Vacuum Interrupters for AC Contactors

A vacuum interrupter uses a high vacuum to extinguish the arc between a pair of contacts. As the contacts move apart, current flows through a smaller area. There is a sharp increase in resistance between the contacts, and the temperature at the contact surface increases rapidly until the occurrence of electrode-metal evaporation. At the same time, the electric field is very high across the small contact gap. The breakdown of the gap produces a vacuum arc. As the alternating current is forced to pass through zero thanks to the arc resistance, and the gap between the fixed and moving contacts widens, the conductive plasma produced by the arc moves away from the gap and becomes non-conductive. The current is interrupted.

AMF and RMF contacts have spiral (or radial) slots cut into their faces. The shape of the contacts produces magnetic forces which move the arc spot over the surface of the contacts, so the arc does not remain in one place for very long. The arc is evenly distributed over the contact surface to maintain a low arc voltage and to reduce contact erosion.

Components of the vacuum interrupter must be thoroughly cleaned before assembly, since contaminants could emit gas into the vacuum envelope. To ensure a high breakdown voltage, components are assembled in a cleanroom where dust is strictly controlled.

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