Some breaker manufacturers and industry standards, such as IEEE 141, recommended surge protective devices on motor circuits fed through vacuum breakers. Such protection was either built into switchgear cubicles or supplied with the motors. However, changes in vacuum contractor technology no longer make this a common practice.
There can be drawbacks to vacuum contactors in motor starter applications. Its contacts, which are designed to infrequently interrupt high current, are not appropriate for the day-in, day-out switching of much lower motor starting currents. As in the combination starter, long familiar to industrial motor users, fault protection is better provided by an upstream fused disconnect switch or breaker while the on-off motor switching is handled by a smaller, simpler “contactor.”
However, conventional contactors tend to wear out through frequent usage, and require energy-consuming operating coils. The vacuum contactor was developed to reduce those disadvantages. Although it was first introduced for medium voltage motor control (2300 to 6900 V), several makes of low voltage (600 to 1500 V) units are now available for 460 V motor control. These are often used at 900 to 1000 V for mine motor service, or for oilfield submersible pumps.
Vacuum Motor Starters
Vacuum motor starters feature two major design differences from the vacuum circuit breaker. First, operation is much more frequent, although at lower current. This reduces both unit size and required operating force. Typical operating life is 1 million operations at rated current.
Second, contact design is altered by changing the contact alloy to prolong the arc when the contacts part to reduce the “current chop” effect, almost eliminating the transient voltage spike. The change of alloy could be unacceptable in a vacuum breaker because it would prolong the flow of fault current (See Fig. 3).
Replacing these starters with vacuum devices lowered cubicle temperature 30°C, cut vibration to one-fourth its former value, and eliminated starter failure for at least 4 years.
Low voltage retrofits can be equally effective. According to an early report, one set of starters for 125 to 200 hp motors was installed in an air-conditioned room. Motor starting frequency ranged from one per hour to one per week. Starter contact life was inadequate. Rapid wear, with occasional welding; required contact replacement every 8 months. A larger size air-break starter would not fit in existing cubicles.
A cost study indicated that replacement of each starter with a vacuum unit would be paid off in contact replacement savings alone within 15 months. The replacement was then complete. From then until the date of the report, no starter troubles or motor winding failures had occurred.
Other applications have highlighted the long operating life to be expected of vacuum starters.
- A metal shear, operated every 8 sec, caused air-break contacts to wear out in 2 months; the associated arc chutes needed replacement after 1 ½ years, still had 20% of its useful life remaining.
- In a Gulf Coast chemical plant, a vacuum contactor was still in good working order after 3 million operations in 3 years.
Freedom from dirt and corrosion helps lengthen contact life. In one mining application, coal dust was a problem for air-break equipment. But a vacuum contactor needed no maintenance. Even annual inspections were eventually discontinued.
Application Considerations
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.