The proposed technique focuses on the non -sinusoidal power supply control strategy aiming to obtain the optimal air-gap flux density distribution. The electrical constraints for the alignment between rotated fundamental and harmonic air-gap flux are illustrated in detail. The supporting simulation is performed to analyze the misaligned angles between fundamental and harmonic air-gap flux for the traditional methods. Different from traditional flux-oriented methods, the improved algorithm for harmonic currents injection is designed based on rotor flux orientated control (RFOC) in fundamental plane and air-gap flux orientated control (AFOC) in harmonic planes. The synchronous and aligned control of the fundamental and harmonic air-gap flux is achieved regardless of load condition. The experimental verification is implemented based on a multiphase driving platform using a 7-phase induction machine (IM) as prototype. Various performance is provided for the proposed control strategy comparing with the traditional ones in terms of induced electromotive force (EMF) waveforms, harmonic currents and advance angles of air-gap flux. The simulated and experimental results validate the theoretical assumption and the effectiveness of the proposed method.

The traditional direct torque control (DTC) strategy utilizes the largest fourteen voltage vectors for a seven-phase induction machine (7PIM) which is directly extended from the three-phase counterparts. This method suffers from the issues of severe harmonic currents and torque ripple for multiphase ones. This paper firstly puts forward the concept of virtual vectors for a 7PIM. This can efficiently eliminate the third and fifth order harmonic currents. However, the DTC strategy based on the virtual voltage vector has insignificant effect on reducing torque ripple. The torque ripple caused by these virtual vectors in different torque and speed conditions is investigated. Analysis shows that there are some additional degrees of freedom while choosing the applied voltage vector. A novel sequential strategy integrated with duty ratio optimization algorithm is proposed to select the optimal voltage vector which can significantly alleviate the torque ripple. Experimental results verified that the proposed method can effectively reduce dynamic and steady-state torque ripple as well as harmonic current. This paper uses a 7PIM for an illustration, but it should be mentioned here that the proposed strategy is applicable for multiphase machines with any number of phases.

Overmodulation (OVM) strategy is widely utilized to enhance the dc-link voltage utilization. For five-phase and six-phase drives, various OVM strategies have been developed to reduce the reference voltage vector in one harmonic subspace (HS) and improve the output current total harmonic distortion (THD) performance. However, the performance of generic OVM strategies can be further improved to deal with the optimization problem of multiple HSs. This article presents a universal sequential optimization scheme and takes a seven-phase induction motor as study example. First, the OVM region is predivided by the principle of fully utilizing available degrees of freedom and the concept of virtual voltage vectors. Then, optimizations are carried out according to the voltage distribution in different HSs, and design guidelines are provided for multiphase drives with more than one HS. Without complex computations, the proposed OVM strategy can easily determine the desired voltage vectors and the corresponding duty ratio, which not only achieves overall good THD performance but also favors for more intuitive implementation. Finally, simulation and experimental results implemented on a seven-phase drive platform validate the feasibility of the proposed OVM strategy.

The proposed technique focuses on the online estimations of multiplane rotor resistances for the multiphase induction machine operating under the flux-oriented frame. The deviations in regard to the steady-state performance caused by the mismatched fundamental and harmonic rotor resistances are analyzed in detail. The reactive-power-based model referenced adaptation system (MRAS) is formulated for the third harmonic plane, which is air gap flux-oriented. The quasi-normalized method with respect to the error terms between the referenced and adjustable models is addressed to improve the adaptive mechanism under various load conditions. Experimental validations have been performed to confirm the effectiveness of the proposed algorithm. The simultaneous online identifications for the fundamental and harmonic rotor resistances are achieved. The approximate convergence processes and steady performance can be obtained for a large range of load conditions by using constant adaptive gains. The estimated results are indirectly verified by the comparisons between command and actual flux. In addition, the dynamic performance for the proposed MRAS is conducted with the sudden change of load. The effects of inverter nonlinearity and temperature rise are taken into account.

This paper focuses on the loss estimation for the multiphase induction machine (IM) operating under fault-tolerant conditions through the field–circuit coupling finite element method (FEM). Both one-phase and two-phase open-circuit faults of a seven-phase IM are researched, and different spatial positions of the fault phases are taken into consideration. The magnitudes and phase angles of the residual phase’s current are deduced based on the principle of equal magnitude of the residual phase currents and unchanged fundamental magnetic motive force (MMF). The magnetic fields’ coupling between the fundamental and harmonic planes is analyzed. Then, the time-stepping electromagnetic fields calculation of the seven-phase IM are carried out under the commercial software Simplorer–Maxwell environment. The transient and steady performance for both the health and fault conditions are obtained based on the rotor field-oriented control (RFOC) strategy. The Joule loss and iron loss are calculated for the torque step and slope responses. The seven-phase motor driving platform is established to verify the numerical calculation results. The proposed method is effective for predicting the loss and designing a reasonable operating range for multiphase IM operating under fault-tolerant conditions considering the thermal balance.

Owing to the severe load torque fluctuation, the speed ripple is a significant concern for the compressor drive system, especially within low frequency range. To address this issue, a zero-pole cancellation (ZPC) based iterative learning control (ILC) method is proposed for speed ripple suppression in this paper. The load torque characteristic is obtained based on the estimated speed harmonics. The essence of the speed ripple is investigated utilizing the compressor mechanical model, which attributes to the conjugate poles effect of the load torque term at the specific mechanical frequency. The zero point of the conventional ILC method is fixed at s=0, which can hardly reduce the dominant poles effect of the torque term. Afterwards, the poles’ matching function is analytically derived to cancel out the dominant poles of the torque term, and then the specific order harmonic can be effectively suppressed. Considering the non-periodic disturbance caused by the operating frequency and temperature variation, the error correction law is designed to avoid the error accumulation and ensure the performance of proposed ILC method. Finally, the effectiveness of the method is validated by experiments through a 750W compressor drive system. Experimental results indicate that the total speed ripples reduce by nearly 60% and each harmonic component of the speed ripple can be effectively suppressed according to the FFT results.