Page 87 - An Introduction To Predictive Maintenance

Machine-Train Monitoring Parameters 77

The second method uses the slip frequency to monitor for loose rotor bars. The passing
frequency created by this failure mode energizes modulations associated with slip.
This method is preferred because these frequency components are within the normal
bandwidth used for vibration analysis.

Running Speed
The running speed of electric motors, both alternating current (AC) and direct current
(DC), varies. Therefore, for monitoring purposes, these motors should be classified as
variable-speed machines. A narrowband window should be established to track the
true running speed.

Slip Frequency
Slip frequency is the difference between synchronous speed and actual running
speed of the motor. A narrowband filter should be established to monitor elec-
trical line frequency. The window should have enough resolution to clearly identify
the frequency and the modulations, or sidebands that represent slip frequency.
Normally, these modulations are spaced at the difference between synchronous and
actual speed, and the number of sidebands is equal to the number of poles in the
motor.

V-Belt Intermediate Drives
Electric motors with V-belt intermediate drive display the same failure modes as those
described previously; however, the unique V-belt frequencies should be monitored to
determine if improper belt tension or misalignment is evident.

In addition, electric motors used with V-belt intermediate drive assemblies are sus-
ceptible to premature wear on the bearings. Typically, electric motors are not designed
to compensate for the sideloads associated with V-belt drives. In this type of applica-
tion, special attention should be paid to monitoring motor bearings.

The primary data-measurement point on the inboard bearing housing should be located
in the plane opposing the induced load (sideload), with the secondary point at 90
degrees. The outboard primary data-measurement point should be in a plane opposite
the inboard bearing, with the secondary at 90 degrees.

5.1.2 Steam Turbines
There are wide variations in the size of steam turbines, which range from large utility
units to small package units designed as drivers for pumps, and so on. The following
section describes in general terms the monitoring guidelines. Parameters that should
be monitored are bearings, blade pass, mode shape (shaft deflection), and speed (both
running and critical).