De-rating of multiphase induction machines due to supply unbalance

Open-loop variable voltage, variable frequency control should offer an effective control strategy in many applications of multiphase machines, but increasing the number of phases can make the machine more sensitive to unbalance in the supply voltage, causing unequal current sharing between phases. This paper uses simple expressions based on an extended symmetric components approach to quantify the degree of current unbalance and provide a way of calculating the de-rating required. The paper shows that the additional stator copper losses are concentrated in the phase with the highest terminal voltage. However, the pulsating torques, rotor losses and total stator losses due to supply unbalance in a multiphase machine are less than for the three-phase case. Experimental results for the same machine when connected with four and six phases show good agreement with the simple analysis method proposed. A more detailed, generalised harmonic analysis algorithm is used to validate the torque and loss calculations.