Electric motor analysis specialist, Megger Baker Instruments, looks at current electrical test methods and trend analysis of the operational health of electric motors designed to support successful predictive maintenance programs.
Predictive maintenance programs
are crucial to an organisation’s
ability to avert unplanned or
unnecessary downtime that can
adversely affect its ability to produce or
operate. Unlike time-based or run-to-failure
approaches to maintenance management,
condition-based programs are ideally geared
to pay for their cost of implementation by
extending the service lives of motors and
rotating equipment, averting costly unplanned
downtimes and minimising the costs of
replacing expensive equipment. Predictive
maintenance programs are most effective
when all available means of measuring
health and analysing health trends of electric
motors, cables, power quality and load are
rigorously implemented.
In other words, the safe and continuous
operation of plants and facilities drives
revenue and profit and depends upon high
motor reliability. Predictive maintenance of
motor systems is a necessity when it comes
to supporting reliability objectives that, in
turn, support an organisation’s business
objectives. The power generation industry,
as an example, ranks at the top of this
requirement for uninterrupted operation and
safe, continuous production. A number of
motors run equipment that is ancillary to the
production or health of a company (e.g., one
of a few rooftop motors for a HVAC system,
which won’t have an immediate impact on
the HVAC system if it stops working). Other
motors, however, are critical to a company’s
ability to conduct business; that is, they are motors that drive such things as conveyor
systems, fluid pumps, or production-line
machinery that a company relies upon to
generate revenue and profit every day.
MOTOR SYSTEMS
Whether used to drive conveyors, pumps,
cooling fans, or any other machinery, motors
are best viewed as core parts of “systems.”
These are often referred to as “motor
systems,” or sometimes “machine systems”
(see Figure 1). These
systems include the
motor, the source of
the motor’s power,
and the equipment or
machinery driven by
the motor.
Today, electric
motor test equipment
is generally
categorised into two
types: static motor
test equipment and
dynamic motor test equipment. The first
type can simulate real world situations when
motors are off-line. The second type is used
for safely acquiring accurate and valuable
health data across a working motor system
or a motor’s in-service environment.
Static motor test data provides visibility
into the integrity and condition of a motor’s
insulation and motor circuit. Modern
equipment helps maintenance technicians
predict or identify imminent failures before
they cause costly unplanned downtime of motors and the rotating machinery they
support. The most effective static test
equipment can test the components of
motors at voltage levels similar to those the
motor will see in its normal operation without
destructive currents.
Static testing should include the surge
test, which is the most effective method of
ensuring the integrity of a motor’s turn-to-turn
insulation. The best static motor analysers
produce trend logs and reports, which
allow technicians to
track any decline or
degradation in a given
motor’s health.
The latest
dynamic test
equipment can locate
and identify problems
that adversely impact
motor health and life
that are on either side
of the motor within
the motor system.
These are generally power-related issues
and load problems, but can include vibration
or circuit condition problems within a motor
while the motor is in-service.
Dynamic motor analysers can often
calculate speed and torque, define rotor bar
problems, and measure distortion. Dynamic
motor testing can also identify several
mechanical issues, such as bearing problems
or motor shaft misalignment. Dynamic
testing helps isolate the mechanical (system)
issues from electrical (internal to the motor) issues, while providing valuable information to
discern the root causes of motor failures.
The goal of a predictive maintenance
program is almost always to reduce
unscheduled downtime. An effective
predictive maintenance program is measured
by how well it works to predict imminent
failures and identify potential problem areas
before they fail, and create expensive recovery
costs for an organisation. They should also
work to determine the root causes of failures
and, ultimately, save money by extending the
service life of motors and rotating equipment.
This is why the electrical testing of motors
is such a critical component of predictive
maintenance. Static and dynamic analysis,
along with trend data acquisition and
analysis, provides the information technicians
need to make good decisions regarding the
use or maintenance of a given motor.
