Vacuum Interrupter for Contactor

Gradual contact wear does occur in a vacuum starter, of course, because the small arcs at each operation vaporize some contact material. Since the contacts themselves are invisible within the individual phase chamber or bottle, some external means of checking wear must be provided—usually by the position of the moveable contact arm or rod projecting from each bottle, when the contacts are closed. A feeler gauge measurement of arm travel reveals the slight change in arm position caused by wear. Typical contactor maintenance instructions prescribe that check at each half millionth operations.

Another cause for concern with any vacuum device is bottle leakage, with subsequent loss of vacuum. Destructive arcing could result in the contacts then opened under load. However, loss of vacuum is more than one of the three-phase bottles is highly unlikely. And if air is present in only one, when the contactor is closed, safe circuit interruption can still be carried out by the other two.

The importance of atmospheric pressure is the basis for the altitude rating of a vacuum contactor. At too high an elevation, air pressure is too low for proper operation. But standard contactors are typically usable up to 10,000 ft. Learn more about how to specify a vacuum contact for high altitude here.

Loss of vacuum can be checked by applying a 1 min overvoltage test across the open contacts of a de-energized starter. The small gap between contacts will break down if air is present. Ask the contactor manufacture for the correct test voltage.

Although the main contacts themselves are remarkably maintenance-free, remember that the complete starter must still include an operating coil, often with an associated rectifier, plus rods, springs, pivots, and all the other mechanical components used with any starter. Those remain exposed to moisture and dirt. Fasteners may loosen, or linkages bind. And auxiliary contacts for low voltage control circuits are also vulnerable.

With those limitations in mind, the user of vacuum motor starters can take full advantage of the benefits of such equipment—especially in hot, dirty, crowded surroundings, with frequent operation and fluctuating load.