Impact of PWM strategies on RMS current of the DC-link Voltage Capacitor of a dual-three phase drive

The major drawback of usual dual three-phase AC machines, when supplied by a Voltage Source Inverter (VSI), is the occurrence of extra harmonic currents which circulate in the stator windings causing additional losses and constraints on the power component. This paper compares dedicated Pulse Width Modulation (PWM) strategies used for controlling a dual three phase Permanent Magnet Synchronous machine supplied by a six-leg VSI. Since the application is intended for low-voltage (48V) mild-hybrid automotive traction, an additional major constraint arises: the compactness of the drive related to the size of the DC-bus capacitor. Thus, the PWM strategy must be chosen by taking into consideration its impact on both, the motor and the RMS value of DC-bus current

Comparison of two-individual current control and vector space decomposition control for dual three-phase PMSM

The relationship between the two-individual current control and the vector space decomposition (VSD) control for a dual three-phase permanent magnet synchronous machine (PMSM) is investigated in this paper. It is found that the VSD control is more flexible in controlling the fundamental current in αβ subplane and the fifth, seventh current harmonics in z1 z2 subplane with different proportional and integral (PI) gains, while the two-individual current control is comparable with the VSD control in having the same PI gains in the αβ and z1 z2 subplanes. It is also found that the two-individual current control may have potential instability issues due to the mutual coupling between the two sets of three-phase windings. If the mutual coupling between the two sets is weak to some extent, then the two-individual current control could have the same dynamic performance as the VSD control without the stability issues. Experiments are conducted on a prototype dual three-phase PMSM to validate the theoretical analysis

Flux-Weakening Control of Dual Three-Phase PMSM Based on Vector Space Decomposition Control

This paper proposes a flux-weakening (FW) control for dual three-phase ermanent magnet synchronous machine (DT-PMSM) based on vector space decomposition (VSD) control, where the output voltage in αβ sub-plane is employed for voltage feedback in the flux-weakening control loop. As the fundamental components are mapped to αβ sub-plane while the 5th and 7th harmonics are projected to harmonic z1z2 sub-plane, the flux-weakening current from this new control in αβ sub-plane is sixth harmonic-free regardless of the 5th and 7th harmonics being resulted from the non-sinusoidal back EMF or inverter non-linearity. The proposed control is compared with the conventional FW feedback control extended for DT-PMSM, where the FW control is applied to the two sets of three-phase windings separately. The experimental results show that the proposed FW control based on VSD is superior to the conventional FW control in terms of reduction in current unbalance and harmonic currents

Enhanced Availability of Drivetrain Through Novel Multiphase Permanent-Magnet Machine Drive

This paper deals with a novel multiphase permanent-magnet (PM) machine drive to enhance drivetrain availability in electric traction applications. It describes the development of new winding configurations for six-phase PM brushless machines with 18 slots and eight poles, which eliminate and/or reduce undesirable space harmonics in the stator magnetomotive force. In addition to improved power/torque density and efficiency with a reduction in eddy current loss in rotor PMs and copper loss in end-windings, the developed winding configuration also enhances availability of drivetrain, in a variety of applications requiring a degree of fault tolerance, by employing it as two independent three-phase windings in a six-phase interior-PM machine, which is designed and optimized for a given set of specifications for an electric vehicle, under thermal, electrical, and volumetric constraints. This paper also describes the design and development of a six-phase inverter with independent control for both sets of three-phase windings. The designs of the motor and the inverter are validated by a series of preliminary tests on the prototype machine drive

Advances in Converter Control and Innovative Exploitation of Additional Degrees of Freedom for Multiphase Machines

Multiphase variable-speed drives and generation systems (systems with more than three phases) have become one of the mainstream research areas during the last decade. The main driving forces are the specific applications, predominantly related to the green agenda, such as electric and hybrid electric vehicles, locomotive traction, ship propulsion, ‘more-electric’ aircraft, remote offshore wind farms for electric energy generation, and general high-power industrial applications. As a result, produced body of significant work is substantial, making it impossible to review all the major developments in a single paper. This paper therefore surveys the recent progress in two specific areas associated with multiphase systems, namely power electronic supply control and innovative ways of using the additional degrees of freedom in multiphase machines for various non-traditional purposes

Control of Asymmetric Permanent Magnet Synchronous Generator Systems

The thesis focuses on the control of asymmetric permanent magnet synchronous generator (PMSG) system, with particular reference to the suppression of its second harmonic (2h) power, DC bus voltage and torque ripples. The asymmetries include the unbalanced resistances, unbalanced inductances, and unbalanced 3-phase back-electromotive forces (EMFs). The mathematical model of the general asymmetries in the PMSG system is firstly presented. The power ripple and torque ripple due to the asymmetries without/with negative-(N-) sequence currents are then analysed in detail. It shows that there are 2h impedances in the synchronous dq-axis frame. Consequently, the N-sequence currents emerge under the conventional current proportional and integral (PI) control, which will result in undesired 2h power, DC bus voltage and torque ripples. To suppress the 2h torque resulted from the N-sequence currents, three typical methods aiming for balanced currents without N-sequence currents are reviewed, evaluated and their relationship is revealed. It shows that all these three methods are capable of suppressing the N-sequence currents as verified by experiments. However, the 2h power and DC bus voltage cannot be suppressed. To suppress the undesired 2h power and DC bus voltage, an improved power control without any sequential component decomposers under general unbalanced conditions is proposed. Its effectiveness is validated by elaborated experiments on a prototype PMSG with inherent asymmetry and deliberately introduced asymmetries. However, the 2h torque is compromised. To solve the 2h torque, power and DC bus voltage simultaneously, the compensation in parallel with the DC bus is investigated in the PMSG system with asymmetric impedances. The undesired 2h power from the PMSG is compensated by the 2h power from the compensation unit. Two topologies of the compensation unit and corresponding control methods are investigated, while the compensation effectiveness is validated by experiments. Furthermore, the compensation unit with external circuits in series with the asymmetric PMSG is investigated. By the compensation in series, the original unbalanced system is modified to a balanced system in theory. Therefore, the N-sequence currents, 2h power, DC bus voltage, and torque ripple can be naturally suppressed. The feasibility of this compensation method is verified by experiments at different speeds and load conditions, although the effectiveness may be slightly affected by the non-linearity of the compensation inductors in practice. Finally, the research of suppressing the 2h DC bus voltage and torque ripple is extended to the dual 3-phase PMSG system with one channel failed. By utilizing the windings, rectifier or inverter in the faulty channel which are still functional, three methods designated as two sets in parallel, two DC buses in parallel and N-sequence currents compensation are investigated, which require minimum extra hardware investment compared with the compensation in parallel and in series