Diagnosis of Static Eccentricity Fault in Line Start ... - IEEE Xplore

2014 IEEE International Conference Power & Energy (PECON)

Diagnosis of Static Eccentricity Fault in Line Start Permanent Magnet Synchronous Motor Mahdi Karami1,3, Norman Mariun1,3, Mohammad Rezazadeh Mehrjou1,3, Mohd Zainal Abidin Ab Kadir2,3, Norhisam Misron3, Mohd Amran Mohd Radzi1,3 1

2

Centre for Advanced Power and Energy Research (CAPER), Faculty of Engineering, Universiti Putra Malaysia Centre for Electromagnetic and Lightning Protection Research (CELP), Faculty of Engineering, Universiti Putra Malaysia 3 Department of Electrical and Electronic Engineering, Faculty of Engineering, Universiti Putra Malaysia Serdang, Malaysia in an electrical motor which can be categorized in three types such as static, dynamic and mixed eccentricity [7]. Due to the importance of eccentricity fault because of its disastrous second failures, several research studies have been performed or ongoing for diagnosis of this fault in various types of electrical motors.

Abstract—In this paper, finite element method is employed for diagnosis of static eccentricity in line start permanent magnet synchronous motor. The motor is modeled with different degrees of eccentricity. Stator current spectrum of healthy and faulty motor are analyzed using power spectral density technique. Amplitudes of harmonic components around fundamental frequency in stator current spectrum are proposed for static eccentricity detection in this type of motor.

A method for diagnosis of static eccentricity in switched reluctance motor (SRM) has been proposed by [8]. The mutually induced voltages in idle phases have been investigated to determine the signature of this fault in SRM. The 2-D finite element method (FEM) has been employed to simulate the SRM with static eccentricity and calculate the mutual fluxes and mutually induced voltages. The effect of this fault on the voltage induced in a specific phase of SRM has been introduced as a proper signature for eccentricity detection.

Keywords—LSPMSM; static eccentricity; stator current; FEM; fault detection.

I.

INTRODUCTION

Despite the induction motors (IMs) are cost effective; they endure low power factor and poor efficiency which violate the new NEMA and IEEE standards for electric motors [1, 2]. Albeit, the permanent magnet synchronous motors (PMSMs) furnish high efficiency with well power factor grade; yet, they need driver to provide the starting capability and operation which is non-economic for single speed applications [2, 3].The line start permanent magnet synchronous motors (LSPMSMs) have been developed with a hybrid rotor to provide the starting torque in addition to high efficiency range which is cost effective since there is no need to feed by inverter for single speed operation condition. Accordingly, the LSPMSM can be suggested as a high potential alternative to IMs. The major advantages of LSPMSM are high efficiency, high power factor, torque production and power density capability [2, 3] which lead to grow the application of this motor in different aspects, recently.

The influence of static eccentricity on the interior permanent magnet (IPM) and surface-mounted permanent magnet (SPM) synchronous motors have been scrutinized and compared in [9]. The simulation and modeling have been performed using FEM to compute the magnetic flux density, electromotive force (EMF), cogging torque and average torque of both motors. It was concluded that the static eccentricity influenced on the magnetic flux density and EMF of IPM is more than SPM. The higher degrees of fault increased the magnitude of cogging torque in SPM while reduced in IPM. The average torque magnitude of both motors increased with progress of fault. Hence, the eccentricity fault shows different behavior based on the type of motor. The objective of this paper is to investigate the effect of static eccentricity fault in LSPMSM, since there is no research work that has been carried out in this case. Accordingly, a three-phase LSPMSM is modeled using FEM at healthy condition and under static eccentricity fault. The simulated steady-state current signals are utilized for spectrum analysis to determine the fault related features in this motor. Then, the current spectrum is analyzed in frequency domain by means of power spectral density (PSD) technique.

However, different stresses during the motor operation lead to stator or rotor failure [4] which must be detected at an early stage to prevent permanent failure and unscheduled downtime. The scientific literatures have reported that the possibility of mechanical faults occurrence is widespread among the other faults in electrical motors and it contains 60% of the faults [5]. Moreover, the statistics clarify that almost 80% of these mechanical faults are due to air-gap eccentricity in the motors. This fact leads to high motivation in eccentricity fault , stator detection, recently [6]. The rotor symmetry axis symmetry axis and rotor rotation axis are located at the center of stator in a healthy motor. Any displacement of one or all of these axes from each other is known as eccentricity fault

II.

The static eccentricity fault happens when stator symmetry axis is non-concentric with rotor symmetry axis while the rotor rotates at its own center. Consequently, a non-uniform

This research is financially supported by Ministry of Education Malaysia.

978-1-4799-7297-5/14/$31.00 ©2014 IEEE

STATIC ECCENTRICITY

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air-gap distributes between stator and rotor where w the position of minimum (and maximum) air-gap versus stator s is motionless and time-independent [7]. The geometrical position of stator and rotor in a motor with static eccentriciity is indicated at Fig. 1. The static eccentricity degree cann be computed as follows [10].

TABLE E I. SPECIFICATION OF THE THREE-PHASE LSPMSM Rated output power (HP) Rated voltage (V) Rated frequency (Hz) Number of Poles Rated speed (RPM) Air-gap length (mm) Number of stator slots Number of rotor slots Effective length of stator coree (mm) Stator outer diameter (mm) Height of stator yoke (mm) Height of stator slot (mm) Height of rotor slot (mm) Number of turns per slot Magnet material Remanent flux density of maggnets (T)

1 is rotor rotation axis and is the static transfer where vector and is the uniform air-gap length. III.

