21-08-2014, 11:12 AM
AC Motor Testing and Predictive Maintenance Seminar Report
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Introduction
The requirement for an inclusive AC induction
machinery standard has been needed that outlines
known condition-based testing technologies and their
capabilities. For the past nine years, an IEEE Power
Engineering Society Standards Committee has been in
the process of developing the “Guide for Induction
Machinery Maintenance Testing and Failure Analysis,”
designated IEEE P1415. At the time this paper was
drafted, IEEE P1415 was in the final stages of voting to
become an IEEE Standard
Electrical Motor Diagnostics Defined
One of the most troublesome areas that has come along
with our modern times is keeping track of definitions.
For instance, on-line can mean using the internet or
while equipment is running. Lately, the concept of
Electrical Motor Diagnostics has been considered as
only the technologies of Motor Circuit Analysis (MCA)
and Electrical or Current Signature Analysis.
Condition-Based Technologies
The following technologies are covered by the
proposed standard:
• AC High Potential: Is a pass/fail test applied at
twice the rated voltage plus 1,000 volts for new
insulation systems and 125-135% of motor
nameplate voltage for existing insulation systems.
• Acceleration Time: Increased or decreasing starting
times may indicate problems with power supply,
motor or load.
• Bearing Insulation: Evaluation of the insulation
integrity of the bearings for purposes of reduced
shaft currents and resulting bearing damage.
Performed following IEEE Std 43-2000
Case #1: DC Thruster Analysis
Electrical signature analysis of DC machines provides a
solid picture of both the driven equipment, motor and
DC drive. In the case of marine thrusters, the DC
allows for variable speed to propellors that are used to
position a vessel. The challenge of analyzing thrusters
is the ability to detect problems in the gears, bearings,
seals and propellor with resulting cavitation, which
directly affect vibration test results.
By using a combination of Voltage Signature Analysis
and Current Signature Analysis (Electrical Signature
Analysis - ESA), the results can be compared and the
driven equipment evaluated. In one recent case,
vibration analysis was performed on a thruster followed
by ESA. ESA identified a strong running speed
signature that would signify an extreme unbalance,
severe misalignment or bent shaft. A review of
vibration data provided additional input that suggested
the result was a bent shaft.
Case #2: Generator Analysis
An operating generator on board an ocean-going vessel
tripped offline, due to high temperature several times
during loaded operations. Infrared thermography
determined that the cooling system was operatingsatisfactorily. MCA was utilized and it was determined
that a developing winding short coupled with an
insulation to ground fault was developing. Following
an online test, performed with ESA, it was determined
that there was a rotating field problem, as well.
Another MCA test was performed and the rate of
insulation failure was determined
Conclusion
The identification of technologies available for motor
condition-based testing, their limits and capabilities
provide a powerful tool for electrical motor diagnostics.
The hoped for publication of IEEE P1415, “Draft Guide
for Induction Machinery Maintenance Testing and
Failure Analysis,” anticipates the need for a full
electrical and mechanical test standard for electric
motor condition testing. Utilizing the combined
capabilities of the technologies, early and accurate
analysis and remaining life estimation can be
performed.