Extending voltage range and reducing torque ripple of five-phase motor drives with added voltage harmonics

As multi-phase (defined as greater than three-phase) drives become more popular and practical, new research in this area investigates potential advantages including lower torque ripple and better power density. The added dimensions of a multi-phase machine leads to a completely different operating nature than standard three-phase machines. It can be shown physically and mathematically that certain harmonics do not contribute to torque production and therefore the torque is not directly tied to the current wave-shape. This paper utilizes this property to demonstrate a substantial increase in voltage range and a reduction in torque ripple through the use of added voltage harmonics. An analysis of a five-phase motor is presented followed by a range of modulation techniques. It is shown that by proper selection of third, fifth, and seventh harmonics, the required dc voltage can be reduced by eighteen percent and the torque ripple can be reduced by nearly sixty percent over traditional methods at the expense of higher current THD; which may not be a disadvantage in certain applications. Further investigation is then carried out in applying a unique space-vector modulation patter to the five-phase motor drive. This further reduces the torque ripple. Detailed simulation and laboratory tests are used to demonstrate this concept.