Identifying Motor Defects
Through Fault Zone Analysis
www.pdma.com
Posted 8-30-04
ELECTRICAL maintenance personnel have for years been limited
to troubleshooting with no more than a fluke and a Megger.
This unfortunately does not provide enough information to allow
most electricians to feel totally confident in determining
if an electrical problem exists or not. A mechanical operator
once said, “If a problem exists with a piece of equipment,
and there is an electric cable within ten feet of it, then
it must be an electrical problem!” If you are involved
with electric maintenance you have probably heard at some time
in your career, “It must be the motor.” If you
are into mechanical maintenance you have likely heard, “It’s
probably the pump. Lets uncouple it.”
This has been an on-going battle and, up to recently, technology
has been primarily developed for the mechanical side. Vibration
shows a two times line frequency (2FL) spike and that must
mean it’s electrical. Right?... Wrong!!!! There are so
many variables producing a 2FL today that removing a motor
from service for an electrical repair due only to a high 2FL
is a mistake. Possibly an expensive one. The best thing you
could hope for is that the repair facility will call back asking “what
do you want done?” to this perfectly good motor.
“RESISTANCE to ground or megger testing is all we need.” I
find this statement hard to believe. How many times as an electrician
have we been nervous restarting a tripped motor after verifying
with our trusty megger that, “The motor is fine.” The
fact is, numerous reasons can exist which causes a motor to
trip that will not be seen by a megger, such as a turn to turn
short. Breakdown in the insulation between individual turns
of a winding can occur inside a stator slot or at the end turn
and be completely isolated from ground. Phase to phase shorts
can occur the same way. If these faults are left unattended,
they can result in rapid deterioration of the winding, potentially
ending in a complete motor replacement. Restarting of a motor
that has tripped should be considered only after these faults
have been factored out.
TROUBLESHOOTING an electric motor that is suspected to have
an electrical problem should not result in the statement, “ The
motor is fine.” Although someone with years of experience
and tons of credibility may be able to get away with such a
simple statement, most electricians won’t find the same
positive response from their supervisor, engineer, or plant
manager. To confidently report the electrical condition of
a motor and ensure that your recommendation is taken seriously,
there are six areas of interest known as Fault Zones that must
be looked at during the troubleshooting effort. Missing any
of these zones could result in missing the problem and losing
credibility in our skills.
The six electric Fault Zones are:
1. Power Quality
2. Power Circuit
3. Insulation
4. Stator
5. Rotor
6. Air Gap
Power Quality: has recently been thrust in the limelight by
utility deregulation and the popularity of AC and DC drives.
With deregulation, competition among utilities has heightened
the concern of penalties from high distortion levels. The variable
frequency drives (VFD’s) and other non-linear loads can
significantly increasing the distortion levels of voltage and
current. How can this distortion be minimized? What equipment
is required, and is the concern purely financial or is equipment
at risk?
First, let’s understand what we are really talking about
when we speak of power quality problems. Voltage and current
harmonic distortion, voltage spikes, voltage unbalance and
power factor are a few of the many concerns when discussing
power quality. Although all of these are important, we will
focus on just a few, beginning with harmonic distortion.
Harmonic Distortion always sounds like such an in-depth concept.
It becomes more elementary if you break it down to the basic
fundamentals. The most common reference in this topic is Total
Harmonic Distortion (THD). THD is the ratio of the root-mean-square
of the harmonic content to the root-mean-square value of the
fundamental quantity, expressed as a percent of the fundamental.
Quite simply it is the RMS value of the signal with the line
frequency(fundamental) removed. A perfect 60 Hz sine wave would
have 0% THD. So anything other then the fundamental line frequency
(60 Hz) would be considered a harmonic distortion.
Common non-linear (switching) loads include computers, florescent
lighting and variable frequency drives (VFD’s) as mentioned
previously. The presence of harmonics in a distribution system
results in excessive heat from the increased current demands.
A load designed to pull 100 amps at full load may draw now
120 amps if the harmonic distortion is high. This additional
current can lead to insulation damage and possibly a catastrophic
failure. Excessive zero sequence harmonics will collect back
at the transformer, leading to overload and possible failure.
These high zero sequence currents return to the source through
the neutral bus, and, if excessive, can generate substantial
heat and even fires. In an effort to avoid such catastrophic
events, many companies are modifying their distribution systems.
