Troubleshooting and Fixing of Inverter Driven In
Contents
1. lt x a Ground Current a E EPEA T 5 O 0 0 10 0u 20 0u 30 0u 40 0u 50 0u 60 0 time s Figure 13 Bearing currents initial situation centrical ground conductor serves as a useful bearing current mitigation G Shaft grounding brush The application of a shaft grounding brush inserts a bypass for the currents in parallel to the bearings But it is evident that this mitigation would protect the motor bearings only whereas the bearings in the driven machine suffer even higher currents due to the lower impedance within the induction motor An additional disadvantage lies in the fact that the brush needs to be maintained IV EVALUATION OF THE MITIGATION TECHNIQUES BASED ON A PRACTICAL APPLICATION The mitigation techniques have been applied to a practical bearing current problem in a drive system of a paper mill Two 500 kW induction motors driving a creping cylinder via a common gear have been investigated comparatively The creping cylinder has the dimension of 5 5 x 5 95 meters and a weight of 150 tones In both motors the bearings have had premature failures due to bearing currents identified to be rotor ground currents 40 30 20 14 current A current A N ON A 15 10 current A ol 0 0 10 0u 20 0u 30 0y 40 0u 50 0u 60 0y time s Figure 14 Bearing currents final solution2. 13 E J Bartolucci and B H Finke Cable design for PWM variable speed AC drives Industry Applications IEEE Transactions on vol 37 pp 415 422 2001
3. A Initial Situation poor grounding conditions The initial situation from the viewpoint of the bearing cur rent phenomenon was characterized by a length of the cable of 250 m and the concentrical shield of the cable was con nected in such a way that the electrical conditions were corre sponding to non shielded cables The grounding measure was effective for currents of very low frequency since the length of the grounding wire was about 10 m to the grounding point of the building In Figure 13 the initial situation of the bearing current problem has been recorded Peak currents of up to 17 A could be measured flowing through the bearings and the operating time for the bearings was extremely shortened B Approach I improved grounding The first approach aimed at improving grounding condi tions As mentioned in the introduction the rotor ground currents are caused mainly by poor grounding of the induc tion motors In this example the induction motor is as usual in paper mills arranged at a solid concrete block and no struc tural grounding such as machine frame was given The im proved grounding was provided by an additional flexible wire from the motor to the grounding point of the building that is much closer than 10 m The improved grounding has reduced the current at the shaft at MP by about 20 So the bearing currents have shown peak values of up to 14 A which still exceed the ac ceptable values significantly C App
4. Figure 10 Shielded Cables as mitigation for rotor ground currents power converter In order to discuss the kind of shield neces sary for this mitigation the equivalent circuit has been en hanced to the model shown in Figure 10 The effectiveness of the shielded cable depends only on the magnetic coupling between the inner conductors and the shield Due to the low frequencies that need to be handled here the use of the very Figure 11 Cable configurations for bearing current mitigation expensive EMC cable shields is not necessary In Figure 11 the cable configurations investigated using the model shown in Figure 10 can be seen The EMC shield not illustrated here has been compared to a configuration with an concentrical ground conductor and the configuration on the right side in Figure 11 the symmetrical cable Using the finite element method the magnetic coupling between the inner conductors and the shield has been calculated and used for a simplorer circuit simulation The results the shield current In relating to the common mode current Icm at the inner conduc tors are shown in Figure 12 It can bee seen that even with the simplified model in Figure 10 the configuration with the con Ish 1 2 Lem 1 0 8 0 6 0 4 0 2 0 atiii kHz 1 10 100 1 000 10 000 100 000 Figure 12 Shielding effectiveness of investigated cable Common Mode Current current A Shaft Current
5. Troubleshooting and Fixing of Inverter Driven In duction Motor Bearing Currents in Existing Plants of Large Size an Evaluation of Possible Mitigation Techniques in Practical Applications S Guttowski S Weber M Schinkel W John H Reichl Fraunhofer Institute for Reliability and Microintegration Gustav Meyer Allee 25 13355 Berlin Germany Berlin Center of Advanced Packaging TIB4 2 1 Gustav Meyer Allee 25 13355 Berlin Germany Abstract The effect of bearing currents in inverter driven in duction motors has been investigated and reported for about ten years Mitigation techniques have been developed and approved by experiments In this paper the special situation of trouble shooting in a given configuration such as existing large plants is discussed The determination of the actual bearing current phe nomenon and the development of the appropriate mitigation technique under given circumstances are shown A new equiva lent circuit diagram for the bearing currents in large drives has been developed and used for evaluation of mitigation techniques Experiences applying the mitigation techniques to a drive suffer ing premature damages due to rotor ground bearing current are presented and discussed using experimental results I INTRODUCTION The application of adjustable speed drives ASD is in creasing due to improved system efficiencies providing en ergy savings and ease in control The development of ad van
