The document discusses various types of motor testing, including electrical tests that evaluate insulation, current leakage, and winding integrity, as well as mechanical tests that examine the rotor for cracks or issues. Electrical tests include insulation resistance testing, polarization index testing, step voltage testing, and surge testing. Mechanical tests involve growler testing to detect discontinuities in the rotor, single-phase rotor testing to find cracked bars, high current rotor testing to identify shorted laminations, and vibration spectrum analysis to evaluate rotor balance and cracked bars by analyzing motor vibrations. Motor testing is recommended to assess integrity, prevent failures, ensure efficiency and safety, and replace damaged motors before irreversible damage occurs.

1. JAVID IQBAL SODAGAR
DGM ELECTRICAL

2. What is motor testing?
The main objective of motor testing is to assess the integrity
of a motor and ultimately prevent the occurrence of
unnecessary failure. Electric motors are highly integrated
machines that can develop faults in many areas and, if left
unattended, a damaged motor can result in dangerous
working conditions. Electrical motor testing evaluates static
parameters such as insulation (the barriers between
winding interconnections/winding to earth), wire damage,
current leakage, and/or dynamic parameters such as
balance, temperature rise, distortion, etc. Mechanical motor
testing often includes evaluating a motor’s rotor for cracks
and shorted laminations. Each test can apply to
most AC and DC motors, but each method of testing is
dependent on the construction and application of the motor
being evaluated.

3. Why test my motor?
It is highly recommended that motor testing be done, as once an
electric motor accrues damage it is often irreversible (known as
“core damage”). A previously-broken motor will never run at
the same efficiency, even if repaired, so testing can make sure
that the motor retains its performance over the maximum value
of its useful life. Electric motor testing is usually the first item
cut from the budget when trying to save money on a project,
but putting time and money aside for motor testing will reduce
failure rates, increase efficiency, and ensure operator safety.
While motor testing is tedious and difficult, it is well worth the
investment, as the bill for testing a motor is always smaller than
the bill to replace a motor (not to mention the losses associated
with delays and system downtime). Services are available that
will do this testing for you, providing a professional evaluation
with none of the hassles of testing your motor on your own.

4. Types of motor tests
This article will look at some common electrical tests and
then at some mechanical tests. Electrical tests involve
measuring the current, resistance, or electrical properties
of motors, while mechanical tests often look for
damage/defects within the rotor which can cause
imbalance. Note that there are many methods for
diagnosing issues within a motor, and the ones
presented in this article are only the most common tests
used. Since there are many ways in which a motor can
fail, there are at least as many (if not more) ways to test a
motor’s integrity. Also, most of these tests are used in
conjunction with each other to verify results, as well as
provide the most accurate picture of motor health.

5. Electrical tests Insulation resistance test
These electrical tests will find issues within the windings of a motor using
resistance. It provides a test of the quality of insulation resistance (IR), which
will start degrading as soon as the motor is in use due to temperature effects.
Motors in harsh environments may need routine checks of the IR to avoid any
failure, as small shorts from moisture or dust can lead to major issues if
undetected. IR testing can highlight dead cables, shorts, loose connections,
open circuits, or any other blatant winding issue that can alter the winding
resistance. These resistance measurements must be compensated for
temperature after each test; this way, the resistance measurements are
standardized when compared over time. This test can also be used to balance 3
phase motors, as the phase-to-phase resistance of each winding can be
compared to see how they differ from one another. Note that these tests are
typically done on off-line motors – that is, motors that have been removed from
their power source, for safety and ease of inspection.
An insulation resistance tester is often used to quickly assess the motor’s
insulation resistance. This device uses a DC voltage to detect insulation
breakdown inside of a motor. Much like a multimeter, If the megger shows a
low resistance value when wired to the motor, it means that the path to ground
has been compromised and the motor must be repaired. Conversely, if the
megger shows a high resistance value, it means that it detects no major leakage
in the wire’s continuity. It is a simple

6. Polarization index test
Polarization index (PI) testers (sometimes known
as dielectric breakdown testers) are used to assess
insulation health, identifying the accumulation of
contaminants as well as physical changes within the
insulation. The test involves positively charging the
motor conductors and negatively charging the frame
for ~10 minutes. The test measures and graphs the
change in current over these 10 minutes, where healthy
insulation will “charge”, or reduce current, while
unhealthy insulation will remain constant. This test has
become increasingly difficult to use as a standalone
acceptance test because of newer insulation systems
but is still useful in conjunction with other tests to
verify results.

