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Case
study
Motor
Current Signature Analysis
Field
studies have shown that up to 20% of all induction motors in use suffer
from problems such as high resistance joints, cracked or broken rotor bars
or air-gap eccentricities. Typically, starting a motor can induce five to
six times the starting current in the rotor and stator windings. This can
result in a number of problems upon start-up.
Effective
techniques can be employed to detect and analyse critical electrical
machinery to prevent failure and loss of production.
Theory
Motor
Current Signature Analysis (MCSA) is a technique used to determine the
operating condition of AC induction motors without interrupting
production. MCSA techniques can be used in conjunction with vibration and
thermal analysis to confirm key machinery diagnostic decisions.
MCSA
operates on the principle that induction motor circuits can, in essence,
be viewed as a transducer. By clamping a Hall Effect Current sensor on
either the primary or secondary circuit, fluctuations in motor current can
be observed.
Research
has shown that when high resistance exists (for example due to broken
rotor bars) harmonic fluxes are produced in the air gap. These fluxes
induce current components in the stator winding that cause modulation of
the supply current at ± the number of motor poles times slip.
Advanced
signal processing techniques extract the modulating frequency and clearly
represent the amplitude relationship of modulating frequency to line
frequency. Knowing this relationship allows you to estimate the presence
and severity of the defect.
The
aptGroup
are able to offer several integrated methods to assess and
track the condition of critical plant induction motors. We can also
supply monitoring tools and software packages with the following features:
>
Testing can be performed online without production interruption
>
Combining MCSA with standard monitoring techniques such as vibration and
temperature (multi-parameter monitoring) to increase decision making
confidence
>
Provide "on the spot" recommendations with no historical
information needed
>
Detect problems such as high resistance joints, cracked end rings, broken
or cracked rotor bars and casting porosity or blow holes in die-cast
rotors
>
Testing can be done on a primary circuit or safely done on low voltage
secondary switch gear
A
summary of AC Induction Motor Monitoring
This
summary outlines the methods of analysing an AC motor's electrical and
mechanical condition based on both electrically induced mechanical
vibration and electrical signals detected with a clamp on ammeter. These
techniques have been applied to motors from 5 to 700 HP.
Mechanical
Vibration
Using
a standard accelerometer placed on the bearing cap, several unique
mechanical vibration signals will be generated by electrical faults in the
motor circuits. One of the more common is a signal at twice line
frequency. If the line frequency is 60 Hz, this signal will be at 120 Hz
or 7200 CPM. If the line frequency is 50 Hz, the signal will be seen at
100 Hz or 6000 CPM. Care must be taken when testing 2 pole motors (3600
RPM or 3000 RPM) that the signal is not twice rotating speed instead of
twice line frequency.
This
two times line frequency signal will be created by any of the following
faults either singly or in combination:
An
Uneven Air Gap Between The Rotor and Stator
As
the poles of the motor pass the narrow gap, the magnetic pull is greater
versus 180 degrees on the opposite side where the gap is the widest. The
number of poles (motor speed) does not change the results, an uneven air
gap will result in a velocity spectrum signal at 2x line frequency for any
size or speed motor.
The
cause of this uneven air gap is often a 'soft foot' caused by an uneven
base plate. As the motor is mounted to the base, the motor housing and
stator are distorted, resulting in the uneven air gap between the stator
and the rotor. Some empirical data seems to indicate that the twice line
frequency signal will appear when the gap clearance exceeds 10% variance.
Soft foot can be confirmed by loosening and tightening one bolt at a time
with the motor running while observing the spectrum. When the soft foot is
loosened, the velocity signal at 2x line frequency will decrease, then
increase as the nut is tightened. At the next shutdown, this foot should
be shimmed to the same lane as the others.
Damage
to the Stator Windings or Insulation
There
are numerous causes to stator damage: manufacturing, environment, or flaws
in the insulation. Any damage to the stator will again create an uneven
magnetic field around the rotor.
This
uneven field will in turn generate an uneven pull on the rotor regardless
of the motor speed and cause a mechanical vibration at twice line
frequency. It is often possible to locate the area of damage with either
an infrared or thermal detector. Often there will be an area on the motor
housing where the surface temperature will be 20 to 30 degrees hotter.
A
damaged stator will also generate a mechanical vibration signal at a
frequency equal to the number of rotor bars times the rotation speed.
