Application Note

3 Fluke Corporation Troubleshooting the HVAC/R refrigeration cycle for mechanical problems
you to detect problems that may
go unnoticed with a single sys-
tem check.
For instance, in a refrigerated
space it is important to ensure
that temperature variations are
minimized. Temperature varia-
tions may result from changes in
load or ambient conditions that
occur over periods of time, so
constant monitoring is called for.
By recording minimum and max-
imum temperatures in key loca-
tions over a period of time you
can be sure that air circulation
and refrigeration capacity meets
the application requirements.
Digital recording thermome-
ters allow you to record mini-
mum and maximum
temperatures over extended
periods of time. Temperature
values can be viewed at any
time by pressing the view but-
ton (recording still continues). If
the HOLD button is pushed, the
recorded MIN/MAX values are
saved and recording stops. The
data is saved until the user
selects a different input or turns
off the instrument. Some DMMs
can also measure MIN/MAX of a
single temperature, plus they
offer the benefit of a 100-hour
relative time stamp to record
when the MIN/MAX occurred.
When selecting a digital record-
ing thermometer look for a
model that can record hundreds
of temperature samples so you
get precise measurement. Addi-
tional features that are helpful
include a time stamp feature,
operator inter
val settings and
dual channels to rec
ord two
temperatures and the same time.
With this type of device you can
record temperature difference
across a coil for extended peri-
ods of time. This feature is espe-
c
ially handy for troubleshooting
erratic problem areas of the
HVAC equipment where time
limitations do not allow the
technic
ian to wait until the
problem occurs.
Compressor valve
performance test
T
o test small hermetic and
semi-hermetic compressors used
for medium and low temperature
applications, the follow
ing
method can b
e used to test
for internal valve leakage.
(see figure 2)
•
Attach a pressure/vacuum
module to a DMM and set the
module to cm/in Hg.
•
Connect the module at the
suction line service port.
•
Close the compressor off from
the low side of the system by
front seating the suction
service valve.
•
Run the compressor for two
minutes.
•
Turn off the compressor and
observe the reading.
The compressor should have
pulled down to at least 16"
(410 mm) of Hg. If the vacuum
reading starts weakening toward
10" (254 mm) of Hg vacuum, the
discharge valves of the compres-
sor may be leaking and will
probably need to be replaced. If
the compressor doesn’t pull a
vacuum below 16" Hg, the suc-
tion valves are weakening and
may need to be replaced. If the
c
ompressor is welded or hermet
-
ically sealed and these c
ondi-
tions exist, a new compressor is
the only possible remedy
.
Caution: Whenever replacing a
compressor with faulty valves,
be sure to diagnose the com-
plete refrigeration system before
and after a new compressor is
installe
d to avoid r
epeated com-
pressor failures.
Troubleshooting compres-
sor electrical motor faults
A clamp meter is a great tool for
troubleshooting electrical motor
faults, especially meters
designed to accurately measure
b
oth ac voltage and ac current
.
These meters allow current to be
measured without breaking into
the electrical circuit. A compres-
sor failure is often caused by an
electrical fault. To check the
compressor for electrical prob-
lems, remove the electrical
terminal cover and check the
following external connections.
1. Check line voltage at the load
center with the compressor
off. Low line voltage causes
the motor to draw more cur-
rent than normal and may
result in overheating and pre-
mature failure. Line voltage
that is too high will cause
excessive inrush current at
motor start, again leading to
premature failure.
2. Check line voltage at the
motor terminals with the
compressor running. The volt-
age should be within 10 per-
cent of the motor rating.
3. Check running current. The
readings should not exc
eed
manufacturers full load rated
amps during heavy load peri-
ods
. L
ow amps are normal
during low load c
onditions
.
Excessive high current may
b
e due to shorted or
grounded windings, a bad
capacitor, a faulty start relay,
or an indication of excessive
bearing fatigue.
Caution: When performing elec-
trical measur
ements on com
-
pr
essors with internal thermal
motor protection devices that
have b
e
en running extr
emely
hot, be sure to give the compres-
sor time to cool down prior to
the electrical test. This will allow
the device to reset to its normal
position.
Figur
e 2. Suction pressure.
Use a multimeter and a pressure module to quick
ly and
ac
curately determine suction pressure.
Compressor
METRIC
ENGLISH
kPa psi
cmHg in Hg
OFF
PV350
PRESSURE /VACUUM MODULE
ZERO