Application Note

4 Fluke Corporation Troubleshooting the HVAC/R refrigeration cycle for mechanical problems
Troubleshooting compressor
electrical motor failures
caused by refrigeration
system problems
Occasionally defective compres-
sors with electrical winding fail-
ures are diagnosed by a service
technician as caused by an elec-
trical system problem. However,
mechanical system failure or
inferior installation and service
practices often cause compressor
electrical problems. These prob-
lems include:
1. Poor piping practices resulting
in oil not adequately return-
ing to the compressor during
the run cycle.
2. High discharge temperatures
creating acids in the oil.
3. Insufficient air flows across
the evaporator and condenser
coils.
4. Extremely low suction
pressures.
5. Liquid refrigerant flooding
back into the compressor.
Diagnosing these refrigeration
system problems and avoiding
compressor failure can be
done effectively using DMMs,
clamp meters, digital thermome-
ters, pipe clamps, infrared
thermometer and pressure/
vacuum modules.
Here are some simple proce-
dures to diagnose these
refrigeration problems:
1. Compressor bearings can fail
or lock up due to poor piping
practic
es, which causes oil
clogg
ing in the system and
results in insufficient oil return
to the compressor. If the
bearings don’t lock-up and
continue to wear during these
conditions, the rotor will
lower into the stator housing,
shorting out the w
indings.
To diagnose this problem,
measure the c
ompressor
amps. They should not exceed
the manufacturer’s full load
ratings. Worn bearings will
cause higher than normal
amps. Inspect the oil level via
the c
ompressor sightglass
.
If there is no sightglass, use
your infrared thermometer to
measure the sump of the c
om
-
pressor housing
. The oil level
can be detected with the tem-
perature probe. The sump
temperature will be different
on the compressor housing at
the oil level
.
Caution: Whenever an oil prob-
lem exists due to poor piping
practices, the correct remedy is
to fix the piping, not to continue
to add more oil to the system.
2. High discharge temperatures
are caused by high head
pressures or high superheat.
The compressor discharge line
can be measured quickly
using the infrared thermome-
ter on a dull section of pipe.
Measure the discharge pres-
sure using a pressure/vacuum
module. Convert the refriger-
ant pressure to temperature
and compare it to the ambient
air temperature. If there is a
temperature difference greater
than 20-30 °F (11-17 °C),
there is either non-condensi-
ble gases in the system or
restricted airflow across the
condenser.
Note: Temperature differences
will vary due to original manu-
facturer’s design and efficiencies.
3. Check for insufficient airflows
across the evaporator using a
digital thermometer. (see fig-
ure 3) Place a bead thermo-
couple on the discharge side
of the coil and on the return
side of the coil. Record the
temperature difference on the
air conditioning unit. Expect
about 18-22 °F (10-12 °C)
temperature difference. On
refrigeration units expect
about 10-15 °F (5-8.5 °C)
temperature difference.
Note: Temperature differences
may vary depending upon initial
de
sign and humidity requirements.
4
.
Extremely low suction pres
-
sures can b
e checked using
the pressure/vacuum module
and your DMM. Install it at the
compressor and record your
suction pressure. Convert
the refrigerant pressure to
temperature using a pressure
temperature (PT) chart.
Measure the retur
n air tem
-
perature b
efore the evapora
-
tor. Compare the refrigerant
temperature to the desired
evaporator return air tempera-
ture. On air conditioning units,
expect ab
out 35-40 °F
(19-22 °C) temperature differ-
ence and refrigeration units
expect
ab
out 1
0-20
°
F (5-11 °C)
temperature differenc
e.
5. Check for liquid refrigerant
flooding back to the compres-
sor by determining the super-
heat using your pressure/
vacuum module and pipe
clamp, along with your DMM.
Check suction pressure and
convert the refrigerant pres-
sure to temperature, using
your PT chart. Measure the
suction line pipe temperature.
Compare the difference of the
two temperatures. If there is
no temperature difference,
then you are bringing back
liquid to the compressor.
If there is a temperature
difference between 10- 20 °F
(5-11 °C), then you have
normal superheat and you are
not slugging the compressor
with unwanted liquid.
Figure 3. ∆T across coils. Record evaporator temperature
differences as well as minimum/maximum temperatures
with a digital thermometer.
Evaporator
Air flow
T1
T2
T1-T2
SETUP
ENTER
CANCEL
MIN
MAX
HOLD
˚C˚F K
52
THERMOMETER
F
TYPE
K
T2
HOLD
T1
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B
.V
.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
F
or more information call:
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ax (9
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ax [phone]
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ess: http://www
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©2005 Fluke Corporation. All rights reserved.
Printed in U
.S.A. 2/2005 2434058 A-US-N Rev A
Fluke. K
e
eping your world
up and running.
Summary
Troubleshooting and ser
vicing refrigeration air con-
ditioning and heat pump systems is a challenge for
any technic
ian, entr
y level or experienc
ed
. Regard
-
less of the size or location of the system, it is
imperative that the ser
vice technician understands
the principles and the tools needed to perform
proper troubleshooting efficiently.