Terminal markings wear off and diagrams go missing. Three resistance readings are enough to identify common, start and run on a single-phase compressor, and the same readings tell you whether a winding is open.
What has to happen before you measure?
Lock out the power, verify there is no voltage, and discharge the run capacitor. Remove the wires from the compressor terminals so other components are not in parallel with your reading.
How do you work out which terminal is which?
Measure all three pairs and write them down. Example:
| Pair | Reading |
|---|---|
| 1 to 2 | 3.5 Ω |
| 1 to 3 | 3.0 Ω |
| 2 to 3 | 0.5 Ω |
- The highest reading is across start and run in series (1 to 2). The terminal left out, 3, is common.
- From common, the higher reading goes to start (3 to 1 = 3.0 Ω, so 1 is start).
- From common, the lower reading goes to run (3 to 2 = 0.5 Ω, so 2 is run).
Check: common-to-start plus common-to-run should equal start-to-run. Here 3.0 + 0.5 = 3.5. If the math does not work, recheck your connections.
What do bad readings look like?
- Open winding: infinite resistance on the pairs that include it. Many compressors have an internal overload between common and the windings; a hot compressor with an open overload reads open until it cools, so let it cool before condemning it.
- Shorted winding: a reading much lower than the manufacturer's specification.
- Grounded winding: any reading from a terminal to the shell (on clean metal) indicates a ground. A standard ohmmeter may miss a weak insulation fault; an insulation resistance tester gives a more reliable answer.
How does the run capacitor fit in?
On a PSC compressor, the run capacitor connects between the run and start terminals. Measure it with a capacitance meter, compare it with the rating and tolerance printed on the case, and replace it with the specified value if it is out of range. A weak capacitor reduces torque and raises current.
Where is this on the exam?
Component testing is sub-task F-15.03 and troubleshooting is F-16.01 in the Red Seal task matrix. Maintenance and service (F) is the largest block of the exam at 25%.
Sources
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.