Different types of bearing currents--the fundamentals.
Bearings tend to be very reliable components in industrial machinery. As a result, when there are failures they tend to get substantial attention. While the vast majority of bearing failures are due to other causes, a small percentage can be attributed to a flow of current through the bearing. Knowledge of these bearing currents has been around for a long time (Pearce 1927). Early cases of bearing currents were typically due to either magnetized shafts, stray flux linking a rotating shaft, or directly applied voltage. With the advent of static rectifier circuits, direct-current (DC) motors and synchronous alternating-current (AC) machines (with static rectifier circuits on the field) were subjected to new sources of bearing currents (Ammann et al. 1988).
With the increased use of inverters to run induction motors at adjustable speeds, more mechanisms have been observed to cause current to flow through bearings. These mechanisms are very similar to those seen with DC motors and synchronous machines fed by static rectifiers. The root causes for additional bearing currents in inverter-fed machines are related to common mode voltage (CMV) and common mode current (CMC) that is imposed on the machine by the inverter (Chen et al. 1996; Erdman et al. 1996). These terms are illustrated and explained in a later section of this paper covering inverter-related sources of bearing currents.
DAMAGE MECHANISM OF CURRENTS IN ROTATING BEARINGS
The passage of current through a rotating bearing typically involves intermittent bursts of current. This is due to the fact that the rolling elements tend to ride a thin layer of lubricant that acts as a relatively weak insulator. Because the electrical "contact" is intermittent, making and breaking connections, small arcs occur as current flow is interrupted and reinstated. Each small arc can melt a tiny amount of bearing race material just at the surface where the race meets the rolling element (see Figure 1). This small defect might be detectable with sensitive vibration monitoring that can focus on specific race defect frequencies associated with the specific bearing geometry and speed of rotation. A single tiny defect, even right in the ball or roller track, does not typically lead to substantial vibration, noise, or bearing failure. The individual damage spots are typically too small to be seen without magnification.
[FIGURE 1 OMITTED]
However, if a large number of these arcs occur, creating a collection of defects, then a secondary mechanism can lead to the sort of damage seen in Figures 2a and 2b. When the bearing race material is melted during an arc of interrupted current, its subsequent solidification results in a change in the metallurgy in that specific volume of material. Bearing manufacturers work hard to create metallurgy with a high degree of "toughness" or resistance to fatigue damage. When the race material is melted due to an arc of current, it solidifies in a form known as untempered martensite. This untempered metallurgy is much more brittle and prone to fatigue compared to the original quenched and tempered bearing material.
When (in the course of normal running) the bearing encounters this untempered martensite material, it may break off tiny pieces, resulting in enlarged defects and also the addition of debris in the bearing. When the bearing is operating with many spots of damaged metallurgy and the presence of debris, it is akin to a vehicle driving over a gravel road that has some bad spots. As the bearing rolling elements encounter a defect, the impact creates higher local stress and greater likelihood of breaking off additional pieces. The process tends to compound, or feed on itself, often creating the severely fluted pattern seen in Figure 2a in a short period of time. If the loads on the bearing are particularly light, it is also possible for the damage to present itself as simply a dull or "frosted" surface (Figure 2b).
[FIGURE 2 OMITTED]
Since other sources (besides current flow) can also cause a fluted pattern as seen in Figure 2a, the typical process to discern if current flow is the root cause of damage involves examining the bearing metallurgy. The microstructure right in the ball or roller track can be examined under a microscope after etching with an acid solution to check for the presence of untempered martensite (Figure 3). A thin layer of untempered material right at the surface, as opposed to the virgin quenched and tempered base material, is the evidence that typically points to current flow as a root cause.
[FIGURE 3 OMITTED]
SOURCES OF BEARING CURRENTS
Bearing currents can be sourced from outside the motor or from internal causes. While the internally sourced bearing currents have been long understood and controlled, it is useful to understand their origins. In fact, there have been cases of corrective actions taken in an attempt to solve bearing currents in inverter-fed motors that resulted in accentuated internally sourced bearing currents.
For AC motors, it has long been understood that a small voltage across a bearing can lead to a substantial flow of current through that bearing (IEEE 1996). While it is possible to directly apply voltage or current to a rotating bearing (such as from static buildup or from electric welding), these are fairly obvious cases. A magnetized shaft or wires looped around the shaft can also be sources of current flow in machinery. The most common source of bearing currents in line-fed AC motors is stray flux, which links the motor shaft. This stray flux is most commonly created by small dissymmetries in the magnetic paths in the motor laminations. These dissymmetries can be the result of material variations, keyways, notches, and other features related to the motor manufacturing. While good practice would minimize these dissymmetries, it is not really possible to eliminate them entirely.
For the case of internally sourced currents due to stray flux, the current is driven by an end-to-end voltage difference on the shaft. As such it is a "circulating" type of current which takes a path through the shaft and both bearings. The motor bearings, endshields, and frame essentially "short across" the shaft voltage, completing the circulating path (see Figure 4).
