When dealing with electric motor bearing failures, maintenance teams are increasingly facing cases where the root cause is not mechanical overload or lubrication issues, but electrical pitting.
This type of electrical bearing damage is especially dangerous because it often develops unnoticed, while gradually causing noise, vibration and premature failure.
Many maintenance technicians ask the same question: Why does the bearing fail again and again in an electric motor, even though everything seems to be installed correctly?
What is electrical pitting and how does it damage bearings?
Electrical pitting in bearings occurs when electrical current flows through the bearing instead of following its intended return path. This is common in electric motors and generators where stray currents are present.
The process typically looks like this:
Small electrical discharges occur, they pass through the rolling elements and raceways, tiny craters form on the bearing surface.
These discharges locally alter the bearing steel structure. A so-called white layer forms – a heat-affected surface zone – which later leads to flaking (spalling), corrugated raceways, increased noise and vibration.
This is why electrical pitting is so dangerous: at first the bearing still runs, but the damage process has already started and cannot be reversed.
Why is electric motor bearing failure common with variable frequency drives?
Electric motor bearing failure is particularly common in VFD-driven applications. Variable frequency drives generate high-frequency voltage components that can easily find a path through the bearing under certain conditions.
In everyday maintenance practice, this usually shows up as:
Increasing bearing noise, rising bearing temperature, higher vibration levels, and early bearing failure.
In many cases, the VFD itself is not the problem. The issue is that the motor and bearing arrangement is not designed to control electrical current paths.
Why a standard bearing replacement does not solve the problem
This is one of the most common mistakes. When electrical pitting is the root cause, the bearing capacity or lubrication quality is usually not the limiting factor.
Replacing the bearing with a standard type often leads to the same result: the electrical current continues to pass through the bearing, and the damage returns after a short time. That is why repeated bearing failures are so common in these applications.
Insulated bearings for electric motors – when are they needed?
A widely accepted industrial solution for preventing electrical pitting is the use of insulated bearings in electric motors.
The principle is simple:
The bearing outer ring is electrically insulated, current is prevented from flowing through the bearing, electrical discharges no longer occur on the rolling surfaces.
The NTN MEGAOHM™ bearings provide at least 100 MΩ insulation resistance at 500 V DC and comply with ISO 492, DIN 620 and JIS 1514 standards. The goal is not a “stronger” bearing, but ensuring that electrical current does not use the bearing as a return path.
Ceramic or resin insulation – what does it mean in practice?
Insulated bearings are available in different designs. The most common types are:
ceramic-insulated bearings, offering high voltage resistance and good thermal stability,
resin-insulated bearings, commonly used in general electric motor applications.
The right choice depends on operating conditions such as load, temperature, duty cycle and electrical stress.
Noisy bearings in electric motors – when to suspect electrical causes
If a noisy bearing in an electric motor shows the following symptoms:
Noise returns shortly after bearing replacement, vibration increases over time, raceways show corrugated or fluted patterns, then electrical pitting should be considered as a possible root cause, not just mechanical wear.
Installation pitfalls – when a good solution fails
Insulated bearings only work properly if they are installed correctly. The NTN catalogue clearly warns against impacts, dropping the bearing, using damaged insulation surfaces, or ignoring mounting dimensions. Poor installation can create unintended current paths that bypass the insulation. For these types, it is advisable to use special tools designed for removing and installing bearings.
If an electric motor bearing fails repeatedly, the real question is often not what is being replaced, but where the electrical current is flowing.
Electrical pitting in bearings is not a visible failure mode, but over time it is one of the most expensive. Prevention is usually far cheaper than repeated repairs.
Source: Bearing Express
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