Maximum torque per ampere and flux weakening control of IPM motors for electrical vehicles

Permanent Magnet Synchronous Motors (PMSM) have long been used in servo motor drives and has lately become more popular in lager motors for industrial applications. Another target application for PMSM is electric vehicles, were its small size and robustness is a big advantage compared to Induction or DC motors. This development of PMSM has been made possible according to the introduction of new magnetic materials like Neodymium Iron Boron (Nd2Fe14B) and Samarium Cobalt (SmCo5). These types of magnets have a high energy density and high resistance for de- magnetization. Previously Ferrite or Aluminum Nickel Cobalt (AlNiCo) magnet had to be used, this made the motors bulky and they were susceptible to demagnetization if the control of the motor current was incorrect. The demagnetization problem was one of the reasons that the id = 0 control strategy was adopted [2], this works well for round rotors but for a salient rotor a couple of their advantages are lost. It limits the motors speed range and the maximum efficiency cannot be reached. To take advantage of the properties of modern magnetic materials, a new approach to controlling the motors has got possible them has to be taken. The current vector control algorithm of an IPMSM in different speed operation regions were explored. Form the theoretical analyses, the optimal behavior of the IPMSM can be achieved by considering two phase control algorithm: maximum torque per ampere (MTPA) control strategy in constant torque region, flux-weakening control strategy in high-speed region. The algorithm provides robust current regulation with maximum efficiency and torque capability for IPMSM. In different regions, the optimum d-axis current command can be calculated according to different control strategy. The q-axis current command is determined from the torque command and d-axis current.