Diagnosis of induction machines by d-q and i.s.c. rotor models

Recently both d-q and instantaneous symmetrical components (i.s.c.) models for faulty cage induction machines have been proposed for numerical simulations of stator instantaneous currents. Applying FFT algorithm to stator current the sideband components at frequencies (1-2s)f can be evaluated. The amplitudes of these components are commonly used as diagnostic index to assess rotor condition in the MCSA diagnostic procedures. With the assumption of infinite machine-load combined inertia value or ideal speed control the speed ripple due to rotor asymmetry will not be present. Steady-state conditions can be considered and only the component at frequency (1-2s)f will appear. If the above assumption does not hold, the (1-2s)f component correspond to the sum of the two sideband lines. In any case the parameterization, i.e. the definition of an asymmetry factor used as diagnostic index to define the fault severity, is a key item. The simplest asymmetry factor may be the definition of an equivalent additional resistance DeltaRr linked to the number of contiguous broken bars. In this paper the proposals about the quantization of DeltaRr found in the literature are reviewed and compared with reference to one broken bar. A new proposal is also presented and its effectiveness is proven.