EASA Shaft Currents 0607
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Dealing with Shaft and Bearing Currents EASA Convention 2007 Minneapolis Convention Center Minneapolis, MN June 25, 2007
Presented by Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MO
DEALING WITH SHAFT AND BEARING CURRENTS By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MO
FIGURE 1: FLUTING
INTRODUCTION This paper addresses key issues related to shaft and bearing currents in electric motors and generators. Critical topics that will be covered include recognizing symptoms of shaft and bearing currents, and determining if damaging current levels are present. Also, possible causes of the damaging current, such as machine dissymmetry and operation on variable frequency drives (VFDs) will be dealt with. Methods of testing to conrm the presence of shaft or bearing currents will be described, as well as how to assess the magnitude of the damaging currents. Further, solutions to eliminate or control shaft and bearing currents, such as insulators, isolators, and ceramic bearings, will be offered. While shaft and bearing currents are not a new problem (papers on the subject date back prior to 1930), what is “new” is the increased understanding of how to identify and solve the problem. Shaft and bearing currents have been described as shaft voltages, circulating voltages, circulating currents and bearing currents. Shaft voltage only becomes a problem when it leads to bearing current and consequential damage to the motor bearings. If this voltage, referred to as “common mode voltage” or “shaft voltage,” builds up to a sufcient lev el, it can discharge to ground through the lubricant lm on the bearings. Current that nds its way to ground through the motor bearings in this manner is called “bearing current.” This paper will primarily refer to the damage phenomenon from shaft or bearing currents as bearing current(s) because it is the current through the bearings (not the shaft) that causes the damage. In cases where the distinction between shaft and bearing currents need to be made, the specic term shaft current(s) or bearing current(s) will be used.
Fluting of the bearing races due to electrical current.
FIGURE 2: FROSTING
RECOGNIZING SYMPTOMS OF BEARING CURRENT All too frequently the rst symptom of bearing current is audible noise from the bearing, indicating it is in advanced stages of failure. Inspection of the bearing after failure may reveal uting of the races (Figure 1), balls or rollers “frosted” (Figure 2), or an overall dull grey or dark “smoky” nish (Figure 3) on both balls/rollers and race ways. The lubricant may also be dark in appearance.
Frosting of the balls/rollers due to electrical current.
CURRENT DAMAGE The appearance of damaged surfaces is related to three major types of current. The rst type of electric current damage is electric pitting (Figure 4). It is mostly
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DEALING WITH SHAFT AND BEARING CURRENTS By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MO
FIGURE 1: FLUTING
INTRODUCTION This paper addresses key issues related to shaft and bearing currents in electric motors and generators. Critical topics that will be covered include recognizing symptoms of shaft and bearing currents, and determining if damaging current levels are present. Also, possible causes of the damaging current, such as machine dissymmetry and operation on variable frequency drives (VFDs) will be dealt with. Methods of testing to conrm the presence of shaft or bearing currents will be described, as well as how to assess the magnitude of the damaging currents. Further, solutions to eliminate or control shaft and bearing currents, such as insulators, isolators, and ceramic bearings, will be offered. While shaft and bearing currents are not a new problem (papers on the subject date back prior to 1930), what is “new” is the increased understanding of how to identify and solve the problem. Shaft and bearing currents have been described as shaft voltages, circulating voltages, circulating currents and bearing currents. Shaft voltage only becomes a problem when it leads to bearing current and consequential damage to the motor bearings. If this voltage, referred to as “common mode voltage” or “shaft voltage,” builds up to a sufcient lev el, it can discharge to ground through the lubricant lm on the bearings. Current that nds its way to ground through the motor bearings in this manner is called “bearing current.” This paper will primarily refer to the damage phenomenon from shaft or bearing currents as bearing current(s) because it is the current through the bearings (not the shaft) that causes the damage. In cases where the distinction between shaft and bearing currents need to be made, the specic term shaft current(s) or bearing current(s) will be used.
Fluting of the bearing races due to electrical current.
FIGURE 2: FROSTING
RECOGNIZING SYMPTOMS OF BEARING CURRENT All too frequently the rst symptom of bearing current is audible noise from the bearing, indicating it is in advanced stages of failure. Inspection of the bearing after failure may reveal uting of the races (Figure 1), balls or rollers “frosted” (Figure 2), or an overall dull grey or dark “smoky” nish (Figure 3) on both balls/rollers and race ways. The lubricant may also be dark in appearance.
Frosting of the balls/rollers due to electrical current.
CURRENT DAMAGE The appearance of damaged surfaces is related to three major types of current. The rst type of electric current damage is electric pitting (Figure 4). It is mostly
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FIGURE 3: DARKENED FINISH
FIGURE 5: DULL FINISH
The darkened nish of the ball on the right was caused by electrical current.
Dull nish and electrical craters in the inner race of a roller bearing.
FIGURE 4: PITTING and races. Crater sizes are small, mostly from 0.00020.0003” (5 - 8 μm) in diameter, regardless of whether the crater is on an inner ring, outer ring or a rolling element. The true shape of these craters can only be seen under a microscope using very high magnication. OTHER SYMPTOMS If a bearing is noisy it is important to remove it from service and dismantle and inspect it prior to complete bearing failure. If the bearing has been destroyed by failure, the evidence of bearing current will also be destroyed and the root cause of the failure will not be determined. Visual indications of shaft voltages include uting or a “picket-fence” pattern on the races of the bearing (Figure 6). The spacing of the uting marks depends on the speed (rpm), bearing diameter, radial load and magnitude of the bearing current. The balls or rollers
Microscopic view of electrical pitting.
related to single crater damage and typically seen in DC applications such as railway traction motors. The size of the crater is from 0.004-0.020” (0.1- 0.5 mm) in diameter and is visible to the naked eye. Such craters are usually produced by a very high voltage source. The next type of damage is uting, which is a pat tern of multiple lines across the inner and outer races (Figure 1). The reason for this uting is mechanical resres onance vibration caused by the dynamic effect of the rolling elements as they roll over smaller craters in the races. Strictly speaking, uting is not a primary mode of failure produced by the current ow through the bear ing itself. Rather, it is secondary bearing damage that becomes visible only after a period of time, and it has the craters as its initial point. The third type of current damage, micro-cratering is the most common type of current damage when the motor is powered by a VFD. The damaged surface appears dull and is characterized by molten pi t marks (Figure 5). Multiple micro-craters cover the rolling elements
FIGURE 6: “PICKET FENCE”
Note the “picket fence” pattern of the bearing outer race. This is often a visual clue to shaft voltages.
