Electrical Testing
Bearing fluting due to shaft currents top and grease contamination bottom Electrical Apparatus Service Association
<p>Bearing fluting due to shaft currents (top) and grease contamination (bottom).</p>

Tip of the Week: Bearing Failure Prevention

Without proper training, electricians may not be the best choice for carrying out motor maintenance.

Electricians often are tasked with motor maintenance, but are not trained in the mechanical end of things. For example, how do you pack grease in a bearing? There is a prescribed method for doing this, and it’s not at all similar to how untrained people do it.

Generally, sending motors to a motor shop for mechanical work internal to the motor is more cost-effective than having electricians do it (if the downtime window permits). Partly, this is because of the training deficiencies (which invariably result in early failures).

And partly it’s because electricians are a limited resource best tasked to electrical work. This dynamic also typically applies to plant mechanics, though presumably they have had instruction on the correct way to pack and install a bearing.

Among the electrical work electricians can do to reduce bearing failure, the following items are particularly important:

  • Eliminate motor grounding. This sounds counterintuitive, unless you understand the NEC definitions of grounding and bonding [Art. 100]. Grounding is a connection to the dirt. It’s crucial for lightning protection, but does nothing to eliminate the differences of potential that can cause current to flow through bearings.
  • Establish a robust bonding system for each motor system. This is what you do instead of driving a ground rod next to the motor. Remember, current does not follow the path of least resistance. We know, from Kirchoff’s Law, that current takes every path before it in inverse relationship to the impedances. Draw a circuit with two resistors, 100 ohms and 10 ohms, in parallel to refresh your memory on this one.
  • Conduct power quality analysis. Good bonding will reduce current through bearings to nearly zero, but you still want to reduce or eliminate that current at its source. Undesired high frequencies from power quality anomalies are especially problematic.
  • Routinely conduct thermographic scans to identify high-impedance connections, as these contribute to a slew of problems (including voltage imbalance if especially bad in one phase). Also use those scans to identify environmental temperature problems and bearings that already are running hot.
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