Investigation of Weighted FOC of Dual Induction Motor Drives Operating under Heavy Load Imbalance

Electric drives used in subway or tramway applications often employ dual traction induction motors connected in parallel and fed by a single converter due to economical reasons. The control can be oriented to the average or weighted flux and torque of the connected motors, as individual flux and torque control of each motor is not possible. In this paper, the control performance related to the weighted flux oriented control of dual induction motors is investigated. A control singularity of weighted FOC arises if one of the motors loses traction capability while a braking torque current set-point is performed. Under these conditions, if the traction-capable motor reaches a specific speed, the weighted flux oriented control scheme reacts in a particular manner. The main flux of the traction-capable motor is significantly reduced while the traction-incapable motor undergoes a shaft speed reversal. The drive ends up not being able to produce steady braking torque. To the best of authors’ knowledge, this effect has not been reported in literature. The existence and characteristics of the control singularity are found and verified in a simulation model consisting of dual induction motors. An experimental test stand consisting of two traction induction motors connected in parallel and fed by a single converter further consolidates the existence and characteristics of the control singularity.