Mitigation Techniques of shaft voltage and bearing current in Inverter Driven Three Phase Induction Motor

Transcription

1 Mitigation Techniques of shaft voltage and bearing current in Inverter Driven Three Phase Induction Motor Darshan Thakar 1, Hemish Choksi 2 and Hemant Joshi 3 1 Institute of Technology,Nirma University,Ahmedabad,India 2 Department of Electrical Engineering, Government Polytechnic, Himatnagar Himatnagar, India 3 Department of Electrical Engineering, Government Polytechnic, Ahmedabad Ahmedabad, India Abstract Bearing currents in induction motors can result in premature bearing failure. In this paper, the sources of inverter driven motor bearing currents is discussed. The current paths are shown to flow through what are commonly considered to be electrical insulators. Currents also are shown to flow through driven equipment. Potential current paths are identified and methods to break these current paths through motor (and installation) modifications are presented. The different failure mechanism inside the bearing failure is also discussed. The Methods to shunt these currents away from the motor or driven equipment bearings through improved high frequency grounding are also discussed. Finally, bearing current remediation methods are discussed for each identified current path and source. Keywords: Induction Motor, Inverter, Variable frequency drive, Pulse width modulation, Bearing Current. 1. Introduction Three phase induction motors are the most widely used motors for industrial control and automation. Hence they are often called the workhorse of the motion industries. They are robust, reliable, less maintenance and of high durability. When power is supplied to an induction motor with recommended specified voltage and frequency, it runs at its rated speed. However many applications need variable speed variations to improve the quality of the product. The development of power electronic devices and control systems has to mature to allow these components to be used for speed control of AC motors control in place of conventional methods. This type of control not only controls the speed of AC motors, but can improve the motor s dynamic and steady state characteristics. Pulse Width Modulation (PWM) variable speed drives are increasingly applied in many new industrial applications that require superior performance. Three phase voltage-fed PWM inverters are recently showing growing popularity for multi-megawatt industrial drive applications. But this PWM drives consist of some disadvantages. Common mode voltage and shaft current is also one of the disadvantages of vfd drives. When a motor is supplied by a balanced 3-phase sine wave supply, each phase voltage is displaced 1200 from its companions. They sum at the neutral point of the motor winding and will add to zero. Add all 3 phases at any point in the 3600 cycle and they will always sum to zero volts at the neutral. For example, in fig.1, at the left-most vertical marking phase A is at zero volts, phase B is 70% negative and phase C is 70% positive, adding to zero volts at the neutral point. Fig.1 Fig.2 Unlike 3-phase utility power, the VFD output has only two states. With only two output states it is not possible to create a completely symmetrical 3 phase waveform and thus an unbalance occurs. The result is an output voltage waveform where the neutral bounces between the plus and minus DC bus levels, creating a very large CMV. Inverter driven motors have the potential for externally sourced bearing currents as a result of the voltage wave shape that is applied to the motor by the inverter. Modern voltage source, Pulse width modulated (PWM) inverters switch a DC bus voltage (V dc ) onto the three phase terminals of the motor in a switching pattern that creates the proper fundamental component voltage and frequency. Since the motor line to line terminal voltage must be either + V dc or V dc, it is not possible to have the three terminal voltages 341

