Peter E. Macheiner

Monitoring of Rotor-Bar Defects in Inverter-Fed Induction Machines at Zero Load and Speed

Rotor-cage fault detection in inverter-fed induction machines is still difficult nowadays as the dynamics introduced by the control or load influence the fault-indicator signals commonly applied. In addition, detection is usually possible only when the machine is operated above a specific load level to generate a significant rotor-current magnitude. This paper proposes a new method of detecting rotor-bar defects at zero load and almost at standstill. The method uses the standard current sensors already present in modern industrial inverters and, hence, is noninvasive. It is thus well suited as a start-up test for drives. By applying an excitation with voltage pulses using the switching of the inverter and then measuring the resulting current slope, a new fault indicator is obtained. As a result, it is possible to clearly identify the fault-induced asymmetry in the machines transient reactances. Although the transient-flux linkage cannot penetrate the rotor because of the cage, the faulty bar locally influences the zigzag flux, leading to a significant change in the transient reactances. Measurement results show the applicability and sensitivity of the proposed method.

Current-controller with single DC link current measurement for inverter-fed AC machines based on an improved observer-structure

In modern industrial applications even low-cost drives have excellent dynamic behavior, which is achieved by field-oriented control combined with high-dynamic current regulation. Usually at least two phase-current sensors are necessary to realize such an operation. In this paper, a new current-control scheme is presented, which enables high-dynamic control of ac machine-line currents with improved accuracy by measuring only the current of the dc-link. The controller is based on three individual and adaptive phase-current observers, which guarantee very accurate phase-current estimates even at low-modulation indices. Thus, operation is possible even at low-speed without modifying the switching pattern of the controller and the necessary modification at zero-speed can be clearly reduced. Different realizations of the proposed scheme are given and a comparison is made to previously published schemes which are also based on a single dc-link current sensor. Measurements performed on an induction-machine drive at different points of operation show the applicability of the proposed structure. It is shown that the long-term accuracy is clearly increased and the maximum deviation of the observer estimate from the actual phase-current is reduced when using the proposed scheme.

Distance laboratory for teaching Industrial Electronics

The paper deals with basic philosophy and structure of remote controlled laboratory for experimentation in electrical engineering. PEMCWebLab project provides the user with a practical experience in industrial electronics education. It is designed based on leading ideas and has clear targets. In this paper laboratory for the course motion control is introduced. First of all description of the system and then two assignments namely calibration of resolver and current control loop design explained. Goal of the experiment, content of the experiment and verification of the achieved learning outcomes is illustrated on the mentioned two examples.

An improved observer-based current controller for inverter fed AC machines with single DC-link current measurement

In modern industrial applications even low cost drives have excellent dynamic behaviour which is achieved by field oriented control combined with a high dynamic current control. Usually at least two phase current sensors are necessary for such an operation. In this paper, a new current control scheme is presented, which enables dynamic control of AC machine line currents with improved accuracy by measuring only the current of the DC link. The controller is based on three individual and adaptive phase current observers, which guarantee very accurate phase current estimates even at low modulation indices. Thus operation is possible even at low speed without modifying the switching pattern of the controller and the necessary modification at zero speed can be clearly reduced. Different realisations of the proposed scheme are given and a comparison is made to previously published schemes which are also based on a single DC link current sensor. Measurement results performed on an induction machine drive at different points of operation show the applicability of the current controller and furthermore enables a grading of different structures.

Extraction and elimination of induction machines inherent asymmetry by lamination material anisotropy

The lamination material used in inverter-fed ac machines shows some non ideal effects like magnetic saturation, hysteresis losses, eddy currents, and anisotropy. This presentation focuses on the effect of magnetic anisotropy which can show up in inverter fed operation and may deteriorate the performance if not considered. The manufacturing process with rolling and punching as well as the demand to reduce the sheet thickness in order to keep losses low support the emergence of an anisotropic distribution of the crystalline energy. Though the anisotropy is smaller than in grain oriented lamination material used in transformers it is still detectable as shown by measurements made on specimen discs. In the behavior of inverter fed ac machines this effect is seen in the transient current slope as shown by measurements performed on an especially manufactured induction machine. In the fundamental wave behavior anisotropy can hardly be detected. As modern drive control methods are mostly based on fundamental wave models it has not been considered up to now. However, in the past years first methods have been proposed to exploit also the high frequency and transient electrical behavior of induction machines, especially in the fields of mechanically sensorless control as well as condition monitoring. The influence of the anisotropy effect on the transient current slope is shown by measurement results obtained from rotor laminations manually stacked in order to enlarge as well as to diminish the anisotropy. The separation of the resulting signal from possible rotor eccentricity is done using additional measurement coils in the machine.

