Page 86 - An Introduction To Predictive Maintenance

76 An Introduction to Predictive Maintenance

• Line frequency
• Loose rotor bars
• Running speed
• Slip frequency
• V-belt intermediate drives

Bearing Frequencies
Electric motors may incorporate either sleeve or rolling-element bearings. A narrow-
band window should be established to monitor both the normal rotational and defect
frequencies associated with the type of bearing used for each application.

Imbalance
Electric motors are susceptible to a variety of forcing functions that cause instability
or imbalance. The narrowbands established to monitor the fundamental and other har-
monics of actual running speed are useful in identifying mechanical imbalance, but
other indices should also be used.

One such index is line frequency, which provides indications of instability. Modula-
tions, or harmonics, of line frequency may indicate the motor’s inability to find and
hold magnetic center. Variations in line frequency also increase the amplitude of the
fundamental and other harmonics of running speed.

Axial movement and the resulting presence of a third harmonic of running speed is
another indication of instability or imbalance within the motor. The third harmonic is
present whenever axial thrusting of a rotating element occurs.

Line Frequency
Many electrical problems—or problems associated with the quality of the incom-
ing power and internal to the motor—can be isolated by monitoring the line
frequency. Line frequency refers to the frequency of the alternating current being sup-
plied to the motor. In the case of 60-cycle power, the fundamental or first harmonic
(60Hz), second harmonic (120Hz), and third harmonic (180Hz) should be
monitored.

Loose Rotor Bars
Loose rotor bars are a common failure mode of electric motors. Two methods can be
used to identify them. The first method uses high-frequency vibration components that
result from oscillating rotor bars. Typically, these frequencies are well above the
normal maximum frequency used to establish the broadband signature. If this is the
case, a high-pass filter such as high-frequency domain can be used to monitor the con-
dition of the rotor bars.