6. avoided risk of bearing damages B Exchange of the power converters and wiring If only the power converter and the wiring need to be ex changed basically the same recommendation can be given as for A But the connections of the concentrical cables at the motor terminals need to be made very carefully since the contact impedance is very important for the effectiveness of the mitigation as shown C Exchange of only the power converter The real challenge for the system designer occurs when only the power converters need to be changed and the wiring and the motors have to remain In this case mostly the cables are not equipped with a shield and the other mitigations need to be considered The first step must be to improve the grounding conditions of the induction motors The low 1m pedance connection between the motors and the frame of the paper mill in combination with good grounding conditions has solved many bearing current problems in the past If these mitigations do not solve the problem additional measures such as output filters have to be considered In either case the rotor ground currents need to be checked for each motor of the system if unexpected downtimes caused by premature damages due to bearing currents shall be avoided VI CONCLUSION The idea of this paper is to provide the industrial user en countering a bearing current problem in an existing plant configuration a guideline for the mitigation development applicable f
7. ced power electronic switching devices has enabled high frequency switching operation and has improved the perform ance of the pulse width modulated PWM inverters of the ASD However the high rate of voltage rise of typical IGBT inverters has adverse effects in the form of motor insulation stress electromagnetic interference and bearing and leakage currents caused by common mode voltages 1 6 A high percentage of process disruptions are due to bearing failures In the meantime a substantial number of mitigation techniques have been developed 1 7 12 However the in dustrial user searching the literature for an immediate solution encountering a concrete bearing current problem in an exist ing plant configuration finds itself abandoned since most of the mitigation techniques are not feasible without enormous changes of the electrical plant configuration This paper ad dresses the practical problems being told to solve a bearing current problem in an existing plant with premature bearing failures due to rotor ground currents in large adjustable speed drives The final conclusions derived from the mitigation method evaluation can be summarized to a guideline that supports the expert responsible for the bearing current suppression in ex isting plant configurations as well as during system moderni zations Il BEARING CURRENT TROUBLESHOOTING Troubleshooting in existing plants of large size means firstly to determine which bearing curr
8. e the rotor ground currents significantly the additional integra tion of such filters often exceed the possibilities during trou bleshooting and fixing Furthermore in many applications the costs for additional filters between the motor and the induc tion motor can not be beard by the operating company C Insulated Bearings Insulating both bearings breaks the path for the rotor ground currents as shown in Figure 6 The implementation of one insulated bearing seems often to be the first mitigation applied to the motor since the damaged bearing needs to be changed anyway However this method may not be accept able for some applications since the shaft voltage still exists and will attempt to find a path to ground elsewhere In many applications an electrical insulated rotor is not desired by the Operating company D Insulated clutch between motor and machine In Figure 7 the effectiveness of an insulating clutch be tween the motor and the driven machine is demonstrated Obviously the clutch opens the path for the rotor ground cur Figure 7 Equivalent circuit diagram for insulated clutch Figure 8 Equivalent circuit for connection between motor and machine rents and the currents on the shaft can be avoided But the integration of an insulating clutch into an existing system means significant changes not only of the electrical but also of the mechanical system parameter and is not acceptable for Operating companies E Low im
9. een the motor and the paper mill could be reduced to about 2 A According to the experiences gained from other bearing current investigations this 1s acceptable for induction motors of such size V DISCUSSION In this particular case the cable were prepared to electri cally add the concentrical grounding conductor So it could be shown to which extend this measure can reduce and as the case may be avoid bearing current problems But mostly this will not be the case In existing systems such as paper mills only the mitigations without any changes in the wiring har ness will be acceptable Rotor ground currents occur often when the electrical drive system in existing plants have been modernized Depending on the electrical system in the plant before the modernization not the complete electrical systems is exchanged but for ex ample only the power converter Depending on the degree of modernization the following guideline for bearing current avoiding can be given A Exchange of the complete drive system In the case of a modernization of the whole system the saf est way to avoid bearing current problems 1s to install cables with concentrical grounding conductor Although the ground ing conditions at the places of the motors are expected to be poor the only additional measure to be adopted is to connect the motors with the grounding point of the building The additional costs due to the special cable are mostly acceptable compared to the