Installing k-transformers, designed to handle the larger loads
generated by harmonics, and increasing the mil size of their
neutral to accommodate larger current levels are two popular
activities. Though these efforts do nothing to diminish the
harmonics, they do reduce the failure risk. Removing the harmonics
requires the installation of filtering mechanisms, such as
zero sequence filters.
Some of the newer VFDs that utilize IGBTs can exceed line
voltage by a tremendous amount in less than a microsecond.
Older class B insulation systems have low tolerance for this
rapid rise time and can fail very quickly. Motors designed
for inverter duty are highly recommended when utilizing drives.
Excessive cable length between the drive and the motor can
create a high impedance mismatch that contributes to high voltage
spikes at the motor connection box. The drive manufacturer
will normally specify the correct cable distance.
General guidelines as stated in Table 3.3.1 of IEEE 519-1992,
recommend <5% voltage THD for systems operating <69kv.
They further recommend the individual harmonic voltage distortion
to be <3%. Figure 1 shows an example of unacceptable levels
of voltage distortion. These high harmonic levels can be seen
on the voltage signal as pulses riding the fundamental frequency
(figure 2).
Figure 1
High 5th and 7th harmonics indicate the presence of a 6 pulse
drive influence on the distribution system. Each of the individual
harmonics should be <3% of the fundamental per IEEE 519-1992.
Figure 2
Figure 2 shows a fundamental 60 Hz voltage signal with 6 pulses
occurring throughout each sinewave. This resulted from an unfiltered
6 pulse drive connected to the distribution system.
Power Circuit: What is a Power Circuit? The power circuit
refers to all the conductors and connections that exist from
the point at which the testing starts through to the connections
at the motor. This can include circuit breakers, fuses, contactors,
overloads, disconnects, and lug connections. A 1994 demonstration
project on industrial power distribution systems found that
connectors and conductors were the source of 46% of the faults
reducing motor efficiency. Many times a motor, although initially
in perfect health, is installed into a faulty power circuit.
This causes problems like harmonics, voltage imbalances, current
imbalances, etc. As these problems become more severe, the
horsepower rating of your motor drops, causing temperatures
to increase and insulation damage to occur. This motor is replaced
many times and the failure cycle begins again. As seen in figure
3, high resistance connections resulting in voltage imbalances
will reduce the horsepower rating significantly.
Reference: NEMA Standards MG 1-14.35
Figure 3
One method of detecting high resistance connections is by
performing phase to phase resistance testing. On a three phase
motor, the three resistance measurements should be nearly identical.
If all three reading are exactly the same, there would be a
0% resistive imbalance. As one or more phases develop a high
resistance the resistive imbalance increases, indicating a
fault.
Some of the fault mechanisms that cause high resistance connections
are:
Corroded terminals
Loose cables
Loose bus bars
Corroded fuse clips
Corroded contacts
Open leads
Different size conductors
Dissimilar metals
Figure 4
Figure 4 shows three different resistance test points which
can be used to determine the actual location of the high resistance
connection. Position X is upstream of the fuses. If the resistive
imbalance is still high, you may want to move to position Y,
down stream of the contactor. If the imbalance is still evident
at position Y, testing at the motor connection box, position
Z, will isolate the motor from the power circuit and determine
which is the problem area.
Insulation Condition: This refers to the insulation between
the windings and ground. High temperatures, age, moisture,
and dirt contamination all lead to shortened insulation life.
It has been said that if plants would just use the space heaters
available to keep the insulation dry, then doubling the life
of our motors would not be out of the question.
Insulation systems today are better than ever and are able
to handle higher and higher temperatures without significant
reduction in life. However, we are still finding ways to destroy
our insulation much earlier than should be expected. Keep in
mind that although insulation is many times involved in a failure,
this fault zone is heavily influenced by other problems. The
power circuit for one can heavily influence the insulation.
If a high resistance connection exists upstream of the motor,
which develops better than a 5% voltage imbalance, and we continue
to run the motor at its normal Hp rating, we will see a shortened
insulation life. Reverse sequence currents developing rotating
magnetic fields in the opposite direction will not only reduce
the torque capability, but can allow the temperature to rise
out of control and exceed even the 150 0C limit on your class
F insulation systems. Was the insulation system the real cause
of the motor failure or was it just a symptom? It is easy to
diagnose the evident insulation failure as the fault mechanism
but it will happen again with a different motor if the problem
is not fixed. Then what will the explanation be?
Again, testing with a megger is not going to tell you everything,
but it is a good start when it comes to insulation testing.