2 add to zero at any instant of time. Most rectifiers that create the DC bus also introduce a common mode voltage to the DC bus itself. The average voltage applied to the motor (over a cycle) is kept at zero, but the instantaneous sum of the voltages at the motor terminals is nonzero. This instantaneous voltage sum is called the common mode voltage (CMV) and exists between the motor windings and the motor ground. For voltage source or current source inverters, the CMV contains high rates of change of voltage with respect to time (high dv/dt) so its frequency content can be in the megahertz (MHz) range. Common mode currents (I) are created due to capacitive coupling of the CMV since I = C dv/dt, where C is the capacitance of the common mode circuit element. Fig.3 Different Path of Bearing Current There are many potential current paths via this capacitive coupling from the motor stator winding to ground. Most of these paths are normally considered to be insulators, for example: stator slot liners, stator to rotor air gap and the bearing grease film between race and ball. Fig. 3 shows an inverter driven motor system with the inverter connected to the motor through a shielded cable and the motor load connected to the motor shaft through a conductive coupling. The motor, cable shield, inverter and load all have electrical grounds as indicated by the downward pointing arrows. All of these grounds are connected together, in some fashion. In Fig. 1, the various current paths of capacitive coupled current are presented. The high dv/dt created in the stator winding couples capacitive with the stator core and frame and with the rotor. The current path marked in dots is a capacitive current coupled to the rotor through the air gap, with a return path to the motor bearings, motor ground connection and finally to the drive ground. Current flow through the bearing is a consequence of two phenomena. Conduction current may flow through the motor bearing if the shaft happens to be shorted to the frame (by bearing ball contact, for example) at the instant that the dv/dt transition occurs in the common mode voltage. Discharge current may flow through the motor bearings if the bearing becomes conductive after first acting as an insulator. Discharge current may also occur when the voltage across the bearing lubricant film exceeds the film break down voltage. The dotted current can not be directly measured without a specially instrumented motor since the entire current path is inside the motor. The dot-dashed (rotor to shaft) current in Fig. 3 is also capacitively coupled from the stator winding to the rotor across the air gap. This current component finds a path that passes through a conductive coupling, and through at least one load bearing, to the load ground and back to the drive ground. The same two bearing current phenomena discussed for the dotted current can occur with the dotdash current, only now the conductive or insulating state of the load bearing will determine the type of current flow. The rotor to shaft current has the potential of creating damage in the load bearing or, for some measured by putting a high frequency current sensor around the motor shaft. The solid line (stator winding to frame/shaft) current path in Fig. 3 indicates capacitively coupled current between the stator winding and the frame. As shown, this current flows through the stator winding insulation (which is capacitively conductive at high frequencies) and, with a poor motor to inverter high frequency ground connection, flows through the motor frame, the motor bearing, the motor shaft, the conductive coupling, the load bearing, the load ground and finally to the drive ground. Current through this path has the potential to damage the motor and load bearings, as well as the motor to load coupling. This current path would also include currents due to a transient voltage difference between the motor frame and the driven equipment that is shorted out by the shafts. The preferred path for all these currents, to reduce bearing damage, is the dashed (or stator winding to ground) path in Fig. 3. Here, no current flows through the motor or load bearings. 2. Effects of electric current going through the bearing The combination of high frequency and capacitive discharge currents is problematic, as it induces shaft voltages and bearing currents. The voltage increases to a certain value where it discharges to ground, usually through the bearings as they are the path of least resistance. The lubricant film in the bearing is a major barrier to cross, and when a threshold voltage occurs that is strong enough to overcome the lubricant film thickness then a discharge occurs. Then the voltage charges up again, like a capacitor would do. It is the same set-up as 342

3 used in electrical discharge machining (EDM), but in motors this is not controlled and leads to electrical erosion. Due to the passage of electric current in the contact zone of rolling elements and raceways heat is generated causing local melting of the bearing metal surface. Craters are formed in the contact area and particles of molten material are transferred and partly break loose. The crater material is re-hardened and much more brittle than the original bearing material. Below the re-hardened layer there is a layer of annealed material, which is softer than the surrounding material. Fig.5 Fluting on inner ring raceway of bearing 2.1 Micro-cratering Because frequency converters are more and more used micro-cratering is by far the most common effect of electric current passage. The damaged surface appears dull, characterized by molten pit marks fig 4. Multiple micro-craters cover the rolling element and raceway surfaces. Crater sizes are small, mostly from 5 to 8 μm in diameter, disregarding whether it is on the inner ring, outer ring or a rolling element. The real shape of these craters can only be seen under a microscope in very high magnification. The dull surface of the ball is a sign of micro cratering. Fig.6 Fluting on inner ring raceway of bearing Fig.4 Micro-Cratering 2.2 Fluting or washboard They are patterns of multiple grey lines across the raceways (fig 5,6,7). They appear shiny and molten. The reason for this fluting is a mechanical resonance vibration caused by the dynamic effect of the rolling elements when they are over-rolling smaller craters. This means that fluting is not a primary failure mode produced by the current flow through the bearing itself. It is a secondary bearing damage that becomes visible only after time and has its initial point from craters. Fig.7 Fluting on inner ring raceway of bearing 2.3 Grease-blackening Current discharges also cause the lubricant in the bearing to change its composition and degrade rapidly. The local high temperature causes additives and the base oil to react, and it can cause burning or charring of the base oil. Additives will be used up more quickly. Thus the lubricant gets Black discolored grease affected by current discharges Fluting or washboard in raceway The dull surface of the ball is a sign of micro cratering almost hard and blackened (fig 8). A rapid breakdown of the grease is a typical failure mode that results from current passage. 343