OFF-LINE TESTING
Static testing (or off-line testing of motors
in their static, powered-down state) is
commonly performed just once in a given
period of months, usually up to a year. It’s also
performed opportunistically during outages
when a motor is shut down for other reasons.
Off-line testing is often used as a quality
assurance measure when receiving new,
reconditioned or rewound motors from a
supplier or motor shop. This is to assure they
work as expected before they are stored or
returned to service. Tests of these motors
serve to prove the motor shop is doing its job
correctly, and they create new baselines for
future trend analysis.
Static motor test equipment can
troubleshoot motor problems or failures. Any
time a problem occurs, the motor involved
should first be tested for insulation integrity.
Out-of-spec voltages, motor loads and
contaminants are examples of problems
that can adversely impact a motor’s internal
insulation.
Typical static tests include winding
resistance, meg-Ohm, polarisation index (PI),
DC step voltage and surge testing. These
tests should be performed in that sequence
with modern, state-of-the-art test equipment.
These surge-test analysers can reproduce
real-world experiences without causing
damage to a given motor’s insulation system,
and underscore the importance of testing
motors at voltage levels and conditions a
motor experiences in normal operation.
Winding resistance tests confirm that
a motor’s phases are balanced; such tests
discern shorts and opens in the motor’s
windings as well as high, out-of-spec
resistance connections. A static meg-ohm test can determine if the motor’s
windings are grounded or contaminated.
The meg-ohm meter is probably the most used test instrument in the field, but it has
its limitations. Meg-ohm testing is usually
performed at voltages slightly above line
voltage.
It is important to note that a meg-ohm test
can determine if a motor is bad, but cannot
confirm it is good. Low meg-ohm results
indicate impending failure, but high meg-ohm
values do not ensure that a motor is free of
other faults. A polarisation index (PI) test can
also confirm poor/degraded insulation within
a motor, but while it can indicate when a
motor’s insulation is old and brittle, it does not
find potential turn-to-turn faults.
A DC step-voltage test involves exposing
the entire winding to a voltage equal to that
commonly seen at start up or shut down,
and looks for weak ground-wall insulation.
Weak or damaged cable problems can also
show up during
this test, and it may
be necessary to
separate the motor
at its junction box to
determine the root
cause of the problem.
DC step-voltage
testing is commonly
performed at double
the line voltage plus
an additional 1000 volts but has no adverse
impacts on the motor or motor insulation
when properly applied.
Lastly, a surge test should be applied once
a motor has passed all the other tests. Surge
testing is the only way to locate weak turn-to-turn insulation. These copper-to-copper
faults are the primary cause of more than
80% of all winding-related failures, and they
will go undetected if not for the surge test.
When allowed to run to failure, most motors
will blow to ground in a slot. That is because
a slot provides a ready path to steel, but most
such shorts will have started as a copper-to-copper/turn-to-turn fault. Locating the weak
insulation before they become hard-welded
faults allows a maintenance professional
time to plan for repairs before a catastrophic
failure causes unscheduled downtime,
expensive repairs and lost production. Once
these turn-to-turn faults have become hard-welded faults, a motor typically only has
about 15 more minutes of service life.
ON-LINE (DYNAMIC) MONITORING
Dynamic or online monitoring is performed
while the motor is powered on and working
within its normal system or application.
Data collection with dynamic motor testers
is safe, fast and non-intrusive. Dynamic
testing provides information regarding power
quality and conditions such as voltage levels,
unbalances and distortion. A small amount
of voltage unbalance, coupled with minor
harmonic voltage distortion, may result in a
NEMA (National Electrical Manufacturers’
Association) derating that will not
be seen with simple
multimeters and
amp probes. Current
levels and unbalances
also affect motor
performance, and
monitoring them is
essential for trending
motor health.
Dynamic testing can and should be
performed more often than off-line testing
with a frequency of testing similar to vibration
analysis.
