Automatic MTPA tracking in IPMSM drives: Loop dynamics, design and auto-tuning

In the control of Interior Permanent Magnet Synchronous Machines (IPMSMs), Maximum Torque Per Ampere (MTPA) based on motor parameters is a common approach to achieve high efficiency and torque density. Parametric uncertainty (e.g. due to identification errors, magnetic saturation or temperature variation) results in undesired deviation from the optimal operating trajectory. To solve this problem, MTPA tracking methods have been proposed, which exploit signal injection to search the minimum current point for a certain load torque, in a closed-loop fashion. For one of these methods, [13], stability of the non-linear dynamics was analyzed, and an upper bound for the convergence time was found, but no explicit method was proposed for the design of the tracking regulator. In this paper this last topic is addressed. By introducing some approximations, the linearized system is calculated and a loop transfer function obtained, which is invariant with the operating point. Thus, by means of a very simple design rule (i.e. suitable for auto-tuning), the MTPA tracking regulator gains can be designed in order to obtain the desired bandwidth. The method has been studied analytically and in simulation, also considering the influence of noise and parametric uncertainties. Finally the technique has been implemented on the hardware of a commercial industrial drive, proving the effectiveness of the proposal.