Test tool for
electrical motors
The four workhorses of motor
troubleshooting and maintenance Technology
at Work
ratio*, such as the Fluke IR66 or
68, is the only safe and efficient
way to go.
Sometimes, the temperature
youre sensing may result from
current unbalance. In that case,
reach for a clamp meter like the
Fluke 337, and trace the cause
because it could be one of
several different factors.
And to detect the most subtle
of all problems insulation
resistance and degradation
nothing beats a MegOhmMeter
like the Fluke 1520. If a currently
functioning motor is about to go
down, your MegOhmMeter can
tell you before its too late.
When the need arises to make
live measurements in a 3 phase
panel observe the following safe
work practices:
1. Wear the
appropriate
Personal
Protective
Equipment
(PPE) for the
measurement at
hand and the
environment.
2. Make sure your
test instrument
is rated for the measurement
environment. In this case
Category III 600V or higher
would be appropriate.
3. Use the three point test
method.
AC electrical motors consume a variety of measurements, from 4. Generally, keep one hand in
40 % of the power generated in checking if power is present at your pocket unless you must
the US. Imagine the overall trou- all legs of a three-phase motor, to use both hands to obtain a
bleshooting and maintenance checking the motor start capacitor proper measurement.
requirements for that much of a single-phase motor, or even
power consumption! To stay on to measuring heat. This application note describes
top of it all, maintenance electri- But to measure temperature on how to use these tools to trou-
cians rely on four principle tools. a large number of motors and bleshoot and monitor four
When problems arise, the first related instruments especially principle indicators: temperature,
tool of choice is usually a digital in live electrical connections a current, voltage, and insulation
multimeter (DMM), such as the non-contact infrared thermometer resistance.
Fluke 87V. A good DMM can take with a large distance-to-spot *Distance to spot ratio calculates from how far
away an infrared thermometer can accurately
measure a particular target area.
From the Fluke Digital Library @ www.fluke.com/library
Temperature Another problem to watch for on The best measurement tool to
a long-term basis is potential detect single phasing is the
A rise in temperature is a good bearing failure. A shaft bearing clamp meter: a zero current
indicator that somethings wrong. will run hot for days perhaps measurement in one phase is a
An infrared thermometer makes before full failure. To avoid clear indication of power loss.
quick temperature comparisons a replacing the entire motor, However, a voltage measurement
snap; press the trigger to take the regularly measure the bearing on the same phase can be mis-
first measurement, save it, refocus housing and compare the result leading, due to induced current
on the second target, and meas- to the baseline temperature. (See from the two remaining live
ure again. One switch running Fluke Application Note 2278935 phases. A 150V ghost voltage
30 degrees hotter than the others for detailed instructions on infra- reading around two 240VAC live
is a sure sign the contacts are red thermometer troubleshooting.) circuits is not uncommon.
failing. To verify whether a fuse is
Current blown, removed the fuse from the
Another root cause for motor circuit and use the resistance
overheating is current unbalance. function on a DMM or a clamp
Use an ac current clamp meter to meter to check the fuse. Once the
check the current draw on each fuse has been removed from the
of the three legs. To determine circuit place the multimeter or
average current, sum the current clampmeter in the ohms () func-
from all three phases and divide tion. Place the test probes at each
by three. Then, calculate the per- end of the fuse. A good fuse will
cent unbalance by subtracting measure as a very low resistance
the actual on one leg from the (less than 10 ohms). An open fuse
average. will show as an OL in the meter
Peeling the onion back further, display.Check and double check
current unbalance can be caused your measurements: many trou-
by several different sources: a bleshooters have been fooled by
power delivery problem, low assuming they know the real
voltage on one leg, or an insula- cause. Dont jump to a conclusion
tion resistance breakdown inside before testing all circuit elements.
the motor windings. In the maxi-
mum case of unbalance, single
phasing, one entire phase is lost
and the fuse blown.
average amps = ph1 amps + ph2 amps + ph3 amps
3
% unbalance on phase 1 = ph1 amps - average amps
x 100
Using an infrared thermometer to quickly average amps
measure bearing heat.
Motor case temperature is Example
another good place to start trou- 1. Measure all 3 phases, add together, and divide by 3.
bleshooting, but you have to
know what is normal, to know 9.9 + 10 + 10.1
= 10
whether your new reading is 3
unusual. Therefore: make a habit
of taking baseline temperature 2. Calculate percent unbalance on phase 3.
measurements of motors when
they are newly installed and run- 10.1 10
x 100 = 1 %
ning well, and then repeat that 10
measurement on a regular basis.
2 Fluke Corporation Test tools for electrical motors
Voltage
When checking for voltage problems, the multimeter is again the tool of choice. First,
check the protection and switchgear for voltage drops. If you dont detect anything
there, you may have a utility delivery problem. A greater than 2 percent unbalance
in power delivered from the utility can harm your motors. Voltage unbalance can be
calculated with this formula
average volts = ph1 volts + ph2 volts + ph3 volts
3
% voltage unbalance on phase 1 = ph1 volts - average volts
x 100
average volts
However: loads do change, and a phase can suddenly be 5 percent lower on one leg.
Voltage drops across the fuses and switches can also show up as unbalance at the
motor and excess heat at the root trouble spot. Before you assume the cause has been
found, double check with a few different tools. In this instance, the IR thermometer
complements both the multimeter and the clamp meter current measurements.
Insulation Resistance Use individually or in
Good
By measuring the resistance of combination
Insulation
motor windings from each other, To summarize, the four basic tools
you can detect deterioration due for motor troubleshooting and
to age, corrosion, dirt, moisture maintenance are:
and excessive vibration, before A digital multimeter to check
Resistance
the motor fails. Disconnect the voltage, resistance and capaci-
(in Megohms) motor from power and then hook tance
up a MegOhmMeter across the A clamp meter to check
Cracked
windings. To measure high current draw
Insulation resistance, apply a high voltage An infrared thermometer for
up to twice the working volt- temperature affects
age. On a 480 V motor for A MegOhmMeter for insulation
example, youd apply 1000 V. resistance measurements
0 Time 10 min Take your reading in megohms. Used in combinations, these
(in minutes) For a 240-480 V rated motor, four tools allow technicians to
300 K ohms is the minimum double-check symptoms and trou-
acceptable resistance. bleshoot the root cause of almost
Since insulation resistance any electrical motor failure.
1000
Aging of After varies with temperature and
Insulation Rewinding
500 humidity, you may need to take
several resistance measurements
over a period of time to get an
100 accurate result. Periodic insula-
Initial tion resistance measurements
Resistance 50 Acceptance
(in Megohms) Test will tell you how healthy your
motors are, and when to replace Fluke. Keeping your world
or re-wind them. up and running.
10
5 Insulation
Failure Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
1995 1996 1997 1998 1999 2000
For more information call:
Year
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa (31 40) 2 675 200 or
Fax (31 40) 2 675 222
In Canada (800) 36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com
2004 Fluke Corporation. All rights reserved.
3 Fluke Corporation Test tools for electrical motors Printed in U.S.A. 9/2004 2395981 A-US-N Rev A
