Some studies on condition monitoring techniques for ...

Ananda Shankar Hati et al. / International Journal of Engineering Science and Technology (IJEST)

Some studies on condition monitoring techniques for on line condition monitoring and fault diagnosis of mine winder motor. Ananda Shankar Hati Junior Research Fellow, Electrical Engineering Department, Indian School of Mines, Dhanbad-826004., India E-mail: [email protected]

Tarun Kumar. Chatterjee Professor, Electrical Engineering Department, Indian School of Mines, Dhanbad-826004, India E-mail: [email protected]  Abstract Survey of existing literature reveals that no serious attempt has been made so far to monitor the health of mine winder motors. The electrical motors are the critical equipment of the mine winders which require constant condition monitoring for planning the right time for their maintenance and thus ensure maximum machine availability. In this research work an online condition monitoring instrumentation system has been developed based on axial flux, current and vibration monitoring technique for mine winder motor. The online condition monitoring instrumentation system is noninvasive in nature and can be connected with mine winder motors which are in operation. The developed instrumentation system would be able to diagnose the health of mine winder motor and the motor fault of incipient nature can be pinpointed by the trend analysis of the frequency spectrum of time varying signal of axial flux, motor current and vibration. Keywords: axial flux, current and vibration monitoring I.

INTRODUCTION

The output of an underground mine and the prospects of its future development largely depends on the shaft handling system and the efficiency of electrical winder. The AC winder motors are slip ring induction motor with liquid controller. These motors are to be maintained properly for their optimum availability and efficiency of operation. A lack of a coherent maintenance strategy can result in loss of motor components/motor, a reduction in operational safety and heavy burden of capitalized losses. Continuous assessment of the winder motor throughout its useful operating life will reduce the down time of the mine winder and increase the efficiency of the winder. The paper will present a novel non-invasive fault diagnosis and condition monitoring instrumentation system for A.C winder motor using axial leakage flux monitoring and current monitoring of mine winder motor. The techniques are expected to provide better maintenance of A.C winder and ensure their efficient and reliable operation. The proposed instrumentation system can monitor and diagnose the various faulty condition of the winder motor like Stator winding interturn short circuit, Wound rotor short circuit, Loss of phase, Unbalanced supply system, Eccentric running/bearing failure. II.

MINE WINDER MOTOR CONDITION MONITORING STRATEGY

Rotor movement in an induction motor is the result of electromagnetic interaction between the airgap flux produced by the 3-phase stator winding and the induced rotor currents. Radial magnetic forces are produced between the rotor and stator surfaces and are proportional to the flux density squared. These forces result in stator core and winding vibration. As faults associated with rotor and stator windings and airgap variations alter the normal airgap flux waveform, quantities which are functions of the airgap flux will also change. This means that stator core vibration, line current and stray flux signals can be used to monitor the condition of the motor. The philosophy monitoring strategy is that if several interrelated signals all indicate a particular fault, the operator is more likely to believe the information in comparison to information obtained from one signal source. Previous research by Thomson et al. [1, 2] has shown that malfunctions such as broken rotor bars/high resistance joints, single-phasing, supply imbalance, and short-circuited coils in the stator winding can be identified by an examination of the signals mentioned above. However, vibration and current were selected as

