Permanent magnet magnetization state estimation using high frequency signal injection

Permanent magnet (PM) magnetization state estimation is important for torque control/monitoring in conventional permanent magnet synchronous machine (PMSM), e.g. interior PMSM (IPMSM) and surface PMSM (SPMSM). Furthermore, this can be critical for variable flux machines (VFM) or spoke-type ferrite magnet motors (STFMM). PM magnetization state can change during normal operation of the machine, either due to changes of the controlled variables (e.g. d-axis current) or due to environmental factors (e.g. temperature). PM magnetization state can be measured or estimated. Measurement of the magnetization state is not easy once the machine is assembled, its estimation being therefore a more viable option. Estimation methods can be divided into back-EMF methods, pulse injection methods and high frequency signal injection methods. BEMF and pulse injection based methods require that the machine is rotating. On the contrary, high frequency signal injection methods can be used at any speed, including standstill. Use of high frequency signal injection methods with NdFeB magnets has already been proposed. However, these methods have not been extended to other magnet types, e.g. SmCo and ferrite magnets. This paper analyzes the use of a high frequency signal injection based method for PM magnetization estimation in SmCo and ferrite magnets. These type of magnets can be found in new machine designs, including VFMs and STFMMs.¹