Alberto Bellini

Quantitative evaluation of induction motor broken bars by means of electrical signature analysis

The paper reports the comparison and performance evaluation of different diagnostic procedures that use input electric signals to detect and quantify rotor breakage in induction machines supplied by the mains. Besides the traditional current signature analysis based on one-phase current spectrum lines at frequencies (1/spl plusmn/2s)f, the procedures based on analysis of the line at frequency 2sf in the spectrum respectively of electromagnetic torque, space vector current modulus and instantaneous power are considered. These last procedures have similar features and the comparison is developed on the basis of instantaneous torque. It is shown that the speed ripple introduces two further terms in the instantaneous torque, decreasing the accuracy of the diagnosis. It is shown that there is a link between the angular displacement of the current sideband components at frequencies (1/spl plusmn/2s)f. This allows a more correct quantitative evaluation of the fault and to show the superiority of the sideband current components diagnostic procedure over the other proposed methods.

On-field experience with online diagnosis of large induction motors cage failures using MCSA

The experience gained by ENEL Produzione (previously the Italian Electric Board) on monitoring the cage condition of large induction motors is reported in this paper. The diagnostic procedure is based on the motor current signature analysis and, in particular, on the two sideband current components near the frequency fundamental line that appear in the current power spectrum when a rotor bar/ring breakage occurs. According to the developed procedure, a diagnostic index obtained from these components is stored and its trend as a function of time allows for the detection of the occurrence of a failure in most cases. This event is clearly shown by the overcoming of a prefixed and triggered threshold. Moreover, machines with particular rotor magnetic structure are considered. In this case, unexpectedly high sideband components appear, even in the presence of healthy cages, and the test procedure was adapted to account for these conditions.

Vibrations, currents and stray flux signals to asses induction motors rotor conditions

Multi-variable monitoring systems for on-line monitoring of induction motors allow larger versatility and robustness. In case of rotor faults, the diagnostic procedure based on sideband current components may fail for the presence of inter-bar currents that reduce the amplitudes of these components. On the other hand the inter-bar currents modify the core vibration; therefore the analysis of both line currents and core vibration signals may increase the effectiveness of the diagnostic procedure. In this paper the features of stator currents and core vibration signals are investigated in order to achieve an accurate and reliable diagnostic system. Moreover stray flux signals are considered to explore the possibility of obtaining further information

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.

Analog synthesis of nonlinear functions based on fuzzy logic

A circuit for the analog implementation of nonlinear functions has been designed. It is based on the fuzzy-logic paradigm mapped into a modular architecture. The basic building blocks rely on the large-signal characteristic of MOS differential pairs to design bell-shaped basis functions and combine them with the center-of-gravity method. The actual input/output (I/O) characteristics are defined by applying proper voltage levels to the high-impedance input nodes of each module. Both a field-programmable processor, where the programming configuration is stored in a digital RAM, and a tool for generating the layout of dedicated circuits, where this connection is hardwired, have been designed using a CMOS 0.7 /spl mu/m n-well technology. A software layer computes the programming values which allow the circuit response to approximate the target I/O relationship. Experiments show that the approximation accuracy is in the range of a few percent r.m.s. of the circuit output range. The application of the system to the synthesis of a nonlinear control law for a DC-DC power converter is discussed.

Assessment of induction machines rotor fault severity by different approaches

The spectral components at frequencies (1plusmn2s)f of stator current are commonly used to assess induction machines rotor conditions in the motor current signature analysis (MCSA) diagnostic procedure. The sum of the amplitudes of these two components can be used as a reliable diagnostic index. A further step is the quantification of the fault severity, i.e. the definition of an asymmetry factor, able to link the diagnostic index to the number of contiguous broken bars. The quantification of the fault severity is a key item, that is not yet fully established. Several machine models were used to this aim: dynamic deep models for faulty cage machines, like mesh rotor models; d-q or symmetrical instantaneous components models. The latter is preferred as it requires a lower number of parameters. Moreover if system constraints dont allow speed ripple as a consequence of the torque unbalance due to rotor asymmetry, only the component at frequency (1--2s)f appears and steady-state forward-backward models can be used in order to compute this component. In this case analytical relationships can be developed under suitable assumptions. A simple asymmetry factor may be the definition of an equivalent additional resistance DeltaRr linked to the number of contiguous broken bars. In this paper some proposals reported in the literature about the quantization of DeltaR r or about the diagnostic index are reviewed and compared with reference to the case of one broken bar. Experiments prove the effectiveness of the authors proposal

An H.261-compatible fuzzy-controlled coder for videophone sequences

In this work we describe an H.261-compatible coder with fuzzy control of the quantization parameters. The quantization parameters are evaluated in order to minimize perceptual distortion and to transmit the images at a fixed bit rate. The fuzzy controller classifies the different areas of the picture according to a set of 28 fuzzy rules using features related to temporal and spatial activity of the sequence. Experimental results show a higher perceptual quality of the new technique when compared to a customary methodology based on the buffer fullness control.<<ETX>>

Induction Drives Diagnosis by Signal Injection: Effectiveness and Severity Classification

The signal injection technique introduced to realize speed sensorless induction drive exploiting rotor asymmetry effects, is an attractive procedure for diagnostic purposes as well. At first it was proposed as alternative technique to detect rotor damage, then its use has been extended to stator short circuit detection. An important issue is the possibility to state the fault severity, linking the amplitude of the novel spectral components induced by the injected signal to the fault entity, avoiding the dumping action of the regulator. In this paper the signal injection technique for rotor bar breakage diagnosis and stator short circuit detection is investigated and compared with the classical MCSA procedures. Simple machine models are presented that allow to state relationships between the fault entity and the amplitude of specific spectrum lines in stator current or current space vector. Experiments and theoretical analysis show that for broken bars this technique has some advantages with respect to MCSA. On the contrary in case of stator short circuits the signal injection techniques is not suitable for diagnostic purposes, being the specific line amplitude only partially affected by the fault

Fuzzy-controlled perceptual coding of videophone sequences

This paper describes a fuzzy quality-controller for video coding systems based on the H.261 recommendation. The picture quality is tuned by a classification of the blocks of each frame of the sequence based on a simplified model of the human visual system. The quantization parameter is evaluated according to this classification making more relevant parts be coded with finer details at the expense of less significant regions. The system is based on a restricted number of fuzzy rules and can be easily reconfigured if a different classification criterion must be adopted. The subjective quality of coded sequences is better than the one obtained by the reference model at the cost of higher computational tasks.

Analog fuzzy audio processor for loudspeaker systems

We describe the application of an analog fuzzy processor to a loudspeaker system. The introduction of this audio processor between the input electric signal and the power amplifier results in a remarkable reduction of the distortion of reproduced sounds. Measurements performed on a commercial loudspeaker, using a demonstration board, show that total harmonic distortion can be reduced by several dBs using the proposed audio processor.