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may have a dull grey or dark “smoky” nish (Figure 3). If the motor speed was varying, the races may also have a frosted pattern. The grease may be black in appearance, due to the burning of the metal that leaves ferrous oxide. Bearing current damage usually initially appears visually in the areas of the bearing that are most heavily loaded. The reason for this is that the lubricant lm will be thinnest in the areas subjected to the heaviest load. For example, in a belt drive application the damage to the bearing will be most pronounced in line with the direction of the belt tension. Regarding lubricant lm thickness, studies have found that a bearing usually has a lubricant lm thickness of 0.2-2.0 μm (0.0000080.00008”) at normal operating speeds. Given this lm thickness, damaging bearing currents can be caused by 60-Hz shaft voltages as low as 0.2–2 V peak. Bearing currents can also cause the lubricant in the bearing to change its composition and degrade rapidly. The locally high temperature causes lubricant additives and the base oil to react, often causing burning or charring of the base oil. Additives will then be used up more quickly and the lubricant becomes hard and blackened. A rapid breakdown of the grease is a typical failure mode that results from bearing current.
the presence of damaging bearing currents. If the shaft to frame voltage exceeds 100 millivolts AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough the degrade the bearings. Another test method, based on NEMA MG1, is to measure the shaft voltage from end to end of the shaft. If the voltage exceeds 300 millivolts AC, bearing damage may occur. The NEMA method uses the same 300 millivolts AC limit for any type bearing. The magnitude of the shaft current can also be an indicator of the presence of damaging shaft current. If a welding cable is touched to both ends of the shaft and a spark is generated, that is an indicator of shaft current, though the magnitude is unknown. If the current can be measured, a value in excess of 20 A is considered an indicator that damaging levels of bearing current are present. VFDs can generate a “common mode voltage” which raises the three phase winding neutral potential signisignicantly above ground potential. For 3-phase motors, the sum of the 3 phases equals zero for sinusoidal power. For a VFD, where each phase is rectied, the common mode voltage is the instantaneous sum of the 3 phases. When the 3- phase output from the drive is rectied, DC is either positive or negative; and the common mode voltage is approximately equal to the RMS voltage. The inherent magnetic dissymmetry can result in shaft and
DETERMINING IF DAMAGING CURRENT LEVELS ARE PRESENT At present there is no known method to measure bearing currents and no practical way to directly measure shaft currents. MeaFIGURE 7: ROGOWSKIsuring shaft current STYLE COIL would require placing a current transformer coil around the shaft inside the motor. In rare cases, with the internal area of the motor accessible, a “Rogowski” coil (Figure 7) can be wrapped A Rogowski-style coil that can around the shaft and be wrapped around a shaft to used to measure shaft measure shaft current. current. The usual way to detect the presence of potentially detrimental shaft and bearing current is to measure the voltage from shaft to ground, that is, from the shaft to the motor frame.
FIGURE 8: CAPACITIVE COUPLING Grease Inner race
Ball bearing Outer race
Shaft Rotor Faraday Shield Rotor to ground
Rotor to winding
s g n i d n i W
SHAFT TO FRAME VOLTAGE Among the challenges in measuring the shaft to frame voltage is that the bearings change from an insulating mode to a conducting mode in a somewhat random manner. Consequently it is necessary to use the highest voltage measured when assessing the possibility of
Stator
VFD in Capacitive coupling between the stator and the rotor supplied from the VFD.
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bearing currents. This common mode voltage oscillates at high frequency and is capacitively coupled to the rotor (Figure 8). The result is pulses as high as 25 volts from shaft to ground, with the current path being through either or both bearings to ground. If a motor is supplied from a VFD, a good practice would be to check the frame to shaft voltage and determine if damaging bearing current may be present.
could be measured. It was found that a current of 5- 200 mA, depending on bearing size, did not cause damage to the bearings. Further, with one bearing insulated the current level was reduced to less than 40% of the value with uninsulated bearings; and in the case of both bearings insulated, the currents were reduced to less than 20%. It was also found that applying lters or reactors between the drive and motor reduced shaft current by 30-50%.
BEARING CURRENT If the shaft voltage caused by a VFD is large enough, current can ow through the shaft and both bearings (Figure 9) and, in some cases, through the shaft and bearings of the load machine. This circulating current may damage the bearings if the magnitude exceeds 320 A, depending on the size of the bearings, the rate of rise of the voltage at the motor terminals and the inverter DC bus voltage level.
RESIDUAL MAGNETISM Bearing currents can result if a shaft has residual magnetism. Residual magnetic eld levels are best meamea sured with the machinery not operating. A gaussmeter can be used in the DC mode to measure the residual magnetic eld level of a shaft. Magnetic eld strength can change if the motor is disassembled because of the alteration in the ux paths. For example, when a coupling is removed, the adjacent shaft magnetic eld level may increase or decrease. Therefore it is best not to disturb any shaft components so that the measured magnetic eld levels are representative of the actual operating conditions. The following table is based on test results compiled by one investigator from more than 200 machines over a 15 year period.
FIGURE 9: PATH OF CIRCULATING CURRENT
Maximum allowable residual magnetic eld levels (mea(mea sured in free air) 2 gau gauss ss
Bear Be arin ing g com compo pone nent nts, s, be bear arin ing g jou journ rnal als; s; se seal als, s, gears and coupling teeth.
4 gaus s
Bearing housings.
6 gau gauss ss
Midd-sh shaf aftt an and rot rotor or co core re ar area ea..
An important factor is that the measured magnetic eld levels are only fringing values and may be considconsid erably lower than the actual levels inside a component. POSSIBLE CAUSES OF THE DAMAGING CURRENT There are three main sources of circulating currents that may pass through the shaft and consequently the bearings. One source is magnetic dissymmetry, another is electrostatic discharges, and the third is capacitive coupling between the stator windings and rotor. Any of these sources may be present independently or simultaneously,, and thus result in bearing currents. taneously Other sources of shaft voltage include supply voltage unbalance, circulating currents in unbalanced parallel circuits of a 3-phase winding, and transient voltage conditions. Less common causes of shaft and bearing currents include non-insulated through-bolts in the rotor or armature, eccentric air gap, and sections of shorted iron in the rotor or stator cores. Any of these sources may
Circulating curent path through the shaft, bearings and frame.
