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The difference is how long they stay in circuit: the run capacitor remains connected with the auxiliary winding for as long as the motor runs; the start capacitor works only for a few seconds while the motor accelerates and is then disconnected.
Comparison
| Feature | Run capacitor | Start capacitor |
|---|---|---|
| Time in circuit | Continuous | Only at start-up |
| Capacitance | Relatively low | Relatively high |
| Construction | Usually film (dry) type | Usually electrolytic type |
| Disconnecting element | None | Centrifugal switch or start relay |
| Contribution | Smooth running, balanced current | High starting torque |
The table describes the general construction; capacitance and voltage values are specific to each motor and the value on the motor nameplate prevails.
Which motor has which?
For light-starting jobs such as fans and circulation pumps, a run capacitor alone is often enough. Compressors starting against pressure, booster pumps or machines starting loaded use both: the start capacitor raises the torque, the run capacitor keeps the motor running.
Why can they not replace each other?
A start capacitor is not built to stay in circuit; if it is not disconnected it soon overheats, swells and can burn the auxiliary winding. Conversely, if a run capacitor is fitted instead of a start capacitor, the motor may fail to start under load. A stuck centrifugal switch causes the same overheating.
For the general function of the capacitor, see what the capacitor does in a single-phase motor. For loaded starts, review the single-phase 220 V electric motor options and see our article on single-phase motor prices. To find out which arrangement your motor has, send photos of the nameplate and terminal box on WhatsApp.