MSM USING FEM MODELING OF THREE-PHASE LSPM

The fault diagnosis method can be reliabble if the practical condition of electrical motor taken innto account for performance analysis during healthy and faulty operation. Finite element based analysis provides a preecise technique for modeling of electrical motors since it includes material characteristic, nonlinearity and complexitiess. Meanwhile, the accuracy of FEM for analyzing the motoor performance is higher than other method such as winding funnction theory [11].

1 415 50 4 1500 0.30 24 16 72 120 45 13 8 139 N38SH 1.235

The heart of fault diagnosis in this paperr is based on FEM which is used to calculate the stator curreent spectrum with eccentricity effect. Hence, the three-phaase LSPMSM is simulated by means of Maxwell 2-D D software. The specification of the modeled LSPMSM haas been shown in Table I. The transient solver with time inttegration approach using backward Euler is utilized to computee the quantities of LSPMSM. Magnetic field distribution insside the motor is calculated by FEM and then, stator current waveform, w induced voltage, windings inductances, torque and sppeed are computed [12]. The 2-D configuration of proposed mootor is displayed in Fig. 2. The non-uniform air-gap magnetic fieldd due to the static eccentricity in motor results in asymmetriccal current, torque and speed. Therefore, several harmonics coomponents will be manifested in magnetic flux, stator current and torque of the motor. Detection of these harmonics frequencies in the stator current leads to nominate a precise fault signaature in LSPMSM. The current spectrum is highly recommendeed signal to be monitored for fault diagnosis, noninvasively; owing to its

Fig. 2. The cross-sectioon of simulated LSPMSM

accessibility and low cost eqquipment [13]. The calculation strategy of air-gap magnetic field f considering the stator and rotor magnetomotive force (MMF) ( in addition to air-gap permeances which present the magnetic flux density distribution in the air-gap is shoown in Fig. 3 [14].

Fig. 3. Air-gap magnnetic field computation Fig. 1. Geometrical spot of stator and rotor in electriical motor with static eccentricity fault

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IV.

SIGNATURE ANALYSIS OF CURREN NT SPECTRUM

The stator current signal of LSPMSM is calculated c by FEM for steady-state operation condition. The current spectrum generated with sampling frequency of 5 kHz for duration of 6.5 seconds which processed using PSD anaalysis for feature extraction. The magnetic flux density diistribution in the proposed LSPMSM is indicated in Fig. 4. Time variation of different quantities of motor at normal operaation is represented in Fig. 5. In order to estimate the static eccentricityy signature at early stages the lower degrees of fault is considered. However, the eccentricity degree up to 10% is negligible inn electrical motors due to inherent eccentricity [15]. The 4-ppole LSPMSM is simulated with 16.6% and 33% static eccentrricity. Fig. 6 shows the frequency spectrum of the stator cuurrent in healthy condition. The PSD of eccentric motor undeer 16.6% and 33% are displayed in Fig. 7 and Fig. 8, respectivelyy.

(aa)

Comparison between PSD of stator current c signals in healthy and faulty motor reveals that 16.6% static eccentricity generates side band components with frequenncies 25 Hz ( ), 75 Hz ( ) and 125 Hz ( ). Amplittude of harmonic components at increases from -52.44 dB in healthy condition to -49.5 dB in eccentric motor, from -54.9 dB to -52.4 dB; and raises from -65.2 dB to -558.4 dB. Therefore, a small degree of static eccentricity manifessts its effect in the stator current which can be detected using thiis feature.

(bb)

The comparison between Fig. 6 and Fig. 8 indicates that the amplitude of harmonic components furtherr increases due to fault severity. The static eccentricity degree of 33% increases the amplitude of to -45.2 dB, to -499.1 dB and to -55.4 dB. Thus, the amplitudes of harmonnic components at , and are an accurate feature forr static eccentricity diagnosis in LSPMSM. By the way, the prroposed feature is capable of predicting the static eccentricity degree accurately. The amplitudes of harmonic components att frequencies , and are presented in Table II.

(C C)

Fig. 5. The time variation profiles off three-phase LSPMSM. (a) current; (b) torquue; (c) speed.

Fig. 6. Normalized PSD of stator currrent spectra of three-phase LSPMSM at healthhy condition

Fig. 4. Magnetic flux density distribution in 4-ppole LSPMSM

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ACKNOWLLEDGMENT The authors would like to express their gratitude to Ministry of Education Malaysiia for financial support through grant number FRGS-5524356 and Universiti Putra Malaysia for the facilities provided duringg this research work. REFER RENCES [1]

[2]

[3]

Fig. 7. Normalized PSD of stator current spectra of LSPMSM L with 16.6% static eccentricity

[4]

[5]

[6]

[7]

[8] Fig. 8. Normalized PSD of stator current spectra of LSPMSM L with 33% static eccentricity [9] TABLE II.

NTED IN FIG. 6-8 AMPLITUDES OF HARMONIC COMPONENTS PRESEN

Feature

Frequency (Hz)

0

16.66

33

25

-52.4

-49.55

-45.2

75

-54.9

-52.44

-49.1

125

-65.2

-58.44

-58.9

V.

[10]

Static Eccentricitty Degree (%)

[11]

[12]

CONCLUSION

In this paper a three-phase LSPMSM is simulated with different levels of static eccentricity using FEM. The stator current signal is calculated for noninvassive diagnosis of eccentricity in this machine. The feature is based on the amplitudes of harmonic components due to static eccentricity c that static in current spectrum. The PSD results clarify eccentricity related-harmonics are eviddent while their amplitudes increase for higher degrees of fault.

[13]

[14]

[15]

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