Efficient Plant November 2017 - 42

RMC

motor testing
Motors with elevated temperatures should be further investigated for cause and repaired or
replaced accordingly.

Measure Motor
Temperatures Correctly
Jim Seffrin
Director, Infraspection Institute

+

WHEN MEASURING MOTOR
TEMPERATURES, KEEP THE
FOLLOWING IN MIND:

 Make certain that all thermometers are within calibration and used properly.
 Motor temperature will vary with load and ambient
temperature. Be sure to record both along with the
motor temperature.
 Elevated temperatures can be caused by electrical
or mechanical defects within the motor and/or
defective installations.
 Motors with elevated temperatures should be further
investigated and repaired or replaced.

+

DETERMINING A MOTOR'S
MAXIMUM OPERATING
TEMPERATURE

WHILE TEMPERATURE CAN BE an indicator of the condition of installed
electric motors, the best location for measuring it is often debated. After all, electric motors differ greatly in design and construction. For example, many people
recommend measuring the motor casing along the stator. This method, though,
doesn't work well for units that are fan-cooled or exposed to external air currents.
For uncooled motors, it can produce varying temperature values, depending upon
the location of the subject temperature readings.
In 1997, a research project led by Infraspection Institute (infraspection.com,
Burlington, NJ) used instrumented motors in a controlled environment to determine the effect of excess force on installed units. One of our primary goals was to
identify a location for collecting reliable temperature data.
The research found that measuring on the exterior of the motor bell-housing
within 1 in. of the output driveshaft consistently produced temperatures that were
within 1 C to 2 C of the motor windings and the output side bearing assembly.
Temperatures taken at the bell-housing were especially useful for fan-cooled motors since this area was unaffected by convective cooling from the fan.

Exceeding the maximum operating temperature of a
motor will shorten the life of its dielectric materials
and decrease the unit's service life. Determining that
crucial temperature requires one to know the motor's
ambient temperature rating and its rated temperature
rise above ambient. Both of these values are generally
found on the nameplate located on the exterior of the
motor casing.
To calculate maximum operating temperature, add
the ambient and rated rise temperatures. Their sum is
the maximum operating temperature for the subject
motor at 100% load.
Example:

Jim Seffrin, a practicing thermographer with more than 30 years of experience in the field,
was appointed to the position of Director of Infraspection Institute in 2000. This article is
based on several of his "Tip of the Week" posts on IRINFO.org. For information on various
workplace topics and infrared-related issues, and various upcoming training and certification
opportunities, email jim@infraspection.com or visit infraspection.com.

It's important to note that some motors specify
insulation class rather than a numeric value for
temperature rise. In such cases, it is necessary to
know the operating limits for the insulation class of the
subject motor. EP

42

| EFFICIENTPLANTMAG.COM

 Rated Ambient: 40 C
 Rated Rise: 90 C
 40 + 90 = 130 C or 266 F

NOVEMBER 2017


http://www.infraspection.com http://www.IRINFO.org http://www.infraspection.com http://www.EFFICIENTPLANTMAG.COM

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