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dealing with shaft and bearing currents - Kentucky Service Co.

dealing with shaft and bearing currentsBy Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MOintroductionThis paper addresses key issues related to shaft and bearing currents in electric motors and generators. Critical topics that will be covered include recogniz-ing symptoms of shaft and bearing currents , and de-termining if damaging current levels are present. Also, possible causes of the damaging current , such as ma-chine dissymmetry and operation on variable frequency drives (VFDs) will be dealt with . Methods of testing to confirm the presence of shaft or bearing currents will be described, as well as how to assess the magnitude of the damaging currents .

dealing with shaft and bearing currents By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MO introduction

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Transcription of dealing with shaft and bearing currents - Kentucky Service Co.

1 dealing with shaft and bearing currentsBy Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MOintroductionThis paper addresses key issues related to shaft and bearing currents in electric motors and generators. Critical topics that will be covered include recogniz-ing symptoms of shaft and bearing currents , and de-termining if damaging current levels are present. Also, possible causes of the damaging current , such as ma-chine dissymmetry and operation on variable frequency drives (VFDs) will be dealt with . Methods of testing to confirm the presence of shaft or bearing currents will be described, as well as how to assess the magnitude of the damaging currents .

2 Further, solutions to eliminate or control shaft and bearing currents , such as insula-tors, isolators, and ceramic bearings, will be shaft and bearing currents are not a new prob-lem (papers on the subject date back prior to 930), what is new is the increased understanding of how to identify and solve the problem. shaft and bearing cur-rents have been described as shaft voltages, circulat-ing 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 sufficient lev-el, it can discharge to ground through the lubricant film on the bearings.

3 current that finds its way to ground through the motor bearings in this manner is called bearing current . This paper will primarily refer to the damage phe-nomenon 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 specific term shaft current (s) or bearing current (s) will be symptoms of bearing currentAll too frequently the first symptom of bearing current is audible noise from the bearing , indicating it is in ad-vanced stages of failure. Inspection of the bearing after failure may reveal fluting of the races (Figure 1), balls or rollers frosted (Figure 2), or an overall dull grey or dark smoky finish (Figure 3) on both balls/rollers and race-ways.

4 The lubricant may also be dark in damageThe appearance of damaged surfaces is related to three major types of current . The first type of electric current damage is electric pitting (Figure 4). It is mostly figure 1: flutingFluting of the bearing races due to electrical 2: frostingFrosting of the balls/rollers due to electrical to single crater damage and typically seen in DC applications such as railway traction motors. The size of the crater is from (0. - 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 fluting, which is a pat-tern of multiple lines across the inner and outer races (Figure 1).

5 The reason for this fluting is mechanical res-onance vibration caused by the dynamic effect of the rolling elements as they roll over smaller craters in the races. Strictly speaking, fluting is not a primary mode of failure produced by the current flow 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 ap-pears dull and is characterized by molten pit marks (Fig-ure 5). Multiple micro-craters cover the rolling elements and races.

6 Crater sizes are small, mostly from (5 - 8 m) in diameter, regardless of whether the crater is on an inner ring, outer ring or a rolling ele-ment. The true shape of these craters can only be seen under a microscope using very high symptomsIf 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 de-stroyed and the root cause of the failure will not be de-termined. Visual indications of shaft voltages include fluting or a picket-fence pattern on the races of the bearing (Figure 6).

7 The spacing of the fluting marks depends on the speed (rpm), bearing diameter, radial load and magnitude of the bearing current . The balls or rollers figure 3: darkened finishThe darkened finish of the ball on the right was caused by electrical view of electrical 4: pittingfigure 5: dull finishDull finish and electrical craters in the inner race of a roller 6: picket fence Note the picket fence pattern of the bearing outer race. This is often a visual clue to shaft have a dull grey or dark smoky finish (Figure 3). If the motor speed was varying, the races may also have a frosted pattern. The grease may be black in appear-ance, due to the burning of the metal that leaves ferrous current damage usually initially appears vi-sually in the areas of the bearing that are most heav-ily loaded.

8 The reason for this is that the lubricant film will be thinnest in the areas subjected to the heaviest load. For example, in a belt drive application the dam-age to the bearing will be most pronounced in line with the direction of the belt tension. Regarding lubricant film thickness, studies have found that a bearing usually has a lubricant film thickness of m ( ) at normal operating speeds. Given this film thickness, damaging bearing currents can be caused by 60-Hz shaft voltages as low as 2 V 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 char-ring of the base oil.

9 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 if damaging current levels are presentAt present there is no known method to measure bear-ing currents and no practical way to directly measure shaft currents . Mea-suring shaft current 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 (Fig-ure 7) can be wrapped around the shaft and used to measure shaft 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 to frame voltageAmong 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.

10 Consequently it is necessary to use the high-est voltage measured when assessing the possibility of the presence of damaging bearing the shaft to frame voltage exceeds 00 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 MG , 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 .


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