Figure 3. A frosted bearing race wall after 5400 hours of continuous
use in a VFD/AC motor system. Early damage typically takes the form
Figure 2. Viewed under a scanning electron microscope, a new bear- of pitting. These fusion craters increase in number and size as each
ing race wall is a smooth surface. As the motor runs, a track eventu- cycle of induced voltage discharges from the shaft through the bear-ally forms where the bearing ball contacts the wall. With no electri- ings to the frame and ground. Soon the entire race is covered with
cal discharge damage, this type of mechanical wear would be the millions of pits. As new fusion craters form over old ones, eventually
only cause of degradation. a “frosted” surface — easily visible to the naked eye — appears.
inverters, all variable frequency drives induce shaft current in
AC motors. The switching frequencies of insulated-gate bipolar
transistors (IGBT) used in these drives produce voltages on the
motor shaft during normal operation through electromagnetic
induction.
These voltages, which can register 70-V or more (
peak-to-peak), are easily measured by touching an oscilloscope probe to
the shaft while the motor is running (see Figure 1).
Once these voltages reach a level sufficient to overcome the
dielectric properties of the grease in the bearings, they discharge
along the path of least resistance — typically the motor bearings — to the motor housing. (Bearings are designed to operate
with a very thin layer of oil between the rotating ball and the
bearing race.)
During virtually every VFD cycle, induced shaft voltage
discharges from the motor shaft to the frame via the bearings, leaving a small fusion crater in the bearing race. These
discharges are so frequent that, before long, the entire bearing
race becomes marked with countless pits known as frosting. As
damage continues, the frosting increases, eventually leading to
noisy bearings and bearing failure.
A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race.
Fluting can cause excessive noise and vibration that is magnified and transmitted by the ducting in heating, ventilation, and
air-conditioning systems. Regardless of the type of bearing or
race damage that occurs, the resulting motor failure often costs
many thousands or even tens of thousands of dollars in downtime and lost production.
Failure rates vary widely depending on many factors, but
evidence suggests that a significant portion of failures occur
only 3 to 12 months after system startup. Because many of
today’s AC motors have sealed bearings to keep out dirt and
other contaminants, electrical damage has become the most
common cause of bearing failure in AC motors with VFDs.
If half of all AC motor failures are due to bearing failure,
almost 80 percent of these are caused by electrical damage to
bearings.
Strategies for Mitigating Shaft Current
Damage
As demonstrated above, electrical damage to VFD/AC motor
bearings begins at startup and grows progressively worse. As a
result of this damage, the bearings eventually fail. To prevent
Figure 4. In a fluted bearing, the operational frequency of the VFD
causes concentrated pitting at regular intervals along the bearing
race wall, forming a “washboard” pattern. This pattern results in
vibration and noise. In an HVAC system, this noise can be transmitted throughout a facility via air ducts.
