Conference Paper

Characteristic signature identification of air-gap eccentricity faults using extended d-q model for three phase induction motor

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The supremacy of three phase squirrel cage induction motors in industrial drives demands accurate and reliable diagnostics for condition monitoring and internal fault detections. Operating stresses on these machines are electrical, mechanical, thermal, magnetic and environmental in nature and might result in internal faults. Avoiding unscheduled maintenance and repair intervention can prevent losses in money, material, manpower and time in process industries. Detection of faults in its early stage becomes an indispensable need especially in critical applications. Mathematical model based simulation studies will support fault signature identification to a great extent. Conventional d-q model of AC machines are not generally used for internal fault diagnoses. In this paper a novel attempt is made for simulating eccentricity related faults by modifying conventional d-q model of three phase induction motor. Characteristic fault signatures were identified in the stator current frequency spectrum for static, dynamic and mixed eccentricity conditions. The increase in magnitudes of these characteristic frequency components with increase in severity of faults is also established through model based simulation studies. The experimental study results presented for static eccentricity in a three phase squirrel cage induction motor clearly validates the modelling approach.

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... The term of the end ring current from the rotor electromagnetic equations in (8) is removed to give Equation (8) a symmetric structure. Considering the assumption above, the MCCM can now be transformed into a two-phase model [32]. ...
... Therefore, with the same degree of eccentricity, the amplitude of the characteristic current component under no-load operation and rated load operation are almost the same. This is consistent with the experimental results in [32]. ...
Full-text available
The motor current signal analysis (MCSA) technique is widely used as a non‐invasive method for detecting mechanical faults in induction motors by capturing characteristic components in the stator line current. However, the threshold of the characteristic component is not clear now, which makes it difficult for MCSA to judge whether the fault occurs or evaluates the mechanical fault severity. The existing model‐based evaluation methods cannot meet the requirement of online condition monitoring because of their slow calculation. To solve these problems, a simplified dynamic motor model under any type of mechanical fault is established, and a formula for the amplitude relationship between the radial vibration of the rotor and fault‐related component in the stator line current is derived. The radial vibration amplitude is related to mechanical fault severity. Using this formula, the MCSA technique can rapidly evaluate the mechanical fault severity according to the amplitude of the characteristic component in the collected stator current. The simulation study results demonstrate the accuracy of the simplified model and formula. The experimental results presented for condition monitoring in a real induction motor clearly validate the evaluation approach.
... Also, an accurate model can be used in model-based fault diagnosis systems to directly identify the fault severity [15]. An attempt has been made for simulating eccentricity faults by modifying conventional d-q model of three phase induction motor in [18], however, this method suffers from rough assumption of sinusoidal distribution for the stator and rotor windings. Multiple coupled circuit model (MCCM) based on the modified winding function theory, also known as the winding function approach (WFA), is the other analytical method for modeling eccentric SCIMs. ...
... 18)-(19) are written by considering that the turn functions of the stator windings are independent of the rotor position and the z-coordinate. The equations versus θ to get exact equation for the derivative inductances to be used to calculate the induced torqueFig. ...
Many mechanical faults cause eccentricity in squirrel cage induction motors (SCIMs), so, it is reasonable to design and implement reliable eccentricity fault diagnosis systems for the SCIMs. Accurate modeling and simulation techniques help to reach this aim more easily. It is common to assume that the eccentricity fault is uniform along the motor length when modeling and simulation that is often far from real situation. In this paper, experimental inspection is performed first to show that having the same severity of eccentricity at one end of the motor, the axially non-uniform eccentricity may produce lower fault indices than the axially uniform eccentricity. Then, 2D modified winding function theory is presented and used to obtain analytic expressions for various self/mutual inductances of the stator windings and the rotor meshes and their derivatives versus the rotor position. The expressions are applicable under various axially uniform and non-uniform eccentricities as well as the healthy condition. Multiple coupled circuit model is used along with these expressions to develop a comprehensive analytic model for the SCIMs. Simulation results obtained by this model are in good agreement with corresponding experimental results and confirm accuracy of the attained modeling and simulation technique.
