New Danfoss VLTÂ® Decentral Drive E cient, reliable and powerful

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Bearing up to electrical erosion Variable speed drives can make life tough for motor bearings. SKF’s communications manager Phil Burge explains that preventing the damage caused by stray currents needs careful motor installation, bearing selection and diagnosis. BEARINGS, BELTS & CHIAINS Stray currents inside motors can be caused by a variety of sources, including rotor eccentricities and asymmetries, uneven air gaps, unbalanced windings or even poor electrical steel homogeneity. However, the increasingly widespread use of variable speed drives has introduced an important new source of these currents. Common-mode voltage – resulting from the inability of PWM (pulse width modulation) techniques used by drives to produce a perfectly smooth sinusoidal output – creates a rapidly changing potential difference between the motor windings and earth. In many motor installations, the bearings provide the path of least resistance for these currents, but following that path can take a severe toll on the bearings. To high-frequency currents, the lubricant film in a rolling element bearing acts as a capacitor. A voltage difference builds up between the bearing casing and the rolling elements until it becomes large enough to overcome the insulation provided by the lubricant. At that point, a discharge occurs. Heat generated by the discharge can cause local melting of the bearing’s metal surface. Craters are formed and particles of molten material are transferred and partly break loose. The crater material rehardens, becoming much more brittle than the original material. Below the rehardened layer there is a layer of annealed material, which is softer than the surrounding material. While the substantial electrical discharges typically seen in DC motor installations can cause craters up to 1mm in diameter and are clearly visible to the A pattern of lines across bearing raceways can indicate that current has passed through the bearing. This effect, known as fluting, is secondary damage that becomes visible over time. Electric discharges can cause the base oil in a bearing lubricant to burn and harden, resulting in poor lubrication AC currents passing through bearings can produce tiny surface craters, a few microns in diameter, which give the rolling elements a dull appearance (left). naked eye, AC applications typically result in the formation of “micro” craters just a few μm in diameter. Over time, however, mechanical vibrations caused by elements rolling over these tiny craters will cause secondary damage, manifesting itself as a series of parallel grey segments along the raceways, known as fluting. The temperature peaks around the discharge area can also cause the lubricant inside the bearing to decompose and degrade, and to become blackened over time. Together, these damage mechanisms lead to premature bearing failure, and to costly downtime. >Difficult diagnosis Bearing currents cause problems in a small minority of VSD installations. It is desirable, however, to find methods of diagnosis that do not rely on inspecting a failed bearing. The complexity and unpredictability of stray currents within motors can make such diagnosis difficult, and the occurrence of problems in an installation can depend on the asmanufactured characteristics of the motor, the details of its installation, and its mode of use. Because electrical erosion typically manifests itself within a few months of a system being commissioned, frequent basic condition monitoring can be an effective way to identify early bearing damage. Detecting stray currents traditionally requires considerable skill and specialist equipment, including high-speed oscilloscopes and special sensors that can be installed on rotating shafts. When correctly set up, the oscilloscope will show peaks on the screen corresponding to each electrical discharge in the bearing. A useful recent innovation in the battle against electrical erosion has been the development of specialised electrical discharge detection devices. These handheld instruments are tuned to monitor changes in the magnetic field around a motor shaft at the specific frequencies associated with electrical erosion, allowing rapid, robust www.drives.co.uk November/December 2010 29
Page 1 and 2: DRIVES & CONTROLS www.drives.co.uk
Page 3 and 4: VLT® Decentral Drive FCD 302 I VLT
Page 5 and 6: DRIVES & CONTROLS Editor Tony Sacks
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New Danfoss VLTÂ® Decentral Drive E cient, reliable and powerful

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