A common schematic-reading mistake is to treat every capacitor as a start-only part. In Copeland's single-phase motor diagrams, a capacitor-start, capacitor-run motor has two different capacitor jobs. This is a circuit-reading exercise for 313A study, not a claim that every compressor has that circuit. Check the actual motor and manufacturer diagram before applying the example to equipment.
What happens to the start capacitor as the motor accelerates?
In Copeland's capacitor-start, capacitor-run example, the start capacitor helps provide starting torque and is intended for intermittent duty. A potential relay has contacts closed before the motor starts. As the motor approaches running speed, voltage generated across the start winding rises; the relay picks up, opens its contacts and takes the start capacitor out of the circuit. The relay senses a winding voltage condition, not a fixed number of seconds after a thermostat call.
For a paper schematic, trace the start-capacitor branch at rest, then redraw it with the relay contacts open. The start capacitor is in the first drawing and absent from the running path in the second. Copeland's start-assist note shows this sequence for specified scroll-compressor applications.
Does the run capacitor also leave the circuit?
No, not in that capacitor-start, capacitor-run design. Copeland describes the run capacitor as remaining in circuit continuously and designed for continuous duty. When the potential relay opens, it removes the start capacitor branch; the run capacitor remains with the motor's auxiliary winding. That distinction explains why “the relay opened” does not mean “all capacitors are disconnected.”
Compare this with Copeland's permanent-split-capacitor diagram: a PSC motor uses a run capacitor without a separate start capacitor and start relay. A schematic that lacks a start branch should not be diagnosed as though the relay failed. The terminal-identification guide covers C, R and S as a separate reading skill.
How would you use this distinction in a study problem?
Suppose a hypothetical question says a single-phase compressor has a start capacitor, run capacitor and potential relay. Ask which branch is present during starting and which remains after the relay opens. Then ask what observation would distinguish a motor that never starts from one that starts but runs poorly. The schematic and nameplate come before any component conclusion; a humming compressor alone does not identify a failed capacitor or relay.
This article is for interpreting a diagram, not for live electrical testing. CCOHS says capacitors can retain hazardous stored energy and directs workers to the manufacturer for the system-specific discharge method. Authorized work requires the applicable lockout procedure and verification. The 313A exam guide gives study context, and the free sample is original, unofficial draft practice awaiting certified-mechanic review.
Sources
- Copeland: Electrical Handbook, single-phase compressor motor diagramsPotential relay and start/run capacitor operation, pages 42-44; checked 2026-10-12
- Copeland: Single-phase scroll start-assist componentsRelay opening start-capacitor branch; checked 2026-10-12
- CCOHS: Lockout/Tag outStored energy and capacitor discharge guidance; checked 2026-10-12
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.