Search

Three phase inverters with switching frequencies above 100 kHz have recently become feasible thanks to the significant progress made in wide band gap (WBG) semiconductor technology. The main drivers behind the intense research and development of these ultrahigh switching frequency pulsewidth modulation (PWM) inverters are the extraordinary gravimetric and volumetric power densities that can be achieved. On the other hand, not as much attention has been devoted to the loads that these inverters supply, quite frequently consisting in ac motors. The impact of sub-MHz PWM and of the associated tens of nanoseconds range switching transients on motor terminal overvoltage, common mode currents, and losses are still to be discussed exhaustively. The authors have thus decided to investigate what happens to induction motors (IM) supplied by inverters having switching frequencies up to 350 kHz. After an introduction on WBG semiconductor devices, the article presents some interesting experimental observations conducted on a 1.1 kW four-pole IM fed by a silicon carbide (SiC) inverter. Further insight is provided by tests on a laminated steel toroid, supplied by a single-phase full bridge SiC inverter. Discussions on the experimental results highlight the previously unreported impact of common mode losses, which become a predominant part of the PWM harmonic losses. [ABSTRACT FROM AUTHOR]

Temperature monitoring is a major issue in induction motor drives, because temperature variations under working conditions may affect the machine lifetime and the control performance as well. The paper presents a technique for on-line monitoring the rotor and stator temperature on field-oriented induction motor drives, based on a new approach to estimate the rotor and stator winding resistances. The proposed approach exploits the injection of small additional current signals to extract the thermal status of the electrical machine, without affecting the normal drive operation. Moreover, it features a low computational effort, does not require other transducers than these already present in a standard field-oriented controlled drive and can be applied on both conventional and sensorless induction motor drives. The stator and rotor temperature estimation can be used either to prevent dangerous overheating and for on-line tuning of the control system. The practical effectiveness of the proposed approach is demonstrated by simulations and experimental tests. [ABSTRACT FROM AUTHOR]

An open-end winding machine configuration and a suitable control strategy for medium-voltage ac motor drives applications are proposed in this paper in order to reduce the distortion of phase voltages in multilevel inverters (MLIs). Differently than standard open-end winding configurations, where two inverters provide active power to both sides of the stator winding, a main MLI supplies in this case the machine on one side, while an auxiliary two-level inverter acts as active power filter on the other side. A high efficiency step modulation manages the MLI, while the auxiliary unit is pulse width modulation (PWM) operated. As the phase current harmonic content is improved, the torque ripple is reduced and the drive efficiency is increased. By exploiting the proposed approach, the apparent switching frequency of the system is that typical of a PWM inverter, although the main unit is operated according to a voltage step modulation strategy. Simulation and experimental results confirm the consistency of the proposed methodology. [ABSTRACT FROM PUBLISHER]

The paper deals with the analysis and implementation of a vector control strategy devoted to integrated multidrive systems used in starter alternator applications with several dc buses at different voltage levels. The electromechanical system of the considered multidrive topology is obtained by splitting the conventional stator winding of an induction machine into different n three-phase subunits, maintaining the same magnetomotive force distribution and sharing the same rotor. Each stator unit together with rotor can be considered a submotor, thus the entire electromagnetic system behavior can be considered as the combination of all submotors contributions. The submotors are suitably supplied through standard three-phase inverters whose technical specifications are also established according to the storage unit voltage levels. First, a detailed mathematical representation of the electromagnetic system has been developed, then the conditions to carry out the field orientation in each generic subdrive are determined. Addressing a starter-alternator system, the integrated multidrive configuration has been experimentally tested, highlighting the capability of the proposed approach to efficiently handle multidirectional power flows among the storage units and mechanical system while ensuring high dynamic performance. [ABSTRACT FROM AUTHOR]

Due to their inner structural redundancy, ac multidrive systems (MDSs) can achieve a fault-tolerance capability with minor circuit modifications. However, some additional components are generally required in order to create emergency current paths. According to a recently presented approach, the amount of additional components can be reduced by exploiting healthy drives to support faulty drives. This can be obtained by connecting together the neutral points of motor stator windings and suitably sharing the neutral current of faulty units among some, or all, healthy units. Following such an approach, three current-sharing methodologies for fault-tolerant MDSs are presented in this paper, which are able to cope with single or multiple failures. They are first theoretically introduced and then validated by mean of simulations and experimental test results. [ABSTRACT FROM PUBLISHER]

The paper presents a method to compute the air-gap flux position in induction motors at very low including zero-stator frequency. A low-frequency (50 ÷ 100 Hz) sinusoidal stationary signal is added to the normal stator variables to provide the machine with a suitable permanent excitation. Such an additional excitation modulates the saturation level of the magnetic core of the machine according to the angular position of the air-gap flux. As a result, a new zero-sequence stator-voltage component is generated that contains useful information about the position of the air-gap flux unaffected by load variation. Such a zero-sequence voltage can be easily employed to provide a wide bandwidth measurement of the air-gap flux position. A key feature of the proposed approach is that a low-frequency (0 ÷ 5 Hz) signal is demodulated, thus avoiding any drawback featured by previous sensorless techniques operating with high-frequency signal injection. [ABSTRACT FROM AUTHOR]

A technique to improve the performance of grid-connected induction motors by exploiting an auxiliary winding set is proposed in this paper. This auxiliary winding features the same distribution of the main winding, although with a reduced number of turns and it is fed by an inverter a fraction of the power in comparison with the rated size of the induction motor. As shown in the paper, through the auxiliary winding, it is possible to set the machine power factor, increasing the efficiency of the power conversion system and mitigating speed oscillations due to torque disturbances. A mitigation of the grid current peaks due to motor start-up is obtainable. First, the proposed technique is theoretically introduced, then a feasibility assessment is accomplished by simulations. [ABSTRACT FROM AUTHOR]