The invention relates generally to bearing currents and, more specifically, to a system and method of monitoring a condition of a bearing.
Rotating machines such as electrical motors typically include bearings that support a shaft, and such rotating machines are typically susceptible to abnormal bearing wear due to bearing current. Typically, rotating machines may be susceptible to various known sources of shaft voltage that can lead to bearing currents such as electromagnetic induction, electrostatic coupling from internal sources, and electrostatic coupling from external sources. The current may manifest itself as an axial flux passing down the center of the motor shaft or as an alternating flux linking the motor shaft, as examples. Bearing currents may arise whether the machine is DC or AC, and whether the machine is a large or a small horsepower motor.
Axial shaft flux is created by unbalanced ampere turns that encircle the shaft, by a broken rotor bar, by residual magnetization, or by an eccentric air gap, as examples. Alternating flux may result from asymmetrical magnetic properties of the stator or the rotor core, or from non-homogeneous steel that causes flux paths in the motor that are not symmetrical, as examples. These currents can cause premature failure of the bearings in the rotating machine by causing an electrical discharge to pass through the bearing balls and races, causing metal transfer therefrom and into the lubricant. This loss of metal leads to pitting, craters, and discontinuities in the surfaces, which leads in turn to increased bearing currents. The increased bearing currents effect tends to increase frictional heating, to increase temperature of the bearing during operation, and ultimately to cause failure of the bearing.
The increased bearing current effect can be mitigated to an extent by addressing the aforementioned drawbacks by improving material properties, component tolerances, and asymmetries of magnetic components, and the like. However, many of the electrical drives used in today's applications are Variable Speed Drives (VSDs) that involve the use of power converters that include fast switching pulse width modulation (PWM) semiconductor devices such as IGBTs and MOSFETs. At the high frequencies of operation (up to several MHz transitions or more), capacitively coupled currents can flow through paths that may normally be considered to be electrical insulators. For instance, currents can flow through magnet wire insulation, stator slot liners, motor air gaps, bearing grease, and stator slot top sticks, as examples. As such, the increased frequencies of operation exacerbate the problem of current flow within the bearings, thus leading to early life failure. Thus, despite efforts to improve motor design and reduce the propensity for early life failure, the bearings nevertheless experience currents passing therethrough that is worsened due to high frequency operation.
Damage progression in a bearing may be indirectly monitored in order to proactively take corrective action to repair or replace the bearing prior to catastrophic failure. However, such methods typically involve the use of brushes or expensive external equipment, and such external equipment may only yield an indirect indication of bearing condition, such as acoustic noise, as an example.
Therefore, it is desirable to provide an apparatus and method of monitoring a condition of a bearing that overcomes the aforementioned drawbacks.