use of the stator current for condition monitoring of bearings in induction motors

Proceedings of the 2008 International Conference on Electrical Machines Paper ID 907

978-1-4244-1736-0/08/$25.00 ©2008 IEEE 1

Use of the stator current for condition monitoring of bearings in induction motors

Lucia Frosini, Ezio Bassi, Andrea Fazzi, Christian Gazzaniga

Dipartimento di Ingegneria Elettrica, Università di Pavia, Via Ferrata 1, 27100 Pavia, ITALY E-mail: [email protected]

Abstract – The aim of this paper is to verify the effectiveness of the stator current analysis for the detection of bearing problems. The paper reports the experimental results on four different types of bearing faults.

I. INTRODUCTION

The idea of this research starts from two main considerations. First, in recent years it has been demonstrated that the stator current analysis can be effective for the fault detection in induction motors [1], especially for problems related to the rotor, such as broken rotor bars [2] [3], air-gap eccentricity [4] and load anomalies [5] [6]. Secondly, the studies on induction motors have shown that the majority of the faults (over 40%) happens in the rolling bearings [7] and also that these faults are not sudden, but progressive, so that a predictive maintenance based on the condition monitoring could be effective in preventing the consequent failures. Therefore, it is worth to investigate the possibility to employ the analysis of the stator current even to detect the bearings problems.

Traditionally, the diagnostics of the bearings is carried out by means of the analysis of the vibration either of the bearings or the motor case. This method requires the use of accelerometers or other vibration sensors and appropriate devices for the signal conditioning. On the contrary, the analysis of the stator current requires the use of a current probe which can also be employed for the diagnosis of the other types of faults.

The stator current analysis for the fault detection of the rolling bearings has been already investigated by other authors [8-16]. The novelty of this paper is given by the experimental analysis of the effects caused in the current spectrum by four different types of bearing faults, some of them not previously considered in the literature.

II. CLASSIFICATION OF THE BEARING FAULTS

Bearing faults can be classified according to the location of the faults (Fig. 1: outer ring, inner ring, ball, cage) and to the type of defect (cyclic or non-cyclic).

When a ball is defective or when it rolls over a defective raceway, it produces an impact against the raceway and generates a detectable vibration. The frequencies at which these vibrations occur are predictable and depend on which surface of the bearing contains the fault, on the geometrical dimensions of the bearing and on the rotational speed of the rotor fr.

outer ring

ballinner ring

cage

Fig. 1 Main parts of a rolling bearing.

Therefore, there is one predictable characteristic fault frequency fv in the vibration spectrum for each of the four main parts of a given bearing, running at a certain rotor speed [8]: Outer ring: ( )α⋅−⋅= cosDdfNf ro 12 (1)

Inner ring: ( )α⋅+⋅= cosDdfNf ri 12 (2)

Ball: ( )( )α−⋅= 2212 cosDdfdDf rb (3)

Cage: ( )α⋅+⋅= cosDdff rc 121 (4)

where N is the number of balls, d is the ball diameter, D is the bearing pitch diameter and α is the ball contact angle, typical equals to 0°. For simplicity, the outer and the inner ring characteristic frequencies can be approximated for most bearings with between six and twelve balls by:

ro fNf ⋅⋅= 4,0 (5)

ri fNf ⋅⋅= 6,0 . (6) The relationship of the bearing vibration to the stator current

spectrum can be determined by remembering that any air-gap eccentricity produces anomalies in the air-gap flux density. This, in turn, affects the inductances of the machine producing stator currents harmonics. Since ball bearings support the rotor, if a bearing defect produces a radial motion between the rotor and the stator, this motion generates stator currents at predictable frequencies fp:

vsp kfff ±= (7)

where fs is the electrical supply frequency, fv is one of the characteristic vibration frequency and k = 1, 2, 3, …

The problem is that these theoretical relationships can be considered as valid only in case of single point defects (cyclic faults). Another important group of faults can be defined as generalized (not localized) roughness and can be included

Proceedings of the 2008 International Conference on Electrical Machines

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among the non-cyclic faults [11]. Another type of fault which is not included in the previous classification is a defect in the protective shield, which generally does not produce an effect similar to the air-gap eccentricity.

