Ferroresonant Regulators

Ferroresonant voltage regulators (sometimes called Ferros) use a principle called saturation to regulate voltage. Saturation is a simple idea. A transformer works by passing current through the windings of the primary, creating electromagnetic flux which induces current into the secondary windings. As current increases in the primary, the total amount of flux in the core increases, thereby increasing the current in the secondary. There is a point at which additional current will not cause more flux. When this happens, no more current can flow from the primary to the secondary of the transformer, no matter how much more current is added to the primary. This is the saturation point.

Ferros are designed to reach saturation quickly. This prevents drastic increases in current from passing to the secondary and then on to sensitive equipment.

Two major benefits of ferros are that they greatly reduce both common mode and normal mode noise. As with all transformers, ferros have a neutral-to-ground bond in the secondary that virtually eliminates common mode noise. Normal mode noise is greatly reduced in ferros because it cannot pass through the saturated transformer core.

But there are several disadvantages to ferros. One of the most important of these is that they are high-impedance devices. This means that ferros react more slowly to demands for current than do other types of transformers. Most manufacturers build additional capacity into their devices to handle this. But, if the ferro is not sized properly, or otherwise meets a demand that it cannot handle, then loads downstream could be starved for power. This is especially true for loads with high inrush currents, like computers.

Another major problem is harmonic distortion. Ferros produce what are called square waves. That is, the wave is clipped at the top and flatter on the sides than a true sine wave. While most manufacturers add filtering devices to the ferro to restore the waveform, these do not always work perfectly. The result is distorted current that can cause problems downstream for sensitive equipment, especially computers.

While the output current of ferros can be distorted, the input current must be a clean 60-hertz sine wave. Even minor variations in the input waveform can cause the transformers magnetic field to collapse. Potential users should be wary if they already have problems with current distortion at their facility, or if they have backup power generators. These generators sometimes produce distorted current that could cause field collapse.

A problem for some potential users is their size and weight. Ferros tend to be big and heavy because of the size of the transformer and the amount of steel needed for the ferro to reach saturation. Ferros also make audible noise and produce heat. Both of these attributes can be problems for most commercial users, as well as for many industrial ones. Ferros are most often used for linear loads in locations where their size, heat generation, and noise level can be tolerated.