The answer to this question appears at the end of this newsletter.


Common Repair Mistakes, Part 4
It can be costly not to address voltage imbalance whenever a motor trips. Typically, what happens is the repair tech doesn't find any ground faults or direct shorts so he decides it's safe to energize the motor and see what happens. The restarted motor runs just fine, the protective device doesn't trip, and all seems well.

This scenario repeats itself on the next shift or maybe days later. All the while, the motor is running too hot but not quite hot enough to trigger its protection device. Or maybe it does trigger the device, but the repair technician "corrects" the problem by tweaking up the settings on an adjustable breaker.

At a plastics plant in Kentucky, a technician encountered repeated thermal overload device trips. His solution was to position a portable fan in front of them. In addition to the obvious safety problem, this effectively upsized the protection thereby leaving the motor unprotected. The actual problem, diagnosed three motors later, turned out to be voltage imbalance.

Physicians refer to symptoms as being chronic (constant but at a fairly low level) or acute (sudden and at a high level). Chronic heat destroys the motor even if never triggers any protective devices. The resulting premature failure will invariably happen on a holiday weekend, a backshift, or during a critical production run.


Operation
NEC in the Facility
The job of sizing motor conductors can be confusing, and it's frequently done wrong. One of the most common mistakes is incorrectly determining the current rating. To get this right, determine the kind of motor you have and work to the applicable requirements as follows:

• Adjustable voltage motors (AC) [430.6(2)(C)].
  
• Corded motors [400.5].
  
• Torque motors [430.6(2)(B)].
  
• General motor applications [430.6(1)].

You fire up that gas heater, but fail to ventilate properly. You don't see the carbon monoxide, and you can't smell it. As your brain gets less oxygen, you are less attentive. Your risk for a lethal contact incident as a secondary consequence of the ventilation problem rises.


Quiz Answer
Answer to Electrical Troubleshooting Quiz
Voltage imbalance can have more than one cause at the same time. To find them all, use a methodical approach. Start with the system one-line, and proceed from the service to the feeder to the individual load (this motor). At the point where you first encounter a voltage imbalance, check the transformer. Most likely, the voltage is balanced on the primary but not the secondary. Look for such things as:

• Poor single-phase load distribution on a panel fed by that transformer.
• Bonding errors, especially grounding posed as bonding (See Article 250, Part V).
• Transformer integrity issues. Conduct standard transformer testing.