Something that people often overlook when it comes to the IEEE
(Institute of Electrical and Electronic Engineers) limits on
resistance to ground is the reference to 40 0C. Simply megger
testing with no regard to temperature will result in resistance
to ground readings, which swing heavily from high to low readings,
depending on the temperature of the windings. Temperature correcting
the readings will not only meet the IEEE testing requirements,
it will give a much better trend as seen in Figure 5.
Figure 5
We must realize that moisture contamination may cause the temperature
corrected reading to be invalid. Ensure the heaters are energized
when the motor is not running to prevent this from happening.
An insulation test that has fallen out of the spotlight is
the Polarization Index test. Applying a constant DC voltage,
in the form of a megger test, for a period of 10 minutes will
result in a gradual increase in the resistance to ground (RTG)
reading. This is a result of charging the insulation system,
much like a capacitor, which causes a reduction in the absorption
current. Per ohms law, I(current) = V(voltage) / R(resistance).
Therefore, the reduction of this absorption current must result
in an increase in the resistance. If we take the ten minute
RTG and divide it by the one minute RTG, a value of 2.0 or
higher is considered acceptable by IEEE. Unfortunately, motors
with unstable insulation systems can give values close to or
greater then a 2.0, but still be defective.
Figure 6
In Figure 6, when the ten minute reading (approximately 600
megohms) is divided by the one minute reading (approximately
300 megohms), the result is 1.94. This nearly meets the IEEE
specification as a good insulation system, and would probably
be accepted in the field. You can see, however, that this insulation
system is very unstable. Always look at the PI Profile and
not just the Index.
A limiting factor about DC resistance to ground testing is
that the DC signal will many times not give the best evaluation
of the true insulation condition. The insulation on a motor
is a natural dielectric material. Therefore it is a poor conductor
of DC. This is good because you don’t want excessive
leakage to ground, but bad in that an insulation system in
a degraded condition may take a bit longer to be identified
using a DC signal or megger. AC however, does not allow the
dielectric to charge and will pass through the dielectric much
easier. This is good because it allows the use of an AC signal
to give much earlier indications of insulation degradation
and bad because it can be destructive, as with an AC Hi-Pot.
Low voltage capacitance to ground tests, however, are non destructive
and very good early indicators of degradation modes in your
insulation systems. These values will be read in pico farads
(pF) and can be effectively trended over time.
Stator Condition: What is a Stator? When we mention the stator,
we are referencing the DC or 3 phase AC windings, insulation
between the turns of the winding, solder joints between the
coils, and the stator core or laminations.
One of the common faults occurring with motor windings is
a turn to turn fault. This occurs when the insulation between
two turns in the same coil breaks down and reduces the coil’s
ability to produce a balanced magnetic field. Unbalanced magnetic
fields result in vibration, which can then cause degradation
of the insulation as well as bearing failures. Localized heating
around the short can also spread to other coils, resulting
in a coil to coil short. Excessive heating will eventually
not only destroy the motor windings, but will also damage the
insulation between the laminations of the stator core.
Another fault that can occur with motor windings is a phase
to phase fault. This results from the insulation breaking down
between two separate phases, usually lying adjacent to each
other in the same slot. A higher difference in voltage potential
tends to make this fault accelerate very quickly. Slot paper
is installed between different phases in the same slot to reduce
the opportunity for leakage between phases.
A turn to turn or a phase to phase short can occur many times
without resulting in an immediate ground fault. Because of
this, testing with just a megger for preventive maintenance
or following a motor trip may not identify the fault. This
could cause a small winding fault to develop into a major catastrophic
failure. Permanent core damage may necessitate replacing an
entire motor.
Testing of the stator can be done by connecting directly at
the motor as well as connecting at the MCC. During the test,
high frequency AC signals are sent into the motor. These signals
produce magnetic fields around the windings which should be
matched between phases. The inductance measurement for each
phase is then compared to the other phases and calculated into
an inductive imbalance. This imbalance minus the influence
of the rotor is used to compare the ability of each of the
phases to produce a balanced magnetic field.
Also during a test, DC signals are sent into the motor. From
these signals the actual resistance of the winding or windings
is measured. The three resistance readings of a three phase
induction motor are compared and calculated to produce a resistive
imbalance. If this imbalance exceeds a predetermined level,
then high resistance connections may exist in the solder joints
between coils.
There are two basic types of stator winding configurations.