The reason that the magnitude of current varies with the size of a bearing is i s that it is more directly related to current density. The current density is the current divided by the rolling element contact area, and can cause damage when it exceeds 1 A/mm2 (645A/in2). The difculty with this tolerance is that the current density cancan not be measured in an actual application because the contact area is normally not known, and can vary. Interesting results were found in experimental studies where the shaft current caused by capacitive coupling between stator and rotor of motors supplied by VFDs
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be present independently or simultaneously, and thus result in bearing currents.
es abutting each other (like slices of a pizza pie). The space between each adjacent lamination segment varies slightly, and each layer of laminations adds to the distortion. The nished core thus amplies the minute distortions since they effectively add up as they distort the magnetic circuit. Smaller cores that use single piece laminations are much less likely to cause magnetic distortion, but it is still possible. A potential reason is that one of the main qualities of magnetic steel is a property termed permeability, which can vary from one part of the circular lamination to another part. The permeability is associated with the ux density at which the core steel will magnetically saturate. Thus, if the permeability of a lamination varies, some parts of it may become saturated and become hotter, leading to thermally induced distortion such as bowing. For this reason, some motor manufacturers shift laminations circumferentially when stacking 2-pole cores, because with the low number of poles, these are the most likely to be affected by changes in permeability.
MAGNETIC DISSYMMETRY Magnetic dissymmetry is due to any differences in the magnetic circuit between stator and rotor. It is often associated with larger motors that have segmented laminations and are supplied from sinusoidal (as opposed to VFD) power sources. The resulting asymmetric ux in the motor results in a low frequency circulating current through the bearings. Although dissymmetry in the core laminations is not the only source of magnetic dissymmetry, it is the predominant cause. Other causes of magnetic dissymmetry include an uneven air gap between stator and rotor, and damage to the stator core laminations such as from a winding ground fault. Magnetic dissymmetry is also common in DC machines, where the individual eld poles and interpoles are likely to have variations in individual air gap. An informal study found shaft currents in most DC machines larger than 10 hp (7.5 kW.) In any motor, including those with one piece rather than segmented laminations, the magnetic core steel is not completely uniform in composition. Because of this, ux paths through the core are not completely symmetrical, resulting in time-varying ux lines that enclose the shaft. These induce a current ow through the shaft, through a bearing, through the frame, and back to the shaft through the other bearing (Figure 9). Slight differences in the spaces between the pieces of segmented lamination cores (Figure 10), and how they stack layer by layer, lead to a lack of symmetry in the stator core. To visualize this, think of one layer of the core laminations as a circle made of many piec-
ELECTROSTATIC DISCHARGES The second major cause of damaging bearing current is somewhat surprising, namely, static electricity. We don’t usually consider static electricity as a source of harmful currents. However, damaging levels of bearing current caused by electrostatic discharges (Figure 11) can occur in applications such as belt drives, paper roll winders and fans or blowers. The larger the machine is (e.g., wind generators, some with 330 ft. (100 meter) blades), the greater the potential bearing current. FIGURE 11: DULL FINISH
FIGURE 10: SEGMENTED LAMINATIONS
Dull nish and electrical craters in the inner race of a roller bearing.
CAPACITIVE COUPLING Capacitive coupling between stator and rotor often occurs when the motor is supplied from a VFD. Voltage associated with the extremely fast switching of insulat-
Segmented laminations, each make up 1/8 of a layer of the stator core.
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though this does not appear to have been proven, it has been found that bearings operating at constant speed are more susceptible to damage from the EDM action associated with electrical current discharges. If the motor and drive are not effectively grounded to each other and to the electrical system ground, the voltage magnitude can increase. That is because it is a common mode voltage, meaning the voltage potential from the motor winding neutral is raised above the normal ground potential level. Also, this common mode voltage oscillates at a very high frequency and capacitively couples the stator to the rotor. The higher the common mode voltage and VFD switching frequency, the greater is the possibility of damaging bearing currents. It is recommended that stranded, low-impedance, ground cable (Figure 13) be used to establish a dedicated common ground path between motor and drive as skin effect is a factor. The higher the frequency, the more alternating current (AC) “travels” on or near the surface (“skin”) of the conductor, so the resistance-to-voltage ow is affected by the surface area of the conductor. Stranded cable has more surface area than a solid conductor, thus provides a lower resistance (impedance) path.
FIGURE 12: PWM WAVEFORMS
Voltage (top) and current (bottom) waveforms of a PWM drive.
ed gate bipolar transistors (IGBTs) in the pulse-width modulated (PWM) output waveform (Figure 12) of the VFD is induced in the rotor and shaft via the air gap between stator and rotor. The IGBT switching frequency, also termed the carrier frequency, is the rate at which it “chops” direct current into simulated blocks of AC in the DC bus of the inverter section of the drive. The induced voltage caused by switching builds up in potential on the rotor until it exceeds the dielectric breakdown capability of the bearing lubricant, resulting in a brief discharge pulse through the bearing to ground. This process continually repeats itself, thus increasing the magnitude of the damage to the bearing. The discharge pulses are a form of electrical discharge machining (EDM) that literally removes material from the surfaces of the affected parts of the bearings (Figure 11). In a similar manner, the voltage associated with the “chopping” of AC into DC power by means of transistors and silicon controller rectiers (SCRs) in a DC drive can cause damaging bearing currents in a DC motor. The higher the switching frequency, the higher the rate of the current discharge pulses and the faster the damage will occur. VFDs are typically audibly quieter at higher switching frequencies; however, the trade-off is that the higher frequencies are more destructive than lower frequencies. When possible, the switching frequency should be adjusted as low as possible without creating unacceptable audible noise levels, preferably avoiding frequencies above 6 kHz. It is usually good practice to use a VFD with a carrier frequency that can be adjusted in increments of less than 1 kHz. That allows ne-tuning of the carrier frequency to the lowest level that provides acceptable operation. The experience of some users and failure analysts has led to speculation that constant speed operation increases the probability of bearing current damage. Al-
FIGURE 13: STRANDED CABLE
Stranded low-impedance cable made specically for VFD applications.
METHODS OF TESTING TO CONFIRM AND ASSESS THE PRESENCE OF SHAFT OR BEARING CURRENTS The most common methods of testing to conrm the presence of shaft and bearing currents are vibration analysis and voltage to frame (ground) analysis. Other techniques that can be used include lubricant analysis and microscopic analysis, though the microscopic anal-
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ysis can only be applied after a bearing has failed. Although diagnostic testing may reveal the presence of potentially damaging bearing currents, a better approach is condition monitoring. Condition monitoring begins with establishing baseline values for the test measurements and then trending these measurements over time. An increased level, that is, an upward trend, indicates probable need for action. The early detection of an upward trend also normally allows time for a planned outage or other course of action. That is, action is taken on a planned rather than a crisis basis.
voltage is that the bearings change from an insulating mode to a conducting mode in a somewhat random manner. Consequently it is necessary to use the highest voltage measured when assessing the possibility of the presence of damaging bearing currents. Shaft voltage can be measured using a digital multimeter (DMM) set to the voltage scale. The impedance of the meter will affect the resulting measurements; higher impedance meters are more accurate. Connect one lead to the frame of the machine. Take a #2 pencil and remove the wood at the eraser end, to expose the carbon “lead”. Use an ohmmeter to verify that the graphite “lead” in the pencil is not broken, by checking the resistance from the former eraser end to the writing tip (point). Attach the other lead of the DMM to the pencil lead, and allow the pencil point to contact the revolving shaft. The contact point should be as close as possible to the bearing (Figure 15).
