The Automatic Voltage Regulator – AVR:
Guide and Comparison

Advantages

The ferro voltage regulator, by its unique design, provides good isolation of load equipment from line noise and surges. The output voltage regulation is also smooth so long as the manufacturer has taken steps to minimize the distortion that is caused by operation in the saturation region. Ferro voltage regulators can be very durable with some manufacturers offering warranties in excess of 10 years.

One unusual operating characteristic of the ferro voltage regulator is that its design can limit the amount of current to the load. When is load current exceeds about 150% of rated current of a ferro voltage regulator, the unit is “pulled” out of the saturation region, the output voltage collapses to a very small value and the current is thereby limited. In some applications, this characteristic is intentionally used to protect loads from high currents, however in other applications this characteristic becomes a substantial disadvantage.

Disadvantages

The principle of magnetism that affords the ferro voltage regulator its voltage regulation capability is also its Achilles heal. Voltage regulators of this type can have electrical efficiencies up to 92% at full load and ideal conditions but at lighter loads the efficiency drops off substantially. The graphic at the right shows a typical range of ferro efficiency versus percent load. It can be seen that at 50% load, the ferro voltage regulator efficiency ranges from 75 to 85%. As a comparison, a typical power transformer would have an efficiency of 90% or more at 50% load.

In applications with high overload or inrush currents, it is common that the constant voltage transformer be oversized by 2 to 3 times the rated load current. This is to permit the ferro voltage regulator to pass enough current before the output voltage collapses as described above. However, oversizing the constant voltage regulator greatly reduces its efficiency. The scales inserted in the lower right of the graphic demonstrate what happens to efficiency when a constant voltage regulator is oversized by a factor of 2 or 3 times the load: at 3X the efficiency even at 75% load only reaches 60 to 75%.