Compressor Windings; External Thermal Protector - Bunn Ultra Technical Training Manual

Compressor Windings and Related Component Testing
Note: Measure the resistance (ohms) between terminal pins C&S and C&R
around ambient temperature of 77° F. Add the resistance between C & S and C &
R. The sum should equal the resistance found between S & R. A 10% deviation
is acceptable.
Continuity must be present between C & S and C & R. If there is no continuity on
start or run winding, there is an interruption within that winding.
Check the C (common) compressor winding to compressor ground terminal/
housing. If continuity is present, one of the compressor windings is shorted to
ground.
Check Compressor Windings for Ground Fault:
Upon removal of the protective terminal cover, if evidence is shown by any lead
or terminal being overheated, it is a good indication that a compressor winding
problem may exist. Follow the recommended step below to check the compres-
sor winding insulation. If a ground fault is detected with the compressor, keep the
power off, thermal protector and potential relay disconnected. A licensed certified
refrigerant technician will need to replace a defective compressor.
Testing Compressor Windings/Insulation with a Megger
A megger is a preferred test over using a typical ohm meter for testing the compressor windings. The megger checks
the insulation factor of a winding making sure it is actually insulated and not leaking current (Ground Fault). An ohm
meter usually produce a low voltage for reading resistance verses a megger uses higher direct voltage source to
measure insulation resistance to detect a breakdown in the motor winding insulation.
Step 1: Disconnect all electrical power to the BUNN ULTRA-2 HP.
Step 2: Access compressor terminal pins and disconnect the wire leads going to the compressor pins.
Step 3: Set the megger to the compressor applicable voltage rating. If the megger you are using only has a few
specific voltage ranges, select the next highest voltage above your compressor volt range.
Step 4: Connect one lead of the megger to the copper suction line or housing. Connect the other lead to one of the
compressor terminal pins (winding).
Step 5: Repeat the procedure for the two remaining terminal pins. If the instrument indicates any resistance less than
2 megohms between any pin and the housing (copper suction line), a ground fault exists. Replace
compressor.

External Thermal Protector

Check for continuity across the thermal protector terminals. Before testing the
external thermal protector, ensure the thermal protector had enough time to cool
off and reset. If no continuity is present across the terminals, replace the thermal
protector.
38
Ultra Training Manual