Vibration analysis Vibration analysis can be used to determine if bearings have uting damage, which may have been caused by discharge currents. A high resolution spectrum of the 2-4kHz range (Figure 14) can indicate an abnormally high band of energy, that is, a hill-like shape as the amplitude gradually increases to a rounded peak and then decreases. As the fault condition progresses over time, vibration levels at the bearing fault frequencies will become apparent. Some vibration analyzers have the ability to measure “spike energy” or g’s of acceleration; each is a means of quantifying impact energy to diagnose impending bearing failure.
FIGURE 15: MEASURING SHAFT VOLTAGE
FIGURE 14: VIBRATION SPECTRUM
Shaft voltage can be measured with a DMM. Note the use of a pencil to contact the shaft.
If the voltage exceeds 100 millivolts AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough the degrade the bearings. Another test method, based on NEMA MG1, is to measure the shaft voltage from end to end of the shaft (using two pencil points and the DMM). If the voltage exceeds 300 millivolts AC, bearing damage may occur. The NEMA method uses the same 300 millivolts AC limit for any type bearing. Note: When a motor is operating from a VFD, the duration of the voltage discharges may be only a few nano-seconds. Conventional DMM’s may consequently indicate much lower voltages than the oscilloscope test described later in this paper.
High resolution vibration spectrum.
Shaft to frame (ground) analysis As mentioned previously, there is no known method to measure bearing currents and no practical way to directly measure shaft currents. Measuring shaft current would require placing a current transformer coil around the shaft inside the motor. The usual way to detect the presence of potentially detrimental shaft and bearing current is to measure the voltage from shaft to ground, i.e., the motor frame. Shaft voltage Among the challenges in measuring the shaft to frame
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of copper cable. The test is completed by measuring the shaft potential to the frame at each of the other bearings. A high impedance oscilloscope is then connected with one lead grounded to the frame and the other lead attached to the shaft brush. It is preferable to use a low-impedance shielded conductor for the oscilloscope leads to minimize electromagnetic interference. This shield should be grounded at one end only. After connecting the leads, the peak voltages are then measured. The second method is an alternative to the oscilloscope, using a high-impedance voltmeter. Both AC and DC voltages should be measured at each bearing. The peak voltage can be estimated by adding the DC level and 1.4 times the AC rms level. This estimated peak voltage, however, may be considerably below the actual peak value. Another alternate method involves measuring the AC voltage with shaft riding brushes contacting opposite ends of the shaft, with the motor operating at rated voltage and speed. The procedure is identical to that of the rst method, with the exception being that a low-resistance ammeter is used in place of the oscilloscope. In this test arrangement, the ammeter is used as a lowimpedance, uncalibrated voltmeter. The meter readings may not be a true indication of the current that might ow when there is a breakdown of the lubrication lm in the bearing(s). This method is useful if a history of similar tests is available for comparison. The IEEE standard does not provide any tolerance or threshold levels for determining if damaging bearing current levels are present. However it suggests that trending of these tests can be useful for detecting increased levels and therefore the increased probability of damaging bearing currents.
Shaft current An “old-timers” method of checking for damaging levels of bearing current was to use a welding cable and touch each end of it to each end of the motor shaft while in operation. If a spark was generated when the second end of the cable was applied to or removed from the shaft it was assumed that damaging bearing current was present. A modern variation of that test is to apply the welding cable as a shorting bridge from shaft end to shaft end and to measure the current through the cable. A current in excess of 20 A is considered an indicator that damaging levels of bearing current are present. IEEE 112 methods The following test procedures are based on three alternative methods of measuring shaft potential for shaft circulating currents described in IEEE standard 112. In motors that have all or all but one bearing insulated, the tests can be used to detect the presence of shaft potential while the motor is operating. First, a shaft riding brush (Figure 16) is used to short out the uninsulated bearing (or one bearing, if all are insulated). The brush is applied to the shaft near the bearing and connected to the frame with a short piece FIGURE 16: SHAFT GROUNDING BRUSH
Lubricant analysis Lubricant analysis utilizes a testing laboratory to analyze the bearing lubricant. Grease sampling is often impractical as there is usually no direct means of obtaining a fresh sample unless the motor is partially disassembled. Grease that is extracted from the relief port is unreliable as the length of time since it last lubricated the bearing cannot be determined. Oil used to lubricate bearings is normally available for sampling by simply accessing the oil ring area of a sleeve bearing machine, or draining some oil from a reservoir of an idle oil-lubricated motor, either rolling element bearing or sleeve bearing. It is not advisable to attempt to draw a sample on an operating motor because of the risk of a rapid loss of oil. If the lubricant analysis indicates the presence of metals from the bearings, electrical discharge machining (EDM) associated with bearing current discharges are a possible cause. There is however, no means of distin-
“Toothbrush” style shaft grounding brush shorts out bearing current path.
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guishing between particles from the passage of electrical current and particles of mechanical wear origin.
Magnetic dissymmetry Magnetic dissymmetry can result in a circulating current from the frame through a bearing, along the shaft, through the other bearing and back to the frame. The circulating current in this case can be eliminated by interrupting the circuit, usually by insulating the opposite drive end bearing (Figure 17). Bearing current due to dissymmetry can be reduced, but not eliminated, by installing a shaft grounding brush. That is, if the motor can be modied to accept such an installation; and if it is economically viable. The brush circuit from frame to shaft provides a low resistance parallel (bypass) circuit (Figure 9) to di vert current from the much higher resistance bearing circuit. The lower the resistance of the brush circuit compared to the resistance through the bearing, the more current is diverted from the bearing. To be most effective, the brush should be located inboard of the bearing, that is, between the bearings. If it is located outboard of a bearing it will be outside of the circulating current “loop” of the motor. Note that the brush resistance is critical to this solution. The special grounding brushes supplied for this purpose are extremely low in resistance (Figure 18). Conventional carbon brushes lack the required low resistance, and so brush manufacturers use brushes that are impregnated with silver. Another type of shaft grounding brush is more like a brush in that it has bristles that contact the shaft surface (Figure 16). Manufacturers of this type of grounding brush usually use silver or gold impregnation in the bristles, for low resistance and to maintain a low resistance lm on the shaft. If the brush path diverting current from the bearing increases in resistance, such as due to contamination or a change in the lm, more potentially damaging cur-
Microscopic analysis Microscopic analysis can be performed by a testing laboratory with that capability, often a bearing manufacturer. If you procure a shop-type microscope you may be able to perform your own analysis. The shop-type microscope can be placed directly on the bearing to magnify the area to be assessed. Electrical current will usually result in molten particles. Damage of mechanical origin often appears as smearing marks with evidence of heating due to mechanical friction. SOLUTIONS TO ELIMINATE OR CONTROL SHAFT AND BEARING CURRENTS The rst step in addressing solutions to shaft and bearing current issues is to determine the type of source that is the probable cause. We have previously identied these sources, and it is worth repeating them, to make clear what they are and then proceed to address eliminating or controlling them. There are three main sources of circulating currents that may pass through the shaft and consequently the bearings. One source is magnetic dissymmetry, another is electrostatic discharges, and the third is capacitive coupling between the stator windings and rotor. Any of these sources may be present independently or simultaneously, and thus result in bearing currents.