[FIGURE 4 OMITTED]
The primary sources of bearing currents in inverter-fed motors can be traced to either common mode voltage (CMV) or common mode current (CMC) contained in the inverter waveforms. In the common inverter types, both voltage source inverters and current source inverters, the basic inverter topology creates both CMV and CMC (Quirt 1988). This is true whether the waveform creation is pulse width modulated (PWM) or another switching pattern, such as square wave or six-step. Figure 5 illustrates the appearance of CMV components in a typical inverter output compared to a line-fed case. These components are typically the result of a combination of how the DC link is created and how that link is inverted to AC.
[FIGURE 5 OMITTED]
When a motor is fed with waveforms containing a CMV component, that voltage is capacitively coupled to the rotating assembly. Essentially, a scaled down replica of the source CMV appears as a voltage on the rotor assembly. The scaling factor is based on the high-frequency capacitances within the motor as illustrated by Equation 1:
[V.sub.b] = [V.sub.CM] x [[C.sub.sr]/[C.sub.sr] + [C.sub.rg] + [C.sub.b]] (1)
[V.sub.CM] = CMV applied to the motor from the inverter
[V.sub.b] = shaft or bearing voltage
[C.sub.sr] = high-frequency capacitance from the stator winding to the rotor
[C.sub.rg] = high-frequency capacitance from the rotor to ground
[C.sub.b] = high-frequency capacitance of the bearings
Figure 6 shows the measured CMV and shaft voltage on an inverter-fed AC motor. The scaling factor of about 25:1 can be seen between the two voltage waveforms, taking into account the scaling of each scope channel.
[FIGURE 6 OMITTED]
Perhaps the most obvious type of bearing current that can flow due to the sort of shaft voltage seen in Figure 6 is one that could be described as a "shaft discharge" type. This amounts to one of the motor bearings "shorting out" the shaft voltage by behaving momentarily in a conductive mode. Similarly, if the motor is coupled via a conductive coupling to driven equipment such as a gearbox, pump, or jackshaft, then such a discharge could also take place in the driven equipment bearings. The traces in Figure 7 show both the voltages and current during such a discharge event.
[FIGURE 7 OMITTED]
The second aspect of inverter waveforms that can cause bearing current is the CMC. Figure 8 shows a typical set of pulses of CMC during operation of a PWM inverter. Every time the output transistors switch on or off, the steep rate of change of voltage leads to capacitively coupled current as predicted by Equation 2. Figure 9 shows the voltage transitions and the current flow in a simultaneous capture of the traces. In this case, the turn off had a steeper slope (higher magnitude of dV/dt) than did the turn on. As a result, a larger pulse of current flowed during the turn off transition.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
I = C x [dV/dt] (2)
where I is the current, C is the capacitance, V is the voltage, and d/dt is the time derivative.
One way that CMC can create bearing current is analogous to the magnetic dissymmetry issue in line-fed motors. Essentially, the capacitively coupled current leaks from the stator winding to the stator core in such a way that it can create a high-frequency version of the end-to-end shaft voltage that can be seen with line-fed motors. This phenomenon has only been seen in larger motors that also have a tendency for bearing currents in line-fed applications (Muetze and Binder 2004).
A more serious issue related to CMC is that the current has to be returned to the source (inverter) via a non-zero impedance. Even with careful cabling to provide a low impedance path for CMC, there is still some impedance in that path. As a result, each time the CMC flows, the motor tends to have a temporarily elevated voltage due to the CMC flowing through the ground path impedance (see Figure 10). While this elevated voltage is only present transiently, there is the opportunity to short out that transient voltage via the motor shaft extension and a load path via the coupled equipment. This current takes a path through a motor bearing to get to the shaft extension, which can cause bearing damage. In fact, measurements have shown this type of bearing current is likely to be a larger magnitude than any other type (Schiferl et al 2002).
[FIGURE 10 OMITTED]
Since the bearing currents from magnetic dissymmetries take a circulating path end-to-end in the motor, the common remediation method is to simply insulate the opposite drive end bearing. This can be done via an isolated hub within the motor endshield (see Figure 11), or the bearing itself can have insulating properties. Two types of bearing insulation may be available. One uses an insulating coating on the bearing surfaces such that it breaks the otherwise conductive path. Another form of bearing insulation uses ceramic components. This most commonly uses ceramic balls in a deep groove ball bearing. The option shown in Figure 11 provides the benefit of using a more "off the shelf" bearing, since the insulation is provided by the floating or isolated hub. Of course, the ideal situation is to minimize magnetic dissymmetries such that there is no need for an insulated bearing. It should also be pointed out that putting a shaft grounding brush on a motor that has an end-to-end shaft voltage from magnetic dissymmetry simply increases the likelihood of failure of the bearing at the end opposite the shaft brush. This is due to the fact that the brush helps to complete the circulating current path. While it may divert current from the bearing at the end where it is applied, the bearing at the other end pays the price with higher current.
