Refrigeration & Air Conditioning Technology 7th Edition

Unit 41 Troubleshooting 1213
3. Check the run winding for correct resistance from
common to run. If the correct resistances are not readily
available, make sure that the start winding and run
winding have different resistances. The start winding
should have much more resistance than the run winding.
Either winding can have some shorted turns in the
motor. Figure 41.31 shows a start winding with shorted,
sometimes called shunted, winding. Figure 41.32 shows
a start winding that has an open circuit.
4. Check for continuity from the run to the common terminal.
If this circuit is open, the winding thermostat may
be open. If the compressor is hot to the touch, let it cool
THE MOTOR TERMINALS ARE INSULATED
WHERE THEY PASS THROUGH THE
COMPRESSOR HOUSING.
RUN
COMMON
START
SOME OF THE WINDINGS ARE
TOUCHING EACH OTHER AND
REDUCING THE RESISTANCE
IN THE START WINDING.
Figure 41.31 A compressor with a shorted, or shunted, winding.
R
C
S
THE START WINDING
HAS AN OPEN CIRCUIT.
Figure 41.32 A compressor with an open start winding.
© Cengage Learning 2013
© Cengage Learning 2013
R
C
S
OPEN CIRCUIT
HEAT
BE SURE POWER IS OFF.
Figure 41.33 Cooling a compressor with water.
RUNNING WATER IS
BEING APPLIED TO
A HOT COMPRESSOR.
for several hours or cool it with water, Figure 41.33.
SAFETY PRECAUTION: Make sure all power is off and do
not stand in water when electrically checking any circuit.•
5. Check the voltage from common to run and from
common to start using two different voltmeters while
starting the motor. This voltage must be within 10%
of the motor’s ratings. For example, many motors are
rated at 208/230. The minimum voltage would be
187.2 V (208 0.90 187.2) and the maximum voltage
would be 253 V (230 1.10 253). This voltage
must be verified as close to the compressor terminals
as possible, but do not use the terminals themselves.
Follow the wiring back to the first junction, such as the
load side of the compressor contactor. It is not unusual
for a loose connection to create a low-line-voltage
problem at the compressor. If there is any discoloration
at the motor terminals, suspect a loose connection.
SAFETY PRECAUTION: Never start a compressor
with the motor terminal cover box removed. If a motor terminal
were to blow out while starting, there is the potential
for a high-velocity blowtorch-type of fire. The electrical arc
may ignite the escaping oil with the refrigerant, propelling
it outward. The box and cover are built to enclose this type
of malfunction.•
The steps for checking a three-phase compressor electrically
are as follows:
1. Check the compressor from all windings to ground
using either an ohmmeter or a megohmmeter as described
previously in step 1. Pay attention to the Safety
Precautions.
2. Check each set of windings from winding to winding—
T1 to T2, T1 to T3, and T2 to T3—on a three-lead motor.
Some larger motors may be dual motors in the same
housing; check both motors. There should be the same
resistance across each motor winding on a three-phase
motor. There should be no circuit from motor to motor
in a dual motor application.
© Cengage Learning 2013