Comparative Investigation of Zero-Sequence Braking Torque in Six-Phase Induction Machines with Different Connections and Winding Designs

DC-signal injection into the secondary xy subspace is commonly used in multiphase drives for online stator-resistance estimation and winding-condition monitoring because it can be decoupled from the fundamental torque-producing excitation. Under open-phase post-fault operation, however, the post-fault current constraints unavoidably couple part of this injected DC signal into the zero-sequence subspace. The resulting zero-sequence current can establish a stationary third-spatial-harmonic field and produce an additional speed-dependent braking torque. This paper experimentally and analytically investigates how this braking effect depends on winding connection and coil pitch in six-phase induction machines. Dual three-phase (D3P), asymmetrical six-phase (A6P), and symmetrical six-phase (S6P) connections are evaluated using two separate 1.1 kW reconfigurable prototypes of the same machine rating and principal geometry but with different stator coil pitches. Isolated rotational tests at several DC-injection levels are used to characterize the braking-torque–speed behavior. A combined rotational–standstill single-subspace characterization procedure is also proposed to obtain practical local estimates of the associated zero-sequence equivalent-circuit quantities. The results show that D3P is much less susceptible to the braking effect because of third-harmonic MMF cancellation, whereas A6P and S6P exhibit substantially higher braking torque. Moreover, 5/6 chording provides an effective passive means of suppressing this undesirable component.