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December 7, 2007 A Penton Media Publication Vol. III No. 23


CONTENTS
Optimize your Predictive Maintenance (PdM) System, Part 2

Motor Maintenance Tip, Part 2

Electrical Troubleshooting Quiz

Common Repair Mistakes, Part 4

NEC in the Facility

Electrical Safety

Answer to Electrical Troubleshooting Quiz


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This twice-a-month
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MRO Insider addresses topics such as:

  • Working with management and supervision
  • National Electrical Code® on the production floor
  • Safety procedures and programs
  • Troubleshooting techniques
  • Equipment maintenance and testing tips
  • Managing motors and generators
  • Trends in training and education
  • Managing energy use


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    Maintenance
    Optimize your Predictive Maintenance (PdM) System, Part 2
    With PdM, you are proactive instead of reactive. Not only is this cheaper and easier, but it also means less downtime -- sometimes a lot less.

    In Part 1, we asked how well you implement three key PdM methods: infrared thermal analysis, ultrasonic surveys, and visual inspection. Here are three more for you to ponder.

    1. Oil analysis. This is an easy and accurate way to gauge the health of your liquid-filled transformers. All you have to do is collect an oil sample and send it to a testing lab. Contact the manufacturer of your transformer if you don't have this testing in place.
    2. Partial discharge (PD) testing. If you have any medium-voltage or high-voltage cables or equipment (switchgear, motors, generators, transformers, etc.), put them on a PD testing schedule. This leakage test is a standard way of identifying insulation degradation and other problems. If you don't have in-house PD testing expertise, consider outsourcing this to a qualified testing firm.
    3. Power monitoring. Don't just install a system and think the job is done. Do you have someone monitoring the power monitor? Do you know how to interpret the information provided? Are you monitoring the points that really matter?

    Motor Maintenance Tip, Part 2
    Bond against power quality (PQ) problems. It's true that harmonics and other PQ problems can cause serious problems. For example, they can damage motor steel laminations, bearings, and insulation.

    This is why many maintenance programs now incorporate PQ measurements, such as waveform analysis and harmonic content readings. However, if you see harmonics on a PQ analyzer, don't immediately go on a PQ crusade.

    Instead, ensure the installation conforms to Part IV of NEC Article 250 and the connections are in good shape. If your maintenance procedure doesn't address this, update your procedure. If you maintain the bonding system, many "typical" PQ problems will never occur on your motors.


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    Repair
    Electrical Troubleshooting Quiz
    A motor keeps dropping offline. It's protected by thermal devices, which have been replaced several times. Using your DMM, you find a 3% voltage imbalance. How do you find the source of the imbalance?

    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)].

    Electrical Safety
    OSHA devotes many pages to portable and stationary heaters (gas and electric). The NEC devotes an entire Article (424) to fixed electric space heaters alone. Why is so much attention given to equipment everybody knows how to use?

    The main reason is people are so accustomed to using this equipment that they sometimes don't stop to apply the safety rules. Familiarity breeds complacency.

    Fire due to nearby combustibles is a danger we can easily visualize, because we can see when combustibles are too close to a heater. But what if you are doing testing or repairs during shutdown and it's really cold?

    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.
    Note that bonding errors can be anywhere on the distribution but are most likely to exist on transformers and process equipment assemblies. Look for driven rods next to these.


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