4 Fig.10 Insulated Bearing Fig.8 Grease Blackening 3. Strategies For Mitigating VFD Induced Shaft Voltages In Motors 3.1 Faraday shield: A conductive shield between the rotor and stator inside the motor would prevent the VFD current from being induced onto the shaft by effectively blocking it with a capacitive barrier. However, this solution is extremely difficult to implement, very expensive, and has been generally abandoned as a practical solution. 3.3 Ceramic bearings: The use of nonconductive ceramic balls prevents the discharge of shaft current through the bearing. As with other isolation measures, shaft current will seek an alternate path to ground possibly through equipment connected to the motor. Such Bearings are very costly and, in most cases, motors with ceramic bearings must be special ordered and so have long lead times. In addition, because ceramic bearings and steel bearings differ in compressive strength, ceramic bearings must be resized in most cases to handle mechanical static and dynamic loadings. 3.4 Conductive grease: In theory, because this grease contains conductive particles, it should provide a continuous path through the bearing and so bleed off shaft voltages gradually through the bearing without causing a damaging discharge. Unfortunately, the conductive particles in these lubricants may increase mechanical wear into the bearing, rendering the lubricants ineffective and often causing premature failures. This method has been widely abandoned as a viable solution to bearing currents. Fig.9 Faraday Shielded Induction Motor 3.2 Insulated bearings: Insulating material, usually a nonconductive resin or ceramic layer isolates the bearings and prevents shaft current from discharging through them to the frame. These forces current to seek another path to ground, such as through an attached pump or tachometer or even the load. Due to the high cost of insulating the bearing journals, this solution is generally limited to larger-sized NEMA and IEC motors. Sometimes, highfrequency VFD-induced currents may be capacitive coupled through the insulating layer and cause bearing damage inside the bearing anyway. Another drawback is the potential for contaminated insulation, which can, over time, establish a current path around the insulation, allowing current flow through the bearings. Fig.11 Conductive Grease 3.5 Grounding brush: A metal brush contacting the motor shaft is a more practical and economical way to provide a low-impedance path to ground, especially for larger frame motors. However, these brushes pose several problems of their own: 344

5 a. They are subject to wear because of the mechanical contact with the shaft. b. They collect contaminants on their metal bristles, which may reduce their effectiveness. c. They are subject to oxidation build up, which decreases their grounding effectiveness. d. They require maintenance on a regular basis, increasing their lifetime cost. Most grounding brushes are installed externally to the motor and require extra space and special mechanical design considerations for the brush mounting. Contact grounding brushes are also subject to severe wear and contamination if installed externally. They also generate high heat at high speeds. It is not suitable to use contact grounding brushes for applications requiring more than 1800 rpm. Their effectiveness may be reduced significantly over a short period of time due to vibration of the brush mounting spring and oxidation of the shaft surface. As a result, such brushes require frequent maintenance. In most applications, the contact brush may often be serviced when the bearings are replaced due to failure. In case of high frequency circulating currents in larger frame motors, it is also important to note that a single contact grounding brush may worsen the bearing current discharge at the bearing location opposite of the grounding brush. This situation forces the use of two grounding brushes on the motor; one on the drive-end and one on the non-drive end. of thousands of discharge points, the SGR channels currents around the motor bearings and protects them from electrical damage. The SGR is a low-cost solution that can be applied to virtually any size AC motor in virtually any VFD application. Fig.13 Shaft Grounding Ring Fig.14 Induction Motor with Shaft Grounding Ring Fig.12 Induction Motor with Shaft Grounding Brush Alternately, one grounding brush may be applied on the drive end when one insulated bearing with a grounding brush on the drive-end. 3.6 Shaft grounding ring (SGR): Applied like a conventional grounding brush, this innovative new approach involves the use of a ring of specially engineered conductive micro fibers to redirect shaft current and provide a reliable, very low impedance path from shaft to the frame of the motor, bypassing the motor bearings entirely. The ring s patented technology uses the principles of ionization to boost the electron-transfer rate and promote extremely efficient discharge of the highfrequency shaft voltages induced by VFDs. With hundreds Grounding the motor shaft is commonly used method due to its simplicity and the relatively low cost of the electrical contact brushes. The idea of the grounding the motor shaft is to provide a lower impedance path to the motor frame than through the bearings. This lower impedance path may be established with the shaft grounding ring. To improve the contact grounding brush performance in the shaft grounding ring, the brush wire material has been replaced with micro-diameter conductive fibers. Figure 13 and fig. 14 show a conductive micro-fiber shaft grounding ring brush and a motor with the micro-fiber shaft grounding ring installed on the drive-end of the motor shaft, respectively. The micro-fibers surround the motor shaft completely to provide full contact with the shaft thereby minimizing the effect of the motor shaft eccentricity or vibration. Micro-fibers in the shaft grounding ring are not spring-loaded on the motor shaft while most contact grounding brushes are spring-loaded. The micro-fiber brushes are either press-fitted into the 345

