Flux-torque cross-coupling analysis of FOC schemes: Novel perturbation rejection characteristics

Abstract

Field oriented control (FOC) is one of the most successful control schemes for electrical machines. In this article new properties of FOC schemes for induction motors (IMs) are revealed by studying the cross-coupling of the flux-torque subsystem. Through the use of frequency-based multivariable tools, it is shown that FOC has intrinsic stator currents disturbance rejection properties due to the existence of a transmission zero in the flux-torque subsystem. These properties can be exploited in order to select appropriate feedback loop configurations. One of the major drawbacks of FOC schemes is their high sensitivity to slip angular velocity perturbations. These perturbations are related to variations of the rotor time constant, which are known to be problematic for IM control. In this regard, the effect that slip angular velocity perturbations have over the newly found perturbation rejection properties is also studied. In particular, although perturbation rejection is maintained, deviations to the equilibrium point are induced; this introduces difficulties for simultaneous flux and torque control. The existence of equilibrium point issues when flux and torque are simultaneously controlled is documented for the first time in this article.

Fundamental Analysis of the Perturbation Rejection Characteristics of Single-Input-Multiple-Output Systems Subject to Multiple Perturbations

“…In this context, it is pertinent to recall that the MSF has been historically used to measure the structural robustness of multivariable systems [8][9][10][11][12][13][14][16][17][18][19][20][21]. As such, ( 0 ) → 1 is a sign of low structural robustness and it is undesired, since in that case the transmission zeros may change from minimum phase to nonminimum phase.…”

“…While the flux operates within this range the motor often operates without problem and the effect of flux variations over the output may not be evident. However, if the flux is driven out of certain neighborhood of the ideal equilibrium point, the machine may bifurcate into another equilibrium point causing an undesired behavior in the other system variables [1,2,16].…”

“…The ICAD framework allows using classical single input single output (SISO) measures of robustness and performance for multiple input multiple output (MIMO) systems. The ICAD framework allows the interpretation of complex control problems in the context of engineering systems [8,16,[18][19][20][21]. Although ICAD was originally proposed as a decentralized control design tool, in this article ICAD is adapted for the study of the SIMO perturbation rejection problem.…”

“…The authors hope that this may aid in bridging the theory-practice gap on such problems. It should be noted that some of the main ideas presented here formally were first explored within the realm of a practical application, in particular, during the analysis of the structural characteristics of the flux-torque subsystem of induction motors [16]. Finally, the acronyms used along the article are shown in Acronyms at the end of the paper.…”

Analysis of the damping characteristics of two power electronics-based devices using ‘individual channel analysis and design’

“…For further information on the ICAD framework, the reader is referred to [6] , where ICAD is presented, its use for multivariable control is discussed in detail, and an emphasis on robust stability is made; [7] , where the structural analysis for general m -input and m -output systems is discussed, the influence of right half-plane zeros on controller design and closed-loop performance is evidenced, and the existence of fixed stabilising controllers in the presence of system uncertainty is established; [8] , where the MSF characteristics for a range of 2 × 2 plants are studied; [9] , where the MSF properties are extended to the general m × m case; and to [10] , where the detailed treatment for 3 × 3 and 4 × 4 plants is examined. The framework has been successfully employed in a number of applications, ranging from submarines [11] , induction motors [12,13] and wind turbines [14,15] .…”