FIGURE 17: INSULATED OPPOSITE DRIVE END BEARING
FIGURE 18: SHAFT GROUNDING CARBON-SILVER BRUSH
Low-resistance silver-impregnated brushes are suitable for use as shaft grounding brushes.
Circulating current due to magnetic dissymmetry can be eliminated by insulating one bearing.
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rent will ow through the bearing. The part of the brush circuit most at risk for increased resistance is the brush to shaft contact surface. As the shaft surface becomes dirty or corroded the resistance of the lm formed in the brush path increases. And higher resistance in the brush circuit results in less current through it, and correspondingly higher current through the bearing. To most effectively protect both motor bearings a grounding brush should be placed on the inboard side of each bearing. Therefore, two brush installations are needed. However, if there is a possibility of circulating current between the motor and the driven equipment, and only one brush is used, it should be on the drive end.
FIGURE 19: INSULATING BOTH BEARINGS
Bearing is electrically isolated to interrupt flow of current.
Electrostatic discharges The second major cause of damaging bearing current is static electricity. Damaging levels of bearing current caused by electrostatic discharges can occur in applications such as belt drives, paper roll winders and fans or blowers. The solutions previously mentioned for magnetic dissymmetry can be used to address electrostatic discharges. In some applications, such as rolls of process product that develop static electricity, a grounded garland-like material contacts the roll to “bleed off” the static electricity.
Both bearings insulated to prevent bearing current.
shaft grounding brushes for both bearings. The bearings can be insulated by a number of different methods including, insulated housings, insulated bearing journals, insulated bearing outer race, insulated bearing inner race, and ceramic rolling elements. It is worth noting that insulating only one bearing, such as the non-drive end is not effective as the current still has a path through the drive end bearing. Conductive bearing greases are also available, using metallic particles suspended in the grease to make it conductive. However there has not been strong evidence indicating that this method is effective. One laboratory study found indications that the conductive material in the grease accelerated mechanical wear and would therefore shorten the life of a bearing. Another key point to put the capacitive coupling issue in perspective is that VFDs are the source. If future drive technology can make advances in this area it may be possible through drive design to eliminate bearing currents due to capacitive coupling.
Capacitive coupling Capacitive coupling between stator and rotor (Figure 8) can occur when the motor is supplied from a VFD. These drives can generate a “common mode voltage” which raises the three phase winding neutral potential signicantly above ground potential. The result is discharges from shaft to ground, with the current path being through either or both bearings to ground. According to the International Electrotechnical Commission (IEC) standard IEC 60034-25, “Guidance for the design and performance of a.c. motors specically designed for converter supply” some of the physical factors and other features of a motor that can lead to shaft voltages and consequently bearing currents include: • Large physical size or high output power of the machine tends to increase the induced shaft voltage. • The physical shape of the motor also has an effect on the induced shaft voltage: short and fat shape is generally better than long and thin motor design. • High pole numbers tend to reduce the induced shaft voltage. • Low running speed and high bearing temperature as well as high bearing load increase the bearing current risk due to thinner lubricant lm.
Insulated bearings The key word in the term capacitive coupling is “capacitive”. What is desired is to have a low capacitance if the circuit is insulated. To achieve this with an insulated bearing it is better to insulate the inner race (journal face) rather than the outer race (housing face) as the inner race being of smaller diameter it is likewise smaller in area. This is an advantage of the hybrid ceramic bearing: The capacitance is based on the small contact area between the rolling elements and the races, and the thickness of the insulation is equal to the diameter
Capacitive coupling solutions Solutions when dealing with capacitive coupling include insulating both bearings (Figure 19), and using
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as the rotor can not be placed on the coated journals in the balancing stand. An adjacent shaft area that is concentric to the bearing journal should be used. The line-loading of the balance stand rollers combined with the rotor weight would probably fracture the ceramic or aluminum oxide coating. If an epoxy is used it must be able to withstand the bearing compression and load forces. Wear-resistant and high-strength epoxy products such as Belzona Supermetal or Devcon Titanium putty are often suitable, but other less durable epoxies should be avoided. Sleeve bearings may be ceramic or aluminum oxide coated on the outer shell to mitigate bearing current. The coating can be applied by the original manufacturer, or as part of a retrot. The oil lm in the bearing also acts as an insulator to prevent or reduce bearing current. Thus coating of the outer race of a sleeve bearing is more effective against bearing currents than coating a rolling element bearing. Care is needed in handling the bearing as the ceramic or aluminum oxide coating cracks easily. Other methods of insulating sleeve bearings include adding Micarta or Glastic (berglass) between the outer shell and the housing (Figure 21), applying Scotchply to the outer shell of the bearing, and applying armature type banding material to the outer shell of the bearing. Except for the last method, these are all usually effective. The banding material tends to delaminate and degrade due to the lubricating oil coming in contact with it; therefore this method is not recommended.
of one rolling element. Whenever insulation is added, heat transfer is reduced compared to when there is metal to metal contact. An inherent trade off with bearing insulation is to use the thickest coating possible without signicantly affecting heat transfer, and consequently causing the bearing operating temperature to increase. Some bearing manufacturers offer bearings with ceramic or aluminum oxide coatings on the outer race. When the outer race rather than the inner race is insulated, a thicker ceramic or aluminum oxide coating, sometimes as thick as 0.012” (300 μm) is used. Be cause these coatings cover a wider area than on the inner race, they are not as effective as most of the other bearing insulating options. If the bearing or bearings in a motor are found to have an insulated coating on the outside of the outer race, and inspection does not reveal evidence of bearing currents, it is reasonable to consider replacing with the same type of bearing. Conversely, if evidence of damaging bearing current is detected, consider using a more effective insulation method. If inspection of an insulated bearing reveals damage from bearing currents, inspect the bearing cap and bracket as an assembly. Undesirable contact between the bearing and the bearing cap or bearing resistance detectors (RTDs) may be bypassing the insulation, rendering it ineffective. A drawback to the ceramic or aluminum oxide coating is that it chips easily. This can occur when a bearing is removed, or if the coating is accidentally struck. Ceramic or aluminum oxide coating of a bearing journal (Figure 20) makes dynamic balancing more difcult
FIGURE 21: INSULATED SLEEVE BEARING
FIGURE 20: INSULATED BEARING JOURNAL
Sleeve bearing outside diameter insulated with Glastic G-9.