Non-invasive detection of rotor cage faults in inverter fed induction machines at no load and low speed

Rotor cage fault detection in inverter fed induction machines is still challenging nowadays as dynamics introduced by the control or the load influence the fault indicator signals usually applied. In addition, the detection is usually only possible when the machine is operated above a specific load level to generate a significant rotor current magnitude. This paper proposes a new method to detect rotor bar defects at zero load and around standstill. The method only uses the standard current sensors already available in modern industrial inverters and is thus non-invasive. It is thus well suited as start-up test for drives. By applying a transient voltage pulse sequence using the switching of the inverter and measuring the resulting current slope a new fault indicator is obtained. As a result, it is possible to clearly identify the fault-induced asymmetry in the machine transient reactances. Though the transient flux linkage cannot penetrate the rotor because of the cage, the faulty bar locally influences the zigzag flux leading to a significant change. Measurement results show the applicability and sensitivity of the proposed method

Modulation of transient reactances of induction machines caused by different types of eccentricity

System reliability is getting more and more important in modern industrial drives. Currently the most frequent source of a drive breakdown is a defect of the bearings, which at the same time is most difficult to detect especially under inverter operation. The exploitation of the transient current slope was shown to give an accurate means to identify the machines transient reactances and thus also to detect possible asymmetries in the airgap. In this paper the influence of different types of eccentricity on the transient current slope of the machine excited by voltage pulses is investigated. Special attention is paid to the influence of combined static and dynamic eccentricity as this combination, due to the non linear behavior of the lamination gives rise to specific modulation that is clearly detectable. The investigation is done at machines with two and four pole pairs. The signal obtained corresponds to the inverse transient reactance. It contains different components of inherent asymmetries in the machine as well as in the case of eccentricity additional modulations that can be extracted. For a machine with two poles static eccentricity leads to a change in the signal offset, whereas dynamic eccentricity rotating with the rotor leads to a modulation with twice the mechanical frequency of the rotor. Mixed eccentricity finally leads to change in signal offset as well as a modulation corresponding to a fundamental wave with respect to the mechanical frequency. For a machine with four poles it is advantageous to use the signals obtained from single phase excitation separately.

Detection of airgap asymmetry in induction machines with different pole pair number using the current step response

In the past years the incident of fault conditions in modern drives has increased. One reason for that loss of reliability is the fast switching of the power electronics applied that puts additional stress to all components of the machine. Besides the stator isolation breakdown and the rotor bar defect the degradation of the bearings has been given increased attention recently from both industry and academia. Bearing defect is the most frequent source of a drive breakdown and at the same time this type of defect is the most difficult to detect. Monitoring methods based on frequency analysis like fourier or wavelet transform are facing problems with the additional harmonics introduced by the inverter, the load, as well as the high dynamic control of the machine itself that have to be separated from the harmonics induced by the bearing defect. In this paper an online method based on the time step response of the machine current is investigated and the influence of the pole pair number on the resulting fault indication signal is discussed. In a machine with one pole pair, eccentricity leads to very distinct changes in the offset values of the resulting signal caused by a change in the phase values of the leakage inductances. This effect diminishes with increasing pole pair number making it necessary to identify other indicators. This other indicators are found in the signal components linked to the slotting of the machine. Their magnitude is increased by a factor 3-4 making it suitable to determine the airgap eccentricity of the machine. Measurement results on machines with different pole pair numbers specially manufactured to accurately change and measure the airgap are presented and evaluation methods to detect defect bearings at different types of machines are suggested.

Comparison of Different Methods to Detect Static Air Gap Asymmetry in Inverter Fed Induction Machines

Monitoring of air gap asymmetry in induction machines is very challenging even considering steady state and ideal line fed operation. Considering also inverter fed and dynamic operation situation gets even worse. In this investigation two different attempts to detect static eccentricity are compared with respect to their sensitivity at different machines. The first method is based on Fourier transform of the phase currents also denoted motor current signature analysis (MCSA) in literature. The second is based on the exploitation of the transient current response of the machine to an excitation with voltage pulses caused by the inverter switching. It is shown that the performance of the MCSA depends on the number of slots per pole whereas the indicator obtained from the transient current response is not affected by this parameter.

Lamination material anisotropy and its influence on the operation of inverter-fed induction machines

In modern inverter-fed machines the exploitation of the transient electrical behavior becomes of increasing interest to obtain additional information on the machine state with speed sensorless flux/rotor position detection schemes or fault condition methods. In the application of these methods it turned out that the anisotropy of the lamination material, which is usually negligible in the fundamental wave behavior of the machines influences the transient control signals and may act as a disturbance during operation. In this paper the anisotropy is determined by hysteresis and torque measurements of lamination sheet as well as by the evaluation of the transient electrical behavior of industrially manufactured inverter fed induction machines.