On a good single-phase compressor, which winding terminals have the highest resistance?

On a good single-phase compressor, which winding terminals have the highest resistance?

• A. Start to Ground
• B. Run to Ground
• C. Run to Start
• D. Run to Run

Answer: (C)

The winding arrangement in a single-phase motor consists of a run winding and a start winding. The run winding carries current continuously, so its DC resistance is relatively low. The start winding has a higher resistance and is only used to kick the motor into motion during startup. When you measure between the terminals where the run winding and start winding connect, you’re effectively measuring the two windings in series, which adds their resistances together. That combined path yields the largest resistance among the possible terminal pairs. Paths to ground are insulation paths and typically show a very high resistance, not the winding resistance, and measuring Run to Run gives just the run winding’s resistance, which is lower. So the highest resistance appears between the run and start terminals.

The winding arrangement in a single-phase motor consists of a run winding and a start winding. The run winding carries current continuously, so its DC resistance is relatively low. The start winding has a higher resistance and is only used to kick the motor into motion during startup. When you measure between the terminals where the run winding and start winding connect, you’re effectively measuring the two windings in series, which adds their resistances together. That combined path yields the largest resistance among the possible terminal pairs. Paths to ground are insulation paths and typically show a very high resistance, not the winding resistance, and measuring Run to Run gives just the run winding’s resistance, which is lower. So the highest resistance appears between the run and start terminals.