7. Step voltage test
The step voltage test ensures that the ground wall insulation
and cables can operate during normal day-to-day voltage
spikes, typically seen during startup/slowdown. It is
performed on a disconnected motor by applying a DC
voltage across all phases, holding it for a predetermined
time, increasing this voltage by some “step”, holding it
again, etc. until the target test voltage is reached. The
leakage of current is plotted after each step, and the
resulting graph will show an indication of the ground wall
insulation condition. If the leakage current rise is less than
double after the test, the motor insulation is fine, but
anything over double suggests weaknesses and the test
should be stopped and the motor inspected for possible
repair.

8. Surge test
The surge test is one of the only tests able to locate weaknesses
within the copper-to-copper insulation, or the area with the
highest rate of electrical failure in motors (over 80% of stator
failures occur at the weak turn-to-turn insulation points). This is
a highly important test to perform, as a motor’s turn-to-turn
insulation dictates motor reliability. These tests send pulses at
increasing voltages up to the target voltage one phase at a time,
generating them in such a way as to simulate startup/slowdown
spikes. Wave patterns are collected from the “surges”, providing
pulse-to-pulse comparisons that can identify weaknesses in the
insulation. If the target voltage is achieved without any change
in frequency on the graph, then the motor’s turn-to-turn
insulation is healthy, but any shifts in waveforms indicate
weakness in that particular area.

9. Mechanical (Rotor) tests
Growler test
The growler test is the first test used to determine
discontinuities of current through a rotor bar, caused by
blown-out wires, loose laminations, or cracking. It is
performed with the rotor completely separated from its stator
and connected to an armature growler– a coil of wire
wrapped around an iron core connected to a source of AC
current. This growler acts as an open-ended transformer that
induces AC power into the rotor’s armature for the purpose
of checking for the presence of shorted turns. The operators
holds a feeler (typically a hacksaw blade) on top of the rotor
and rotates the rotor around, looking for any area where the
feeler vibrates, or “growls”. If this happens, it means that
there is some issue with the generated magnetic field, and
therefore some mechanical issue with the rotor. Note this test
can be very dangerous as it uses an open-ended transformer
design, so having skilled technicians is imperative.

10. Single-phase rotor test
The single-phase rotor test is used to search for cracked
rotor bars and is performed with the motor still
together, but disconnected from power. When a rotor
bar is cracked, current will not be induced in it,
changing the current to the rotor. The test applies
single-phase power to the motor and the tester slowly
rotates the rotor, where an analog meter monitors one
phase for any fluctuations in current draw. If no
changes in amps are found, then there are no
discontinuities, but any increase or decrease in stator
current suggests one or more cracked rotor bars are
present.

11. High current rotor test
By applying a high current through the shaft of the rotor
when separated from the motor, thermal scans of the outer
diameter can highlight shorted laminations. Any shorts will
manifest as “hot-spots” on the thermal image, which means
that any current through the rotor will unevenly heat those
points. This uneven distribution of heat can cause rotor
bowing and imbalance, as well as premature rotor bar
cracking.

12. Current spectrum analysis
This test is done while the motor is at 50-100% load, and
measures the back EMF caused by the rotor into the stator
windings. This back-current is a function of the motor's poles
and slip frequency (if slip is present), and will appear on
spectrum analysis graphs as “side-band” peaks around the
supply line frequency (60HZ in North America, 50Hz in
Europe). If these peaks are especially large, they suggest the
presence of a number of broken rotor bars which can be
determined from the ratio of the side-band frequency to the
supply frequency. This test is one of the most accurate and
reliable tests on rotors for bar damage.

13. Vibration spectrum analysis
While under load, both the normal working motor and any
broken rotor bars will vibrate with some frequency.
Vibrations in a motor modulate at a rate equal to the number
of poles times the slip frequency, and vibration analyzers are
used to see changes in vibrations potentially related to
malfunctions. Any cracked rotor bars will increase the
amplitude of vibrational frequency when the load is
increased. Vibration analysts can take data as the rotor spins
and split the vibration frequency up into its component
frequencies (via Fast Fourier Transform analysis) to highlight
evidence of cracked rotor bars. This is a high-level test that is
used in balancing rotors as well and requires specialized
technicians to perform. Specialized service providers can
perform this sort of service, where even working rotors can be
balanced to perform quieter and without fluctuations.