Using Flux and Current for Robust Wide-Speed Operation of IPMSMs

Abstract

Deadbeat-direct torque and flux control (DB-DTFC) provides accurate and robust torque control, is well suited for dynamic loss optimization, and can utilize the full inverter volt-second hexagon. However, when operating at the voltage and current limits, the DB-DTFC control law becomes more complex and parameter sensitive since it relies on accurate flux trajectories that minimize losses while respecting voltage and current limits. This paper proposes techniques to operate at the voltage limits even if it is infeasible to achieve the torque command in one switching period. When the torque command is infeasible due to the voltage limits the proposed techniques allow DB-DTFC to transiently go into overmodulation to deliver a fast torque response. Furthermore, it is very crucial for direct torque controlled drives to guarantee that the torque command does not exceed the current limits. This paper proposes a robust method to integrate current limits into the DB-DTFC control law. If the torque command requires more than rated current, then the torque is automatically saturated to the maximum feasible torque. This technique does not only enable the drive to operate at the current limit, but also allows for operation in the deep field-weakening region. The paper evaluates the proposed methods in a quasi-steady-state operation by comparing current loci over a wide speed range with inaccurate parameters. Moreover, the paper evaluates speed and torque transients at current and voltage limits using step responses below and above the base speed.