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

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