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According to different clamping methods, nine modes (C_MODE1-9) of the traditional discontinuous pulsewidth modulation (DPWM) for neutral-point-clamped three-level converter have been revealed. Since traditional DPWM cannot balance neutral point (NP) voltage under the full range of modulation index (MI) and power factor (PF), two extended modes (E _MODEs) are proposed in this paper, which can balance NP voltage. The proposed extended discontinuous pulsewidth modulation (EDPWM) is characterized by using C_MODEs and E_MODEs alternately.Therefore, EDPWM can balance NP voltage under the full range of MI and PF. Since the switching numbers under C_MODE and E_MODE in one switching cycle are two and three, respectively, the switching loss under EDPWM is lower than that under other pulsewidth modulation (PWM) strategies. The performance of EDPWM is analyzed and compared with traditional DPWM, space vector PWM (SVPWM) and virtual SVPWM (VSVPWM) in terms of switching loss, waveform quality, and the ability of NP voltage control. Finally, the feasibility and superiority of the proposed method are verified by experiments.

Virtual space vector pulsewidth modulation (VSVPWM) for the neutral-point (NP)-clamped three-level inverter is briefly reviewed in this paper, as well as its carrier-based realization. The carrier-based VSVPWM (CB_VSVPWM) is featured with dual modulation waves and a single carrier. Although CB_VSVPWM can make NP voltage unfluctuating in theory, it does not have the NP voltage recovery ability. In practice, due to the effect of nonidea factor effects, NP voltage may shift. Thus, active NP voltage control (ANPVC) is necessary. To achieve that, three ANPVC approaches are proposed based on CB_VSVPWM in this paper. The NP voltage control ability of CB_VSVPWM with ANPVC is analyzed. Finally, comprehensive experiments are carried out to verify the feasibility of the proposed method.

The three phases of brushless dc motor (BLDCM) are involved in energy conversion during the commutation interval. A novel commutation analysis method of the BLDCM based on coordinate transformation theory is proposed in this paper, though this coordinate transformation theory is popular in the control of three-phase sinusoidal motors, such as induction motors and permanent magnet synchronous motors. The method is intuitive and easy to understand compared with the method based on circuit solving. Combined with the voltage vector diagram of the inverter, commutation control to reduce commutation torque ripple under low speed (LS_RCTR) and high speed (HS_RCTR) is deduced. However, under a heavy load and high-speed condition, long commutation time may result in commutation failure if these commutation controls to RCTR is adopted. To avoid that, commutation time must be controlled. Then, commutation control to reduce commutation time under low-speed (LS_RCT) and high-speed (two cases, named HS_RCT1 and HS_RCT2) are proposed. The improved control of BLDCM presented in this paper can coordinate commutation torque ripple and commutation time in full speed range. Finally, the feasibility and effectiveness of these methods are verified by experiment.

Nowadays, studies of the control of brushless dc motor (BLDCM) are mainly focused on motor state. However, it may also work in generator state. Current hysteresis control (CHC) is very suitable for the motor control, because it can directly control current. In this paper, a multimode CHC for BLDCM in motor and generator state is presented. It can not only complete commutation successfully but also reduce commutation torque ripple. Moreover, a novel transition control method based on the background of CHC with fast response is proposed to carry out commutation and the transition processes simultaneously. Finally, the effectiveness and insensitivity to the parameters of the proposed control algorithm are verified by experiments.

Two parallel grid-connected inverters can increase system capacitance effectively. In order to exert maximum efficiency of parallel grid-connected inverters, an optimal power allocation (OPA) method is proposed. Firstly, the power model of every inverter is established. Compared with the equal power allocation (EPA) method, theoretically, higher overall system efficiency can be achieved with the OPA method. Secondly, the overall system efficiency can be calculated with easily measurable electric parameters, and the online optimization realized by adopting an existing method, such as the hill-climbing method. Lastly, the feasibility of the OPA method is verified by simulation and experimentation.