AC Electric Motor Faults For Vibration Analysis Diagnostics
Alternating current (AC) induction motors
operate by creating a rotating magnetic field from currents
flowing throughout the stator windings. In the rotor assembly,
this stator current induces current flow in the rotor bars.
If current is passed through a conductor positioned perpendicular
to a magnetic field a force is produced. The direction of
the force acts perpendicular to both the direction of the
magnetic field and the current flowing through the conductors.
This force will make the rotor rotate, effectively chasing
the rotating magnetic field.
Alternating current (AC) induction motor windings are designed
to create pairs of magnetic poles. Like pairs will repel
each other when the magnetic fields line up pole to pole,
while opposite pole pairs will attract each other. The electromagnet
polarity will reverse as the AC power source continuously
alternates the current through the coils. This changing
pattern of polarity generates torque. The result is motor
shaft mechanical rotation.
AC motor formulas
Motor Synchronous Speed (NSYNCH)
NSYNCH Pole Pairs
3600 2
NSYNCH = 120 x (Line Frequency, Hz 1800 4
# Poles 1200 6
900 8
NSYNCH = (7200, CPM) 720 10
# Poles 600 12
| Slip Frequency (FSLIP)
FSLIP = (NSYNCH - Actual RPM)
Pole Passing Frequency (FPOLE)
FPOLE = (FSLIP x (# Poles))
Rotor Bar Passing Frequency
FRBPF = (RPM x (# of Rotor Bars)) |
 |
AC Motor Rotor Electrical Faults
(Type I)
The AC motor rotor electrical faults is
a result of broken or cracked rotor bars, rotor laminations
or shorted end rings, or defective or loose rotor bar joints.
These faults can also be traced to cast squirrel cage motors
because of casting (porosity) problems.
These faults create a series of amplified 1x RPM harmonic
peaks. The harmonics usually have low amplitude sidebands
sets spaced at pole passing frequency. The normally present
peak at 2x line frequency is either found at very low amplitudes
or absent.
To resolve these closely spaced sidebands a high resolution
spectrum is required. To detect this type of motor fault,
the 24000 CPM spectrum with a 6400 FFT line resolution will
also be used.
Severity is not usually judged by amplitudes levels, rather
by the simple presence of the pole pass sidebands in the
vibration spectrum. Deterioration over time is tended to
be indicated by increased numbers of sideband sets. To confirm
this fault and determine its cause, motor electrical testing
(motor current testing) is recommended.
FSLIP = (NSYNCH - Actual RPM)
FPOLE = (FSLIP x (# of poles))
A normal AC motor signature ought to include a rotor bar
passing frequency (RBPF) response. This response takes place
when each rotor bar passes slight disruptions within the
magnetic field because of the electrical current’s
path through the stator/rotor geometry. An elevated RBPF
is created when the rotor bars pass a significant disruption
in these fields produced when the current path is altered
or distorted from broken or cracked rotor bars, loose or
defective rotor bar joints, improperly brazed rotor bar
joints, porous rotor bar castings, or electrical arcing
across the rotor bar to end ring interfaces.
RBPF is recognized as an integer multiple of rotor speed,
and will usually have sidebands spaced at 2x electrical
line frequency (7200 CPM). Rotor eccentricity problems can
be indicated by additional (less dominant) sidebands sets
spaced at 1x RPM.
AC motor may have from 35-96 rotor bars, so an FFT spectrum
with sufficient frequency range will be necessary to capture
this fault. 1x and 2x RBPF will be captured by a 360000
CPM with a 1600 FFT line resolution, and will provide enough
resolution to define the 2x LF sidebands.
Severity is usually judged by changes in RBPF amplitudes
over time. Amplitude variation with load is common.
FRBPF = (RPM x (#of rotor bars))