Insulated housings Insulating the housing (Figure 22) is frequently done by the original manufacturer and by rebuilders as a ret-
A potential drawback to ceramic coatings is that they chip easily.
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able and are expensive even when compared to hybrid ceramic bearings. The insulation thickness of the hybrid bearing is the diameter of the rolling element (i.e., the ceramic ball or roller). The surface area is limited to the small contact area between the balls and the races. The ceramic rolling element therefore results in an insulation thickness much greater than that of coated race options, and with a much lower capacitance. At one time the load rating of hybrid ceramic bearings was typically less than that of comparable conventional all-steel bearings. However, many manufacturers now offer hybrid bearings with dynamic load capacity equal to or in some cases greater than conventional steel bearings. It is good practice to verify the dynamic load rating of a ceramic bearing that is to be used in place of a conventional steel bearing. Another advantage of ceramic rolling elements is that they are lighter than steel. Consequently, the ceramic bearing is capable of higher operating speeds than comparable steel rolling element bearings.
FIGURE 22: INSULATED HOUSING
Insulated rolling bearing housing.
rot method of providing bearing insulation. The thicker the housing insulation, the lower the capacitance, and consequently, the potential bearing current is lower. However, thicker insulation inhibits heat transfer, so this method always requires a judgment in thickness to be used.
Other solutions Where it is possible, other solutions include insulated couplings (between motor and driven equipment or accessory such as tachometer and motor), insulating the bearings of the driven equipment, or adding shaft brushes to the driven equipment. In belt drive applications, the belts may or may not effectively diminish the capacitive shaft voltage of the motor.
Hybrid ceramic bearings The use of ceramic rolling element bearing offers a simple, though relatively expensive, way to insulate the bearings. These bearings are termed “hybrid ceramic bearings” (Figure 23) because although the rolling elements are ceramic, the inner and outer races are of hardened bearing steel composition. Completely ceramic bearings, with ceramic races as well as rolling elements, are made but are not normally readily avail-
Precautions with insulating bearings Insulating both motor bearings is normally effective in eliminating bearing currents in the motor. However, doing so can create a current path between the motor shaft and the connected equipment, e.g., fan, pump, gearbox. If that is the case, damage due to bearing current can occur in the driven equipment and accessories such as tachometers. Solutions in such a case include an insulated coupling or adding a shaft grounding brush to the drive end bearing of the motor. The coupling interrupts the shaft current path and the grounding brush creates a bypass circuit for shaft and bearing current, and signicantly reduces the shaft voltage. When the bearing housing is insulated, provision must be made to insulate the end faces of the bearing. Axial changes in bearing location could cause the bearing to contact a bearing cap, which if not insulated would bypass the bearing insulation path. Common methods of insulating the bearing face are to apply an insulation material such as Micarta or Glastic (berglass) to the face of the bearing caps on each side of the bearing.
FIGURE 23: CERAMIC BEARING
Shaft grounding brushes Adding grounding brushes to motors with insulated
Hybrid ceramic roller and ball bearings
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bearings is not the primary application for shaft grounding brushes. They are more commonly used “standalone” to divert shaft current away from the bearings. Although they can be effective in VFD applications, insulating the bearing circuit is usually the preferred method. That is because the shaft grounding brush, as a bypass or parallel circuit, will always allow some current to pass through the bearings. To be effective, shaft grounding brushes must maintain a very low resistance (actually low impedance) circuit between motor frame and shaft. The magnitude of the shaft current in most cases does not create a very high current density in the grounding brush. That in turn can lead to excess lm formation, with the lm being of increased resistance compared to the bare shaft surface. As the lm resistance increases, the grounding brush circuit carries less current, and the bearing more current. The result is more rapid degradation of the bearing due to the increased rate of electrical discharge machining (EDM), and premature bearing failure. Conventional carbon-silver (Figure 18) or bristle-type (Figure 16) shaft grounding brushes also represent a maintenance issue as they wear over time and must be replaced, and should be inspected periodically. There is a recent development in shaft grounding brush technology that uses an atmosphere of microscopic carbon bers to create a conductive path from frame to shaft. The device is similar in appearance to a shaft lip seal, and is known as the Aegis “shaft grounding ring™” (Figure 24).
Faraday shield One other method of dealing with capacitive coupling is to install a Faraday shield in the stator. This method typically uses a grounded copper foil material on top of the stator windings. It creates an electrostatic shield that reduces the magnitude of currents caused by the capacitive coupling between stator and rotor. The Faraday shield is of complex and fragile construction, making it impractical and uneconomical for mass-produced motors. Grounding The quality of the ground between motor and drive is also a critical factor. The higher the common mode voltage and VFD switching frequency is, the greater the possibility of damaging bearing currents. One alternative is to reduce the switching frequency of the VFD. In many applications the switching frequency can be reduced without negatively affecting drive or motor performance. A partial solution that can reduce bearing currents is to use shielded and stranded ground cable (Figure 13) to establish a dedicated common ground path between motor and drive as skin effect is a factor. The higher the frequency, the more alternating current (AC) “travels” on or near the surface (“skin”) of the conductor, so the resistance-to-voltage ow is affected by the surface area of the conductor. Stranded cable has more surface area than a solid conductor, thus provides a lower resistance (impedance) path, and the shielding helps reduce the high frequency effects. Filters and chokes Installing lters between the VFD and motor also helps reduce the magnitude of bearing currents, but does not eliminate the current. The lters modify the VFD output waveform so as to minimize the common mode voltage. The main drawbacks to lters are their cost and the complexities associated with their installation. Adding lters, or chokes (modify the current waveform), also helps reduce transient voltages that could overstress the motor windings and increase audible noise.
FIGURE 24: SHAFT GROUNDING RING
Carbon-micro-ber shaft grounding brush.
