Section 6Electrical System
6-38
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.
THREE 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 L1 and L2, between L2 and L3,
and between L3 and L1 should all be equal.
3. If the overload is open, there will be open
readings between L1 and L2, between L2 and L3,
and between L3 and L1. 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.
NOTE: Scrape the 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.
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