Electrical System Section 66-40 Part No. 80-1100-3COMPRESSOR ELECTRICAL DIAGNOSTICSThe compressor will not start or will trip repeatedly onoverload.Check Resistance (Ohm) ValuesNOTE: Compressor windings can have very low ohmvalues. 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 closedand the resistance readings will be accurate.SINGLE PHASE COMPRESSORS1. Disconnect power from the cuber and remove thewires from the compressor terminals.2. The resistance values must be within publishedguidelines for the compressor. The resistancevalues between C and S and between C and R,when added together, should equal the resistancevalue between S and R.3. If the overload is open, there will be a resistancereading between S and R, and open readingsbetween C and S and between C and R. Allow thecompressor to cool, then check the readings again.THREE PHASE COMPRESSORS1. Disconnect power from the cuber and remove thewires from the compressor terminals.2. The resistance values must be within publishedguidelines for the compressor. The resistancevalues between L1 and L2, between L2 and L3, andbetween L3 and L1 should all be equal.3. If the overload is open, there will be open readingsbetween L1 and L2, between L2 and L3, andbetween L3 and L1. Allow the compressor to cool,then check the readings again.Check Motor Windings to GroundCheck continuity between all three terminals and thecompressor shell or copper refrigeration line. Scrapemetal surface to get good contact. If continuity ispresent, the compressor windings are grounded and thecompressor should be replaced.Determine if the Compressor is SeizedCheck the amp draw while the compressor is trying tostart.COMPRESSOR DRAWING LOCKED ROTORThe two likely causes of this are:• Defective starting component• Mechanically seized compressorTo 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 isseized. Replace the compressor.B. If the pressures move, the compressor is turningslowly and is not seized. Check the capacitorsand start relay.COMPRESSOR DRAWING HIGH AMPSThe continuous amperage draw on start-up should notbe near the maximum fuse size indicated on the serialtag.The voltage when the compressor is trying to start mustbe within ±10% of the nameplate voltage.Diagnosing Capacitors• If the compressor attempts to start, or hums and tripsthe overload protector, check the startingcomponents before replacing the compressor.• Visual evidence of capacitor failure can include abulged terminal end or a ruptured membrane. Do notassume a capacitor is good if no visual evidence ispresent.• A good test is to install a known good substitutecapacitor.• Use a capacitor tester when checking a suspectcapacitor. Clip the bleed resistor off the capacitorterminals before testing.
