Four compressors, each powered by a 400-hp motor, provide plant air. Lately, the plant’s been experiencing low pressure episodes. One effect is pneumatically-powered lifting apparatus that have dropped product, causing costly breakage.
These low pressure episodes occur when the motor overloads open on one or more of the compressor motors. The breakers don’t trip and the compressor units check out, so the maintenance response has been to just replace the overloads. The system engineer also set up an “overload blown” alarm to facilitate rapid enough replacement to prevent a pressure drop.
How can you determine the appropriate root cause repair?
You have a good clue in the fact that the overloads, not the breakers, are opening. In non-motor circuits, the breaker does the jobs of overload and fault protection, but in motor circuits these jobs are split between motor overloads and the branch/feeder breaker. Thus, you can rule out a fault condition.
It’s not enough to ask, “What is causing the motor overload condition?” You must also ask why it is chronic and why it affects all of the motors rather than just one. It’s unlikely that all four motors have insulation failure issues, but you should have a motor repair shop test run a full assessment on one motor at a time to rule this out. Protect plant air pressure by renting a temporary compressor unit.
The most likely cause is low power factor, but other power anomalies may also contribute. Use a power analyzer to see what you’re up against.