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the most suitable parameters because of the ease with which accelerometers and current transformers can be installed in an industrial environment. Airgap eccentricity and unbalanced magnetic pull (UMP) have been researched since the beginning of 20th century; hence there is an abundance of published literature on the subject. Many of the classical papers have concentrated on the calculation of the airgap field as a function of eccentricity [3] and others have identified the principal factors causing UMP, [4]. The design of rotor assemblies, critical speeds, slot combinations, windings, vibration problems and the calculation of vibratory forces and acoustic noise have all been researched in relation to airgap eccentricity and UMP [5-13]. However, the research has not been directed to the development of online diagnostics, but it could be argued that parameters were identified which were certainly functions of airgap eccentricity and UMP and that this should suffice for the development of an online monitor. Rai [14] verified that vibratory forces at frequencies of 50 Hz, 100 Hz, and 200 Hz can vary due to eccentricity/UMP; however, these components can also change due to other malfunctions such as changes in dynamic imbalance [15]. Leonard and Thomson [16] have verified that the 100 Hz, 200 Hz and 300 Hz components in the stator frame vibration are functions of interturn winding faults, single-phasing and voltage supply imbalance. Consequently, fault discrimination is not possible by monitoring these components in isolation. Ellison and Yang [13] verified from tests carried out in ananechoic chamber that slot harmonics in the acoustic noise spectra from a small-power induction motor were functions of static eccentricity. However, the application of noise measurements in an industry to detect eccentricity is not practical due to the number of motors operating in close proximity and too high background noise levels. Verma and Natarajan [17] have studied the changes in the airgap field as a function of static eccentricity using search coils in the stator core, but the installation of airgap search coils is neither practical nor economic for monitoring the condition of auxiliary drive motors which are already in service in power stations. Binns and Barnard [18] monitored the airgap flux and core vibration together and concluded that the use of two signals provided useful information for machine analysis, but they did not present results for controlled variations in static or dynamic eccentricity. Penman et al. [19, 20] have clearly verified that axial flux monitoring can identify stator winding faults and have also shown that changes in the axial flux spectrum occur due to eccentricity [20]. However, from the literature search carried out concluded that a choice of noninvasive monitoring techniques for detecting airgap eccentricity in industrial installations was necessary. Consideration of methods suitable for the detection of such eccentricity variations led to the selection of motor current and stator frame vibration as the appropriate signals for processing and investigation. These quantities have the advantage that they can be easily obtained by noninvasive measurement techniques. The application of these techniques for fault diagnosis of mine winder motor is an essential part of the research work. It is apparent that the perfect electrical machine should produce no net axial flux because the currents flowing in the end windings of both the rotor and stator circuits should be perfectly balanced, under fault-free conditions. This, of course, is never the case, for there are always small asymmetries in both the material and geometric aspects of the magnetic and electric circuits of any machine. This results in every machine producing a small, but measureable net axial leakage field. The proposed method of monitoring depends on the fact that fault conditions represent a gross change in the local electric or magnetic circuit behavior and may therefore be identified by the effect such changes produce in the axial leakage field If a coil is wound around the shaft of an electrical machine, it will have an induced voltage which can be simply related to the axially directed fluxes, by Faraday's Law. These fluxes, which are present in all electrical machines, arise because of asymmetries in the electric and magnetic circuits of the machine due to tolerances in the manufacturing process. This axial leakage flux can be sensed not only by a coil wound around the shaft of the machine but by any simple symmetrical arrangement of coils at the machine end, or by other sensing devices such as Hall probes. A single coil or probe outside the machine casing can also be used, although at the expense of a reduced signal level. The method can be used to identify a wide range of fault conditions. To date, as discussed in References 21-23, it has proved possible to identify, and discriminate between, conditions such as: (i) Broken rotor bars (ii) Stator winding interturn short circuits (iii) Wound rotor short circuits (iv) Loss of phase (v) Negative phase sequence in supply lines (vi) Eccentric running.

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III.

AXIAL FLUX HARMONICS.

