Advances in semiconductors
and power electronics technologies
have made adjustable-
speed drives (ASDs) with
pulse-width modulation
(PWM) technology a popular
choice for many motor-driven
processes. Originally, ASDs
with PWM inverters had
switching rates between 1 and
8 kHz. To eliminate the audible
motor noise caused by such
low switching frequencies,
manufacturers developed ASDs
with switching frequencies as
high as 20 kHz. The faster
switching frequencies did
reduce motor noise, but they
also created a side effect:
excessive motor-shaft voltage
and current that can discharge across the motor bearings.
ASDs that use insulated-gate bi-polar transistors (IGBTs) as high-frequency switches are most
likely to cause bearing-discharge current. The high switching frequency and fast rise times of an
IGBT inverter output can cause induced voltage in the rotor to be capacitively coupled to the
motor shaft. As shown in Figure 1, this shaft voltage can exceed the dielectric strength of the
lubricant in the shaft bearings. The resulting current flows from the shaft, through the bearing
lubricant, and to the grounded motor frame, pitting?or fluting?the bearing races. The resulting
high rolling resistance leads to premature failure of the shaft bearings. This PQTN Application
provides ASD users with some pragmatic insights into the nature of the problem caused by
bearing discharge currents and suggests preventive or corrective actions to avoid the problem.
What To Look For:
Audible motor noise and vibration are usually the first obvious symptoms of premature bearing
failure. Because excessive noise and vibration can be symptoms of other motor problems,
maintenance personnel frequently misdiagnose problems caused by fluting. Usually, the current
arcing across the bearings will first damage the smaller idle bearing opposite the end of the shaft
connected to the motor load. Idle-bearing failure can occur as soon as six months after the motor
has been installed.
Discharge current may also damage the bearings of other equipment connected to the motor
shaft, such as direct-connected tachometers and gear boxes. In many cases, discharge current will
damage the tachometer bearings instead of the motor bearings because the smaller tachometer
bearings offer the path of least resistance. Damaged tachometer bearings can cause the tachometer
to vibrate, resulting in an erratic signal from the tachometer.
A shaft voltage as low as six volts can cause arcing through the bearing lubricant, depending
upon the type of bearing lubricant and the clearing between the race and the ball bearings.
Excessive shaft voltage can be verified in two ways. One way is to measure the shaft-to-ground
voltage, which requires a specialized shaft-monitoring device. If motor bearings have already
failed, then inspect the bearing races for fluting. If the bearing races look similar to those in
Figure 2, and the ASD driving the motor is a PWM-type with a switching frequency above
10 kHz, then most likely the shaft voltage is excessive.
HOW TO PREVENT PREMATURE BEARING FAILURES
Install a Shaft Grounding System Minimizing the
magnitude of the shaft voltage reduces the chance of
electrical arcing through bearing lubricant. Grounding
the motor shaft with a system of brushes creates a low-
impedance path to ground for otherwise damaging
discharge currents. A number of brush systems are
commercially available. Soft carbon brushes are
usually not suitable because they may create a nonconductive
film that prevents electrical contact between
the brushes and shaft. Brushes made of special materials?
such as brass and stainless steel?do not create
this film. Also, a sealed grounding system is recommended
for a clean-room environment, which may be
contaminated by airborne particles from a standard
grounding-brush system. During every routine maintenance,
ensure that the brushes are in electrical contact
with the shaft, regardless of the type of grounding
system you select.
Install Insulated Motor Bearings - Although insulated
motor bearings stop the flow of discharge current
through the motor bearings, they do not prevent damage
to the bearings of other shaft-connected equipment,
such as tachometers and fans. Also, the voltage
on the shaft of a motor with insulated bearings and
without shaft-connected equipment may pose a risk of
a mild shock to anyone who touches the rotating shaft.
Decrease the ASD Switching Frequency - The
switching frequency of most PWM ASDs can be set by
the operator. By decreasing the switching frequency of
an existing ASD, you can prevent the premature failure
of motor bearings. Although shaft voltages will be
Quick Check List
-Look for signs of premature bearing failure such
as audible motor noise, vibration, and localized
heating.
-Look for fluting in the races of damaged
bearings from motors and other shaft-
connected equipment such as tachometers,
pumps, fans, and gear boxes.
-Reduce shaft voltage by installing a shaft
grounding system.
-Stop current from discharging through motor
bearings by replacing existing motor bearings
with insulated bearings or decreasing the
switching frequency of the ASD that drives the
motor.
-Purchase new motors with bearings guaranteed
against premature bearing failure.
present at lower switching frequencies, industry
experience to date indicates that problems caused by
discharge current begin mostly when the ASD switching
frequency is greater than 10 kHz. Therefore, if
shaft grounding systems or insulated bearings are impractical
options, then avoid using switching frequencies
above 10 kHz. If a higher switching frequency
must be used, then purchase motors with warranties
against bearing failure caused by discharge current.