III. STATE OF THE ART

The stator current analysis for the fault detection of the rolling bearings has been investigated in the last years [8-16]. One of the first research is [8], where the authors test two types of fault: i) hole drilled through the outer race and ii) indentation produced in both the inner and outer surface. For both faults the vibration and current spectra are analyzed, both in case of loaded and unloaded motor. The first faulty condition reveals fo and 2fo components in the vibration spectrum, os ff ± and os ff 2± components in the current spectrum. The second fault highlights fo, 2fo and fi components in the vibration spectrum, os ff ± , os ff 2± and is ff ± components in the current spectrum. The authors state that the characteristic fault frequency components are relatively small when compared to the rest of the current spectrum: the largest components occur at multiples of the supply frequency and are caused by saturation, winding distribution and supply voltage. However an evaluation of the amplitude of these largest components in the different cases (healthy and two types of faulty conditions) is not shown. The characteristic fault frequency components decrease in full load condition because of the damping produced by the mechanical load. In [10] two inner race faults (spalls and drilled hole) are analysed and the authors point out a problem related to the experimental simulation of the bearing faults: the act of disassembling, remounting and realigning the test motor can significantly alter the vibration and current spectra. The results show that for both defects the characteristic fault frequency components are clearly visible only in the vibration spectrum and not in the current spectrum. In [11] the difference between “single point defect” and “generalized roughness” is pointed out: the vibration and current spectra are analyzed for ten bearings with microscopic pitting on all surfaces and microscopic scratches on the rolling elements and cage, caused by shaft current injected through the bearings. This research suggests that this type of fault produces unpredictable and broadband changes in the vibration and current spectra.

In [12] different sizes of holes drilled in the outer race of different types of bearings are considered and sophisticated methods to process and analyse the vibration and current spectra are used (notch filter, time-frequency analysis, Mahalanobis distance). In [13] a new formulation for the current spectral analysis is proposed for the detection of bearing failures in induction motors driven by frequency power converters. The fault analysed is on the outer race and the authors show an increase in the characteristic fault frequency components of the current spectrum; however the scale of the amplitude of the current spectra is different in the two cases. In [14] both an inner and an outer raceway defect are analyzed: they show some differences in the amplitudes of the current spectrum of an induction motor at full load, but the characteristic fault frequency components do not stand out so

clearly among the other ones. In [15] a Wiener filter is used to extract the mechanical information contained in the electrical current to detect a ball defect, but the authors do not show any comparison between healthy and faulty current spectra. In [16] the Fast Fourier Transform and the Continuous Wavelet Transform are applied to the current of a permanent magnet synchronous machine in order to detect bearing faults.

IV. THE ANALYZED BEARING FAULTS

The experimental set-up consists of a three-phases induction motor fed by the mains and coupled with a brake [17] [18]. This type of motor is normally used in pumping systems for domestic appliances and its main data are reported in TABLE 1. The value of the load torque can be imposed and measured by means of the control unit and the visual display unit of the brake. A powermeter collects, for each phase of the motor, the rms value of current and voltage and the measurement of the active power. The current of one phase of the motor is gathered by means of a current probe, in order to analyse it in the frequency domain, with the probe connected by means of a BNC interface to a personal computer with an acquisition card and LabVIEW software. The motor is equipped with two rolling ball bearings, type NSK 6205Z, with nine balls, lubricated with grease (Fig. 2).

TABLE 1 INDUCTION MOTOR DATA

Rated power 2.2 kW Number of pole pairs 2 Supply voltage 400 V Supply frequency 50 Hz Number of stator slots 24 Number of rotor bars 18 Air-gap length 0.5 mm

Fig. 2. NSK 6205Z ball bearings.

The aim of this work is to test different types of bearing faults, a few of which are similar to those analyzed by other authors while other are different. This will allow to investigate the effectiveness of the use of the stator current analysis for their detection. As a first case, a crack in the outer race has been considered as an example of a very serious fault; this is a single-point defect and can be caused by wear from fatigue (Fig. 3). This condition is close to the complete breaking of the bearing, so, if a diagnostic tool does fail in detecting it, probably it will not able to detect any cyclic fault in the outer ring. Second, in order to have a comparison with the results presented in the literature, a fault already investigated by other researchers, i.e. a hole in the outer race [8] [12], has been reproduced. In fact, a “perfect” fault like that shown in Fig. 4 cannot occur during the working of a bearing, but a similar


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defect can be caused by circulating currents, so this hole does in fact represent a magnification of defects that can really happen in the bearings.

Fig. 3. Fault #1, crack in the outer ring. Fig. 4. Fault #2, hole in the outer ring.

As a third case, a fault not yet considered in previous researches has been investigated, i.e. a deformation of the protective shield (Fig. 5). This fault can be produced by errors during the assembly and can be considered as a cycling fault, even if it does not produce effect like air-gap eccentricity. So, the expectancy is that it will not arouse particular changes in the current spectrum. Finally, we have produced a corrosion of the bearing, which can be caused by humidity of the environment and can be considered as a generalized roughness (non-cyclic fault, Fig. 6).

Fig. 5. Fault #3, deformation of the protective shield.

Fig. 6. Fault #4, corrosion.

