Static and Dynamic Motor Testing as Part of a Predictive
Maintenance Program by
Timothy M. Thomas
presented at IMC-2005 The 20th International Maintenance
Motors are a part of a “system” which includes the incoming
power, the load and, of course, the motor itself. Maintaining
the safe and profitable operation of plants and facilities,
demands a high degree of motor reliability. The power generation
industry ranks at the top of this requirement for uninterrupted
operation and safe, continued production. This paper seeks to
present the most current, effective and widely accepted methods
of electrically testing and trending the operational health of
electric motors. The benefits and features of various modern
electrical test equipment and testing methodologies will also be
Predictive maintenance programs are only effective when all
available means of measuring and trending the condition of
electric motors, cables and switch gear are utilized. Modern
test equipment exist to both simulate “real-world” situations in
static testing and acquire safe and complete dynamic data in the
motor’s natural environment.
Static data helps define the motor’s insulation integrity and
modern equipment is capable of aiding technicians in predicting
imminent failures before they become catastrophic. Effective
static test equipment is capable of testing 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 insuring the integrity of the
copper-to-copper insulation. Modern test equipment will
provide trending logs and reports allowing technicians to track
the decline in the motors health.
State-of-the-art dynamic test equipment can locate power related
issues and load problems as well as motor condition problems.
The most modern of equipment can calculate speed and torque,
define rotor bar problems and measure distortion. Dynamic
testing of electric motors is a relatively new field that has
huge potential and is growing rapidly. Numerous mechanical
issues are being identified including bearing problems,
mechanical looseness and many other concerns. Dynamic testing
will help separate mechanical from electrical issues and
provides extensive information regarding the root-cause of motor
An effective Predictive Maintenance Program must include both
static and dynamic motor testing if the program is to be
successful. Each defines specific areas of concern regarding the
motor system, each has its limitations, and each lends support
to the other.
What are we really after?
The goal of a predictive maintenance program is always the same.
Reducing unscheduled downtime by predicting imminent failures
and identifying problem areas, determining the root-cause
problem of failures and, ultimately, to save money.
Electrically testing motors is a major part of any well
organized and executed predictive maintenance program. On-line
and off-line testing and trending provides valuable information
technicians need to make accurate decisions regarding the motors
health. These technologies provide very different information as
each looks at very different areas of concern. Both technologies
are required to have a complete picture of operational health of
Static or off-line testing is usually performed once a year or
during outages with the motor shut down. Off-line testing is
also used as a quality assurance tool when first receiving
reconditioned or rewound motors from the motor shop before they
are stored or returned to service. Testing these incoming motors
provides proof the motor shop is doing its job properly and
becomes the new base-line for future trending. Off-line
equipment can also be used as a troubleshooting tool. Any time a
problem has occurred the motor involved should be tested for
insulation integrity. Overload situations, contaminate issues
and voltage problems can compromise the insulation.
Off-line testing includes winding resistance, meg-ohm,
polarization index, high potential and surge testing. The tests
should be performed in that sequence with modern,
state-of-the-art test equipment. Equipment is manufactured today
that can adequately reproduce “real world” experiences with out
causing damage to the motors insulation system. It is important
to test motors at voltage levels and conditions they will see in
their normal, day-to-day operation.
Winding resistance tests confirm the phases are balanced finds
shorts and opens in the windings as well as high resistance
The meg-ohm test can determine if the 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. The meg-ohm test can determine if a motor is bad but
can not confirm the motor is good. Low meg-ohm results are an
indication of impending failure but high meg-ohm values do not
insure a good motor. Performing a polarization index test can
further confirm poor insulation systems and will indicate when
the insulation is old and brittle but, again it does not find
potential copper-to-copper faults.
A high potential or dc step voltage test raises the entire
winding to a potential voltage equal to that seen at start up
and shut down and looks for weak ground wall insulation. Weak or
damaged cable problems will also show up during this test and it
may be necessary to separate the motor at its junction box in
order to determine where a problem lies. High potential testing
is usually performed at twice line voltage plus 1000 volts.
HiPot testing is not destructive when applied properly.
The final test we would conduct, once the motor has passed all
the other tests is the surge test. Surge testing is the only way
to locate potential copper-to-copper faults. Copper-to-copper
faults are the main cause of over 80% of all winding related
failures and they will go undetected if not or the surge test.
Most motors, when allowed to run to failure will “blow” to
ground in a slot, because that is where they can get to steel,
but most will have started as a copper-to-copper fault.
Locating these potential faults before they become hard welded
faults allows the technician time to plan the required repairs
before a catastrophic failure causes unscheduled down time,
expensive repairs and lost production. Once these
copper-to-copper faults have become hard welded faults, the
motor’s life is less than 15 minutes.
Dynamic or on-line testing is performed while the motor is
operating within its normal environment. The collection of data
is safe, fast and non-intrusive. On-line testing can and should
be performed more often than off-line testing with the usual
frequency similar to vibration analysis. The concept is a new
but rapidly growing technology and its capabilities are only
limited by its age. Besides the apparent electrical issues the
technology can monitor, many mechanical issues are also
perceptible with the collected data. Torque and current spectra
have proven useful in determining bearing faults, looseness and
eccentricity. The motor is part of a machine system with three
links; power condition, load and the motor. The on-line
equipment available today provides information about all three.
Many motor problems are created by the load or by poor supply
power and many times the “root cause” of the failure goes
undetected. The ability to acquire and define torque provides
the on-line user to separate mechanical from electrical issues.
On-line 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 de-rating that will not
be seen with simple multi meters and amp probes.
Current levels and current unbalances also affect motor
performance and monitoring them is essential when trending motor
Another major issue with electric motors is the condition of
their rotors. Modern on-line testers will be able to 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 making
During normal operation a motor’s rotor is stressed by its load.
The “torque ripple” provides a picture of those stresses and is
an indicator of many mechanical problems. Cavitations and belt
flapping are easily seen in the torque ripple signature.
Defining other mechanical issues earlier and with more certainty
is constantly on going in research and development labs and new
progress is being made continuously.
On-line testing provides efficiency information allowing the
technician to make wise and practical decisions when it is time
to repair or replace a motor. Improving efficiency by just 2%
may results in thousands of dollars in energy cost every year.
Off-line testing measures the integrity of the motor’s
insulation system. On-line testing provides information about
the power condition, the load and the motor. Together they
present a picture of the motors health and provide technicians
with sufficient information required to accurately diagnose and
predict imminent failures. Electrical testing is an essential
part of a complete predictive maintenance program.
Timothy M. Thomas,
Senior Applications Engineer
Baker Instrument Company
4812 McMurry Ave.
Fort Collins, CO 80525