Circuit models for multiphase coupled inductors are summarized, compared, and unified. Multiwinding magnetic structures are classified into parallel-coupled structures and series-coupled structures. For parallel-coupled structures used for multiphase inductors, the relationships between: 1) inductance-matrix models, 2) extended cantilever models, 3) magnetic-circuit models, 4) multiwinding transformer models, 5) gyrator-capacitor models, and 6) inductance-dual models are examined and discussed. These models represent identical physical relationships in the multiphase coupled inductors, but emphasize different physical aspects and offer distinct design insights. The circuit duality between the series-coupled structure and the parallel-coupled structure is explored. Design equations for interleaved multiphase buck converters based on these models are streamlined and summarized, and a simplified equation showing the relationships between current ripple with and without coupling is presented. The models and design equations are verified through theoretical derivation, SPICE simulation, and experimental measurements.

Unified Models for Coupled Inductors Applied to Multiphase PWM Converters

Fault-tolerant control is normally required to achieve continuous operation under postfault modes in some critical fields. Regarding the remedial strategy for open-end-winding permanent magnet synchronous machines (OW-PMSMs) in the case of open-phase fault, the typical way is to reconfigure the controller after the fault is detected. Different from those mainstream control strategies, which account for the postfault situations by reconfiguring controllers or modulators, the fault-tolerant predictive control strategy proposed in this article handles the asymmetry produced by open-phase fault in a different manner. By combining the predictive control strategy with an embedded vector-resonant control scheme, the proposed strategy can achieve effective regulation of OW-PMSMs in both pre- and postfault operations. Controller reconfiguration and switching between pre- and postfault control modes based on fault detections are thus avoided. Besides, with the rotor flux harmonics taken into account and benefiting from the proposed zero-sequence controller, the triple-frequency component in the zero-sequence current is successfully suppressed to reduce torque ripples while the fundamental-frequency component in the zero-sequence current is properly retained under postfault operations, which is required in order to obtain an uninterrupted rotating magnetic field after one phase is open circuited. Experimental validations are conducted to verify the effectiveness of the proposed strategy and its superiority over conventional voltage compensation methods.

Open-Phase Fault-Tolerant Predictive Control Strategy for Open-End-Winding Permanent Magnet Synchronous Machines Without Postfault Controller Reconfiguration

Fast switching characteristic of wide bandgap devices enables high switching frequency of power devices and thereby, can facilitate high fundamental frequency operation of electrical machines. However, with the switching transition times in orders of tens of nanoseconds, the high dv/dt is observed across the switching device. The high dv/dt experienced by the switches, and consequently by the machine, can degrade winding insulations or bearings over a period of time. Therefore, it is imperative to maintain the dv/dt below recommended values depending on the machine insulation. The dv/dt across the devices can be adjusted by varying the gate resistance. A high value of gate resistance, however, introduces additional switching losses on the device. Using different dv/dt filtering techniques can also help to control the dv/dt on the machine terminals. These techniques do not increase the switching losses on the device. However, it introduces additional losses in the filter resistors and also increases the cost of the system. In this paper, an analysis based on the impact of gate resistance on the dv/dt across the machine, and the corresponding losses is carried out. An analytical dv/dt filter design strategy is proposed to limit the dv/dt to a particular value. With the proposed design scheme, the value of each filter component can be easily obtained, and filter losses can be estimated accurately. Lastly, a comparison is performed on the basis of efficiency between these two techniques.

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Analytical Study of SiC MOSFET Based Inverter Output dv/dt Mitigation and Loss Comparison With a Passive dv/dt Filter for High Frequency Motor Drive Applications

This paper presents an efficiency evaluation of three silicon carbide (SiC) inverter topologies: two-level (2L) voltage source inverter (VSI), 3L neutral-point clamped (NPC), and 3L T-type inverter. Their efficiency is evaluated for powertrains rated at 400 V and 800 V, and using SiC MOSFET devices rated at 650 V and 1200 V operating at a switching frequency of 30 kHz. Firstly, the efficiency is evaluated at different operating load currents, on a per-unit scale. Secondly, the efficiency curves are mapped into torque-speed 2D maps of 120 kW interior permanent magnet (IPM) motors. Thirdly, the resulting efficiency maps are employed in an electric vehicle (EV) model, in order to study the performance of the three inverters on standard drive cycles. At the vehicle level, the energy consumption of the vehicle using the studied inverters is analyzed. It is found that 3L SiC-based inverters are most competitive in the 800 V powertrain. When compared to VSI, NPC and T-type offer 0.6% and 1.2% energy consumption savings. In 400 V, only T-type enjoys 0.9% energy savings over VSI.

Efficiency Evaluation of 2L and 3L SiC-Based Traction Inverters for 400V and 800V Electric Vehicle Powertrains

One of the recent trends in power electronics field is to achieve high efficiency and compact size by using wide bandgap power devices such as silicon carbide (SiC) or gallium nitride switches. This paper compares the efficiency, the motor terminal voltage quality ( dv/dt ), and the common mode current of the SiC-based variable frequency drives (VFDs) according to the inverter topologies with and without the output filters. For this purpose, both the 2-level (2L) and 3-level (3L) topologies with SiC devices are built and tested. Here, the well-known 2L half-bridge inverter is compared with the well-known 3L T-type neutral point clamped inverter. A dv/dt filter and a sine wave filter have been discussed to satisfy the National Electrical Manufacturers Association standards as well. The experiments have been carried out with a 380-V/60-Hz, 3.7-kW induction motor and the custom designed power stages. Additionally, the experiments with both a 3-m and a 50-m cable length between the VFD and the induction motor are performed and reported.

