Optimize your Predictive
Maintenance (PdM) System, Part 3
Motor Maintenance Tip, Part
NEC in the Facility
Answer to Electrical
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Working with management and supervision
National Electrical Code® on the production floor
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The designations "National Electrical Code” and “NEC” refer to the
National Electrical Code®, which is a registered
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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:
- Vibration monitoring. Vibration monitoring (VM) allows you
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
- 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.
- 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
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
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
also make your manual maintenance processes leaner by eliminating a
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A production line motor trips its temperature overloads
at least once per day. After about an hour, they can be reset. The
draws the expected current, supply voltage is correct on all three
phases, power factor is good, and harmonics are minimal. What should
do? What is the likely problem?
The answer to this question appears at the end of this
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
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.
- 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
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
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
a different duty cycle (e.g., periodic). Do you still size the
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
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
motor? When confronted with such a question, see the Note in Table
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.
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
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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