Primoz Bajec

A Study of a Hybrid Filter

This paper presents results of a comparative study of two possible hybrid filter topologies, comprised of a passive and active stage, which can be implemented in any general dc supply distribution system. The main filter task is to mitigate current dynamics in the dc distribution system in order to prolong the operational life of delicate dc supplies, i.e., fuel cells, and to reduce the electromagnetic interferences between sensitive electronic circuits connected to the distribution net. The active stage is incorporated into the passive part in order to: 1) improve its insufficient attenuation in the low-frequency range and 2) source or sink any surplus energy flow between the dc source and load in case of low-frequency current dynamics. Two active stage topologies are proposed, analyzed, and evaluated in detail: 1) an active filter based on a single-leg inverter and 2) an active filter based on an electronic smoothing inductor. A special focus is on achieving attenuation at different voltage levels of the auxiliary supply and at different coupling inductances. The impact of the current ripple generated by the switching mode of the active filter operation is discussed as well. The active filter based on the single-leg inverter offers superior attenuation, particularly in the low-frequency range where attenuation is improved nearly for 15 dB compared to the passive filter.

Extending the low-speed operation range of PM Generator in automotive applications using novel AC-DC converter control

This paper outlines a case study on an integrated starter-generator-torque-booster (ISGtB) for a hybrid propulsion system, with a particular focus on the generator operation in the low-speed range. The propulsion system consists of an internal combustion (IC) engine and a brushless dc machine and is intended to drive a street scooter. The complex functionality of the ISGtB, some outstanding characteristics of modern IC engines, stringent demands on the generated supply voltage, and operational principle of the selected electrical machine are the reasons for our research on the generator operation. An ac-dc converter and its control are proposed. By utilizing MOSFET transistor reverse conduction characteristics, the idea of synchronous rectification, and the principle of boost switching conversion, several improvements in the generator characteristics are accomplished. The proposed solution, enabling each of the three-phase converter legs to operate as an autonomous boost converter with synchronously driven upper transistors instead of using body diodes, extends the generator operation range below the nominal rotational speed and offers simple yet efficient energy flow control.

Cost-Effective Three-Phase PMSM Drive Tolerant to Open-Phase Fault

This paper presents a low-cost fault-tolerant system for open-phase fault (OPF) in a power-converter-fed permanent-magnet synchronous machine. The proposed fault-tolerant system is based on field orientation control with additional fault tolerance functionality. A current predictive method for OPF detection is presented, together with an estimation of the threshold level for detection. The proposed method is based on the prediction of stator current for the next sampling interval. Furthermore, a new method for postfault operation of the machine is proposed. For optimal performance of the complete drive, a prefiring angle is introduced in order to avoid the temporary generation of negative torque. This improvement increases the average generated postfault electromagnetic torque, and consequently, it reduces the mechanical stress on various machine parts. The proposed fault detection and postfault operation solutions were simulated in MATLAB, and they were also tested on an experimental setup. The results show several advantages of the proposed fault-tolerant solution, like its short fault-detection time, substantial robustness against variation of machine parameters or load fluctuations, and negligible implementation costs, since no hardware modifications are needed. The fault detection algorithm does not require high computing power, and it operates well even during transients.

