The two large compressors for plant air are housed in a small cinder block structure tacked onto the main building. The motors have each been replaced twice in the past five years. After the first replacement, problems, such as inexplicable motor overload blowouts, started occurring. Those led to the second replacements, which solved nothing. And the motors run hot.

There's been some conjecture that the cinder block housing is insufficient, resulting in condensation in the windings. However, you've noted the housing isn't what has changed and is probably not the source of the problem. What troubleshooting steps should you take next?

You should strongly suspect motor installation errors. One common problem is the misuse of solderless connectors. These connectors are designed to grab and mechanically connect the wires as you twist the connector on. If you twist the wires together first, the mechanical connection is poor; it's likely to become a high-impedance connection.

The poor connections resulting from this misuse usually exhibit different impedances, which means voltage imbalance on the motor. That would explain the "run hot" part.

  • Disassemble the connections.
  • Conduct insulation resistance testing and AC resistance testing on the branch circuit wiring.
  • Before reconnecting the motor, clean the wires with copper cleaning compound.
  • Check the entire installation for bonding errors. This is load side equipment, so nothing should be "grounded" (see Art. 100 definition).

To determine where to look next, conduct vibration analysis and power quality analysis while the compressor's in the loading phase.