10. ent phenomena causes the bearing failures Therefore a short review of the known bearing current phenomena followed by a discussion how the actual phenomenon can be determined will be given here A Short Review of Bearing Currents Phenomena The different types of bearings currents have been de scribed very well by the authors in 8 So at this place only a short review focusing on the specific aspects necessary for this paper will be given The common mode voltage caused by the feeding inverter leads to currents due to the unavoid able parasitic capacities within the induction motor In Fig ure 1 the allocation of these parasitic capacities within an induction motor model can be seen Four types of bearing currents the small capacitive currents the electrostatic dis Figure 1 Parasitic capacities within of an induction motor Figure 2 Causes and paths for circulating bearing currents charge currents EDM the circulating bearing currents and the rotor ground currents have been explored so far In this paper the focus lies on high power applications so not only the small capacitive currents but also the electrostatic dis charge currents EDM can be neglected Circulating bearing currents are known to be caused by the high frequency current flowing from the stator to ground As shown in Figure 2 this current excites a circular magnetic flux Poir Which induces an additional shaft voltage thus causing circular bearing cur
11. gerald Modeling of motor bearing currents in PWM inverter drives Industry Applications IEEE Trans actions on Vol 32 pp 1365 1370 1996 6 J Erdman R J Kerkman D Schlegel and G Skibinski Effect of PWM inverters on AC motor bearing currents and shaft voltages Ap plied Power Electronics Conference and Exposition APEC 1995 7 P J Link Minimizing electric bearing currents in ASD systems Industry Applications Magazine IEEE vol 5 pp 55 66 1999 8 A Muetze and A Binder Experimental evaluation of mitigation techniques for bearing currents in inverter supplied drive systems in vestigations on induction motors up to 500 kW International Electric Machines and Drives Conference IEMDC 2003 9 J Zitzelsberger and W Hofmann Effects of modified modulation strategies on bearing currents and operational characteristics of AC in duction machines Industrial Electronics Society 2004 10 A Muetze A Binder H Vogel and J Hering Experimental evalua tion of the endangerment of ball bearings due to inverter induced bear ing currents IEEE IAS Annual Meeting 2004 11 R F Schiferl and M J Melfi Bearing current remediation options Industry Applications Magazine IEEE vol 10 pp 40 50 2004 12 D Hyypio Mitigation of bearing electro erosion of inverter fed mo tors through passive common mode voltage suppression Industry Ap plications IEEE Transactions on vol 41 pp 576 583 2005
12. in an existing plant with premature bearing failures in large adjustable speed drives So the mitigation techniques need to be discussed in terms of their suitability for a concrete bearing current problem in an existing plant configuration The mitigation techniques taken into account are A Modulation scheme with low common mode voltage B dv dt Filter at the inverter output C Insu lated bearings D Insulated clutch between induction motor and machine E Low impedance connection between induc tion motor and machine F Shielded cables between inverter and induction motor and G Shaft grounding brush The pos sibility of low resistance lubrication has been left out due to the lacking availability A Modulation scheme with low common mode voltage Several modulation schemes for adjustable speed drives with common mode voltage reduction have been presented Figure 6 Equivalent circuit diagram for insulated bearings The effectiveness of those schemes has been approved by simulations and be practical experiments From the trouble shooting point of view this approach can not be considered to be a useful mitigation since the idea of changing the modula tion scheme within an existing plant is not feasible B dv dt Filter at the inverter output The effectiveness of different types of filters at the inverter output has been investigated carefully in 10 Although fil ters that reduce the dv dt of the line to earth voltage reduc
13. ion mo tor The capacities Cys Cyr Crp Cp and Cy are the parasitic capacities within the motor as shown in Figure 1 In order to discuss the phenomenon rotor ground current the diagram has been extended by the impedances Zim and Zgpm as well as the capacity Cy pm The impedance Z i which is the imped ance of the real current path from the induction motor to the ground is the cause of current flowing through the bearings to the driven machine The capacity C and the impedance Zg pm represent the currents path through the bearings of the driven machine to ground In this diagram the two parallel paths of the common mode current from the motor frame to ground can be seen and the influence of the mitigation measures on these paths can be evaluated The measures against the rotor ground currents have in common that the current path through the bearings of the motor and the driven machine needs to be made more unalluring so the dangerous current flows somewhere else B Measurements for Phenomena Determination The procedure necessary to determine the kind and exact amount of current flowing through the bearings of an induc tion motor has been described based on the results of several research projects Although the methods applicable to the machines especially dissected for the experiments can not be applied to the motor of the large adjustable speed drives in an existing plant being told to solve the bearing current prob lems Knowing tha
14. or the industrial practice For this reason the explored bearing current phenomena and the developed miti gation techniques have been reviewed The mitigation tech niques have been applied to a practical bearing current prob lem if a large drive The effectiveness of the mitigation tech niques has been determined by measurements So the appro priate mitigation technique for this specific bearing current problem could be derived The results of this investigation have been derived from a bearing current problem of a con crete drive but the cognitions are of general usefulness VII REFERENCES 1 A von Jouanne H Zhang and A K Wallace An evaluation of mitigation techniques for bearing currents EMI and overvoltages in ASD applications Industry Applications IEEE Transactions on vol 34 pp 1113 1122 1998 2 S Chen T A Lipo and D W Novotny Circulating type motor bearing current in inverter drives IEEE IAS Annual Meeting 1996 3 D Busse J Erdman R J Kerkman D Schlegel and G Skibinski System electrical parameters and their effects on bearing currents Applied Power Electronics Conference and Exposition APEC 1996 4 D F Busse J M Erdman R J Kerkman D W Schlegel and G L Skibinski The effects of PWM voltage source inverters on the me chanical performance of rolling bearings Industry Applications IEEE Transactions on vol 33 pp 567 576 1997 5 S Chen T A Lipo and D Fitz
