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Electrical System
COMPRESSOR ELECTRICAL
DIAGNOSTICS
The compressor will not start or will trip repeatedly
on overload.
Check Resistance (Ohm) Values
NOTE: Compressor windings can have very low
ohm values. Use a properly calibrated meter.
Perform the resistance test after the compressor
cools. The compressor dome should be cool enough
to touch (below 120°F/49°C) to assure that the
overload is closed and the resistance readings will
be accurate.
SINGLE PHASE COMPRESSORS
1. Disconnect power from the cuber and remove
the wires from the compressor terminals.
2. The resistance values must be within published
guidelines for the compressor. The resistance
values between C and S and between C and R,
when added together, should equal the
resistance value between S and R.
3. If the overload is open, there will be a resistance
reading between S and R, and open readings
between C and S and between C and R. Allow
the compressor to cool, then check the readings
again.
Check Motor Windings to Ground
Check continuity between all three terminals and
the compressor shell or copper refrigeration line.
Scrape metal surface to get good contact. If
continuity is present, the compressor windings are
grounded and the compressor should be replaced.
Determine if the Compressor is Seized
Check the amp draw while the compressor is trying
to start.
COMPRESSOR DRAWING LOCKED ROTOR
The two likely causes of this are:
• Defective starting component
• Mechanically seized compressor
To determine which you have:
1. Install high and low side gauges.
2. Try to start the compressor.
3. Watch the pressures closely.
A. If the pressures do not move, the compressor
is seized. Replace the compressor.
B. If the pressures move, the compressor is
turning slowly and is not seized. Check the
capacitors and start relay.
COMPRESSOR DRAWING HIGH AMPS
The continuous amperage draw on start-up should
not be near the maximum fuse size indicated on the
serial tag.
The voltage when the compressor is trying to start
must be within ±10% of the nameplate voltage.
Diagnosing Capacitors
• If the compressor attempts to start, or hums and
trips the overload protector, check the starting
components before replacing the compressor.
• Visual evidence of capacitor failure can include
a bulged terminal end or a ruptured membrane.
Do not assume a capacitor is good if no visual
evidence is present.
• A good test is to install a known good substitute
capacitor.
• Use a capacitor tester when checking a suspect
capacitor. Clip the bleed resistor off the
capacitor terminals before testing.
Diagnosing PTCR’s
See “PTCR Diagnostics” on the next page.