The fluxes are a result of the winding currents and so their spectral content may be related directly to the harmonic components present in the winding currents. These currents are, in turn, induced by the airgap flux, so it is clear that the axial flux signal produced by a given winding will contain the harmonic components induced in that winding by all of the airgap fluxes. While the rotor is at rest, its reference frame is related to that of the stator by a simple angular displacement, and so the airgap spectrum seen by stator and rotor windings will be identical. Once the rotor moves, however, its spatial reference frame rotate at (1 — s)/p rad/s relative to the stator, so the airgap flux components will be frequency shifted. Here  is the angular frequency of the supply, p is the number of pole pairs in the machine, and s is the fractional slip. The harmonic components of flux density B due to a balanced 3-phase stator winding, are given by the well known expression cos cos 5 cos 7 cos 11 1 where is the angle between the stator datum and an arbitrary point on the rotor. These components are obviously at supply frequency only. To derive the harmonic currents induced in the rotor winding, it is necessary to transform these components into the rotor reference frame. If we let be the angular separation between an arbitrary point on the rotor, with respect to the rotor datum, and the rotor datum is separated from the stator datum by an angle , then  2 , whence  3 where r is the angular velocity of the rotor. Using eqns. 1 and 3, and the usual definition of slip it is easy to show that the time series of the harmonics of flux density in the rotor frame of reference is given by, cos cos 6 5 5 cos 7 6 7 4 The frequencies of the currents induced by these harmonic fields are obviously slip dependent. Now, as stated previously, the perfectly manufactured machine would produce no axial flux. In practice, minor rotor and stator asymmetries always exist, and as a result these harmonics will be present in the flux leakage from the rotor windings. In addition, the existence of such asymmetrics will result in yet more airgap harmonics, aside from those given in eqn. 1. It is these harmonics which one relate to machine abnormalities, and are therefore of the greatest importance in fault condition. Under normal operation, such components are small in magnitude, but any significant change may be indicative of a fault condition. IV.

CURRENT AND VIBRATION HARMONICS FOR AIRGAP ECCENTRICITY OF MOTOR.

Airgap eccentricity can occur in the form of static or dynamic eccentricity. In the case of static eccentricity, the position of minimum radial airgap length is fixed in space. For example, static eccentricity can be caused by stator core ovality or incorrect positioning of the rotor or stator at the commissioning stage. Provided that the rotor-shaft assembly is sufficiently stiff, then the level of static eccentricity should not change. Dynamic eccentricity occurs when the centre of the rotor is not at the centre of rotation and the minimum airgap revolves with the rotor. This means that dynamic eccentricity is a function of space and time. Dynamic eccentricity could be caused by a bent shaft, mechanical resonances at critical speeds, or bearing wear and movement. It is also possible that high levels of static eccentricity can produce unacceptable levels of UMP which can result in shaft flexing and dynamic eccentricity thus increasing the risk of a rub between the rotor and stator. A. CURRENT HARMONICS FOR AIRGAP ECCENTRICITY OF MOTOR The analysis is based on the rotating wave approach whereby the magnetic flux waves in the airgap are taken as the product of permeance and magnetomotive force (MMF) waves [24]. This means that the airgap field is complex and comprises the following components: (a) fundamental (b) stator and rotor MMF harmonics (c) stator and rotor slot permeance harmonics (d) airgap eccentricity permeance harmonics (e) permeance harmonics due to saturation. In the following analysis, the specific permeance  is termed as the permeance. The permeance of an airgap bounded by a slotted stator and a smooth rotor is given by ∞



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n cos n Sθ

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where n = any integer, st = stator, θ space variable, rad and S = number of stator slots The permeance of an airgap bounded by a slotted rotor and a smooth stator is given by ∞



n cos n R θ

,

t

6

where rt = rotor, t = time variable, s, = rotational speed, rad/s andR = number of rotor slots, The resultant of these two permeances can be expressed as the product of a constant and the values of the two permeance [25]. The permeance of a slotted rotor and stator can therefore be expressed by ∞

∞

Λ

Λn , n cos x

,

,

n R

n S

7

In the presence of static eccentricity, the radical airgap length is a function of space only. Assuming a smooth stator and rotor the permeance will be ∞

Λn cos n θ

Λ

8

where s, se = static The magnitude of the harmonic permeance waves due to static eccentricity are found by Fourier analysis to be μ 21 √ 1 ε′ 9 Λn ′ g ε′ √ 1 ε′ where n 1, 2, 3 … … … … … .., μ = permeability of free space, H/m, g' = effective mean airgap length in the presence of sloting, m and ε′ = effective relative eccentricity = e/g' Eqn. 5 shows that as the level of static eccentricity increases, the magnitude of Λn will also increase. The radial airgap length in the presence of dynamic eccentricity is a function of space and time. The permeance can thus be represented as ∞

Λn cos n

,

Λ

10

where d, de = dynamic Saturation can be represented by a permeance wave with twice the number of poles and twice the frequency of the fundamental wave [26] because the airgap becomes effectively larger in the regions of maximum flux density. Hence, the permeance of a smooth and concentric airgap combined with the effects of saturation is expressed as ∞

