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28 Aug 2019

PREDICTIVE MAINTENANCE FOR MOTORS

Electric motors provide the muscle for almost every industrial operation, as well as for
critical utilities, infrastructure and transportation. Failure of a single motor can bring an
entire production line to a standstill, and unplanned outages can be extremely costly.
With this in mind, I strongly believe that industry could save an enormous amount of
money by implementing predictive maintenance programs for motors.

At the heart of such a program is condition monitoring, for which there are many
components, including vibration analysis, thermography and lubrication analysis.
According to studies by industry organizations, electrical failures account for
approximately 40% of motor outages, so monitoring the electrical condition of motors is
a high priority.

Broadly, there are two types of electrical condition monitoring for motors: on-line and
off-line. On-line monitoring is performed while the equipment is running and works by
capturing currents and voltages at a high sampling frequency. Armed with this
information, intelligent analyzers can reveal issues relating to the power supply (e.g.
brown-outs, harmonic content, VFD noise, unbalanced phases, power factor and startup
transients), and motor condition (e.g. broken rotor bars, bearing failures, overcurrent,
eccentricity).

In addition, the analyzer can record instantaneous torque, revealing issues with the
mechanical load, such as overload conditions, transient peaks, etc., allowing the motor-
machine system to be tuned for increased motor life and improved efficiency. In many
cases, online analysis can reveal whether a problem lies with the power supply, the
motor or the load – which is often difficult to determine by other means.

While online monitoring can reveal a lot about a motor’s operating conditions, it is
important to also include offline testing in a predictive maintenance program. Offline
testing involves carrying out electrical tests during a motor’s scheduled downtime, with

the primary objective of testing the motor’s insulation systems. This is important as most
electrical failures in motors start as an insulation failure, so getting a handle on
insulation condition is an important predictor of whether a motor is likely to fail in the
future. If the prediction is that it will, the maintenance staff can then plan ahead to
replace the motor at a convenient (or least inconvenient!) time, thereby avoiding the
pain and cost of unplanned outages.

A motor has several insulation systems, all of which can be assessed by applying
industry-standard electrical tests. Often, the insulation resistance tests carried out on
motors are simple, low voltage tests that provide a good initial estimate of the insulation
between the windings and ground (the “ground wall” insulation). However, low voltage
tests only go so far: if a failure is found at low voltage, the motor has already failed! High
voltage testing – much like pressure testing a tank – allows insulation weaknesses to be
uncovered long before they become issues at normal operating voltages. This is true
predictive condition monitoring.

There are two main types of high voltage testing: DC and surge. DC, or hi-potential (“hi-
pot”) testing assesses whether ground wall insulation is aging and is especially useful if
results can be compared over time. Surge testing, using short-duration, high voltage
impulses, is the only way to expose insulation weaknesses between the turns of a
winding – which is where most electrical faults start. Finding these before they
deteriorate and become a fault at operational voltages is what predictive maintenance is
all about, and is the key to minimizing costly and disruptive unplanned motor outages.

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