[FIGURE 11 OMITTED]
Since bearing currents due to inverter waveshapes come in several variations, the remediation methods are similarly diverse. For the case of high-frequency circulating-type currents, because the path is the same as that for internally sourced circulating currents, the simple insulation of the opposite drive end bearing is sufficient.
For the case of discharge-type currents, the choice is either to block the current path with an insulator or to short out the voltage with a parallel conductor. If the motor is coupled to the load via a conductive coupling, then protection should be considered not only for the motor bearings but also for the coupled equipment bearings. Any individual bearing can be protected by bearing insulation--either in the way the bearing is mounted (see Figure 11) or in the bearing construction itself. This would apply to either motor bearings or coupled equipment bearings. Since the discharge event can be at any bearing, simply interrupting the current path at one bearing provides no protection of any other bearing. So, in that sense it is quite different than remediation for circulating type currents. An insulating coupling can block the motor shaft voltage from being transmitted to the coupled load.
For the case of CMC causing a jump in motor frame voltage that gets shorted out by the coupled equipment, clearly an insulating coupling is one way to avoid this possibility. When an insulating coupling is not feasible, as is often the case for high torque loads, other methods must be used. Since insulating all of the bearings in the coupled equipment is a rather onerous requirement, that is usually not seen as a solution. Rather, the approach is often to simply "bond" the stationary parts of the motor and coupled equipment together. In that way, the bond connection tends to hold the two bodies at the same voltage potential. Even if there is some potential difference that is shorted out by the shaft extension, the bond connection provides a low impedance path for equalizing currents to take rather than flowing through bearings and shafts. Because of this need to maintain a low impedance bond path between the motor and coupled equipment, this bond is typically created using either flat braided strap conductors or a finely stranded conductor. The fine stranding helps to maintain not just low resistance but also low impedance to high-frequency pulses of current that are expected to flow through the bond. It should also be noted that applying a motor shaft ground brush can increase the currents of this type that are seen in the coupled equipment bearings (Figure 12a). This is due to the brush creating a very low impedance path from the frame to the shaft. This path can then carry current to ground via the coupled equipment bearings. Similarly, in the case of applying a shaft brush on a motor that utilizes a single insulated bearing to prevent circulating-type bearing currents (Figure 12b), the brush can again be an enabler for the bearing currents by reducing the impedance in the current path.
[FIGURE 12 OMITTED]
SUMMARY AND CONCLUSIONS
Bearing currents in electric motors are not new. Internally sourced circulating currents within the motor have been known to exist on line-fed motors for many years. Externally sourced, inverter-induced bearing currents due to CMV are relatively new and present new challenges. Fortunately, these CMV- and CMC-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. A summary of current flow paths was presented here to illustrate potential issues. Once the types of currents that exist in a given system are known, remedies are available for each one. Proper grounding/bonding is a key to minimizing transient voltages and 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 a motor bearing in order to prevent increased current flow through other bearings in the system (see Figure 12b).
Elimination of bearing damage in inverter-driven motors requires a thorough understanding of the inverter-/motor-driven equipment system. High-frequency current paths are not always easy to identify, but proper identification is key to providing an appropriate remediation method.
Ammann, C., K. Reichert, R. Joho, and Z. Posedel. 1988. Shaft voltages in generators with static excitation systems--Problems and solution. IEEE Transactions on Energy Conversion 3(2):409-19.
Chen, S., T.A. Lipo, and D. Fitzgerald. 1996. Source of induction motor bearing currents caused by PWM inverters. IEEE Transactions on Energy Conversion 11(1):25-32.
Erdman, J.M., R.J. Kerkman, D.W. Schlegel, and G.L. Skibinski. 1996. Effect of PWM inverters on AC motor bearing currents and shaft voltages. IEEE Transactions on Industry Applications 32(2):250-59.
IEEE. 1996. IEEE Std-112-1996, Shaft Currents and Bearing Insulation, Section 9.4. New York: Institute of Electrical and Electronics Engineers.
Muetze, A., and A. Binder. 2004. Calculation of circulating bearing currents in machines of inverter-based drive systems. Proceedings of the 39th IEEE IAS Annual Conference 2:720-26.
Pearce, C.T. 1927. Bearing currents--Their origin and prevention. The Electric Journal XXIV(8):372-76.
Quirt, R.C. 1988. Voltages to ground in load-commutated inverters. IEEE Transactions on Industry Applications 24(3):526-30.
Schiferl, R.F., M.J. Melfi, and J.S. Wang. 2002. Inverter driven induction motor bearing current solutions. IEEE Petroleum & Chemical Industry Conference Proceedings.
Michael J. Melfi is a consulting engineer in the Advanced Technology Group of Baldor/Reliance Electric, Richmond Heights, OH.
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|Author:||Melfi, Michael J.|
|Date:||Jul 1, 2008|
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