Λn cos n

,

Λ

2p

11

where sa = saturation and p = number of pole pairs Combining eqns. 7, 8, 10 and 11 in the way in which the permeance due to slotting were combined gives the total permeance as ∞

Λ

∞

∞

∞

∞

Λn , n , n , n , n

,

cos n R

n S

n

n

2n p θ

n R n ω 2n ω t 12 where ω = angular supply frequency, rad/s The magnetomotive force produced by the current flowing in the stator and rotor windings consists of a series of space and time harmonics. This can be represented by (neglecting phase angle and skew) ∞

∞

Fnθ , nω cos nθ pθ

θ, t ω

θ

∞ ∞

nω ω t

∞

,

cos

13

∞

where θs = stator space harmonic and s = stator time harmonic The flux density distribution in the airgap is given as the product of the permeance and the MMF. Combining eqns. 8 and 9 gives the resulting expression as: ,

,



cos

,

where 2

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2

, ,

,

,

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

14 2

, 2

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The flux density distribution varies in both space and time and the time component gives expressions for predicting the frequency content of the flux density waveform. These expressions are expressed as follows: 1 2 15 1

2

16

where = frequency of flux density or current slot harmonic. Hz As the harmonic fluxes are moving relative to the stator, they should induce corresponding current harmonics in the stationary stator winding. Hence it should be possible to detect airgap eccentricity by analyzing the stator current spectrum. B. VIBRATION HARMONICS FOR AIRGAP ECCENTRICITY OF MOTOR The radial force waves acting on the stator core structure are proportional to the square of the flux density waveform [27]. In terms of the force per unit area, the force wave distribution can be determined from: , 17 , 2 where radial force per unit area, N/m2. Substituting eqn. 10 into eqn. 13 gives: ,  cos

,



18

,

′ ′ ′ ′ ′ ′ ′ ′ where 2 ′ and  2 ′ The time component of this expression gives an equation for predicting the harmonic content of the vibration forcing function and is expressed as: 1 ′ ′ ′ 2 ′ 19

where = frequency of vibration slot harmonic, Hz These harmonic forces acting on the core will cause vibration of the same frequency to be transmitted to the surface of the stator core. Hence the surface vibration signal will contain frequency components characteristic of static and dynamic eccentricity. A preliminary study by Thomson et al. [28] has shown that one of the principal slot harmonics in the stator frame vibration changed as a function of static eccentricity. It was observed that the change in the monitored parameter was a function of the transducer position around the periphery of the frame. The initial results were encouraging and have led to a full investigation into the effectiveness of stator frame vibration monitoring for detecting static and dynamic eccentricity. V.

EXPERIMENTAL SETUP:

A special fault simulation test setup of model mine winder motor was designed to investigate the following malfunction: a) Interturn short circuit of stator winding, b) Wound rotor short circuit, c) Single phasing, d) Unbalanced supply system and e) Eccentric running/bearing faults. 10 kW, 3-phase 440V, 50Hz, 1440 r.p.m. slip-ring induction motor was used as a model winder motor. The model winder motor was designed to simulate various electrical faults which normally may occur in stator and rotor circuit of a winder motor. The model winder motor was loaded from no load to rated load with the help of a d.c generator which was coupled with model winder motor. DC generator was loaded with a resistive load box. A 16 channel PC based DAS was connected with the model winder motor to monitor continuously the stator currents of three phases and axial leakage flux, speed, temperature, supply voltage of three phases, stator frame vibration using the sensors like CT, PT , search coil, tacho generator and accelerometer. The time varying signal of motor stator current, vibration and axial leakage flux are captured by DAS and is transformed to frequency spectrum using FFT software.

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  Fig. 1.: Online condition monitoring instrumentation system for model mine winder motor.

VI.

TEST RESULTS

A. AXIAL FLUX HARMONICS ANALYSIS

Fig. 2: Healthy Motor at full load.

Fig. 3: Single phasing of the motor at full load.

