The plant manager calls you into her office and tells you about a months-old motor failure problem in an area of the plant you’re not familiar with. Though this area makes up about 10% of the production floor, it’s responsible for 90% of the motor failures.
Maintenance had sent the motors out to be examined by a motor shop. The shop suggested there were grounding problems, so maintenance drove ground rods for each motor. But that did not solve the problem.
The plant manager says there’s a promotion for you if you figure this out. Where should you start?
Ask that motor shop exactly what they meant by “grounding” problem. They probably meant the motors were serving as low-resistance paths for undesired current. This would cause pitting in the bearings, resulting in motor failure. It would also mean the plant manager is correct about a root cause. But the solution has nothing to do with grounding (load side grounding — earth connection — serves no electrical purpose).
Next, you need answers to these two questions:
1) Where is the undesired current coming from? A likely culprit is the 277V lighting; thoroughly inspect the neutrals in those circuits.
2) Why is significant current flowing through the motors, rather than around them? Per Kirchhoff’s Law of Parallel Circuits, we know that electricity flows along all paths before it and does so in inverse proportion to the impedance. If electricity is flowing through motor bearings, you solve that problem by establishing very low resistance bonding paths around the motors.