V. THE EXPERIMENTAL RESULTS

A. Healthy bearings The method employed to test the effectiveness of the stator

current analysis for the bearing fault detection lies on the experimental comparison between the spectra of the current in healthy and faulty case and in different load conditions (no-load, about half-load, full-load). So, some experimental tests have been carried out in a first phase of the procedure with both healthy bearings, while similar tests have been then realised on the same motor with one damaged bearing, according to the four cases analyzed in the previous paragraph. For each type of test, three acquisitions are gathered: the current is measured with a sample rate of 4000 samples per second, each acquisition lasts 2 s and the spectrum is calculated as arithmetical mean of the three acquisitions. An example of result of this data processing is reported in Fig. 7,

where all the odd harmonic components are well-defined in the no-load current even in case of healthy bearings. At full load, a the current spectrum presents a more jagged aspect (Fig. 8).

Fig. 7. Experimental current spectra, healthy bearing, no-load.

Fig. 8. Experimental current spectra, healthy bearing, full-load.

B. Faults #1 and #2 on the outer ring Since the first two considered faults affect the outer ring of

the bearing, the characteristic frequencies fo of the vibration and the correspondent predictable frequencies fp of the stator currents are calculated by means of the relationships (5) and (7). The rotational frequency of the rotor fr is collected by the control unit of the brake during the tests in different conditions of load and fault. These frequencies are reported in TABLE 2.

The tests with the fault #1 are characterized by a different acoustic noise compared to the healthy condition, which reveals that this defect affects the operating of the motor. In the current spectra at no-load (Fig. 9), there is a considerable increase in the 3rd and 7th harmonic components, whereas the fundamental one remains constant; the absolute value of this increase corresponds to about 10 mA, which is about the 80% of the value of the respective harmonics in healthy condition. Among the calculated predictable frequencies in TABLE 2, only one appears in the faulty condition (407.5 Hz). Another component is excited at about 1100 Hz. This behaviour is certainly related to the presence of the fault, which produces an asymmetry in the distribution of the air-gap of the motor and therefore influences the stator currents. There is also an increase in other odd harmonic components, but this variation is rather small.


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These results differ from those expected from the literature and can be justified by the presence of noticeable odd harmonic currents in healthy conditions, which makes difficult the identification of other harmonic components of slight amplitude. However, the variation in the current spectrum caused by the fault #1 permits the detection of the defect, even if in a different manner from the forecast: in fact it does not allow to isolate the part of the bearing which is damaged, but is sufficient to highlight the presence of a fault. This is a first step for the diagnostics of the motor.

TABLE 2 PREDICTABLE FREQUENCIES IN VIBRATION AND CURRENT FOR FAULT #1, #2.

No-load Half load (4 Nm) Full load (7.5 Nm) fr 49.7 48.9 47.7 fo 178.8 176.0 171.5 fp (k = 1) 228.8 226.0 221.5 fp (k = -1) 128.8 126.0 121.5 fp (k = 2) 407.5 402.0 393.1 fp (k = -2) 307.5 302.0 293.1 fp (k = 3) 586.3 578.0 564.6 fp (k = -3) 486.3 478.0 464.6 fp (k = 4) 765.1 754.1 736.2 fp (k = -4) 665.1 654.1 636.2

Fault #1Healthy motor

407.5 Hz

3rd

harmonic 7th

harmonic

~1100 Hz

Fig. 9. Experimental current spectra, healthy and #1 faulty bearing, no-load.

In the current spectra at load (Fig. 10, Fig. 11) the components at predictable frequencies (TABLE 2) are not visible, the 3rd harmonic remains practically constant, whereas almost all the other odd harmonics increase. This increase is a signal of the distortion of the current caused by an asymmetry of the air-gap distribution and therefore it can be related to the presence of a defect. The problem is to verify whether this effect can be produced also by other type of fault.

By summarizing the results obtained in the experimental tests related to the fault #1, we can say that: i) the current spectra in case of fault show changes compared to the healthy case; ii) these changes vary according to the load condition; iii) these changes are different from those expected from the literature.

The differences between the load and no-load tests are relating to the presence of the coupling joint, which can add further vibrations in the operating of the motor or damp the effect due to the bearing fault. So, for diagnostic purposes, the

results obtained at no-load are more reliable and can be considered as directly linked to the presence of the defect.


Fig. 10. Experimental current spectra, healthy and #1 faulty bearing, half load.


Fig. 11. Experimental current spectra, healthy and #1 faulty bearing, full load.