Besides electrical issues with motors
that the technology can monitor, many
mechanical issues with a motor and its
system are also discernible with data that
dynamic analysers can collect. Torque and
current spectra have proven to be highly
useful in determining mechanical issues,
including bearing faults, looseness (vibration
or misalignment) and eccentricity. Again,
considering a motor is part of a system with three components (power source,
load source, and the motor itself), a good
dynamic analyser provides relevant
condition information about all three. Many
motor problems are created by adverse or
mismatched loads or poor supply power.
Without a means of analysing data from
monitoring across a motor system, the
true root cause of motor failure often goes
undetected. The ability to acquire and
define such adverse impacts as torque
provides a maintenance professional the
means to separate the mechanical from
the electrical issues, improve decision-making concerning repair or replacement,
and otherwise extend the service life of the
motor.
Dynamic testing provides health
information about motor systems across
power source, motor and load source. It
monitors power quality and conditions such
as voltage levels, unbalances and distortion.
Current levels and current unbalances also
affect motor performance, and monitoring
them is essential for analysing motor health
trend data.
Another challenge with electric motors
is tracking the condition of their rotors.
Today’s dynamic motor analysers help
predict rotor bar failures or potential failures
if the load is relatively steady. A pump, fan
or blower operating at a steady frequency
will show very clear rotor bar signatures
that make rotor fault diagnosis easier.
During normal operation, a motor’s rotor
is stressed by its load. Torque waveform
analysis provides a picture of those
stresses, and when they reach levels out of
spec for the motor, they can be an indicator
of a number of mechanical problems.
Cavitation and belt flapping, for example,
are easily seen in a torque waveform
signature. Motor analyser manufacturers
continue to improve upon the ability of test
equipment to discern other mechanical
motor system issues earlier and with
greater accuracy.
Nearly all modern static and dynamic
testers are portable. Static testers can
be used in a shop or easily carried into
the field. Dynamic testers are by nature
used in the field (wherever running motors
are located), but often test via a motor
control centre. However, emerging new
technology has spawned a dynamic motor
analysis tool known as an online motor
analyser that is permanently installed. The
concept is to perform all of the same tests
a portable dynamic motor tester does, but
with the additional benefits of continuous
monitoring and viewing the status of a given motor from a central office location
– or for that matter, anywhere in the world
with a PC and a good Internet connection.
This technology enables maintenance
professionals to make better decisions
faster than the “spot-testing” method of
testing that is characterised by route-running
once every few months to yearly. It captures
information that cannot be captured in a
single testing session performed with a
portable tester. Alerts can be set to flag
maintenance
professionals
of the need to
investigate or replace
critical motors the
online analyser
is monitoring.
Moreover, the trend
data from months of
monitoring provides
valuable insight that
informs predictive
maintenance
planning and helps
prioritise resources and actions. Finally,
because the monitoring is effectively
performed remotely, online dynamic
analysers all but eliminate safety hazards
associated with testing in-service motors in
the field.
Dynamic monitoring also provides
efficiency information allowing maintenance
professionals to make wise and practical
decisions when confronted with choices to
repair or replace a given motor. Improving
efficiency by just 2% may result in thousands
of dollars in excessive annual energy costs.
CONCLUSIONS
Static and dynamic testing of electric
motors is critical for successfully
implementing predictive maintenance
programs. Static testing is the most
effective means of measuring the integrity
of the motor’s insulation system and can
be used for quality assurance when a
motor is out of service. Dynamic testing
provides valuable information about
motor systems, including power condition,
load, and the motor,
including physical
aspects that can
affect the life or
operation of the
motor. Online motor
monitoring adds
the dimension of
gathering motor
system health data
at regular intervals
throughout the
day, 365 days a
year. Combined,
they present a comprehensive picture of
motor and motor system health that can
be a foundation for successful predictive
maintenance programs. They provide
the full spectrum of motor condition
information required to diagnose and
predict imminent failures accurately
and, as a result, solidify electrical motor
testing’s place as an essential part of
a complete predictive maintenance
program.
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