When a capacitor is switched on, a very high, very short inrush current flows. A capacitor duty contactor closes early-make contacts with damping resistors first, which limit this surge, then the main contacts. Choose it by the capacitor's kVAr at your voltage.
The problem: inrush current
A discharged capacitor behaves almost like a short circuit for a moment. In a panel where other capacitor steps are already on, the inrush can reach many times the rated current. On a normal contactor this pits and welds the contacts and stresses the capacitor, shortening the life of both.
How a capacitor duty contactor works
- Auxiliary contacts close first and connect the capacitor through damping resistors, which limit the inrush.
- A moment later the main contacts close and short out the resistors.
- The resistors are then disconnected, so they do not heat up during normal running.
Comparison
| Capacitor duty contactor | Normal contactor | |
|---|---|---|
| Inrush limiting | Built-in damping resistors | None |
| Selected by | kVAr at 415/440 V | Amps (AC-3) for motors |
| Contact life on capacitors | Long | Short: pitting and welding |
| Use | APFC panels, capacitor banks | Motors and general loads |
Selecting one
Match the contactor's kVAr rating to the capacitor step it switches, at your supply voltage: for example an L&T MO C20 for a 20 kVAr step at 440 V. Choose the coil voltage to suit the APFC controller's output, typically 240 V or 415 V AC.
Products
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Values are typical guidance for Indian 230 V single-phase and 415 V three-phase supplies. Always follow the equipment nameplate, the manufacturer's instructions and your licensed electrician, who is responsible for the final selection and installation.