The fault #2 has then been considered not only to obtain experimental results comparable to other researches, but also to analyse another fault that, at least theoretically, produces effects similar to the fault #1. In the current spectra at no-load (Fig. 12, Fig. 13), any predictable frequency fp seems to be excited by the faulty condition. Similarly to the previous fault, there is an increase in the 3rd and the 7th harmonics and a component excited at about 1100 Hz. So, the experimental results seem to confirm the theoretical considerations previously reported and to validate the possibility to employ the signature current analysis to detect cyclic fault in the outer ring of the bearing. In the current spectra at full-load with fault #2 (Fig. 14, compare with Fig. 11), the first odd harmonics remain almost constant (there is only a small increase in the 7th harmonic), whereas the amplitude of the odd harmonics at high frequency increases noticeably. So, even for this fault, the synthesis of the experimental tests is that some changes in the current spectra compared to the healthy case do in effect arise, but they differ from the results reported in the literature and vary according to the load condition.


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3rd

harmonic 7th

harmonic

~1100 Hz


Fig. 12. Experimental current spectra, healthy and #2 faulty condition, no-load.


fp =128.8 Hz

fp =307.5 Hz

fp =407 .5Hz

fp =228.8 Hz

Fig. 13. Experimental current spectra, healthy and #2 fault, no-load (close-up).


Fig. 14. Experimental current spectra, healthy and #2 faulty bearing, full load.

C. Faults #3 on the protective shield The fault #3 consists in a deformation of the lateral

protection of the bearing: its effect could be considered as cyclic, even if it affects the behaviour of the motor in a different way with respect to the previous faults.

The indentation in the protective shield does not get in touch with the rolling elements, because the internal cage is interposed between them: the defect arouses only a rubbing which increases the dynamical friction in the bearing, but does not cause any variation of the air-gap.

The experimental current spectra at no-load show a little increase in the amplitude of some harmonics and a component excited at about 1100 Hz (Fig. 15). The tests in load conditions do not provide any diagnostic information: the spectra are practically superimposed and no one component appears far from the profile of the healthy motor (Fig. 16).


~1100 Hz



Fig. 16. Experimental current spectra, healthy and #3 faulty bearing, full-load.

D. Faults #4: corrosion This fault has been realized by maintaining the bearing

plunged in water for three weeks. At the end of the treatment, the first signs of the corrosion of the inner and outer rings appear. The products of the corrosion would interfere with the balls during the rotation of the shaft, so causing vibrations.

As far non-cyclic faults, they are more difficult to detect with respect to the faults previously analysed. In fact, a localized defect on one ring arouses impulsive excitation at every contact with the rolling element. In this situation, the debris due to the corrosion process move themselves inside the


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bearings, shifted by the passage of the balls; the consequent vibrations are directly linked to the positions of the debris and therefore they unlikely repeat at the same frequency. So, it is impossible to analytically define some predictable frequencies to detect in the vibration or in the current spectra.

In the experimental current spectra (Fig. 17, Fig. 18) there is only a slight increase of some odd harmonics at high frequencies, more pronounced at full-load. The conclusion could be that the fault #4 is not so serious to harm the operating of the motor or that non-cyclic faults are not suitable to be detected by the current signature analysis




Fig. 18. Experimental current spectra, healthy and #4 faulty bearing, full-load.

VI. CONCLUSIONS

The cyclic faults in the outer ring cause evident effects in the current spectrum, even if these effects are different from what is commonly reported the literature: i) at no-load, there are small changes at the expected frequencies, a component excited at about 1100 Hz and the considerable increase of the 3rd and 7th harmonics; ii) at load, there are no changes at the expected frequencies and the increase of odd harmonics at high frequency. The deformation of the protective shield, a cyclic fault that does not produce any eccentricity of the rotor, shows

a little increase of some odd harmonics and a component excited at about 1100 Hz, but only at no-load. A non-cyclic fault like the corrosion arouses only a slight increase of some odd harmonics at high frequencies, more pronounced at full-load. So, for the cyclic fault in the outer ring the stator current analysis seems to be effective for the condition monitoring of the bearings. In the other cases, this method needs to be supported by other signals analysis. Future work could be consider more sophisticated techniques for the current processing, the measurement of vibration to find predictable frequencies for the new faults here analyzed and the extension of the results obtained for the faults related to the outer ring to all the cyclic faults, especially to those related to the inner ring.

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[8] R.R. Schoen, T.G. Habetler, F. Kamran, R.G. Bartheld, “Motor bearing damage detection using stator current monitoring”, IEEE Trans. Industry Applications, 31(6), pp. 1274-1279, 1995.

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[17] A. Fazzi, “Utilizzo della corrente di statore di una macchina a induzione per monitoraggio e diagnostica”, M.Sc. thesis, Pavia, Italy, 2007.

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use of the stator current for condition monitoring of bearings in induction motors

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