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.