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IEC 60252-1: AC Motor Capacitors — The Fundamental Difference Between Start and Run Capacitors
Start vs. Run Capacitors: One Pursues Instant High Current, the Other Pursues Long-Term Reliability
IEC 60252-1:2013 specifies AC motor capacitor requirements. Single-phase induction motors cannot self-start — without a rotating magnetic field, the rotor merely vibrates. Capacitors create the phase-shifted second winding current for the rotating field.
| Parameter | Start Capacitor | Run Capacitor |
|---|---|---|
| Type | Electrolytic (high capacitance) | Metallized polypropylene film |
| Capacitance | 50–300 μF | 2–50 μF |
| Duty | Start only (1–3s), then centrifugal switch disconnects | Continuous after starting |
| Key Requirement | High C, low duty, must never be continuously energized | Self-healing, low loss, long life |
The self-healing miracle: In metallized polypropylene capacitors, a local breakdown instantly vaporizes the metal coating around the fault point (like a fuse blowing), isolating the defect — the capacitor continues operating, only slightly reduced in capacitance. A run capacitor may experience hundreds of self-healing events over its lifetime. When cumulative capacitance loss exceeds 5% of nominal, the capacitor is considered end-of-life.
TN Lab — Wrong start capacitor = motor stalls. Wrong run capacitor = motor runs inefficiently and overheats.