Bi-directional power converter for wide speed range integrated starter-generator

The paper outlines a case study on an integrated starter-generator-torque-booster (ISGtB) for automotive applications, representing an electric part of a hybrid propulsion system of a street scooter. Requirements for a high power density of the electrical machine led to a permanent magnet brushless DC machine (BLDC) with an external rotor. The complex functionality of the ISGtB, some deciding characteristics of modern two-stroke internal combustion (IC) engines and the nature of the selected electrical machine with linear dependence between back EMF and rotational speed dictate the properties and capabilities of the switch-mode power converter (SPC) that must support all operating modes of the ISGtB and bi-directional energy flow. The research work involved the extensive study on different suitable SPC topologies, including also control strategies, in the sense of starting torque, motor as well as generator operation mode of ISGtB. The selection of the SPC topology was made on the basis of theoretical statements, computer simulations and measurement results on several ISGtB prototypes. It consists of a bi-directional buck/boost converter and of a common known six-step inverter, as an executing stage. The topology of the bi-directional buck-boost converter is derived mainly with regard to the conversion ratio between the supply battery voltage level and DC link voltage level. The instantaneous value of the transferred electric power is tuned to the torque/speed characteristic of the IC engine and, in the opposite energy flow direction, to the requested output power in the generator operation mode. The deciding parameter when selecting the executing stage of the SPC was the torque/current ratio of the BLDC machine for different inverter topologies and phase winding connections. A particular attention was paid also to an efficient operation of the ISGtB on the boundaries of the operation range, such as providing a low impedance current path at the starting maneuver and extending the low-speed generator mode. The phenomenon of reverse conduction of MOSFET transistors was effectively employed, thus improving the overall SPC efficiency and extending the ISGtB operation range. Measurement results clearly reveal the functionality of ISGtB over the whole rotational speed range of the prototype set-up. The employment of reverse conduction of MOSFET transistors significantly influences the torque characteristic in the starter mode of operation. The proposed inverter control principle in the generator operation mode of the ISGtB evidently improve the transition from generator to torque-boosting operation mode.

Detection of induction motor squirrel cage asymmetry using dynamic torque spectrum analysis

In the paper, an alternative method of induction motor squirrel cage asymmetry detection is presented. The detection is made possible by a modern measurement system and effective computer software, which is specially designed for induction motor torque characteristic measurements using run-up test. Precise and accurate torque monitoring gives us the ability to calculate and further on to evaluate the dynamic torque spectrum, which provides more complex insight into the motor operation and discloses possible malfunctions. The paper contains a theoretical establishment of the consequences of the rotor asymmetry on the motor operation. The theory is based on the observation of additional stator current harmonic components, generated in the case of a damaged rotor, which reflect out as the additional torque components, speed variation and in common as a reduced motor performance. In order to evaluate the proposed detection method a set of induction motors with deliberately damaged squirrel cages was made. The calculated dynamic torque spectrum for different rates of the rotor asymmetry clearly reveals the additional low frequency components that confirm the theoretical statements.

Novel AC-DC converter control principle for automotive BLDC generator in low-speed range

The paper outlines a case study on an Integrated Starter-Generator-torque-Booster (ISGtB) for automotive applications, with a special focus on low speed generator operation mode. The discussed hybrid propulsion system consists of an internal combustion (IC) engine and a BLDC machine and is in its origin intended to drive the street scooter. The complex functionality of ISGtB, some deciding characteristics of modern two-stroke IC engines, stringent demands for the generated supply voltage and the nature of the selected electrical machine with linear dependence between the back EMF and rotational speed make the generator operation mode a point of interest. A sophisticated AC-DC converter and its control are introduced that enable different rectification possibilities, mainly due to the used power MOSFET switches and their characteristics. Two improvements of the low speed generator operation were studied, the synchronous rectification through activation of the MOSFET channels during the reversal of the drain voltage and the operation of the AC-DC converter as a BOOST converter with the use of BLDC phase windings. Measurement results reveal that the proposed AC-DC converter and its control, where each converter leg operates as autonomous BOOST converter with synchronously triggered upper transistors (T1, T3, T5) instead of the use of body diodes (D1, D3, D5), lead to an improved low speed generator characteristic and offer simple, yet effective energy flow control.

Operation of PMSM after single open-phase fault

Recently, the reliability and safety of electrical drives became very important, especially in critical applications, such as in automotive, aerospace and military industry. For such systems, it is desirable to detect and identify different kind of faults and, if possible, to continue operating after the fault occurs. After single open-phase fault is detected and identified, it is desirable to continue operating with reduced power. In this paper, modified direct torque control method for post-fault operation of permanent magnet synchronous motor is presented, without the need of changing the topology of power converter. In order to avoid negative torque operation, pre-firing angle is also introduced. The proposed method has been simulated and tested in Matlab/Simulink.