Figure 2 illustrates the axial flux spectrum of healthy mine winder motor at full load condition. The FFT spectrum of other faulty conditions of the motor under test are shown in Figures [3, 4, 5, and 6]. In fig 3. 100 Hz component of the spectrum (second harmonic) shows a marked increase and this fact is well supported by the theory. It is a common knowledge that during single phasing condition of a 3-phase induction motor, the rotating magnetic field due to the presence of negative sequence component of stator induces approximately twice the supply frequency current in the rotor circuit and it gets reflected in the search coil output. The single phasing condition of the motor is shown in figure 3 when one supply phase may get disconnected either due to blown out fuse or loose contact at the contactor controlling the motor.

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Fig 4 Stator winding interturn short circuit of the motor at full load.

Fig 5 Wound rotor interturn short circuit at full load of the motor.

Figure 4 shows the condition when rotor winding of the slip ring induction motor was deliberately shorted to simulate the interturn short circuit condition. The FFT spectrum of figure 4 shows a typical signature which can be identified easily to pinpoint the nature of the fault. Figure 5 shows the fault condition where few turns of the stator winding of the induction motor were shorted under half full load condition. The figure shows a sharp rise of the fundamental component i.e. 50 Hz. Figure 6 shows the insulation failure condition of motor stator winding at full load. The fault condition was simulated by grounding the stator winding turn through a resistance. Fig 6 Insulation failure of the stator winding at full load.

B. STATOR CURRENT AND VIBRATION SLOT HARMONICS ANALYSIS The line current was monitored via a clip-on current transformer and the signal preprocessed using a high pass filter to reduce the magnitude of the dominant 50 Hz component. An accelerometer was used to sense the vibration around the periphery of the motor's outer frame which was an interference fit with the stator core assembly. I.

STATOR CURRENT SLOT HARMONICS

In general, eqns. 15 and 16 can be used to predict the frequency content of the current signal. Dynamic eccentricity component (de) as per eqn. 15. 1 2 where f1 = 50 Hz., p = 2 nrt = 1, ns = 1, nd = 1, nsa = 0, s = 0.02, R = 28. 711.5 . 21 Principal slot harmonic (psh) as per eqn. 15. 1 2 where f1 = 50 Hz., p = 2 nrt = 1, ns = 1, nd = 1, nsa = 0, s = 0.02, R = 28, nd = 0 736 . 22 Dynamic eccentricity component (de) as per eqn. 15. 1 2 where f1 = 50 Hz., p = 2 nrt = 1, ns = 1, nd = 1, nsa = 0, s = 0.02, R = 28. 760.5 . 23

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(a) Uniform airgap

(b) 60% static airgap eccentricity Fig. 7 Current spectrum

Fig 7 (a) shows the frequency spectrum of the motor current with uniform airgap. Frequency spectrum is obtained by transforming the time varying signal of the motor stator current using FFT software. It has been observed that the principal slot harmonics (i) is at 736 Hz. and the magnitude is 38.2 dB. The sidebands i.e. dynamic eccentricity components (ii) and (iii) are at 711.5 Hz. and 760 Hz. as per calculations shown in eqn. 21 and 23 and the magnitude of the side bands are 4.7 dB and -0.6 dB respectively. Fig 7 (b) shows the frequency spectrum of the stator frame vibration with 60% airgap eccentricity. It has been observed that the principal slot harmonics and the sidebands i.e. dynamic eccentricity components are at same frequency i.e. 736 Hz., 711.5 Hz. and 760 Hz. respectively. But the magnitude of the psh and the sidebands are more compared to the same harmonics of the motor with uniform airgap. The respective magnitude of psh (iv) is 51.1 dB and side bands (v) and (vi) are 26.6 dB and 34.1 dB respectively due to 60% airgap eccentricity. II.

