Sensorless speed measurement for n-phase induction machines under open-phase fault by means of rotor slot harmonics

Multiphase induction machines (MIMs) are attractive for safety-critical applications, because they are able to operate under open-phase fault. On the other hand, algorithms for sensorless speed estimation improve reliability and decrease cost and maintenance. The approach based on rotor slot harmonics (RSHs) is especially popular, due to advantages such as its independence from time-varying parameters. In healthy machines, usually the fundamental or any harmonic component has ideally equal magnitude in all stator phases. The previous RSH-based techniques rely on this characteristic. Conversely, under open-phase fault there is current imbalance, and such imbalance varies with the load. Thus, the existing RSH-based methods are unsuitable for open-phase fault, and alternative ones should be developed. This paper addresses sensorless speed estimation based on RSHs for MIMs under open-phase fault. The main RSHs in faulty situation are studied. Then, accordingly, a speed estimation technique for MIMs under open-phase fault is developed, able to work with good accuracy despite the variable current imbalance. Experimental results are provided.