Static Eccentricity Fault Analysis in Inverter Fed Induction Motor using Finite Element Method

One of the common faults encountered in three phase induction motors (IM) is the eccentricity fault. In this work, a study on the electromagnetic characteristics of open-loop voltage source inverter fed IM is carried out for healthy and static eccentricity fault condition with the assistance of Finite Element Analysis. The importance of electromagnetic field analysis is it contains the data about the position of stator, rotor and mechanical parameters of IM. Thus the strategy of monitoring airgap magnetic fields and current are often used for the diagnosis of faults in IM. Detection of eccentricity fault at the incipient stage is difficult because the changes that the fault would introduce in the motor terminal quantities are negligible unless the severity is very high. A comparative study is carried out for healthy and eccentric fault motor with the help of ANSYS Maxwell Finite Element Analysis tool. Electromagnetic field parameters such as speed, current, flux distribution over the machine and in the air gap are analysed for healthy and faulty motor.


INTRODUCTION
Induction motors are generally utilized in the ventures for variable speed applications because of their ease of operation, high starting torque, and low maintenance cost and speed variation. Inverter fed induction motor is employed in the vicinity where there is a need for speed control. 80% of the mechanical faults occurring in the induction machines lead to eccentricity fault [1]. There are three sorts of eccentricity faults namely static, dynamic and mixed eccentricity fault. In static eccentricity the rotor hub is uprooted from the stator pivot; in any case the rotor pivots around its hub. The dynamic eccentricity, when the rotor pivot is dislodged from the stator hub, yet it turns around the stator hub. Blended issue happens when both the static and dynamic fault exist together in a machine.
The root cause of eccentricity faults are sudden increase or decrease in load when repetitively happens, manufacturing defects, stator core ovality and mechanical resonance. The eccentricity fault in electrical machine will lead to non-uniform air gap length. If the eccentricity fault is not diagnosed at the earlier stage, will have impact on the performance of the motor and finally damages the stator core, winding, and the rotor body which will result in huge financial loss. The mechanical fault occurs due to failure of mechanical parts in the motor which covers nearly 50-60% of overall faults in induction motor. The critical and severe among these is bearing and eccentricity fault. The analysis and detection of the air gap eccentricity is carried out with the help of stator line current spectrum in  [1]. The effects of eccentricity on stator and rotor core sheets get reflected in the windings as vibration. One of the popular methods to detect the eccentricity fault is Motor Current Signature Analysis (MCSA). In the practical scenario, the eccentricity fault gets transformed with the axial coordinate of the motor and leads to inclined eccentricity. The change in the axes are due to inappropriate position of the bearing, skewed rotor, bearing scraped area, skewed burden shafts, mechanical reverberation at basic speed and mechanical burden asymmetry which prompts the particular fault. The commonly occurring eccentricity fault has the axes stagnant in parallel with each other which also ensures that there is no much significant change in uniform air gap distribution. But if the parallelism is lost then it ends up in inclined eccentricity [2]. Even in completely perfect technical conditions, there might be a low percentage of eccentricity exists within tolerance level. The noteworthy change in static eccentricity fault is the direction of insignificant air hole length is timeinvariant just as the worth and heading of the lopsided attractive draw. Unusualness shortcoming is likewise brought about by an inappropriate area of the stator and rotor at the assembling stage or by stator ovality [3]. The nearness of shifting air hole prompts outspread quality of electromagnetic beginning known as the uneven attractive force. Some different reasons for eccentricity fault are bowed shaft, bearing wear, misalignment, and mechanical reverberation. Infrequently in reality, a wide range of erraticism shortcoming that is static, dynamic and blended unpredictability issue will, in general, happen together at once. Indeed, even in another motor additionally has the eccentricity shortcoming because of getting together and development strategies. Plan circumstances and working qualities additionally add to unpredictability deficiency [5]. To distinguish the shortcoming precisely, the best possible and exact displaying should be set up [6].
Static eccentricity degree (SED) is the proportion between the separation of the hub of chambers of rings and air-gap length. Eccentricity shortcoming is depicted as far as SED as, SED = Distance of hub of chambers of the internal eccentric ring/air-gap length of the motor The mentioned fault causes vibration and sounds in the present sign because of mechanical pressure, the rotor and stator impact when unconventionality deficiency rate increments. The shortcoming in mains fed IM can be recognized utilizing FFT range of stator current [4]. Yet, when the machine is taken care of from an inverter, MCSA doesn't hold well in light of the fact that the inverter harmonics have impacts on the frequency spectrum. This paper deals with the Finite Element Method (FEM) which is used to analyze the electromagnetic signatures of IM during static eccentricity fault condition and compared with healthy motor. The use of FEM for analyzing the electromagnetic parameters under healthy and faulty condition is a favored approach. Because FEM provides more accurate information during fault condition contrasted with the expository technique which utilizes direct properties [3]. FEM includes non-linearity such as magnetic saturation, actual properties of magnetic materials, windings placed in actual slots etc., FEM is used for analyzing IM faults such as stator winding faults [7] [8] [9], broken rotor bar faults [10] [11] and bearing faults [12].

