Multi-Pole Winding Behavior in Multiphase Motors Under Current Harmonics Operation

Multiphase machines can be operated in interesting operation modes which are not possible when using their three-phase counterparts. Besides fault-tolerant and multimotor operation, torque enhancement by injecting harmonic currents is another of their very important properties. Torque enhancement by injecting harmonic is usually conditioned by the existence of spatial harmonics, typical for concentrated rather than distributed windings. An improved theory of the torque enhancement principle is presented in this paper. The theory is based on the inherent multi-pole behavior of multiphase stator windings supplied by harmonic currents, producing non-zero MMF. As a result, additional torque is produced even with sinusoidal MMF in distributed windings where negligible space harmonics exist. These conclusions are proved by both theoretical analysis and experimental results. Experimental tests were performed on a nine-phase star-connected induction machine with a rated power of 15 kW when supplied by individual fundamental, 3 $^\mathrm{rd}$ and 5 $^\mathrm{th}$ harmonic or by their combinations.