Andrea Toscani

Thorough Understanding and Experimental Validation of Current Sideband Components in Induction Machines Rotor Monitoring

The MCSA diagnostic procedures for the rotor diagnosis of induction machines are mostly based on the sideband lines near the supply frequency line that appear in the input current spectrum. The left component (1-2s)f is the effect of rotor backward rotating field caused by the cage rotor asymmetry and it varies with the asymmetry degree and with the rotor current, i.e. with the machine load. The right component (1+2s)f is caused by the speed ripple created by the left component, being the combined machine-load inertia the main parameter that spreads the asymmetry effect into the two sideband components. Theoretically it can be shown that the sum of the two components is not affected by the speed ripple effect and therefore this sum may be used as an effective diagnostic index to state rotor conditions. The above results can be experimentally validated only once a suitable test set-up is realized that allows changing the inertia value. To this aim a test bed was designed so that the virtual inertia of the mechanical system can be chosen by the user. This allows to prove the theoretical claims and to obtain a thorough understanding about the effect of inertia on sideband components, improving the performance of rotor condition monitoring techniques

Mixed-Mode PWM for High-Performance Stepping Motors

This paper describes the realization of a high-performance drive based on stepping motors, with a specific reference to the optimal choice of pulsewidth modulation. An original modulation technique is proposed that allows, in conjunction with current feedback, to achieve high performances together with a reduced current ripple. A prototype digital signal processor-based drive was realized for a small-size stepping motor.

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

Validation of a Faulted Rotor Induction Machine Model With an Insightful Geometrical Interpretation of Physical Quantities

Many proposals about induction-machine broken-bar detection can be found in the literature. Disturbances that affect phase currents, current space vector, power, fluxes, torque, and speed have been utilized to diagnose mains-supplied machines. Nevertheless, there is still disagreement about how the fault signatures of the different variables correlate with the actual fault and how they depend on the operating conditions. This paper presents a simple but effective analytical representation of induction machines with rotor faults, based on a synchronous reference frame and on Steinmetzs phasors. The developed relationships can be visualized through equivalent circuits, feedback loop block diagrams, or geometrical loci, allowing a deep understanding of the underlying physical phenomena, particularly as functions of inertia and of the machine parameters. The insight thus gained helps to enhance the quantification of fault severity with respect to the usual diagnostic technique based on the sum of the sidebands, clearly defining for the first time the field of validity of such technique. The analytical results are verified through simulations and experiments.

Mixed mode PWM for high performance stepping motors

This paper describes the realization of a high performance drive based on stepping motors, with specific reference to the optimal choice of PWM modulation. An original modulation technique is proposed that allows to achieve high performances together with a reduced current ripple. A prototype DSP based drive was realized for a small size stepping motor.

A new method to discern mechanical unbalances from rotor faults in induction machines

Intensive researches have been spent with the aim of separating load torque unbalance and rotor faults effects in induction machines by devoted diagnostic procedures. Recently, current space vector active and reactive components due to these troubles are considered for their different relevance in correspondence of the two events: the torque unbalance seems to affect more the active component than the reactive one, vice versa the rotor faults. In this paper it is shown that the space vector components amplitude and their phase displacement must be considered, being the latter the main parameter: if the active component leads or lags the reactive one by less than π/2 a rotor fault is present, otherwise a torque unbalance is present. Regarding the components amplitude, it depends, overall, besides on trouble severity, on the operating conditions and on the combined rotor-load inertia.

Severity Assessment of Rotor Faults in Closed Loop Induction Drives by Different Approaches

In this paper different procedures are validated to diagnose and assess the severity degree of rotor faults in closed loop controlled induction machines. The possible diagnostic indexes are investigated through spectral analysis of manipulated and controlled variables. Common control schemes have been considered to evaluate the diagnostic robustness towards operating condition and regulator parameters. Voltage injection was considered as well as a possible tool for diagnostics by analyzing the amplitude of spectral lines arising in motor input current spectrum. Experimental results guide the selection of suitable analytical models for the faulty machine. The experiments are obtained with a DSP based current controlled voltage source inverter (CCVSI) supplying a 1.5 kW induction machine in which three different rotors differing in asymmetry degree were used.

Vibrationless alignment algorithm for incremental encoder based BLDC drives

Incremental encoder based BLDC drives are widely employed in industry whenever smooth and efficient operation is needed in the whole speed range. Their weak spot lies in the need for a phase of initial alignment. Such task is traditionally performed by forcing the motor through a series of fixed phase supply configurations, causing strong vibrations which can be intolerable for certain applications. In this paper a novel algorithm that allows determining the initial rotor position (sector) and aligning the encoder while minimizing vibrations and oscillations is presented.

Fault tolerant digital control of 2 MVA parallelable frequency converters for harbor applications

This paper deals with the design of a fault tolerant digital control of high power, parallelable frequency converters for harbor application. With the aim to yield a high degree of system availability the control algorithm must be able to automatically switch from parallel to single mode operation in case of fault. Scalability is a further key feature of the proposed control strategy: in order to increase the output power rating, the control strategy can parallel more than two converters increasing, in turn, the availability of the whole system as well. An inclusive set of MATLAB simulations shows that the proposed control algorithm makes the whole power system complying with the specifications in terms of THD, power efficiency and dynamic response. A full set of measurements, performed on a scaled converter prototype, confirmed the simulation results.

A geometrical interpretation of current space vector components due to induction machines rotor faults

This paper presents a deep investigation on the rotor fault signatures of induction machines. Phase current and current space vector sidebands, spectral components of flux, torque and speed are analyzed as functions of inertia, load and machine size. Analytical expressions among the physical quantities, validated by simulation and experimental results, are developed using a synchronous reference frame. These expressions, together with geometrical loci showing the vector relationships among the quantities involved, allow a thorough understanding of the underlying physical phenomena. A test-bed able to realize a variable virtual inertia allows the experimental validation as well.