The causes and effects of VFD-supplied motor shaft and bearing currents, as well as measures to counteract them, have been exhaustively covered in literature. However, as a drive and motor supplier, we frequently receive requests for information on this topic. So a summary seems in order…
Basically, shaft currents are induced because of the high frequency of the voltage pulses sent to the motor from the VFD. Recall that a pulse-width modulated VFD creates a synthesized sine wave by firing its output transistors many thousands of times per second. These pulses form high-frequency waves sent along the motor cable to the motor. Since impedance is inversely proportional to frequency, the capacitances of the cable and motor present little or no impedance to these high-frequency pulses. As a result, circulating currents can readily flow in the motor shaft. With sufficient magnitude, and in the absence of corrective measures, these shaft currents can pass through the bearings and races to the motor frame, causing bearing pitting or “fluting” – regular, tightly spaced grooves on the bearing races – via a process referred to as Electric Discharge Machining (EDM). Ultimately, these irregularities will cause bearing failure.
What can be done to reduce the likelihood of premature bearing failure? Corrective measures fall into two main categories, which for simplicity’s sake I will call electrical and mechanical, and they each present advantages and disadvantages. Some common ones are as follows:
- Drive output filtering: Most commonly used are referred to as dV/dt filters, dV/dt representing the rate of rise of the voltage pulse with respect to time. This is basically an inductive impedance introduced to slow the rate of rise of the voltage pulse. By reducing voltage stresses imposed on the motor, effective dV/dt filters have been shown to reduce common mode currents by up to 30%. And common mode currents are one of the primary causes of bearing damage.
- Reduces the magnitude of voltage spikes at the motor terminals, so it also helps to maintain winding and motor insulation integrity.
- In most applications, can be safely used where motors leads are 150 meters or more in length
- Reduces the harmonic content of the drive output
- More costly than many mechanical methods (see below)
- Requires additional space near the drive for mounting (typically installed external and adjacent to the drive)
- Insulated bearings: a non-conductive material is used to isolate the bearings. Due to expense and to the relatively lower probability of bearing problems on small motors, insulated bearings are normally used on IEC frame 315/NEMA frame 500 or larger motors. Insulated non-drive end (NDE) bearings are often standard, with insulated drive end (DE) bearings typically optional.
- Provides incremental protection at a modest price
- Requires no additional floor space to install
- Subject to wear/contamination over time, resulting in insulation failure
- “Transfers” the stray currents to other connected components, for example direct-coupled loads, which may then have to be protected in some fashion
- Grounding brushes: a conductive brush is mounted between the shaft and grounded frame to provide a low-impedance path to ground. The brush is typically metallic and spring-loaded to maintain contact with the shaft.
- Provides a low-impedance path to ground when properly connected, thus “removing” bearing currents before they form
- Subject to wear, contamination, and/or oxidation, increasing the ground path’s impedance and defeating the purpose of the brush
- Requires more frequent maintenance than other methods
- On larger motors, more than one brush may be required to protect the total bearing/race circumference
- Often requires additional installation footprint and/or shaft extensions
- High heat transfer during operation, making their use in motors above 1800 rpm problematic
- Shaft grounding ring: A ring with conductive micro-fibers is installed around the motor shaft and bonded to ground, bypassing the bearings.
- Highly conductive, very low impedance surface area provides good ground path
- Rate of wear is very low, requiring minimal maintenance and longer life
- Low rate of heat transfer; can be used on high-speed motors
- Small size of conducting fibers allows current transmission via corona discharge, meaning that (1) degree of contact with the shaft is less critical and (2) oil, grease, and dirt are less likely to inhibit operation
- Requires minimal space for installation
- Can be somewhat more expensive than other mechanical means
Note that there are other electrical and mechanical measures which have been tried over the years and found problematic and so are not mentioned above. For instance, electrostatically-shielded motors are available but tend to be too costly for anything but specific applications; and electrically conductive grease, which theoretically would bleed off charge slowly to prevent bearing voltages reaching a level where EDM could occur, has not been found to function consistently, being subject to wear, contamination and breakdown.
I hope this discussion has shed some light on measures which can be taken to enhance motor bearing life. If you have any questions or comments, please visit our Comments section. Or you can contact us at firstname.lastname@example.org or visit our website, www.joliettech.com. Thanks for reading, and please join me next month for another column.
As always, thanks for reading –