Bearing damage resulting from Electrical Discharge Machining (EDM) isn’t new.
In some cases, this fault has been either overlooked in the haste to get the equipment back into service, or it has been misdiagnosed and mistaken for another problem. Only in the last few years has this fault begun to be recognized as a major cause of premature bearing failure in certain types of electric motors.
EDM (or fluting, as it is more commonly known) is the passage of electrical current through the bearing. This fault is also known as frosting, electrical pitting, and electric arc damage. The following two conditions must exist for current to flow:
1- there must a voltage potential,
2- there must be a path to ground.
Current takes the path with the least resistance, and in most cases, this is through the motor bearings. However, it also is possible for the path to be through the bearings of a connected component, such as a gearbox or a tachometer
(see Figures below).
The bearing’s lubrication plays an important role in determining the path of current flow.
The lubrication acts as an insulator allowing the shaft voltage potential to build until it is greater than the breakdown level of the lubricant film or metal-to-metal contact occurs. As the voltage level exceeds the breakdown voltage level or breakdown threshold of the bearing lubrication, the lubrication begins to oxidize.
The oxidation of the bearing lubrication results in the breakdown of the lubrication and creates a pipeline or pathway for the shaft current to flow through.
Current flow through the bearing results in a repetitive electrical arcing phenomenon between the bearing components, which causes localized heating to the extent that metal is removed, damaging the bearings.
The threshold or level at which the breakdown of the lubricant begins is not constant.
Changes in bearing lubrication, humidity, temperature, and bearing component clearances will change the breakdown threshold. Bearing component clearances change with the roughness of the component surface. For example, the surface of the ball bearings may appear to be smooth when viewed by the naked eye, but magnified, small peaks and valleys can be seen. The thickness of the lubricant oil film at a peak will be less then it would be at a valley.
All electrically-driven machines have some level of AC and/or DC shaft voltages present.
Though typically insignificant, excessive shaft voltages become a problem. Once the electrically- induced damage has started, normal bearing degradation takes over, but electrical arcing may continue.
The rate of failure can vary from a few months to a few years depending on the amount of shaft voltage present, the resistance of the bearing, the distance between the bearing ball and raceway, the type of lubrication, and the type of bearing. There are three sources of shaft voltages and currents: 1) electromagnetic, 2) electrostatic, and 3) external voltages supplied to rotor windings.
The rotor eccentricity, static and dynamic, produces mainly a two-pole harmonic in the airgap conductance. For a two-pole machine, in interaction with the fundamental mmf, a homopolar flux density is produced. This flux is closing axially through the frame, bearings, and shaft, producing an a.c. shaft voltage and bearing current of frequency f1 (for static eccentricity) and Sf1 (for the dynamic eccentricity).
Rotor eccentricity, homopolar flux effects or electrostatic discharge are known causes of bearing (shaft) ac currents in power grid fed IMs. The high frequency common mode large voltage pulse at IM terminals, when fed from PWM inverters, has been suspected to further increase bearing failure.
Examination of bearing failures in PWM inverter-fed IM drives indicates fluting, induced by electrical discharged machining (EDM). Fluting is characterized by pits or transverse grooves in the bearing race which lead to premature bearing wear. When riding the rotor, the lubricant in the bearing behaves as a capacitance.
The common mode voltage may charge the shaft to a voltage that exceeds the lubricant’s dielectric field rigidity believed to be around 15Vpeak/μm. With an average oil film thickness of 0.2 to 2 μm, a threshold shaft voltage of 3 to 30 Vpeak is sufficient to trigger electrical discharge machining (EDM).
A PWM inverter produces zero sequence besides positive and negative sequence voltages.
These voltages reach the motor terminals through power cables, online reactors, or common mode chokes. These impedances include common mode components as well.
The behavior of the PWM inverter IM system in the common voltage mode is suggested by the three phase schemata in Figure below.
The common mode voltage, originating from the zero sequence PWM inverter source, is distributed between stator and rotor neutral and ground (frame):
Csf – is the stator winding-frame stray capacitor,
Csr – stator – rotor winding stray capacitor (through airgap mainly)
Crf – rotor winding to motor frame stray capacitor
Rb – bearing resistance
Cb – bearing capacitance
Zl – nonlinear lubricant impedance which produces intermittent shorting of capacitor Cb through bearing film breakdown or contact point. With the feeding cable represented by a series/parallel impedance Zs, Zp, the common mode voltage equivalent circuit may be extracted from 21.12, as shown in Figure below. R0, L0 are the zero sequence impedances of IM to the inverter voltages. Calculating Csf, Csr, Crf, Rb, Cb, Zl is still a formidable task.
Consequently, experimental investigation has been performed to somehow segregate the various couplings performed by Csf, Csr, Crf. The physical construction to the scope implies adding an insulated bearing support sleeve to the stator for both bearings. Also brushes are mounted on the shaft to measure Vrg.
Grounding straps are required to short outer bearing races to the frame to simulate normal (uninsulated) bearing operation. (Figure below)
In region A, the shaft voltage Vrg charges to about 20Vpk. At the end of region A, Vsng jumps to a higher level causing a pulse in Vrg. In that moment, the oil film breaks down at 35 Vpk and a 3 Apk bearing current pulse is produced.
At high temperatures, when oil film thickness is further reduced, the breakdown voltage (Vrg) pulse may be as low as 6–10 volts. Region B is without bearing current. Here, the bearing is charged and discharged without current.
Region C shows the rotor and bearing (Vrg and Vsng) charging to a lower voltage level. No EDM occurs this time. Vrg = 0 with Vsng high means that contact asperities are shorting Cb.
The shaft voltage Vrg, measured between the rotor brush and the ground, is a strong indicator of EDM potentiality. Test results on Figure below show Vrg, the bearing current Ib and the stator neutral to ground voltage Vsng.
An indicator of shaft voltage is the bearing voltage ratio (BVR):
With insulated bearings, neglecting the bearing current (if the rotor brush circuits are open and the ground of the motor is connected to the inverter frame), the ground current IG refers to stator winding to stator frame capacitance
Csf . With an insulated bearing, but with both rotor brushes connected to the inverter frame, the measured current IAB is related to stator winding to rotor coupling (Csr). In contrast, shortcircuiting the bearing insulation sleeve allows the measurement of initial (uninsulated) bearing current Ib . Experiments as those suggested in Figure below may eventually lead to Csr, Csf, Crf, Rb, Cb identification.
Reducing the shaft voltage Vrg to less than 1–1.5 Vpk is apparently enough
to avoid EDM and thus eliminate bearing premature failure.
To do so, bearing currents should be reduced or their path be bypassed by
larger capacitance path; that is, increasing Csf or decreasing Csr.
Three main practical procedures have evolved so far
-Properly insulated bearings (Cb decrease)
-Conducting tape on the stator in the airgap (to reduce Csr)
-Copper slot stick covers (or paint) and end windings shielded with
nomex rings and covered with copper tape and all connected to ground
(to reduce Csr)
Various degrees of shaft voltage attenuation rates (from 50% to 100%) have been achieved with such methods depending on the relative area of the shields.
Shaft voltages close to NEMA specifications have been obtained.
The conductive shields do not notably affect the machine temperatures.
Besides the EDM discharge bearing current, a kind of circulating bearing current that flows axially through the stator frame has been identified.
Essentially a net high frequency axial flux is produced by the difference in stator coil end currents due to capacitance current leaks along the stack length between conductors in slots and the magnetic core. However, the relative value of this circulating current component proves to be small in comparison with EDM discharge current.