorless control of a permanent magnet synchronous generator (PMSG) for wind turbine applications is presented. This kind of generator has many advantages, such as: high efficiency, high power density and low maintenance requirements. To improve these characteristics in the whole wind generator system a sensorless scheme is proposed, thereby avoiding problems of electromagnetic interferences and failures in the position sensor.

Model Predictive Torque Control for Torque Ripple Compensation in Variable-Speed PMSMs

This paper presents a new and simple finite-control set model predictive control strategy to reduce the torque ripple in permanent-magnet synchronous machines (PMSMs). The method is based on minimizing a cost function that considers the flux linkage torque harmonics obtained from a discrete-time model of the machine. The power converter switching state that minimizes this cost function is selected and applied during a whole sampling period. Additionally, it is proposed to mitigate the other source of torque ripple, known as cogging-torque, using a feed-forward signal applied to the torque control loop. A hybrid method that uses the output information from an observer and look-up table is presented to obtain a good cogging-torque estimation and thus an accurate mitigation of this disturbance torque at low rotational speed. Experimental results demonstrate the good performance of the torque ripple compensation methods presented in this paper.

Dead-Time and Semiconductor Voltage Drop Compensation for Cascaded H-Bridge Converters

Compensation of nonlinear effects generated by dead-time and semiconductors voltage drop has been widely studied in the literature about two-level converters. This paper takes a closer look at those analyses for multilevel converters, specifically for a cascaded H-bridge (CHB) converter modulated with phase-shifted pulse width modulation. The interaction between the cells is analyzed for a generic n-cell converter, from which a general expression for the distortion generated by the nonlinearities has been obtained. Based on this expression, a compensation signal for the overall converter is calculated on each sample time, which is a fraction of the triangular carrier used to modulate each cell, thus significantly improving the quality of the output voltage signals. Experimental validation of the proposed compensation method is presented using a three-phase four-cell CHB converter prototype of 5.5 kW.

Design aspects and experimental results of a high power factor induction heating system

In this paper, a design method and some experimental results of a high power factor induction heating system is presented, which is formed by a non-controlled rectifier, a DC-DC buck converter and a resonant inverter using IGBTs. The design method considers the load parameters, the load power and the frequency as input data. Then, electric variables and parameters are calculated according to a theoretic group of relations. Digital simulations of the main electric variables are developed and finally a 15 kW prototype is tested at the laboratory to validate the simulation results. Conclusions and comments about the application of this method for a high power level are presented.

Experimental results of electric protections for a controlled current source resonant converter

In this paper, the design of electric protections for a controlled current source resonant converter and the corresponding experimental results are presented. This converter was designed for an induction heating current controlled system and is formed by a non-controlled rectifier, a DC-DC buck converter and a current source resonant inverter using IGBTs. The electric protections are focused to overvoltage, overcurrent and overheating protections and some strategies for those special cases of failures in semiconductors. Experimental results are shown for a 15 kW laboratory prototype.

Control of an induction machine fed by a Cascade Multicell converter under fault

Multilevel converters offer good quality load waveforms, which is clearly beneficial to its operation. A significant drawback that affects to all multilevel topologies is the high number of components required, which is related to a high fault probability. Among all the multilevel topologies, cascade multicell converters (CM), characterized by high modularity, allow for power to be easily increased. Moreover, this characteristic also allows any faulty module (cell) to be isolated, so the load can be fed by the remaining operational cells. Modifications to the modulation scheme, based either on the use of redundant states or on modified references have been proposed in literature, avoiding the unbalanced operation. This work presents the transient operation from normal to fault operation when modified references are used and an induction motor is driven by the converter. Modifications to the motor control scheme are proposed based on the use of field weakening to achieve operation and on-line speed reference modifications. In this way, hard torque pulsations are avoided, while the maximum power available from the converter is used.

Novel anti-windup scheme for stator flux control in surface permanent magnet machines

The control of a synchronous permanent magnet machine by means of direct control of stator flux using a novel Anti-Windup scheme is presented. The proposed strategy achieves torque control through load angle imposition and ensures maximum torque per ampere operation through the careful adjustment of the stator flux vector magnitude. The flux controller is implemented using a novel anti-windup scheme that allows the correct relation between d and q voltage components. The stator voltage is generated using space vector modulation (SVM) in a standard voltage source inverter.