VIBRATION CURRENT SLOT HARMONICS. In general, eqn. no. 19 can be used to predict the frequency content of the vibration signal. For dynamic eccentricity component (de) as per eqn. 19. 1 ′ ′ ′ 2 ′ = 2, ′ = 1, 861.5 . Principal slot harmonic (psh) is as per eqn. 19. 1 ′

where f1 = 50 Hz., p = 2,

1,

′

′

= 1, s = 0.02, R=28 24 2

′

′

= 2, ′ = 1, ′ = 1, s = 0.02, 886 . 25 Dynamic eccentricity component (de) as per eqn. 19. 1 ′ ′ ′ 2 ′

where f1 = 50 Hz., p = 2,

1,

′

where f1 = 50 Hz., p = 2,

1,

′

= 2, 910.5

′

= 1, .

′

′

0, R=28

= 1, s = 0.02, R=28 26

(b) 60% static airgap eccentricity (a) Uniform airgap Fig. 8 Stator frame vibration spectrum

Fig 8 (a) shows the frequency spectrum of the motor vibration with uniform airgap. Frequency spectrum is obtained by transforming the time varying signal of the stator frame vibration using FFT software. It

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has been observed that the principal slot harmonics (i) is at 886 Hz. and the magnitude is 88.8 dB. The side bands i.e. dynamic eccentricity components (ii) and (iii) are at 861.5 Hz. and 910 Hz. as per calculations shown in eqn. 24 and 26 and the magnitude of the sidebands are 63.2 dB and 71.2 dB respectively. Fig 8 (b) shows the frequency spectrum of the stator frame vibration with 60% airgap eccentricity. It has been observed that the principal slot harmonics and the sidebands i.e. dynamic eccentricity components are at same frequency i.e. 886 Hz., 861.5 Hz. and 910 Hz. respectively. But the magnitude of the psh and the sidebands are more compared to the same harmonics of the motor with uniform airgap. The respective magnitude of psh (iv) is 112.3 dB and side bands (v) and (vi) are 89 dB and 87.1 dB respectively due to 60% airgap eccentricity. VII.

CONCLUSION

It has been observed that any fault in stator winding of the mine winder motor can be very well detected by axial flux monitoring technique. Bearing failure/eccentric running of the motor can be detected by current and vibration monitoring techniques. The magnitude of psh and sidebands of motor current and vibration frequency spectrum is recorded continuously. Increase in magnitude of psh and sidebands indicate off centre running of the motor. 16 channel data acquisition system along with transducers signal conditioner and FFT software would be a very useful instrumentation system for on line condition monitoring and fault diagnosis of mine winder motor. REFERENCES. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

