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There are 8 parameters in the first group that are called electrical parameters. These are the
network equivalent parameters and are correlated to the physical parameter of the motor, like
inductances, resistances, etc. They are sensitive to electrical faults developing in the motor. eMCM
evaluates and analyzes the differences between the model parameters at any instant and the
average value of the same parameters that are obtained during the learn stage. These differences
are normalized with respect to their standard deviations obtained during the learn stage. Hence the
values indicate the number of standard deviations they are away from the average values obtained
during the learn stage. If they exceed threshold values, then an alarm is given. The changes in their
values are associated with the faults that are developing in the system. As an example, an isolation
problem in winding will affect the parameters associated with resistances. Their change will allow
eMCM to detect the isolation problem at an early stage. Though they are primarily used to detect
electrical problems, they also can indicate mechanical problems as well. As an example, an
imbalance or gear problem would cause dynamic eccentricity in the air gap. This eccentricity will
cause a change in the induction parameters and therefore in the model parameters. By monitoring
the changes in these model parameters, imbalance can be detected at an early stage. This
eccentricity eventually affects the bearing eventually leading to its damage. Therefore, detection of
eccentricity at an early stage can prevent further damages.
The electrical parameters are further classified in two groups, E1 (internal) and E2 (external)
parameters. Electrical parameters 1-4 (E1) indicate problems associated with rotor, stator, winding
etc. while 5-8 (E2) indicate electrical supply problems such as voltage imbalance, isolation problem
of cabling, capacitor, motor connector, terminal slackness, defective contactors, etc.
3.7.2 Mechanical Parameters
The parameters in the second group are sensitive to mechanical faults such as load imbalance,
misalignment, coupling and bearing problems. They are called Mechanical Parameters 1-12. These
parameters are obtained from the frequency spectrum of the electrical signals by extracting
information from the line current and voltage supplied to a motor. The variances in the stator-rotor
air gap are reflected back in the motor’s current through the air gap flux affecting the counter
electromotive force. Therefore, current carries information related to both mechanical and
electrical faults. Hence, faults will exhibit a change in the frequency spectrum of the current evident
in specific frequencies.
eMCM uses the power spectral density (PSD) obtained from the differences between the expected
current obtained from the model and the actual current. These differences include only
abnormalities generated by the motor. Therefore, they are immune to the noise or harmonics
present in the supply voltages. The mechanical parameters indicate the power level of the
difference between measured and estimated current at frequencies at which they occur in terms
of number of standard deviations. If they exceed the threshold value, which is 8, an abnormality is
indicated.
The mechanical parameters (M1-M12) correspond to the 12 maximum values obtained in the
frequency spectrum. These parameters are also used for diagnosis. The frequencies at which they
occur indicate the type of fault, i.e., an imbalance, loose foundation, oil whip, fan blades, inner or
outer race of bearing, etc. These parameters are provided to the user for trending and diagnostic
purposes.
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