The first is wye (or “Y”) connected and the second
is delta connected. To more fully understand what the inductance
readings are telling you, a simple understanding of the winding
configuration can help.
Figure 7
A “Y” configuration winding with a turn to turn
short will result in two low inductance readings and one high
inductance reading, when looking at phase to phase inductance.
Figure 8
A delta configuration winding with a turn to turn short will
result in one low inductance reading and two high inductance
readings, when looking at phase to phase inductance.
Rotor Condition: This refers to the rotor bars, the rotor
laminations, and the end rings of the rotor. In the 1980s,
a joint effort between EPRI and General Electric showed that
10% of motor failures were due to the rotor. The rotor, although
a small percentage of the motor problems, can influence other
fault zones to fail. When a motor is started with a broken
or cracked rotor bar, intense heat is generated around the
vicinity of the break. This can spread to other rotor bars
and destroy the insulation around the nearby laminations. It
can also effect other parts of the motor. What is just a few
millimeters away from the rotor? The stator! Stator insulation
can not hold up to the intense heat developed by the broken
rotor bar and will eventually fail. Unfortunately, many times
broken rotor bars are not easily seen without technology and
it may be missed as the root cause of failure. This will result
in a motor rewind, and replacement of bearings, but not a rotor
repair. When the motor returns to service, it has the same
problem all over again, just with new insulation to destroy.
One method of testing the rotor condition is the Rotor Influence
Check (RICTM). What is a RIC? The RIC is a test performed on
AC induction, synchronous, and wound rotor motors which illustrates
the magnetic coupling between the rotor and stator. This relationship
indicates the condition of the rotor and air gap within the
motor.
The Rotor Influence Check is performed by rotating the rotor
in specific increments (determined by the number of poles)
over a single pole group, and recording the change in inductance
measurements for each phase of the 3 phase motor. For proper
resolution, 18 inductance measurements per pole group are recommended.
To determine the number of poles in a motor use the following
equation.
F = NP / 120
F = Line Frequency (normally 60hz in US)
N = Speed of the motor in RPM
P = # of poles
Recalculated: P = 7200 / RPM
Example: A motor with name plate RPM = 1780 would have how
many poles?
P = 7200 / 1780
= 4 poles
Without historical data, a RIC must be performed to provide
any information about the standard squirrel cage induction
rotor. Faults such as broken rotor bars or damaged laminations
can exist even if the balance of inductance is low. If you
are basing the decision to perform a RIC only on how high the
balance of inductance is on the baseline test, you could be
overlooking late stages of rotor bar defect.
Figure 9
Figure 9 shows the expected inductance changes for a rotor
with broken rotor bars. Note the erratic inductance values
at the peak of the sinewaves for each phase. Broken rotor bars
cause a skewing in the field flux generated by and around the
rotor bars. A normal rotor would have no skewing or erratic
inductance patterns, as seen in Figure 10.
Figure 10
Rotor / Stator Relationship: This relationship references the air gap between
the rotor and stator. If this air gap is not evenly distributed around the
360 degrees of the motor, uneven magnetic fields can be produced. These magnetic
imbalances can cause movement of the stator windings, resulting in winding
failure, and electrically induced vibration, resulting in bearing failure.
A faulty relationship between the rotor and stator is also called an eccentricity.
Figure 11
The first type is called static eccentricity. Figure 11 and
Figure 12 show an example of what static eccentricity looks
like, physically and inductively. This type of eccentricity
is caused by problems like a misaligned endbell or the shaft
sitting low in the bearing. The physical result is that the
shaft is always in the same place out of the electric center.
Figure 12
The inductive result is the variation in peaks of the sinewave
as seen in Figure 12.
Figure 13
The second type of eccentricity is called dynamic eccentricity.
This results when the rotor does not stay in one place but
is allowed to move within the space of the stator as seen in
Figure 13.
The inductive result is the movement of all three inductance
values up or down, depending on which phase is closest to the
rotor at a given degree rotation. This is seen in Figure 14.
Figure 14
In conclusion, the term “The motor is fine” is
just not enough to ensure that we are taken seriously and that
a true assessment of the motor health has been performed. If
the decision is up to you as to what to do in a troubleshooting
or diagnostic situation, look at the whole picture. If at all
possible, do not make a quick decision. Break the system down
into its individual fault zones, test each fault zone completely
with every technology available to you, and finally make your
recommendations written or verbal using the terminology used
in fault zone analysis to express your confidence and capabilities.
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