Abbey Blessed with Phase Solution

Nuns of the Santa Rita Abbey, near Tucson, Ariz., bake bread in a special oven. Imported from France, the oven needs 3-phase power for a continuous process. These aren't just any ovens, and they don't produce just any bread. The Abbey's bread, used for Catholic Communion, is distributed all over the United States.

When the Abbey wanted to purchase the special oven in 1998, it asked its power company, the Sulphur Springs Valley Electric Cooperative, for three-phase power. Unfortunately, only a 14.4-kilovolt single-phase distribution line served this remote area. To run a three-phase distribution circuit out to the Abbey would have cost more than $200,000. As a result, other solutions were investigated.

One problem was that because of the length of the single-phase power line, any solution might cause feeder voltage dips, which could not be tolerated. The cooperative's specification was that the solution could not produce more than a 3% voltage dip.

The cooperative collaborated with a supplier to couple a 60-horsepower single-phase motor with a conventional 3-phase generator. An added refinement was that the stator of the driving motor was wound in two parts, allowing each half of the stator to be sequentially energized. The configuration permitted the start current to be kept to a tight limit that is very close to the motor's running current.

In selecting the 3-phase generator, designers made sure to choose one that could comfortably deal with transient voltages associated with the oven's three-phase load. Testing, which was conducted before shipment, verified that the motor-generator came within the specified 3% limit. The unit has the ability at full load to provide a two-second ride-through, with output frequency dropping to 55 Hertz.

From Pure Power, Winter 2001.