Derating of Multiphase Induction Machines Due to Supply Imbalance

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 imbalance in the supply voltage, causing unequal current sharing between phases. This paper uses simple expressions based on an extended symmetric-component approach to quantify the degree of current imbalance and provide a way of calculating the derating required. This 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 imbalance 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 generalized harmonic analysis algorithm is used to validate the torque and loss calculations.