Analytical maximum torque per volt control strategy of an interior permanent magnet synchronous motor with very low battery voltage

Abstract

The operation of an interior permanent magnet synchronous motor included in an electric power steering system is related to two demanding requirements: (i) the low-voltage DC source pushes the motor to a deep flux weakening region, and (ii) the motor is so optimised that it can withstand only few cycles at nominal torque. Owing to that, maximum torque per ampere (MTPA) and maximum torque per volt (MTPV) current reference generation strategies are common in this type of application. Most of the published MTPA or MTPV strategies are applied to standard voltage motors, so stator resistance is typically neglected, leading to simpler equations. Other works consider the stator resistance but, as the resulting equations are complex, look-up tables or numerically adjusted polynomials are employed in current generation tasks. This work presents analytical expressions allowing the exact computing of current references. These expressions include stator resistances. The battery voltage is considered as an input variable, together with motor speed and reference torque, and direct and quadrature current references are the output variables. Contrary to look-up tables or numerically adjusted polynomials, the proposed expressions can take into account any parameter variation during real-time operation. Simulation and experimental results validate the proposed approach.