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December 21, 2007 A Penton Media Publication Vol. III No. 24

Optimize your Predictive Maintenance (PdM) System, Part 3

Motor Maintenance Tip, Part 3

Electrical Troubleshooting Quiz

Reducing Repair Variability

NEC in the Facility


Answer to Electrical Troubleshooting Quiz

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

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  • 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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    Optimize your Predictive Maintenance (PdM) System, Part 3
    Using PdM puts you in the position of fixing small problems on a prepared basis, rather than doing triage on big ones when an emergency strikes. In Parts 1 and 2, we asked how well you implement six PdM methods. Here are the final three in this series:
    1. Vibration monitoring. Vibration monitoring (VM) allows you to intervene before real damage occurs in running machinery, especially if you use a monitoring system. Rather than waiting for things to bend or break, you get an alarm when vibration exceeds a preset limit. VM also shows where you are wasting energy to vibrate the load instead of rotate it.
    2. Power quality analysis. Today's power quality analysis (PQA) tools provide a wealth of useful information you can use to eliminate PQ-related failures. To tap that wealth, you must assess your PQA needs and then address them through a combination of automatic monitoring and periodic manual testing.
    3. Insulation resistance (IR) testing. This may be the most under-rated, highest ROI PdM test available. It's a bit different from other PdM tests, because you don't do it live. If you aren't doing IR trending on your feeder cables, feeder distribution transformers, major motors, and critical systems, you aren't performing effective maintenance.

    Motor Maintenance Tip, Part 3
    In a 3-phase motor, a small difference in voltage between the phases (measured phase to ground) of the supply causes a large rise in heat. Generally, a 3.5% voltage imbalance brings about a 25% rise in motor temperature.

    The standard recommendation is that you have no more than 2% difference in voltage between phases. Motor damage is almost guaranteed beyond this point.

    Most discussion of heat damage concerns the winding insulation. However, other parts are not immune. Motor bearings fail if the grease cooks and dries. Many misdiagnosed bearing failures could have been prevented if an undiagnosed voltage imbalance had been corrected. If bearing temperature-monitoring shows high temperature, suspect voltage imbalance.

    Downtime from voltage imbalance can easily justify automatic power analysis for your motors. A well-planned power monitoring system that looks at individual feeders will usually do the job.

    With automated systems in place, you can fix a motor-killing problem without waiting to discover actual damage during a scheduled PM. You can also make your manual maintenance processes leaner by eliminating a step.

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    Electrical Troubleshooting Quiz
    A production line motor trips its temperature overloads at least once per day. After about an hour, they can be reset. The motor draws the expected current, supply voltage is correct on all three phases, power factor is good, and harmonics are minimal. What should you do? What is the likely problem?

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

    Reducing Repair Variability
    In the typical plant, operators develop preferences for who handles their repairs. Although it's true that people aren't interchangeable parts, it's also true that repair techs should provide a level of service that isn't markedly different from tech to tech. Operators should feel comfortable and confident with any tech. If they don't, then petty complaints and phantom problems will undermine your maintenance and repair efforts. Some tips:

    • Develop standardized troubleshooting procedures, based on best practices. Find out what works best on specific equipment, document that, and train everyone in those procedures.
    • Teach interpersonal skills. People's emotions definitely color their perception of reality, and how the repair tech speaks to them influences their emotions. Yet, the typical maintenance organization doesn't train its techs on how to interact in a manner that leaves operators feeling respected and confident that they are being taken care of.
    • Teach interview skills. The hardest part of the job for some techs is getting an accurate description of the problem from the operators. Some people seem gifted at this, while others beat their heads in frustration. It's a learned skill, so make sure everyone learns it.

    NEC in the Facility
    When you size the conductors for a continuous duty motor, you select a conductor with an ampacity that is 125% of the motor's full-load current rating [430.22(A)]. But some motors operate in a different duty cycle (e.g., periodic). Do you still size the conductor at 125%? The answer is no. The inrush current required to start a motor subjects the conductors to higher heat loading.

    Your ampacity calculations must account for the duty cycle and rating of the motor. This is where Table 430.22(E) comes in handy, because it provides you with the necessary multipliers for any combination.

    Sometimes, a motor can be continuous duty but just not in all modes of operation. An example would be a batch tank impeller motor. When in mix mode, the motor runs continuously. In batch storage mode, however, perhaps the motor runs intermittently. So, which way do you classify the motor? When confronted with such a question, see the Note in Table 430.22(E).

    Look carefully at your safety manual. Often, such manuals are filled with legalese and OSHA excerpts. They are often bulked out with information, such as administrative procedures, of no relevance whatsoever to the intended user. For safety manuals to be of real value, they need to be in plain language and apply directly to the work and circumstance of their intended users.

    Quiz Answer
    Answer to Electrical Troubleshooting Quiz
    The motor isn't cooling properly. You should use a thermal imaging camera to look at the temperature profile. If this problem is fairly recent, ask, "What has changed?" Sometimes, what hasn't been changed is the problem. On a production line, a motor is often tucked into a tight place and has cooling air supplied to it via filtered ducts. Check the air filter(s).

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