Again, in the area of stator damage, the magnetic field will be weakened
and therefore stronger 180 degrees away. As each rotor bar passes this
area of higher strength, the bar will be mechanically pulled in that
direction.
Typically
induction motors will have between 45-55 bars in the rotor but this can
vary greatly depending on the manufacturer. Since the number of rotor bars
can vary greatly, it is most important to establish a procedure that any
time a motor is down for repair, the actual number of rotor bars are
counted and recorded for future reference. It is also important to record
the full bearing model number so that the bearing frequencies can be
accurately determined when analysing for bearing degradation.
The
user can verify that the vibration is electrically induced by shutting off
the motor while observing the velocity spectrum in the analyser mode. The
moment the power is removed, the distorted magnetic field is instantly
collapsed and the twice line signal will disappear. If the signal does not
disappear but slowly degrades, then the user knows there is some type of
mechanical problem.
There
is no agreed upon amplitude of concern if the twice line frequency signal
is present in the velocity spectrum. It is generally agreed that it is not
desirable to have any signal at 2x line frequency, however it is often
seen. Generally accepted limits are between 0.04-0.06 IPS at 2x line
frequency. One case using enveloped acceleration, where the 2x line
frequency was trended over six months, showed an increase from 0.4 Env G's
to 1.6 Env G's, when the motor failed. However after the motor was
repaired, the amplitude started at 0.8 Env G's and has remained fairly
level to the present.
The
first occasion was most probably a damaged stator with soft foot. After
repair the soft foot is still present, though somewhat
different because of a different torque on the mounting bolts.
Sidebands
As in most vibration signals, the presence of sidebands around
fundamental frequencies is a measure of increasing severity as the
sidebands increase in number and amplitude. Some of the sideband energy
that may be seen will be pole pass frequency, (number of poles times slip)
and slip, (nominal speed minus actual speed). At the rotor bar pass
frequency (number of rotorbars times actual motor speed) it is possible to
see sidebands of 2x line frequency. In trouble shooting, the user may find
it necessary to increase the resolution to either 1600 or 3200 lines of
resolution to be able separate these sidebands.
Analysis
of AC Motor Current
The technique of evaluating the motor condition by performing an FFT of
the motor current has been verified many times over the past 6 years. And,
although it is often referred to as a method to detect broken rotor bars,
the fact is that it detects abnormal high resistance in the rotor circuit.
In other words, bad solder joints, loose connections and damaged rotor
bars. In
all the cases, the motor must be at 70-75% load.
Observations
of Other Motor Problems
High efficiency induction motors obtain their higher efficiency, and use
less electricity, by two methods- a smaller air gap and thinner insulation
on the windings. If the owner installs these motors on the same
transformer circuit that has DC motors installed, it is possible for the
DC motor silicon control rectifiers (SCRS) to back feed onto the AC
circuit and induce high voltage spikes into the motors.
The
reduced insulation will rapidly deteriorate and lead to a reduced motor
life. Field results have shown as much as a 50% reduction in the life of
the motor due to such an occurrence. DC motor problems will be seen at the
SCR firing frequency, 6 times line frequency. If this frequency is seen,
check connections, SCRs, control cards, and fuses.
Enveloped
AC Motor Current
When the motor current from a motor with a damaged rotor circuit is
enveloped, the resulting spectrum will show energy at the actual pole pass
frequency. For example, at 0.8 Hz, not as a sideband of the 60 Hz signal
or 59.2 Hz. Initial research has shown there is a relationship between the
pole pass frequency amplitude as a ratio to the overall amplitude of an
FFT spectrum taken with an Fmax of 25 Hz.
Typically,
in a good motor, this will be a very low amplitude signal and will not be
seen in an enveloped spectrum. So, the frequency will have to be
calculated to locate it. Initial data has shown a good motor will have a
ratio of 5% or less but as damage increases, this percentage will
increase. See the example with broken rotor bars ( Figure 5).
Also
harmonics of slip frequency are additional indicators of damage. Initial
testing has shown this to be a very sensitive method and will detect very
early degradation in the rotor circuit.
By
utilising velocity and enveloped acceleration in conjunction with motor
current analyses, users can dramatically increase their success in
trending, analysing, and evaluating the condition of AC induction motors.
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