Constant voltage transformers (CVTs) or ferroresonant transformers produce a constant secondary (output) voltage for varying primary (input) voltages within a certain range. With CVTs, the saturation of the ferrous core induces ferroresonance, a phenomenon that increases the inductive reactance relative to the capacitance reactance. When the iron cores of constant voltage transformers (CVTs) are saturated, the relatively large changes in magnetic current cause only very small changes in magnetic flux. Since the winding current and magnetic flux are proportional to the input voltage and the output voltage, respectively, relatively large changes in input voltage cause only small changes in output voltage.
Depending upon their design, constant voltage transformers (CVTs) or ferroresonant transformers are used with either single-phase or three-phase power. CVT product specifications include operating frequency range, maximum primary voltage rating, maximum secondary voltage rating, power rating, and operating temperature. Operating frequency range is the range of frequencies over which constant voltage transformers (CVTs) meet all guaranteed specifications. The maximum primary voltage is the device’s input voltage range. The maximum secondary voltage is the output range. Power rating is expressed as the sum of the VA (volts x amps) for all of the secondary windings.
Constant voltage transformers (CVTs) or ferroresonant transformers differ in terms of approvals and features. In Europe, products may comply with the RoHS and WEEE directives or meet requirements from TUV or VDE. In North America, the American National Standards Institute (ANSI) and the Canadian Standards Association (CSA) publish applicable CVT standards. Constant voltage transformers (CVTs) that meet IEC requirements or U.S. military standards (MIL-STD) are also available. In terms of features, some constant voltage transformers (CVTs) are waterproof or include current limiting protection. Others have a NEMA enclosure or are designed for outdoor use.
Selecting constant voltage transformers (CVTs) or ferroresonant transformers requires an analysis of mounting styles and form factors as well as applications. Some devices are designed for mounting on a chassis, disk or disk, pole, or printed circuit board (PCB). Others mount on concrete pads or incorporate the RJ-45 form factor with magnetics to insure high common-mode noise immunity and signal integrity. H-frame mounting is used in environments where high vibration or shock is present. In terms of applications, most constant voltage transformers (CVTs) are used with rectifiers, power supplies, or voltage regulators. Often, ferroresonant transformers that provide voltage regulation are called constant voltage regulators, ferroresonant voltage regulators, or ferros.