6 housing or mounted on the motor faceplate with two small screws. This micro-fiber brush construction provides several advantages over the spring-loaded contact brushes. First, the wear rate of micro-fibers is very low, due to an insignificant force between the light weight micro-fibers and the shaft. As a result the heat generation of the fibers during operation is negligible, at even high rotating speeds. Tests at up to 14,000 rpm for 3000 hours have shown negligible wear indicating much longer life at the lower rotating speeds found in HVAC applications. Micro-fiber brushes are said to have no speed limitation. Second, the current limits of micro-fibers are in general much higher than that of a conventional brushes or a solid carbon brush since microfibers can provide a large real contact area on a sliding surface, thereby greatly improving the efficiency of shaft voltage discharges. Third, the microfibers create a corona discharge at the tip of the fibers when the fibers are over a charged surface. Recent hard-disc manufacturer reports that currents can flow to a small diameter point without an intimate contact even though the surface voltage is less than 1 volt under a special circumstances. Since the corona discharge is through the air molecules, the discharge can also take place in the media of water or dirty grease. This current transfer mechanism due to electrolysis or ionization of a surrounding medium allows the micro-fiber brush to be maintenance-free for the life of a motor, even when the shaft has oil or grease on it. 4. Conclusion Inverter induced bearing currents due to common mode voltage are relatively new and present new challenges. Fortunately, these common mode voltage induced bearing currents are not seen in a vast majority of inverter driven motor applications. Proper remediation methods depend upon a thorough understanding of the potential current paths in a given installation. Diagrams of current flow paths were presented here to illustrate potential issues. Once the types of currents that exist in a given system are known, remedies were presented for each one. Proper grounding is a key to shunting currents away from paths that flow through motor or driven equipment bearings. It was pointed out that caution should be taken when adding a shaft grounding brush to an inverter driven motor bearing in order to prevent increased current flow through other bearings in the system. Elimination of bearing damage in inverter driven motors requires a thorough understanding of the inverter/motor/driven equipment system. Acknowledgments The authors acknowledge gratefully the support provided by all individuals who are directly or indirectly involved in this work. References [1] C. T. Pearce, Bearing Currents Their Origin and Prevention, The Electric Journal, Vol.XXIV, No.8, pp , August [2] IEEE, Std , Section 9.4, Shaft Currents and Bearing Insulation, New York, NY: IEEE. [3] J. M. Erdman, Russel J. Kerkman, David W. Schlegel and Gary L. Skibinski, Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages, IEEE Transactions on Industry Applications, Vol.32, No.2, pp , March/April [4] Quirt, Richard C., "Voltages to ground in Load- Commutated Inverters," IEEE Transactions on Industry Applications, Vol 24, No. 3, pp , May/June, [5] S. Chen, D. Fitzgerald and T. A. Lipo, Source of induction motor bearing currents caused by PWM inverters, IEEE Energy Conversion, Vol.11, No.1, pp.25-32, March [6] GAMBICA / REMA Technical Guide.Motor Shaft Voltages and Bearing Currents Under PWM Inverter Operation Variable Speed Drives & Motors. Technical Report No. 2. 2nd Edition : [7] Gao Qiang, Xu Dianguo A New Approach to Mitigate CM and DM Voltage dv/dt Value in PWM Inverter Drive Motor Systems /07 IEEE [8] H. William Oh,Adam Willwerth Shaft Grounding-A Solution to Motor Bearing Currents. ASHRAE Transactions Vol. 114, Part 2. SL ,2008 [9] R. F. Schiferl,M. J. Mel_, J. S. Wang Inverter Driven Induction Motor Bearing Current Solutions. IEEE Paper No. PCIC