15. pedance connection between induction motor and machine Being encountered by a rotor ground current problem to be solved the electrical connection between the induction motor and the machine seems to be very promising As it can be seen in Figure 8 the additional path between the motor frame and the driven machine housing lays in parallel to the current path though the bearings and the shaft This mitigation can be applied very easily and it leads to substantial improvements as long as the connection does not exceed a certain length In several applications where other mitigations were eliminated the bearing currents could have been reduced by factor 2 The main disadvantage lies in the fact that the connection has to be maintained F Shielded cables between inverter and induction motor The length of the connection between the inverter and the motor has an immense impact on the risk of bearing currents The use of shielded cables is known to reduce the rotor ground currents as well as the circulating currents 7 8 13 However the higher cost due to the shielded cables always needs to be motivated and furthermore shielded cables are not feasible in all applications In Figure 9 the equivalent circuit diagram extended by the elements of the shield is shown It has been assumed that the shield has been contacted on both the motor frame as well as the grounding point of the Figure 9 Shielded Cables as mitigation for rotor ground currents
16. rents Rotor ground currents are caused by poor grounding condi tions that can be found especially in existing plants with older electrical systems In such configurations the rotor ground impedance may be significant lower than the grounding of the stator housing The unavoidable common mode current gen erated by the high dv dt of the common mode voltage flows to substantial portion through the bearings of motor via the rotor and the driven machine to ground In Figure 3 the current paths of the rotor ground current Figure 3 Current paths of the rotor ground currents Motor Star Point Motor Frame Figure 4 Equivalent circuit diagram for rotor ground currents can be seen whereas the rectangle solid on the right side in the figure shall mention the driven machine where the current is flowing through In Figure 4 an extension of the commonly used equivalent circuit diagram for the bearing current phe nomena is shown Especially for the troubleshooting and mitigation evaluation described in this paper this extension has proofed to be very useful In addition to the commonly used equivalent circuit diagram in this single phase diagram the whole system has been taken into account The power converter is represented by the common mode voltage source Vem The cable between the power converter and the motor has been included by the elements Re Le and Ceg The ele ments R and Lw represent the winding of the induct
17. roach 2 low impedance connection The mitigation described in section I E was the second approach to reduce the bearing currents that were measurable at MP3 So an additional flexible cupper meshwork was con nected between the induction motor and the frame of the paper mill In this particular case the shaft between the motor and paper mill was fairly short so this measure was success ful This additional wiring has reduced the remaining bearing currents by factor 2 which basically confirms the experience gained from other bearing current investigations D Approach 3 cable shield application The cables in this particular paper mill were designed with concentrical ground conductor but by mistake the connection with the motor was not realized according to the require ments So it was possible to determine the influence of the cable in a real system Although the connection made subse quently did not reach the impedance that could had been reached by correct installed wirings In Figure 14 the currents flowing into and out of this inves tigated induction motor are shown Due to the special situa tion of subsequently connected shielding the common mode current could be determined separately In systems were the concentrical grounding conductor has been installed correctly the current probes can determine only the differential current of the inner conductors and the shield The current flowing through the bearings and the shaft betw
18. t the small capacitive currents and the electrostatic discharge currents do not appear in large induc Figure 5 Investigation of rotor ground currents in existing plants tion motors the investigation can be focused on the circulating currents and the rotor ground currents But only the rotor ground currents can be measured in existing systems In Figure 5 the measuring points for the rotor ground cur rent investigation can be seen The common mode current flowing into the induction motor is determined at measuring point M indicates the amount of current that is flowing out of the motor at the other current terminals such as the motor grounding the cable for the temperature monitor the cable for the shaft encoder and of course the shaft to the driven machine The current at measuring point M evaluates the effectiveness of the motor grounding which might be very helpful for explaining the current flowing through the shaft Whereas the measuring points M and M are easily to realize the measuring point M can be realized only by using special Rogowski Coil current probes since the measurements have to be taken during operation of the motor II MITIGATION TECHNIQUES Mitigation techniques for inverter driven bearing currents have been developed and presented by different authors The question underlying in this paper focuses on the feasibility of the mitigation methods for troubleshooting being told to solve a bearing current problem
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