Abstractâ This paper proposes a novel criterion for precise diagnosis of rotor broken bar fault in squirrel-cage induction motor at different operating modes.
2012 IEEE International Conference on Power Electronics, Drives and Energy Systems December16-19, 2012, Bengaluru, India
A New Criterion for Rotor Broken Bar Fault Diagnosis in Line-start and Inverter-fed Induction Motors using Hilbert-Huang Transform Jawad Faiz, Senior Member, IEEE, Vahid Ghorbanian and Bashir Mahdi Ebrahimi Center of Excellence on Applied Electromagnetic Systems, School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran Abstract— This paper proposes a novel criterion for precise diagnosis of rotor broken bar fault in squirrel-cage induction motor at different operating modes. The criterion is extracted from the energy value of the Hilbert-Huang transform of the faulty motor current. Impact of the line-start and direct torque control modes are investigated at different fault levels, load torques and reference speeds. The new saturated winding function method is also used in order to include the non-linear characteristic of the motor core materials. Keywords— Induction motor, broken bars fault, inverter-fed motor, Hilbert-huang transform, Criterion
I.
INTRODUCTION
Rotor broken bars fault is one of the common faults in induction motors which causes time and frequency oscillations and imperfects in electrical and mechanical variables of the motor [1, 2]. High saturation adjacent to the breakage location distorts the air gap flux density and generates current frequency pattern of (1±2ks)fs [3]. Frequency distortion of the motor variables leads to the fluctuation of the time domain signals [4]. It is observed that applying a proper processing method to the frequency, time or time-frequency parameters of the faulty motor can be used as a tool for rotor broken bar fault diagnosis. In [1, 3], the concept of sideband components has been used and normalized amplitude of the above-mentioned frequency pattern of the line current of the motor has been introduced as index. Since the behavior of induction motors highly depends on load variations, the effect of the load variations is also investigated [5]. However, the extracted spectrum varies with changing window size in fast Fourier transform (FFT). The frequency domain indexes can be applied to large induction motors in order to diagnose the fault at start-stop modes [5]. Combination of Hilbert transform (HT) and FFT has introduced an index over low frequency that enables to diagnose the broken bars fault in the no-load condition as well [6]. There are high frequency time harmonics in the inverterfed motors which seriously affect the amplitudes of indexes. On the other hand, the operating point of the inverter-fed
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motor widely changes due to the controllable nature of the drive. So, the absolute values of the motor variables vary. Therefore, introducing an index which can precisely diagnose the fault under different operating modes (line-start and inverter-fed) and conditions (fault level, load torque and reference speed) is inevitable. Using the id and iq components of the current leads to an appropriate indexes at this field [7]. Moreover, the index value highly depends on the reference speed of the drive. Voltage related indexes are introduced in [7]. However, application of such indexes is not recommended due to the dependency of the motor voltage variations upon the electrical network oscillations as well as drive parameters. One of the major drawbacks of the frequency-domain indexes is ignoring the time-dependent characterization of the fault. Rotor broken bars also leads to the time domain oscillations. Fluctuations of the current components in the synchronous reference frame and stator current curve envelop index have been considered as time domain indexes [8, 9]. Both indexes enable to diagnose the motor fault level with suitable accuracy. The fault information is clearly present in time and frequency domains. Therefore, considering only the time oscillations caused by fault leads to neglecting the frequency information and vice versa. Based on this concept, if both time and frequency information of the fault can be used, a more precise index will be introduced. One of the wide applicable transforms in time-frequency domain is the wavelet transform [2, 10]. So, it has wide application in the analysis of the transient signals [10]. It is worthy to mention that dependency of the value of this index in the wavelet transform on the mother wavelet is the drawback of these set of indexes. Another frequency-time domain transforms is the HilbertHuang transform (HHT) [11]. The basis of the HHT is calculation of the instantaneous frequencies components over different times. A defined index has not been presented in [11] and only time-frequency domain variations have been utilized to diagnose the fault. Advantage of HHT is that it has a constant routine and on contrary to the wavelet and short Fourier transform does not depend on other parameters. In this paper, the precise model introduced in [1] is used for modeling the induction motor with rotor broken bars fault. In
this model, the effect of overall core saturation and local saturation due to the rotor broken bars have been included in the winding function method (WFM). The HHT is applied to the current of healthy and faulted motor (obtained by simulations and experiments) and instantaneous energy (IE) is introduced as an efficient criterion for the rotor broken bars fault diagnosis in different operating modes and points. IE covers the whole frequency bands due to the fault over different times. Therefore, all fault information is usable in order to diagnose the fault precisely. II. Saturated Winding Function Method The WF-based method evaluates the windings inductances using magneto-motive force (mmf) distribution. It is possible to include the effects of all spatial harmonics of the motor mmf, skew of the rotor bars, and the stator slot [1]. The only drawback of the method is the lack of studies on applying the saturation effects under different fault types. Since the amplitudes of the side-band components directly depend on the core saturation level, ignoring the saturation causes a significant error at the fault diagnosis. So, the precise saturated winding function method which is validated experimentally in [1] is used to study the broken bars at different modes. The fundamental of the WFM has been thoroughly presented in [1]. Therefore, applying the saturation is going to be discussed here. In order to apply the core saturation effect and variations of the air gap flux density to the WF model in the healthy and broken bar motor, the idea of decreasing the mesh current is used. Such idea for modeling the broken bar fault is based on the experimental results. The criterion used in this method is the normalized amplitude of left sideband of frequency pattern (1±2s)fs in the motor current spectrum. At this end, proportion of the current that eliminated from the mesh (β) for conformity of the simulated left sideband amplitude with the corresponding test result is evaluated at the minimum load and line-start mode with several broken bars. Then, by obtaining a mathematical expression fitted to the obtained data, a defined and regular relationship between the number of rotor broken bars and the corresponding coefficient is obtained in order to apply to the WF model. Fig. 1 shows the variations of β versus the fault level. The related fitted equation is as follows: β = 0 . 8035 e ( − 0 . 03853 × N ) − 0 . 7634 e ( − 0 . 2185 × N ) 0.5
Betha
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Fitted curve Experimental
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Ii = α × Ii
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2 3 4 Number of broken bars
Fig. 1. Variation of β versus the number of rotor broken bars
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