Direct equivalent circuit parameter extraction for a brushless doubly-fed induction machine with a nested-loop rotor winding from a finite element model

Abstract

The Simplified Equivalent Circuit Model (SECM) is well established as a tool for the performance analysis of the Brushless Doubly-Fed Induction Machine (BDFIM). However, the SECM typically relies on a constant-parameter approximation, which, in theory, contradicts the machine’s inherent velocity-dependent nature, given its typical nested-loop rotor structure that couples magnetically with a large portion of the air-gap field harmonics’ spectrum. This paper investigates the physics behind this by directly deriving its parameters from rigorous power-balance equations and a time-harmonic finite-element model. The study identifies a self-compensating mechanism: although the extracted rotor leakage reactance and effective turns ratio exhibit significant variations near the natural speed, their combined interaction minimises the impact on terminal quantities. Furthermore, the paper investigates the validity determinants of the constant-parameter SECM in double-feed mode and explains the factors of the conditional accuracy.