Effects of high-order space harmonics on the operation of five-phase induction machines under unbalance.

The paper addresses the effects of high-order space harmonics on the steady-state performance of five-phase induction machines operating under unbalance. We show that the airgap harmonic fields with orders higher than three, although usually disregarded, can produce a significant increase in the torque pulsation and in the Joule losses. We propose a model based on symmetric components which is used to demonstrate that the field produced by each rotor harmonic current is related to two stator sequence currents. Further, the proposed model allows determining additional losses and harmonic torques produced by airgap harmonic fields with orders higher than three, which up to now have not been addressed elsewhere. As a case study, we applied the model to two five-phase induction machines with different designs and assessed the influence of high space harmonics under operation at steady state with one open phase in the stator. The model has been validated by comparing analytical results with results obtained through finite element analysis. Finally, the model validation was also based on experimental results obtained from tests with two prototypes under many different conditions.

Induction distribution of five-phase induction machines under open-phase fault.

The main objective of the paper is to determine the induction distribution of five-phase induction machines operating under unbalanced steady state, focusing on the operation with two opened stator phases. Firstly, we present an analytical model based on symmetric components which includes the third harmonic component of the airgap induction. The model is then used to determine the induction distribution in the airgap and also in the main iron parts, i.e. cores and teeth of the stator and rotor parts. The analytical model is thoroughly validated through two-dimensional Finite Element Analysis (FEA) and using extensive experimental data obtained from tests taken on a prototype machine of 5.5 kW; experimental data, as well results from FEA, showed a close agreement with theoretical data, which confirms the accuracy of the proposed model. The results indicate that when two phases are opened and with no control strategy to compensate the fault, the induction distributions can change significantly, predominantly in the stator and rotor teeth, as a result of additional induction harmonic components. These changes in the field distribution of stator and rotor teeth, in turn, can increase the associated magnetic losses as they are no longer evenly distributed among the teeth, thus potentially creating hot spots inside the machine. The model proposed can, therefore, serve as a basis to assess the overall magnetic losses and the load capacity under fault, given that the field distribution, as well as its associated effects, is essential to establish the machine capacity under unbalance.