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EASA Convention 2007
June 25, 2007
Dealing with Shaft and Bearing Currents 2007 EASA Convention
Minneapolis, MN
Tom Bishop Electrical Apparatus Service Association St. Louis, MO
Introduction • Address key issues related to shaft and bearing currents Electric motors and generators
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• Critical topics include Symptoms of shaft and bearing currents
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Determining if damaging current levels are present
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Minneapolis, MN
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EASA Convention 2007
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Introduction • Critical topics also include Causes of damaging current such as
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• machine dissymmetry • operation on variable frequency drives (VFDs)
Methods of testing
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Assessing magnitude of damaging currents
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Solutions to eliminate or control shaft and bearing currents
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Introduction • Shaft and bearing currents problems not new !
Papers on the subject date back prior to 1930
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Identifying and solving for these currents are “new”
• Shaft and bearing currents synonymous terms
Minneapolis, MN
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Shaft voltages
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Circulating voltages
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Circulating currents
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Bearing currents
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EASA Convention 2007
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Introduction • Shaft voltage only a problem if: !
It leads to bearing current and
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Consequential damage to motor bearings
• Terminology clarification: shaft and bearing currents !
Damage is due to bearing current(s)
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If distinction is needed, specific terms will be used • Shaft current(s) • Bearing current(s)
Bearing current symptoms • First symptom often audible noise Indicates advanced stages of failure
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• Inspection after failure Fluting of races
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Balls or rollers “frosted”
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Bearing current symptoms • Inspection after failure !
Overall dull grey or dark “smoky” finish on both balls/rollers and raceways
• Remove noisy bearing from service !
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Inspect prior to complete bearing failure Evidence of cause may be destroyed by failure
Current damage • Appearance relates to: Electric pitting
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Fluting
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(not shown here)
Micro-cratering
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Electric pitting • Mostly related to single crater damage Typically seen in DC motor applications
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• e.g., traction motors
Crater diameter 0.004-0.020” (0.1- 0.5 mm)
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Visible to naked eye
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Usually produced by very high voltage source
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Fluting • Multiple lines across inner and outer races • Cause: Mechanical resonance vibration Dynamic effect on rolling elements
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• Passing over small craters in races
Fluting is secondary bearing damage
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• Not produced by current flow through bearing • Originates in craters
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Micro-cratering • Most common current damage when motor powered by VFD
• Damaged surface appears dull !
Characterized by molten pit marks
• Micro-craters cover rolling elements and races !
Crater sizes 0.0002-0.0003” (5 - 8 µm) diameter
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Need very high magnification microscope to see
Other symptoms • Visual indication of shaft voltages !
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Minneapolis, MN
Fluting or “picket-fence” pattern on bearing races Fluting marks spacing depends on • Speed (rpm) • Bearing diameter • Radial load • Magnitude of bearing current. If motor speed was varying races may have frosted pattern
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EASA Convention 2007
June 25, 2007
Determining if damaging current levels are present • At present: !
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No known method to measure bearing currents No practical way to directly measure shaft currents • Need CT around shaft inside motor
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Measure voltage from shaft to ground (frame)
Rogowski coil
Shaft to frame voltage • Shaft current probably high enough to degrade the bearings if: !
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Minneapolis, MN
Shaft to frame voltage exceeds 100 mv AC for ball or roller bearings Shaft to frame voltage exceeds 200 mv AC for sleeve bearings Shaft end to end voltage exceeds 300 mv AC limit for any type bearing (NEMA MG1)
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EASA Convention 2007
June 25, 2007
Shaft to frame voltage • VFDs can generate “common mode voltage” Raises winding neutral significantly above ground potential
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• Oscillates at high frequency • Capacitively coupled to rotor
Shaft to frame voltage • “Common mode voltage” !
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Minneapolis, MN
Pulses as high as 25 volts shaft to ground • Current path through either or both bearings Good practice: • Check frame to shaft voltage on VFD motors
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EASA Convention 2007
June 25, 2007
Bearing current • Current can flow through the shaft and both motor bearings Possibly through shaft and bearings of load machine Current may damage bearings if >3-20 A
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Bearing current • Experimental studies found: One bearing insulated vs uninsulated bearings
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• Current level was reduced to < 40%
Both bearings insulated vs uninsulated bearings
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• Current level was reduced to 100 millivolts AC for ball or roller bearings
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Shaft to frame voltage >200 millivolts AC for sleeve bearings
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Shaft voltage measurement • Shaft current probably high enough to degrade the bearings if: !
continued
Shaft end to end voltage >300 millivolts AC for any type bearing • Per NEMA MG1
• Motor operating on VFD ! !
Voltage discharge duration a few nano-seconds DMM voltage indication may be low • Use oscilloscope test described later
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Shaft current measurement • Magnitude can be indicator of damage !
“Old-timers” method: • Short circuits circulating current • If spark is generated when welding cable is touched to both ends of shaft
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If current in cable can be measured: • Current >20 A is considered indicator of damaging levels
IEEE 112 methods • Three alternative methods of measuring shaft potential for shaft currents Motors with all or all but 1 bearing insulated
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All can be used with motor operating
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• Standard does not provide damaging current level tolerances or threshold levels !
Minneapolis, MN
Suggests trending of tests can be useful
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EASA Convention 2007
June 25, 2007
IEEE 112 methods • First method: !
Shaft riding brush and oscilloscope
• Second method: !
Shaft riding brush and highimpedance voltmeter
• Third method: !
Shaft riding brushes and ammeter
Shaft riding brush and oscilloscope • Brush shorts out uninsulated bearing • Measure shaft voltage to frame at other bearings !
Use high impedance oscilloscope
• Use low-impedance shielded conductor for oscilloscope leads ! !
Minimizes electromagnetic interference Ground shield at one end only
• Measure peak voltage values
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Shaft riding brush and highimpedance voltmeter • Alternative to oscilloscope method !
Uses high-impedance voltmeter in place of oscilloscope
AC and DC voltages measured at each bearing
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• Peak voltage can be estimated by adding DC level to 1.4 x AC rms level • Estimated peak voltage may be considerably less than actual peak value
Shaft riding brushes and ammeter • Measure AC voltage with shaft riding brushes contacting opposite ends of shaft Alternative to oscilloscope method
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• Uses low-resistance ammeter in place of oscilloscope
• Ammeter used as low-impedance uncalibrated voltmeter
• Method useful if history of similar tests is available for comparison
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Lubricant analysis • Utilizes testing laboratory to analyze bearing lubricant
• Grease sampling often impractical Usually no direct means to obtain fresh sample without disassembly
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Lubricant analysis • Lubricating oil normally available for sampling !
