S. Foti

A new approach to improve the current harmonic content on open-end winding AC motors supplied by multi-level inverters

In open-end winding AC motor drives, terminals of stator phases are normally connected between two inverters, normally, of the same type and size. In the present paper, an alternative configuration is studied where the machine is supplied exploiting two different converters: a main multi-level inverter and an auxiliary two-level inverter. The last operates as active power filter in order to eliminate undesired low-frequency voltage harmonics produced by the multi-level modulation process. Furthermore, the two level inverter is operated at a rather low voltage, thus it can be equipped with power MOSFETs. Consequently, a voltage PWM at a remarkably higher frequency is allowed, if compared with standard multilevel inverters. According to the proposed approach, the phase current harmonic content is remarkably reduced, the torque ripple is mitigated while the drive efficiency is increased. Numerical simulations and experimental results confirm the consistency of such new methodology. A particular emphasis is given to stator current harmonic content and power losses analysis as well as to specific implementation issues.

THD and efficiency improvement in multi-level inverters through an open end winding configuration

A new approach, based on a current controlled Asymmetrical Hybrid Multilevel Inverter (AHMLI), is proposed to reduce power losses and output current distortion on Multi-Level Inverters (MLI) used in AC motor drives, STAT COM devices, Photovoltaic and Wind generators. A key feature of the proposed approach is that the AC machine (motor or transformer) is operated in an open-end winding configuration, being supplied on one end by a main Multi-Level Inverter and, on the other side, by an auxiliary Two-Level Inverter (TLI). The MLI controls the main active power stream; it operates at a low switching frequency and can be equipped with very low on-state voltage drop IGBT devices. The auxiliary TLI is instead operated according to a conventional high frequency two-level PWM technique and acts as an active power filter providing only a null-average power to the AC machine. As the DC bus voltage of the TLI is remarkably lower that that of the main MLI, the auxiliary inverter can be equipped with low switching losses IGBTs, or even Power MOS devices. Simulations and experimental results confirm that using the proposed approach, the phase current harmonic content is remarkably reduced, the efficiency is increased and in motor drive applications, the torque ripple is mitigated.

An Open-End Winding Motor Approach to Mitigate the Phase Voltage Distortion on Multilevel Inverters

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.

An Open-end Winding approach to the design of multi-level multi-motor drives

An Open-end Winding approach to the design of multi-motor drives exploiting multi-level inverters is presented in this paper. A key feature of the proposed approach is that a single multi-level inverter is tasked to provide the entire active power required by the system, while a two-level inverter acts as a power filter, providing a zero average active power. Moreover, a staircase voltage modulation technique is adopted on the multi-level inverter in order to minimize the switching losses, while the two-level inverter is PWM operated to accurately shape the motor currents. The proposed solution produces a much lower stator current distortion if compared to a conventional MLI switching at the fundamental frequency, while it requires less power switches and generates lower power losses than a PWM multi-level inverter featuring the same THD. Experimental results obtained on Multiple Motor Single Converter and Multiple Motor Multiple Converter systems are provided in order to confirm the consistence of the proposed approach in terms of efficiency maximization and motor voltage THD minimization.

Overvoltage mitigation in open-end winding AC motor drives

An over-voltage mitigation technique for open-end winding electrical drives with long power cables is proposed in this paper. In particular, a suitable dwell time is included in the switching patterns of the two inverters to minimize the over-voltage occurring at the stator terminals of this kind of systems. No passive RLC networks are required, thus avoiding additional costs and extra power losses. The proposed approach is first theoretically introduced, then validated by numerical simulations and experimental tests. Finally its effectiveness is compared with that of conventional passive RC filters.