Performance Comparison Between Two-Level and Three-Level SiC-Based VFD Applications With Output Filters

This article proposes an average periodic delay-based repetitive controller for power factor correction (PFC) converters undergoing grid frequency variations. When the grid frequency is changed, a conventional repetitive controller requires an interpolation method or additional memory to achieve frequency adaptability. Furthermore, frequency adaptability can also be realized using a variable sampling frequency; however, this requires additional computation effort and a multirate digital control system. In this article, with a fixed sampling frequency and memory length, the proposed repetitive controller, which generates the average periodic delay unit, maintains its harmonic compensation performance under grid frequency variations. In the proposed method, the control variables are updated depending on the phase of the grid voltage to reflect the grid frequency deviations in the average period delay unit. Because no complicated software modification is necessary, the proposed scheme can be easily plugged in the existing control algorithm. The theoretical analysis and stability issues of the proposed scheme have been discussed in detail to ensure stable operation and disturbance rejection. Both the simulations and the experiments on a 1.5-kW PFC converter have been carried out to verify the effectiveness of the proposed repetitive control.

Average Periodic Delay-Based Frequency Adaptable Repetitive Control With a Fixed Sampling Rate and Memory of Single-Phase PFC Converters

Gallium nitride (GaN) devices have been widely adopted to achieve high efficiency and high power density as alternative solutions to silicon devices. When the GaN power devices are used for variable frequency drive (VFD) systems, the high dv/dt pulses at the motor terminal, which induce shaft voltage and common mode current, should be carefully considered to ensure system reliability. Although the high dv/dt issues can be mitigated with a dv/dt filter method, it leads to performance degradation depending on cable length. Meanwhile, a matrix converter also becomes a next-generation power converter for the VFDs which has a regeneration capability and unity power factor. Thus, this paper discusses the GaN-based matrix converter for the VFD as a study case considering the motor terminal voltage quality and the common mode current. Also, a sine wave filter is adopted to cope with the terminal voltage quality irrespective of cable length. The optimized design procedure of the sine filter considers practical issues. Experimental results are presented to suggest a suitable solution for the GaN-based VFDs in accordance with the cable length.

Analysis and Design of a Sine Wave Filter for GaN-Based Low-Voltage Variable Frequency Drives

One of the recent trends in power electronics field is to achieve high efficiency and compact size by employing wide-band-gap (WBG) devices such as a Silicon Carbide (SiC) or Gallium Nitride (GaN) metal oxide semiconductor field effect transistors (MOSFETs). This paper compares the efficiency, the motor terminal voltage quality (dv/dt), and common mode current of the SiC-based variable frequency drives (VFDs) according to the inverter topologies and the use of the output filters. For this purpose, both the 2-level (2L) and 3-level (3L) topologies with SiC devices are built and tested. Here, the well-known 2L half-bridge inverter is compared with the well-known 3L T-type neutral point clamped (TNPC) inverter. A dv/dt filter and a sine wave filter have been discussed to satisfy the national electrical manufacturers association (NEMA) standards as well. The experiments have been carried out with a 380V/60Hz, 3.7kW induction motor. Additionally, the experimental results with both a 3m and a 50m cable length between the VFD and induction motor are suggested.

Performance Comparison Between Two-Level and Three-Level SiC-Based VFD Applications with Output Filters

This paper presents a torque ripple mitigation strategy for the fault tolerant permanent magnet synchronous motor (PMSM) drive system, in which an auxiliary phase is employed. It is shown that the torque ripple whose frequency is twice the fundamental frequency is induced in the PMSM undergoing an open circuit fault on a certain phase. In order to compensate the torque ripple, current controllers are implemented in the dual synchronous reference frame (SRF), in which the rotating frequencies of the two SRF are different. In more detail, one of the SRFs rotates in two times of the fundamental frequency to actively compensate the torque ripple in the steady state while another one is synchronized to the fundamental component. Compared to traditional methods, the proposed algorithm effectively extends the operating speed of the PMSM where the torque ripple can be compensated. Both the simulations and the experimental results confirm the usefulness of the proposed torque ripple mitigation strategy.

Seunghoon Baek

Papers

Average Periodic Delay-Based Frequency Adaptable Repetitive Control With a Fixed Sampling Rate and Memory of Single-Phase PFC Converters

Performance Comparison Between Two-Level and Three-Level SiC-Based VFD Applications With Output Filters

Analysis and Design of a Sine Wave Filter for GaN-Based Low-Voltage Variable Frequency Drives

Performance Comparison Between Two-Level and Three-Level SiC-Based VFD Applications with Output Filters

Dual SRF based torque ripple mitigation strategy for a fault tolerant PMSM drive system