CO-SIMULATION OF IM MODEL IN ANSYS MAXWELL WITH SIMPLORER
The investigation of eccentricity flaw is completed on a three-phase IM with the help of ANSYS Maxwell and Simplorer. The motor is taken care of from a three-phase PWM inverter. Figure 1 shows the IM model in ANSYS Maxwell FEA instrument, the structure subtleties of which are appeared in the Appendix.
Static eccentricity fault is incorporated in the IM model in ANSYS Maxwell for a fault degree of 0.15mm (43%). Figure 2 delineates the ANSYS Simplorer model of SPWM controlled inverter fed

RESULTS
The study is completed on a 3φ, 380V, 7.5kW, 4 pole IM with 48 stator openings and 44 rotor spaces. The created FE model of IM is provided from a three-phase, PWM inverter with the switching frequency of 2 Khz in open circle condition. The motor is begun without any torque and at t=0.5s rated torque of 49 Nm is applied. Figure 3 shows the speed of IM under healthy and eccentric fault condition. During eccentric fault condition, the oscillation in speed slightly increases while running at rated load compared to healthy motor as seen from Figure 3(b). Since the variations in speed ripples are very less, the oscillations alone cannot be treated as a fault parameter. Hence the static eccentricity with a high fault severity level of 43% induces slight oscillations in rotor speed.     Figure 6(a) shows the radial flux density distribution for healthy induction motor with respect to radial airgap distance at rated load condition. As indicated by Ampere circuital law, the result of the length of a component and attractive field power over a shut way gives the current encased in that surface. In the event that the current over a surface is thought to be steady, at that point attractive field power is contrarily relative to the length of a component. The attractive transition thickness is given as the result of relative porousness and attractive field force. Consequently, the air hole transition thickness experiences twisting when there is an adjustment in air hole length. The circulation of transition around the airgap is balanced with four half-cycles and the noise present in the waveform is expected stator and rotor spaces, non-sinusoidal winding dispersion and attractive immersion. Figure 6(b) portrays the outspread motion thickness of an acceptance engine with the flightiness of 0.15 mm. The twisting in the airgap motion thickness is because of flightiness shortcoming of 43% seriousness. Notwithstanding the above-said harmonics, more noise is initiated because of static eccentricity flaw. Figure 7 delineates the spatial FFT range of airgap outspread motion thickness for the solid and flawed motor. Aside from the central part, different noises in solid motor range are because of openings, windings and inverter harmonics. The expansion in consonant substance because of static erraticism can be seen from Figure 7(b) contrasted with solid motor range.  The flux distribution of a fine fettle IM appears in Figure 8. It is seen from Figure 8(a), the dispersion of magnetic lines over the solid motor is balanced over the post pitch and each shaft is found at an attractive pivot of 360˚/p geometrical degrees, where p is the number of posts. Whenever moment the bend of the periphery is πD/p for all the shafts, where D is that the inner distance across of the stator. Figure 9 shows the attractive field conveyance of IM under the static unusual condition with a seriousness level of 43%. It tends to be seen from Figure 9(a), asymmetry happens in the attractive field appropriation because of unpredictability flaw, the attractive tomahawks position of the posts become temperamental and each shaft length changes around πD/p when arriving at the shortcoming district of the rotor during the pivot. The length of the attractive posts of the flawed machine experiences an intermittent variety round the rotor during motion turn. From the conveyance of attractive fixation in Figure 9(b) it is seen that the motion thickness becomes uneven bringing about immersion inside the rotor and stator teeth close to the dislodged position because of issue. At the point when the issue seriousness expands, the asymmetry in transition lines and motion thickness dispersion over IM increments.

CONCLUSIONS
Thus, electromagnetic qualities of PWM inverter fed IM is broke down for solid and eccentricity flaw of degree 43% utilizing FEM model. Since the issue is exceptionally hard to recognize utilizing stator current range and speed for inverter took care of IM. An examination on the attractive field circulation (Flux Lines and Flux Density) of the solid and flawed motor model is completed. The examination shows the distortion in the hub of attractive posts prompting immersion in the rotor and stator teeth close to the dislodged locale. The spiral airgap transition thickness as for the airgap separation of sound and broken IM has been thought about. The examination shows the consonant substance in spatial FFT range of spiral airgap transition thickness is increased because of static unpredictability flaw.