By accessing oil ring area of sleeve bearing
Draining some oil from reservoir of idle oillubricated motor
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Lubricant analysis • If lubricant analysis indicates presence of bearing metals EDM from bearing current discharges possible
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No means to distinguishing between:
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• Particles from passage of electrical current • Particles of mechanical wear origin
Microscopic analysis • Usually performed by testing laboratory Often at a bearing manufacturer
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• Shop-type microscope Perform your own analysis
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Identify electrical vs mechanical damage
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Solutions for bearing currents • Determine source that is probable cause • Three main sources of circulating currents Magnetic dissymmetry
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Electrostatic discharges
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Capacitive coupling between stator and rotor
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• Any or all of these may be present
Magnetic dissymmetry • Circulating current from frame through bearings and shaft
• Eliminate by interrupting circuit Commonly by insulating opposite drive end bearing
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Magnetic dissymmetry • Bearing current due to dissymmetry
• Reduce by installing shaft grounding brush Low resistance parallel (bypass) circuit
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Locate brush between bearings
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Magnetic dissymmetry • Brush resistance is critical to this solution
• Must be extremely low in resistance Carbon brush impregnated with silver
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Gold bristles contact shaft surface
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Magnetic dissymmetry • More resistance in brush path = more bearing current !
Brush to shaft contact surface is critical
• Circulating current between motor and driven equipment !
Brush at drive end bearing
Electrostatic discharges • Caused by static electricity Damaging current can occur in applications such as:
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• Belt drives • Fans/blowers
• Solutions for magnetic dissymmetry can be used
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Capacitive coupling • Motor supplied by VFD “Common mode voltage”
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• Raises winding neutral far above ground potential
Discharges shaft to ground
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• Current path through either or both bearings to ground
Capacitive coupling • Physical factors and motor features can lead to shaft voltages and bearing currents !
Large physical size or high output power of machine • Tends to increase induced shaft voltage
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Physical shape of motor affects induced shaft voltage • Short and fat shape better than long and thin motor design
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Minneapolis, MN
Low number of poles
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EASA Convention 2007
June 25, 2007
Capacitive coupling solutions • Insulate both bearings ! ! ! ! !
Insulated housings Insulated bearing journals Insulated bearing outer race Insulated bearing inner race Ceramic rolling elements
Capacitive coupling solutions • Shaft grounding brushes for both bearings • Conductive bearing greases !
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Metallic particles suspended grease make it conductive No strong evidence indicating this method is effective • Laboratory study found conductive material in grease accelerated wear
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Capacitive coupling solutions • Insulating only one bearing not effective Typically non drive end bearing
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Current path through drive end bearing
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• VFDs are the source Future drive design may eliminate bearing currents due to capacitive coupling
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Insulated bearings • Goal: Low capacitance in insulated circuit
• Better: Insulate inner race !
Smaller diameter = smaller area
• Less desirable: Insulate outer race !
Larger area
• Alternative: hybrid ceramic !
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Minneapolis, MN
Smaller contact area Thicker insulation
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EASA Convention 2007
June 25, 2007
Insulated bearings • Insulation reduces heat transfer compared to metal to metal contact Inherent trade off
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• Use thickest coating possible • Thicker coating inhibits heat transfer – Bearing temperature increases
Insulated bearings • Ceramic or aluminum oxide bearing outer race coatings Typically thicker than inner race coating
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• Sometimes as thick as 0.012” (300 µm) • Cover a wider area than inner race
Not as effective as most other bearing insulating options
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Insulated bearings • Ceramic or aluminum oxide drawbacks: !
Chips easily e.g., removal
Balancing more difficult if journal coated
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• Epoxy coatings !
Must withstand bearing compression and load forces • Belzona Supermetal or Devcon Titanium putty often suitable
Insulated bearings • Sleeve bearing insulation Ceramic or aluminum oxide coat outer shell
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• More effective than coating rolling element bearing • Ceramic or aluminum oxide coating cracks easily
Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Insulated bearings Other methods for sleeve bearings
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• Micarta or Glastic • Scotchply • Armature type banding material – Delaminates and degrades due to lubricating oil – Not recommended
Insulated housings
• OEM or retrofit method • Thicker housing insulation Lower capacitance
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• Bearing current lower
Inhibits heat transfer
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Hybrid ceramic bearings • Simple way to insulate bearing !
Relatively expensive • Cost is decreasing
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Ceramic Rolling elements • Inner & outer races steel
Hybrid ceramic bearings • Ceramic rolling element insulation thickness greater than coated race !
Capacitance also lower
• Dynamic load capacity ! steel !
Capable of higher operating speeds
• Completely ceramic bearings !
Minneapolis, MN
Expensive and not readily available
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EASA Convention 2007
June 25, 2007
Other solutions • Driven equipment protection Insulate bearings
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Shaft brushes on driven equipment
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• Belt drive applications Belts may or may not effectively diminish capacitive shaft voltage of motor
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Precautions with insulating bearings
• Insulating both motor bearings Can create current path between motor shaft and connected equipment
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Minneapolis, MN
Options include • Insulated coupling • Grounding brush drive end of motor
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EASA Convention 2007
June 25, 2007
Precautions with insulating bearings
• Insulated bearing housing Must insulate end faces of bearing
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Insulation material • Micarta • Glastic (fiberglass)
Shaft grounding brushes • Used to divert shaft current away from bearings Parallel path -- does not eliminate current As film resistance increases bearing current increases Maintenance and replacement issues
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• To divert current away from driven machine Add to motors with insulated bearings
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Minneapolis, MN
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EASA Convention 2007
June 25, 2007
Shaft grounding brushes • Recent development: Carbon fiber “brush” Similar in appearance to shaft lip seal
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• Known as Aegis “shaft grounding ring™”
Atmosphere of microscopic carbon fibers
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• Conductive path frame to shaft
Faraday shield • Solution for capacitive coupling ! !
Shield installed in stator Grounded copper foil on top of windings
• Creates electrostatic shield !
Reduces magnitude of currents caused by stator-rotor capacitance
• Complex and fragile construction ! !
Minneapolis, MN
Impractical Uneconomical for mass-production
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EASA Convention 2007
June 25, 2007
Grounding • Quality of ground between motor and drive a critical factor !
Use shielded and stranded ground cable • Dedicated path motor to drive • More surface area than solid = lower impedance • Shielding reduces high frequency effects
Filters and chokes • Modify VFD output waveform !
Minimize common mode voltage
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Reduce bearing current magnitude
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Reduce transient voltages that: • Overstress motor windings • Increase audible noise
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Potential drawbacks: • Cost • Complexities of installation
Minneapolis, MN
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