Conference Paper
In recent years squirrel cage induction motor (SCIM) appears to be very cost-effective and energy efficient if combined with advanced power electronic inverters. Many industrial problem solutions related with varying speed applications recognized them due to their catchy characteristics like manageable speed-torque, reasonable performance, operational control and satisfactory efficiency etc. The major problems associated with pulse width modulation inverter (PWM) based SCIM are internal and external sourced bearing currents, harmonic productions and fast switching inverter action. This paper presents mathematical modelling based on direct-quadrature transformations of three phase sinusoidal PWM inverter fed SCIM. Direct control of voltage has been analyzed in latest Matlab environment using dynamic simulation technique. Dynamic simulation tool is not only beneficial in verifying the design steps of motor driven system but also reduction of fault can be achieved in construction and testing of prototypes. Issue of noise creates instability throughout the system which is reduced in the designed model by input voltage control. The simulated model shows encouraging output with reduced harmonic content. Early fault detection is possible to avoid the catastrophic conditions of motor which surely goes in economic benefit of industries. This piece of work may be useful for other circuits based on induction phenomenon and results can also be referred for wound machine applications.
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This paper discusses use of air-gap torque spectra as a means of identifying faults in cage rotors. Being dependent on both stator and rotor currents, the torque is very sensitive to faults in the rotor. Through a comparative study using a detailed machine model and the standard dq model, the paper shows that the characteristic frequencies generated by a particular fault are preserved even if the standard dq model is used for estimation of air-gap torque. This is validated through a practical hardware implementation for online spectrum estimation of air-gap torque using TMS320C31, where several faulted cage rotors were used for study.
Conference Paper
Three phase squirrel cage induction motors are the most popular motors in industries. Electrical, magnetic, mechanical, thermal and environmental stresses during operating conditions lead to internal faults in it. There are no reliable non-invasive tools available for early diagnoses of internal faults. Hence, these internal faults are likely to be left undetected in its early stage, leading to unscheduled maintenance, process shutdown and huge financial loss in industries. Early detection of faults helps to save resources by avoiding process shutdown/repair of machines. Hence, there is a need for a reliable non-invasive condition monitoring system for three phase squirrel cage induction motors. Condition monitoring involves non-invasive acquisition of signals, processing, fault signature extraction, decision making on the presence and type of faults. This paper reviews the current trends in internal fault diagnosis of induction machines and identifies future research options. A statistical analysis of the results of motor current signatures obtained from a motor with rotor bar faults is done to study the relative effect of fault severity and load variation on the current signature.
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Medium-voltage (MV) induction motors are widely used in the industry and are essential to industrial processes. The breakdown of these MV motors not only leads to high repair expenses but also causes extraordinary financial losses due to unexpected downtime. To provide reliable condition monitoring and protection for MV motors, this paper presents a comprehensive survey of the existing condition monitoring and protection methods in the following five areas: thermal protection and temperature estimation, stator insulation monitoring and fault detection, bearing fault detection, broken rotor bar/end-ring detection, and air gap eccentricity detection. For each category, the related features of MV motors are discussed; the effectiveness of the existing methods are discussed in terms of their robustness, accuracy, and implementation complexity. Recommendations for the future research in these areas are also presented.
A new multiple coupled circuit model is presented for simulation of induction machines with both arbitrary winding layout and/or unbalanced operating conditions. The model is derived by means of winding functions. No symmetry is assumed. The parameters of the model are calculated directly from the geometry and winding layout of the machine. The behavior of an induction machine during starting is simulated using this model. The results are shown to be in good agreement with the solution obtained by a conventional d-q model for symmetric conditions. The new model is then extended to the solution of a wide variety of fault conditions such as broken bars and end rings and open or short circuited motor coils
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  • Oleg Krause
  • Scott D Wasynczuk
  • Sudhoff
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