Application Note

3 Fluke Corporation Optimizing electric-motor efficiency – Part II
Potential savings and return on investment
(ROI)
Production savings from correcting bad connec-
tions, poor grounding and shorts to ground can
be calculated if you know the cost of downtime
for the process affected. Left uncorrected, a loose
or corroded connection, poor grounding or a short
to ground could blow a fuse and take down an
entire production process.
Insulation resistance
Failure to find degraded insulation in elec-
tro-mechanical equipment can lead to motor
failure and lost production. The best method is to
integrate regular insulation checks into your pre-
ventive maintenance schedule.
Measuring insulation resistance directly
requires the services of an experienced electri-
cian or power specialist and the delivery of a high
voltage, low direct current signal to de-energized
systems. Both megohmmeters and insulation mul-
timeters can supply such test current and pinpoint
breakdown strength of electrical insulation. Before
doing any insulation resistance testing, you must
isolate from the circuit under test any electronic
controls and other devices. Be sure to lock out and
tag the disconnect to the motor starter.
When a motor is having problems and you are
certain the voltage supply is nominal, use a mego-
hmmeter or an insulation multimeter to check for
insulation degradation.
The DOE recommends testing winding resis-
tance phase to phase and phase to ground in
motors twice a year.
2
Since insulation resistance
varies with temperature and humidity, you may
over time need to make several winding resis-
tance measurements to get meaningful results. It’s
best to track insulation resistance measurements
over-time, regardless, so that you’re not caught by
surprise. Determining what is a “good” or “bad”
reading is depends on circumstance, however
Fluke does offer an insulation resistance chart.
3
General guidelines
•
At less than 25 % difference, the equipment is
probably still operating acceptably (leakage
clamp okay)
•
At 25 % to 50 %, most professionals recommend
additional and more frequent testing (using an
insulation resistance tester)
•
At >50 %, most professionals would view this
as indicative of potential problems, though
equipment may continue to run for some time
before failure.
Ground testing line and load circuits at the starter
will identify the resistance to ground of the starter,
line circuits to the disconnect, and load lines to
the motor and starter windings. General thresh-
olds: ac devices can safely operate at not less than
two megohms to ground and dc devices can safely
operate at not less than one megohm to ground.
When measuring the resistance of a three phase
motor between the load legs of the starter, you
should see high resistance and roughly equivalent
measurements between phases.
Measuring insulation resistance can also be
done while equipment is operational, by using a
leakage clamp meter to measure leakage cur-
rent from the device. Leakage clamps, such as
the Fluke 360, have specially designed jaws
that eliminate the influence of adjacent current
conductors and minimize the effects of external
magnetic fields, even at low currents. Leakage
currents can be an indicator of the effectiveness
of insulation on conductors. High levels of leakage
current may be present in circuits where either
the insulation resistance is low, or electronic
equipment with filters have been used. Leak-
age currents can cause disruptions of the normal
operation of equipment and installations. The
big advantage of leakage current testing versus
insulation testing is the ability to measure during
normal operations.