From time to time, customers have some problem involving motor and control behavior.
Often, the problem will have to be solved by an electrical contractor or engineer, who will prescribe some change in customer equipment or operating practice. But we can often help point the investigation in the right direction. That’s best done in accordance with a few basic rules.
Never mind what the trouble is; just as a physician follows a certain diagnostic procedure, so should the electrical troubleshooter. We’re not talking here about the behavior of the utility distribution system or apparatus, but about customer problems. Try to work along these lines:
- What exactly are the symptoms? “Things just don’t work right” isn’t enough. Measured conditions — how much (or little), how often, what, when, where — are the details needed.
- How long has this particular trouble been occurring — just started this week, or been happening on and off for years?
- No matter how seemingly insignificant, what changes in equipment, operations, or maintenance practices took place about the same time the trouble began?
- How accurate is the evidence of trouble? If meters were used, what kind were they, and how were they connected? Readings are often useless because meters were of the wrong type, range, or accuracy to give good results.
- What are the ratings — nameplate voltage, current, etc.— of all components involved in the problem (e.g., motors, contactors, fuses, circuit breakers)? Don’t assume that the equipment voltage rating matches the circuit.
- What’s the circuitry involved? Is a three-phase transformation open delta or open wye rather than a full set of three transformers? If a motor trips off unexpectedly, or won’t start, just how is it controlled? Basic one-line and schematic diagrams should be available, showing how things are today, not as they were long ago.
- What specific actions have been taken to correct the trouble? What were the results?
There’s no intent here to “place blame” or “find fault.” The object is to get a clear picture of what is happening so that someone can learn why. Motors and controls sometimes do mysterious things, but violating physical laws is not one of them (power factor cannot “exceed” 100%; a motor doesn’t run hotter at a lighter load). If the evidence shows an impossible condition, something’s wrong with the evidence.
Richard L. Nailen, P. E.