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July 10, 2006 A Prism Business Media Publication Vol. II No. 13



CONTENTS
Maintaining the Motor
Environment, Part Two


Electrical Troubleshooting Quiz

Replacing Motor Drives

Replacing Motor Starters

NEC at the Facility

The Practical Implications of OSHA 1926.416

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
    Maintaining the Motor
    Environment, Part Two

    Motors are critical components of larger systems. You don't run a motor just to run it. You run a motor at a certain speed and at a certain time to accomplish a specific goal. A mixer motor might run on high speed for 1 minute, and then maintain a low speed to keep the mixture from separating. A conveyor motor may stop and start to move parts from station to station.

    All of these applications require properly functioning motor controls, or they fail. At a minimum, you should perform these motor control circuit tests on a regular basis:

    • Test the e-stops for operation. This is the most critical part of motor control maintenance, because e-stops protect people.
    • "Walk down" the circuitry and look for loose connections, foreign matter, and discoloration.
    • Check fasteners, including mounting hardware. If the controls are in an enclosure, check the backplane and any other mounting surfaces.
    • Manually engage the starter. If you measure more than 0.1 ohms across the closed contacts, replace all contacts in the starter.
    • Perform an insulation resistance test to ground of starter, from the power side to ground. Repeat from the load side to ground. If you have less than 2 megohms, replace the starter.
    Perform any preventive maintenance required for the motor drive.



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    Repair
    Electrical Troubleshooting Quiz
    By Mark Kenyon, low-voltage drives engineer, ABB, Inc., New Berlin, Wis.
    A motor drive that performed flawlessly for several years has suddenly failed. This was on a critical line, so you had a spare. You swap out drives, and quickly bring the line back up. But then the new drive fails. You open it up and see bulging capacitors. What could have caused this?

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

    Replacing Motor Drives
    Motor drives are reliable, but they can fail. Often, the point of failure is a transistor (SCR in the older drives) -- typically an IGBT. A quick replacement, and you're up and running, but the failure can be more severe, requiring a total drive replacement.

    A drive upgrade is almost always beneficial. Motor drives keep getting better, smaller, and cheaper. Each new generation solves or reduces some problem in the previous generation.

    With all the various motor drives on the market, how can you pick the right one? Contact your electrical supplier or the drive manufacturer for assistance, and have this information ready:

    • Nameplate data from the motor
    • Nameplate data from the existing drive
    • Cable distance between the drive and the motor
    • Photo of motor and drive, as installed
    • Brief description of the application
    • Brief description of the failure and what you found per the troubleshooting steps in the manual.
    With this information, your distributor or manufacturer can help you make the best purchase. Now is also a good time to check the environment of that drive. Look for voltage imbalance and other issues in the power supply. Ensure the "ground" wiring to the drive is properly bonded so electricity has a path back to the source (not to a ground rod).

    Replacing Motor Starters
    Mass changeouts provide a great upgrade opportunity. For example, suppose changes in your company's products have resulted in a project: You've got to reconfigure a conveyor system and some other equipment. Part of this project will involve replacing several 10-hp motors with 25-hp motors. Simply replacing the existing starters with larger ones might be short sighted.

    You may want to look at what your electrical supplier has in the way of starters or other motor controls. You may find some connectivity features, for example, that allow you to provide input via your network to an operator console. Or you may decide on a different model of starter that provides more compactness and will allow you to centralize starters in a common cabinet. If you've got individual starter enclosures scattered all over the production floor, consider consolidating these into a central location to reduce footprint clutter. You could do that with NEMA starters or with IEC starters.


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    Operation
    NEC at the Facility
    What's the difference between Art. 280 and Art. 285, and what does it mean for your facility? To answer that, you need to know the difference between surge arresters and surge suppressors.

    Surge arresters limit surge voltages by discharging or bypassing the current. Surge suppressors limit transient voltage by diverting or limiting the current. These are distinctly different methods that accomplish distinctly different goals.

    Difference in application: Surge arresters prevent the voltage from rising above a level the suppressors can handle. Surge suppressors then reduce the voltage to a level the equipment can handle.

    The typical application for surge arresters is before the service (e.g., a spark gap arrester), to prevent large transients from entering the facility. Surge suppressors (e.g., TVSS device), which are applicable only to circuits of 600V or less, are used downstream of that.

    From an engineering standpoint, applying Art. 285 without any Art. 280 protection exposes the TVSS to more energy than it can handle. To avoid sacrificing both equipment and TVSS, start with a one-line and note the points where each kind of protection applies. For TVSS, you will probably need multiple units at staged energy levels for cost-effective protection.

    The Practical Implications of OSHA 1926.416
    This passage focuses on the idea of protecting employees from energized equipment. Some of the key points include:

    • If you must work near a circuit with which you could make contact, de-energize it and then provide proper grounding or guarding.
    • When removing fuses, use a tool rated for the voltage.
    • Keep workspaces and walkways clear of cords.
    • Don't use frayed cords.
    • Don't fasten extension cords with staples, hang them from nails, or suspend them with wire.

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    Quiz Answers
    Answer to Electrical Troubleshooting Quiz
    You might be tempted to assume both drives failed for the same reason, but that's not what happened. The observation that "it performed flawlessly for years" is a clue that the first drive failed probably due to a power event or other transient cause. This is also a clue as to the age of the replacement drive. Capacitors hold a charge for only so long. Before putting a spare drive online, you need to put the DC link capacitors through a recharging process. Consult your drive manual or the manufacturer for information on how to do this.

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    Copyright 2006, Prism Business Media. All rights reserved. This article is protected by United States copyright and other intellectual property laws and may not be reproduced, rewritten, distributed, re-disseminated, transmitted, displayed, published or broadcast, directly or indirectly, in any medium without the prior written permission of Prism Business Media.