THOMSON, W.T., LEONARD, R.A., DEANS, N.D., and MILNE, A.J.: 'Condition monitoring of induction motors for availability assessment in offshore installations'. Proceedings of 4th Euredata Conference, Venice, Italy, 1983. THOMSON, W.T., LEONARD, R.A., DEANS, N.D., and MILNE, A.J.: 'Monitoring strategy for discriminating between different types of rotor defects in induction motors', 18th UPEC Proceedings, University of Surrey, Guildford, Surrey, UK, 1983, pp. 241-246. SWANN, S.A.: 'Effect of rotor eccentricity on the magnetic field in airgap of a nonsalient pole machine', Proc. IEE, 1963, 110, (5), pp. 903-915. BINNS, K.J., and DYE, M.: 'Identification of principal factors causing unbalanced magnetic pull in cage induction motors', Proc. IEE, 1973,120, (3), pp. 349-354. CRAWFORD, W.G.: 'Unbalanced magnetic pull and the mechanical stability of rotating electrical machines', Eng., 1951, 171, pp. 504-505. BRADFORD, M.: 'Unbalanced magnetic pull in a 6-pole induction motor', Proc. IEE, 1968, 115, (11), pp. 1619-1627. SUMMERS, E.W.: 'Vibration in 2-pole induction motors related to slip frequency', Trans. Amer. Inst. Electr. Eng., 1955, pp. 69-72. KOVACS, K.P.: 'Two-pole induction motor vibrations caused by homopolar alternating fluxes', IEEE Trans., 1977, PAS-96, (4), pp. 1105-1108. FREISE, W., and JORDAN, H.: 'Unilateral magnetic pull in 3-phase machines'. CEGB CE-Trans 7836, 1983 from ETZ-A, 1962, 83, (9), pp. 299-303. JORDAN, H., RODER, G., and WEIS, H.: 'Under what circumstances may mechanical vibrations of the stator core be expected at supply frequency in 4-pole, 3-phase asynchronous machines'. ERA Translation IB 2578 from Elektrie, 1967, 21, (3), pp. 91-95. BELMANS, R., GEYSEN, W., JORDAN, H., and VANDENPUT, A.: 'Unbalanced magnetic pull in three-phase two-pole induction motors with eccentric rotor'. IEE Conf. Publ. 213,1982, pp. 65-69. WRIGHT, M.T., GOULD, D.S.M, and MIDDLEMISS, J.J.: 'The influence of unbalanced magnetic pull on the critical speed of flexible shaft induction machines', ibid., 1982, pp. 61-64. ELLISON, A.J., and YANG, S.J.: 'Effects of rotor eccentricity on acoustic noise from induction machines', Proc. IEE, 1971, 118, (1), pp.174-184. RAI, G.B.: 'Airgap eccentricity in induction motors'. ERA Report 74-1188, ERA Technology Ltd., Leatherhead, Surrey, UK, 1974. ERSKINE, J.B.: 'A users view of noise and vibration aspects of AC induction motors'. IEE Colloquium Digest 1978, pp. 52-64. LEONARD, R.A., and THOMSON, W.T.: 'Vibration and stray flux monitoring for unbalanced supply and interturn winding fault diagnosis in induction motors'. Proceedings of 1st UK International Conference on Condition monitoring, University College of Swansea, Swansea, UK, April 1984, pp. 340-354. VERM A, S.P., and NATARAJAN, R.: 'Effects of eccentricity in induction motors'. Proceedings of International Conference on Electrical machines, 3, Budapest, Hungary, Sept. 1982, pp. 930-933. BINNS, K.J., and BARNARD, W.T.: 'Some aspects of the use of flux and vibration spectra in electrical machines'. Proceedings of Conference on Applications on time-series analysis, University of Southampton, Southampton, UK, 1977, pp. 71.1-71.12. PENMAN, J., HAD WICK, J.G., and BARKER, B.: 'Detection of faults in electrical machines by examination of the axially directed fluxes'. Proceedings of 3rd International Conference on Electrical machines, Brussels, Belgium, 1978, pp. R/5-1-R/5-1O. PENMAN, J., HADWICK, J.G., and STRONACH, A.F.: 'Protection strategy against the occurrence of faults in electrical machines'. IEE Conf. Publ 185, 1980, pp. 54-58. PENMAN, J., HADWICK, J.G., STRONACH, A.F.: ‘Protection strategy against the occurrence of fault in electrical machines’. IEE. Conf. Publ. 185, 1980, pp.54-58. PENMAN, J.,DEY, M.N., and SMITH, J.R.: ‘A new approach to the protection of industrial of industrial drives’ Trans. IEEE-IAS Winter Meeting, San Fransisco, CA, USA, 1981, pp. 1266-1270. PENMAN, J., and DEY, M.N.: ‘A multifunctional machine monitoring system’. UPEC 19, Universities of Dundee and Aberdeen, Dundee, UK, 1984, Paper 14.4. YANG, S.J.: 'Low noise electric motors' (IEE Monographs in Electrical and Electronic Engineering, Oxford Science Publications, 1981). HELLER, B., and JOKL, A.L.: ‘Tangential force in squirrel cage induction motors', IEEE, Trans, 1969, PAS-88, (4), pp. 484-492. HELLER, B., and HAM AT A, V.: 'Harmonic field effects in induction machines' (Elsevier Scientific Publishing Company, 1977). CARTER, G.W.: 'The electromagnetic field in its engineering aspects' (Longmans, 1967). THOMSON, W.T, LEONARD, R.A., MILNE, A.J., and PENMAN, J.: 'Failure identification of offshore induction motor systems using online condition monitoring'. Proceedings of 4th National Reliability Conference, Birmingham, UK, 1983, pp. 2C/3/1-2C/3/11.

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