Magnetization and Demagnetization Energy Estimation and Torque Characterization of a Variable-Flux Machine

This paper examines the required energy for the magnetization and demagnetization of magnets in a spoke type AlNiCo-based variable-flux machine and studies the torque characteristics of this machine at different magnetization levels. The low coercivity magnet in this machine can be magnetized or demagnetized using a short-time current pulse with negligible Ohmic loss. An advanced method is proposed to estimate the required energy for magnet demagnetization or magnetization to a specific level. A test procedure is developed to measure the energy that is injected to the variable-flux machine during the demagnetization and magnetization procedures. Since this machine has the ability to operate at various magnetization levels, it is of great importance to obtain the torque characteristics such as torque mean value, the peak to peak value of the torque, as well as the torque ripple, at different operating conditions. A static torque measurement test procedure using a variable speed drive system is developed to measure the torque waveform of the variable-flux machine at different magnetization levels. The verified finite element model of the variable-flux machine is used to analyze the harmonic content of the back-emf and the no-load air gap flux density at different levels of magnetization.