A six-level asymmetrical Hybrid Photovoltaic Inverter with inner MPPT capability

This paper presents a three-phase six-level Asymmetrical Hybrid Multi-Level Photovoltaic Inverter (AHMLPI) for medium voltage grid connected applications. A Low to Medium Voltage Transformer is operated in an open-end winding configuration. Thus the low voltage winding is supplied on one end by a three-phase three-level Neutral Point Clamped Inverter (NPC) and, on the other side, by an auxiliary Two-Level Inverter (TLI). A key feature of the proposed approach is that the NPC controls the main active power flow extracted by PV strings, while also accomplishing the MPPT (Maximum Power Point Tracking), thus avoiding the need of additional DC/DC power converters. The NPC operates at a low switching frequency, according to a step modulation, and can be equipped with very low on-state voltage drop IGBT devices. The auxiliary TLI instead operates at low voltage, according to a conventional high frequency two-level PWM technique, in order to accomplish a twofold task. First it controls the inverter output current. Second it acts as an active power filter in order to compensate low order harmonic components caused by the NPC step modulation. As the DC bus voltage of the TLI is remarkably lower that of the main NPC, the auxiliary inverter can be equipped with low switching losses IGBTs, or even Power MOS devices. Simulations results confirm that, compared to conventional systems, the proposed approach, features a remarkably reduced phase current harmonic content is and an increased efficiency.

Asymmetrical hybrid unidirectional T-type rectifier for high-speed gen-set applications

The paper deals with a high efficiency boost rectifier for high rotational speed Gen-Set applications. It is tailored around a permanent magnet synchronous generator (PMSG) featuring an open end winding configuration. According to such a configuration the generator stator windings are connected on one side to a three level T-type unidirectional rectifier, and on the other side, to an auxiliary two-level inverter. The multilevel rectifier is operated at the fundamental frequency, while, the two-level inverter is driven by a standard sine PWM technique, in order to suitably shape the currents produced by the electric generator. The entire system is equivalent to a six-level multilevel converter. As the main stream power is handled by the multilevel rectifier operating at low switching frequency, a higher efficiency is achieved compared to a similar multilevel rectifier driven through a PWM technique, even considering extra losses due to the auxiliary inverter. The last can be supplied through an active source or a floating capacitor, and plays a key role in balancing the voltage across the DC Bus capacitors of the T-type rectifier, making unnecessary additional circuits. Suitable control solutions have been implemented considering either an active source, either a floating capacitor. Simulation results confirm the effectiveness of the proposed configuration.

Over-voltage mitigation on SiC based motor drives through an open end winding configuration

An original overvoltage mitigation technique is presented in this paper for Sic based motor drives, exploiting an open-end winding motor configuration. According to the proposed solution over-voltage occurring at the terminals of the motor phases is mitigated by driving the two inverters through a modified switching pattern including a suitable dwell time. No passive RLC networks are required, thus avoiding additional costs and extra power losses. A comparison between IGBT and SiC MOSFET equipped motor drives is first provided, highlighting the difference in terms of power cable critical length and other factors affecting peak terminal over-voltages. The proposed approach is then theoretically introduced and its consistency is finally assessed by simulations and experimental results.

Self-sensing control of open-end winding PMSMs fed by an asymmetrical hybrid multilevel inverter

This paper deals with a self-sensing control for Open-end Winding Permanent-Magnet Synchronous Motors fed by Asymmetrical Hybrid Multilevel Inverters. In this kind of system, a main multilevel inverter (MLI) supplies the PMSM on one side, while an auxiliary two-level inverter (TLI) acts as active power filter on the other side of the machine. The MLI works at a low-switching frequency and high voltage while the TLI is pulse width modulated at low voltage. A sensorless high frequency injection method is adopted to detect the rotor position at low and zero-speed. The high frequency voltage injection is performed by the TLI in order to keep low and constant the MLI switching frequency, thus minimizing power losses. At high speed, the rotor position is instead detected by estimating the motor back-EMF. The proposed self-sensing approach gives both high efficiency and high control performance in all the operating speed range. Experimental results confirm the effectiveness of the self-sensing implementation.

Sensorless field oriented control of multiple-motors fed by multiple-converters systems

On Multiple-Motor systems fed by Multiple-Converter systems a set of electric motor drives share the load through a common mechanical coupling. Such a coupling is exploited in this paper to realize a sensorless field oriented control, based on a quite common back-emf technique to estimate the rotor flux angular position and an original approach to correct the estimation errors. The technique is based on the injection of very low frequency, sinusoidal signals on the current references of one of the motor drives. If the estimated rotor flux position is correct, the drive output torque does not show variations. Differently, if the estimation is incorrect, a torque ripple is generated at the frequency of the injected signal, whose effects on the system speed are easily compensated by the main speed controller. However, due to the mechanical coupling, an additional component of the reference torque current at the frequency of the injected signals is generated on all the other drives. By minimizing this component, the rotor flux position estimation can be effectively corrected, as well as errors in estimating the stator and rotor resistances.