Square-wave drive for synchronous reluctance machine and its torque ripple analysis

Synchronous reluctance machine (SynRM) can be perceived as a special type of permanent magnet synchronous machine (PMSM), and shares similar control method, i.e. the sine-wave current drive with sinusoidal phase currents. In this paper, square-wave drive, which is employed for permanent magnet brushless (BLDC) motors, is employed for the SynRM, in order to economically reduce the cost of rotor position sensor. It is revealed that the torque density and efficiency are slightly sacrificed, whereas torque ripple is deteriorated, proving the SynRM with square-wave drive still promising for the cost-sensitive application if torque ripple is not considered as a critical issue. To further investigate the additional pulsating torque under square-wave drive, mathematical model based on a-b-c phase inductance and d-q axis inductance are established, together with the time-stepping FE calculated currents. It is concluded that the harmonics in the currents tend to cause non-sinusoidal variation of the magnetic reluctance, which can be represented as additional inductance harmonics. The harmonics of the current and inductance interact with each other, thus undesirable torque ripple components are produced.