Components and Failure Modes
of the major issues from the various
components of the motor system shall be reviewed in order to provide an
understanding of the types of faults and the technologies used to detect them.
As an overview, this may not encompass all of the modes of failures you may
Starting from the
incoming power to the load, the first area that would have to be addressed is
the incoming power and distribution system. The first area of issue is the power
quality then transformers.
Voltage & Current
harmonics: Voltage limited to 5%THD & current to 3%THD.
Current harmonics carries
the greatest potential for harm to the
electric motor system.
Over & Under
voltage conditions. Electric motors are designed for +/- 10% variation from the
Voltage unbalance is the difference between the phases. The relationship between
the voltage & current unbalance varies from a few times to many times
current unbalance as related to voltage unbalance based upon motor design. This
can be as high as 20 times.
The lower the power factor from unity, the more current the system
must use to perform work.Signs of
a poor power factor are the dimming of lights when heavy equipment starts.
Overloaded system: Based upon the capabilities of the
transformer, cabling and motor.
Primary tools used to detect problems with the incoming power are
power quality meters, Electrical Signature Analysis (ESA) and voltage and
current meters. Knowing the condition of your power quality can help identify a
great many "phantom" problems.
Transformers are one of the first
critical components of the motor system. In general transformers have fewer
issues than other components in the system. However, each transformer usually
takes care of multiple systems both in the electric motor as well as
other systems. Common transformer problems include: (oil filled or the dry
Insulation to ground faults
Electrical vibration/mechanical looseness
Test equipment for monitoring the health of transformers includes
Analysis (MCA) for grounds, loose/broken connections and shorts
ESA for power quality and late stage faults
Infrared Analysis & Ultrasonic for looseness and severe
Insulation testers for insulation to ground faults
Controls & Disconnects
motor control or disconnect provides some of the primary issues with electric
motor systems. The most common for low and
medium voltage systems are:
Bad contacts including pitted, damaged, burned or worn
Bad starter coils on the contactor
Bad power factor correction capacitors which normally results in
significant current unbalance.
test methods for evaluating the controls include infrared, ultrasonic, volt/amp
meters, ohm meters and visual inspections.
Infrared, MCA and ESA provide the most accurate systems for fault detection and
before and after the controls
problems are rarely considered and, as a result, provides some of the biggest
headaches. Common cable problems
Thermal breakdown due
to overloads and age
Contamination which can be even more serious in cables that pass
underground in conduit
Phase shorts can occur as well as grounds
Physical damage is often a problem in combination with other
the Motor supply side the problems can be broken down as
Under or over voltage conditions-8%
most common equipment that covers these areas includes MCA, Infrared and
mechanical & electrical components
Primary Mechanical problems
Bearings- General wear,
misapplication, loading or contaminants
Bad or worn shaft or bearing
General mechanical unbalance, looseness and
Vibration Analysis is the
primary method of detection of mechanical problems in electrical motors. ESA
will detect late
stage mechanical problems as will Infrared and Ultrasonic.
Winding shorts between
conductors and coils
Insulation to ground
Air gap faults, including
Rotor faults including casting
voids and broken rotor bars
MCA will detect all of the faults
early in development. ESA will detect late stage stator faults and early rotor
Analysis will detect late stage faults, insulation to ground will only detect
ground faults which make up less than 1 % of motor system faults, surge testing
will only detect shallow winding shorts and all other testing will only detect late
Coupling (Direct and belted)
The coupling between the motor
and the load provide opportunities for problems due to wear and the application.
Belt or direct drive
Belt tension issues are more
common than most think and usually result in premature bearing
The most accurate system for
coupling fault detection is Vibration Analysis. ESA and Infrared Analysis will
severe or late stage faults.
Load (Fans, pumps, compressors,
The load can have numerous types
of faults depending on the type of load. The most common are worn parts,
unbalance and bearings.
Test instruments capable of
detecting load problems include Vibration Analysis, Infrared Analysis, ESA and
There are two common applications
for electric motor system testing. These include:
components or the complete system as it is newly installed or repaired. This
will help you
avoid infant mortality disasters.
of test results for system reliability. Using test such as MCA, Vibration
Analysis & Infrared Analysis, potential faults can be trended over the
long term, detecting many faults months in
equipment available from Postma & Postma is listed in the following