A Summary of AC Induction Motor Monitoring

A Summary of AC Induction
Motor Monitoring
By Dr. Bob Jones
This paper summarizes the methods of
analyzing an AC motor’s electrical and
mechanical condition based on both
electrically induced mechanical vibration and
electrical signals detected with a clamp on
ammeter. It will also provide the Microlog user
with suggestions and hints on analyzing motor
problems. The author has applied these
techniques 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
two pole motors (3600 RPM or 3000 RPM) that
the signal is not twice rotating speed instead of
twice line frequency. Verify the frequency by
placing the cursor on the signal and on the
Microlog pressing the 1x RPM button or Set
Speed with the PRISM software.
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

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
The cause of this uneven air gap is often a
‘soft foot’ caused by an uneven base plate.