Troubleshooting Motors : Testing Winding Resistances

 Testing Winding Resistances on a Split Phase Motor

• Discussing winding resistances: Run winding and Start winding.
• Measure from R to C for run winding resistance.
• Measure from S to C for start winding resistance.
• Measure from S to R to get the sum of the run and start windings (e.g., 10 ohms).

Actual Compressor Measurement

• R to C and S to C labels are often under the compressor/motor cover.
• Start winding resistance is higher than run winding resistance.
• Example: 4 ohms for run winding and 6 ohms for start winding should sum to 10 ohms for S to R.

Motor Failure Categories

1. Open Winding

   • Infinite resistance in one winding indicates it's open/broken.
   • Infinite resistance across both windings indicates an internal overload.
   • Example: Single phase compressor with an open run winding shows OL/infinite resistance across R and C.
   • Ensure start winding is not on overload by checking for measurable resistance.

2. Shorted Winding

   • Shorts can be caused by acid and debris in compressors.
   • Example: Single phase compressor with shorted windings shows zero ohms across R and S.
   • Zero or close to zero ohms (e.g., 1-2 ohms) indicates a shorted winding.

3. Shorted Winding to Ground

   • No circuit should be detected from any motor terminal to ground.
   • Use the copper suction line or process stub for a good ground connection.
   • Measure resistance between each motor terminal and ground.
   • A reading of at least three million ohms is expected; anything less indicates a grounded motor winding.
   • Grounded motor winding typically causes breaker trips or fuse blows.
   • Ensure to check before replacing fuses or resetting breakers.